Advances in hybrid rice technology

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ADVANCES IN HYBRID RICE TECHNOLOGY

Proceedings of the 3rd International Symposium on Hybrid Rice

Edited by S.S. Virmani, E.A. Siddiq, and K. Muralidharan

Introduction: IRRI's role and vision for hybrid rice

G.H.L. Rothschild

Achieving self-sufficiency in rice production and maintaining price stability are important objectives in low-income countries, where rice as the staple food provides the basis for national food security and generates employment and income for poor people. Asia produces and consumes 90% of the world's rice. Most rice-growing countries in this region have done remarkably well in meeting their rice needs over the past three decades using green revolution technologies. But the future poses a major challenge. By 2030, the world must produce 70% more rice than it produced in 1995 to meet demand created by increasing populations and rising incomes. This production increase must be achieved on less land, with less labor, less water, and less pesticide, and it must be sustainable. Increasing the yield potential of rice varieties is considered an important strategy for meeting this challenge.

In the 1970s, Chinese scientists amply demonstrated that the use of hybrid rice could increase rice yields in China by 15-20%. Hybrid rice is now used extensively in China. But hybrid rice from China was neither adapted to tropical conditions nor was it available freely to countries outside China. Therefore, IRRI began research in 1979 to explore the potential of adapting this technology for the tropics. Soon, it was concluded that hybrid rice offered an important option to increase varietal yields in the tropics. This encouraged several tropical rice-growing countries to develop this technology either independently or in collaboration with IRRI and/or China. Currently, 17 national programs are involved in developing hybrid rice: Bangladesh, Brazil, Colombia, Egypt, India, Indonesia, DPR of Korea, the Republic of Korea, Japan, Malaysia, Myanmar, Pakistan, the Philippines, Sri Lanka, Thailand, the United States, and Vietnam. Hybrid rice research programs in national agricultural research systems (NARS) are in different stages of development. Although China has the strongest national network in hybrid rice, programs in Brazil, India, Japan, and the United States are well established. Other countries may take 3-5 yr to develop such programs. Several private companies in Brazil, India, Japan, and the United States have developed a strong research base in hybrid rice while some are still developing this and others are involved solely in seed production activities.

Hybrid rice technology in India: problems and prospects

R.S. Paroda

Rough rice production in India has exceeded 100 million t annually since 1988. Total production in 1995 was almost 122 million t, with yield averaging 2.9 t ha-1. Irrigated rice yields in northern India can reach 5.5 t ha-1. But the future poses challenges. Despite the rice yield increases of the 1970s and 1980s and the stable production achieved in the 1990s, the yield ceiling of irrigated rice must be raised again. This production increase must be achieved from less land, with less labor, less water, and fewer pesticides. It must also be sustainable. Productivity in less-favorable rainfed environments and in rice-based systems must also be increased, while protecting the environment and the natural resource base. Simultaneously, rice production must be made profitable for farmers so that they do not join the rapidly expanding, highly explosive communities of urban poor. To meet this challenge, India-IRRI collaboration, with sponsorship from the United Nations Development Programme (UNDP) and technical support from the Food and Agriculture Organization of the United Nations (FAO), has led to the commercialization of hybrid rice technology in India. About 60,000 ha were planted to rice hybrids in 1996, boosting the average yield ha-1 by 15% over that of today's modern varieties. This chapter discusses the problems and prospects of hybrid rice cultivation in India and the strategies adopted to increase production.

Prospects for hybrid rice in tropical Asia

P.L. Pingali, M. Morris, and P. Moya

Demand for rice in Asia continues to rise, although the supply has begun to level off as productivity gains from Green Revolution technologies show signs of becoming exhausted, especially in irrigated zones. Without an immediate shift in the yield frontier for rice and increased rice production, many countries risk that rice supplies will not keep up with demand. Recent breakthroughs in tropical hybrid rice technology provide some hope for sustaining future production growth. This chapter assesses the economic potential for hybrid rice in tropical Asia. The objective is to examine the determinants of and constraints to hybrid rice adoption, taking into account technical, economic, and institutional factors. Lessons are drawn from the only successful case of hybrid rice adoption-in China-and implications are spelled out for the likely future dissemination of hybrid rice technology elsewhere in Asia. In addition, the development of the global maize seed industry is scrutinized for patterns that may be relevant to the emerging hybrid rice seed industry. The chapter also discusses a set of key policy issues that will have to be addressed if hybrid rice is to realize its potential to significantly increase rice production in tropical Asia.

Hybrid rice breeding in China

L.P. Yuan

From 1976 to 1995, hybrid rice helped China to increase rice production from 129 million t to 200 million t annually. Hybrid rice varieties yield on average 6.6 t ha-1 compared with 5 t ha-1 for conventional rice varieties. Hybrid rice seed production technology has been well developed to achieve a nationwide average seed yield of 2.4 t ha-1. Most existing commercial rice hybrids belong to the category of intervarietal hybrids based on the CMS system. Future emphasis is on developing two-line rice hybrids using PGMS and TGMS systems. To increase the yield potential of hybrid rice in China, emphasis is given to intersubspecific hybrids (indica/japonica, indica/javanica, and japonica/javanica). The discovery of QTLs for yield in wild rice species has opened up a new avenue for raising the heterosis level by using distant genes.

Hybrid rice research and development in the tropics

S.S. Virmani

Hybrid rice technology has enabled China to increase its rice production significantly during the past 20 years. This technology also offers an economically viable option for increasing rice yields beyond the level of semidwarf inbred rice varieties in the tropics. During the past 16 years, IRRI, in collaboration with several NARS, has developed this technology for the tropics and helped India, Vietnam, and the Philippines to start commercializing it. Several other countries in the tropics are also developing this technology. By the year 2000, about 3 million ha are expected to be covered with rice hybrids, which should produce about 3 million t of extra rice (worth $450 million) annually. This chapter highlights the current status of research and development, the major constraints experienced, and strategies to expedite the development of hybrid rice technology in the tropics.

Ushering in an era of hybrid rice in India

E.A. Siddiq and M. Ilyas Ahmed

Using a goal-oriented and time-bound research network at 12 centers across the country launched in 1989, four public-bred rice hybrids were released in India in 1994. Later, three more public-bred hybrids were released. The private sector also markets more than six other hybrids. These hybrids have a convincing yield advantage of about 1 t ha-1 over the highest-yielding inbred cultivars with similar maturity duration. The additional yield of hybrid rice results in an additional net profit of US$100 (Rs. 3,500) ha-1. During the 1996 wet season, hybrid rice was planted on 50,000 ha. Because hybrid rice seed production technology has been optimized over the past 5 years, seed yields of 1.5-2.0 t ha-1 are now being obtained. Five public-sector seed agencies and about 10 private seed companies are engaged in large-scale hybrid rice seed production. In the 1995-96 dry season, 1,300 t of hybrid seed were produced. Having pure seed of parental lines available for public-sector seed agencies and obtaining the proper synchronization of parental lines are some of the problems encountered in large-scale seed production. To sustain hybrid rice technology, attempts are being made to enhance heterosis by using diverse parental lines, developing cytoplasmic male sterile lines with better outcrossing traits, incorporating resistance to major pests and diseases, and improving cooking and eating quality characteristics. Efforts are also under way to develop hybrids suitable for northwestern India--basmati rice hybrids and hybrids suitable for the favorable shallow lowland ecosystem. The large-scale adoption and further spread of hybrid rice depends primarily on the economic attractiveness of the technology. This chapter considers various issues, options, and strategies for future research and development and discusses prospects for the large-scale adoption of hybrid rice technology in India during the 21st century.

Using tropical japonica germplasm to enhance heterosis in rice

G.S. Khush, R.C. Aquino, S.S. Virmani, and T.S. Bharaj

The current level of heterosis (15-20% or 0.75-1.0 t ha-1) in indica rice hybrids developed for the tropics is economically viable, but a higher level would be more attractive. Studies at IRRI have showed a higher heterosis for yield in tropical japonica/indica crosses than in indica/indica crosses. Increased heterosis for yield in tropical japonica/indica crosses is possible only if they exhibit normal spikelet fertility. Several tropical japonica rice cultivars that possess the wide compatibility (WC) gene have been identified. Suitable parental lines in improved tropical japonica germplasm possessing the WC gene are being bred at IRRI. These parental lines would help to develop heterotic tropical japonica/indica hybrids. We discuss the progress made in improving tropical japonica germplasm and strategies used at IRRI to develop tropical rice hybrids with enhanced yield heterosis.

The wide compatibility system: current knowledge of its genetics and use for enhanced yield heterosis

H. Ikehashi and J. Wan

The genetic basis for hybrid sterility is presented with methods to analyze it. Several independent loci have been identified that cause partial abortion of female gametes carrying an allele in heterozygotes. Similar genetic mechanisms were also identified for male gametes that affect pollen fertility. The hybrid sterility in indica/japonica crosses was mostly attributed to an S-5 locus. Therefore, a single wide compatibility allele can be used to obtain fertile hybrids between the two groups. But hybrid sterility in crosses between rice cultivars from the Indian subcontinent and other areas was controlled by several loci, as detected in Basmati 370 crosses. In hybrid rice breeding, hybrid sterility genes in cytoplasmic male sterile lines affect the screening of potential restorers but not maintainers. It is important to analyze hybrid sterility genes in maintainers to achieve enhanced heterosis in hybrid rice breeding.

Breeding and characterizing indica PGMS and TGMS lines in China

M. Tongmin, L. Chunhai, Y. Guocai, and L. Xinggui

Hubei photoperiod-sensitive genic male sterile (PGMS) rice Nongken 58S, which was used as the donor of genic male sterile genes, was crossed, backcrossed, and multicrossed with various target indica varieties. Sterile plants were selected under different environments. In the past 15 years, eight thermosensitive genic male sterile (TGMS) lines and four PGMS lines were developed. The characteristic response of these lines to photoperiod and temperature was studied under various environments in the field and in phytotrons. The results showed that most of them had a completely stable sterile period of about 45 days at Wuhan. The temperature-sensitive stage ranged from the differentiating stage of the pollen mother cell to the early ripe stage of pollen. The critical temperature points (CTP) of fertility alteration varied in different TGMS lines: 26.5 °C (daily mean) for W6154S, W6184S, W6111S, W6417S, and W8103S; 25.5 °C for W9046S and W9056S; and 24 °C for W91607S. The fertility expression of PGMS lines was controlled simultaneously by photoperiod and temperature. PGMS line W7415S had a CTP of 26 °C and a critical photoperiod point (CPP) of 13.5 h. The CTP and CPP for W9451S and W9461S were 24 °C and 14.0 h, and for W9593S they were 24 °C and 13.0 h, respectively. A practical and effective procedure for breeding PGMS and TGMS lines has been established.

Advances in two-line hybrid rice breeding

L. Xinggui, S.S. Virmani, and Y. Rencui

The discovery of environment-sensitive genic male sterility in rice led to the development of a simpler and more efficient two-line hybrid breeding system compared with the cytoplasmic male sterility or three-line system. Several elite photoperiod-sensitive (PGMS) and thermosensitive genic male sterile (TGMS) lines have been developed in China. The commercial two-line hybrids developed using these lines occupied about 330,000 ha in 1996. Multiplying PGMS and TGMS lines in a pure form requires some special handling. These methods have been developed in China. Similar methods of seed production have been adopted for both two-line and three-line hybrids. The key point is to determine the time when a PGMS or TGMS line will show complete sterility for about 1 mo at a given location. Seed yield of two-line hybrids in China is 2.25-3.0 t ha-1. Under tropical conditions, in which daylength differences are marginal, the TGMS system is considered to be more useful than the PGMS system. Genetic analysis at IRRI confirmed the monogenic recessive control of the TGMS trait. The TGMS gene of the IRRI mutant, IR32364 TGMS, was found to be nonallelic to the TGMS genes identified in China (tms 1) and Japan (tms 2). IRRI has developed some TGMS lines in indica rice that possess the tms 2 gene; these lines are being evaluated in national agricultural systems. As expected, the TGMS system gave a higher frequency of heterotic hybrids than the CMS system. This chapter also discusses constraints and the future outlook for two-line hybrid rice breeding.

Improving parental lines to increase efficiency of hybrid rice breeding: some new approaches

X.C. Liu, S.S. Virmani, and B.C. Viraktamath

Because of the potential of hybrid rice to increase rice production and productivity, many countries are currently working to exploit the benefits of this technology. The major problems in hybrid rice breeding are the limited number of parental lines with specific desirable traits, a lower frequency of maintainers and restorers among elite breeding lines, the narrow genetic base, a lack of resistance to biotic stresses, and poor grain quality of some parental lines. Therefore, improving parental lines must be an integral part of hybrid rice breeding to develop heterotic hybrids and improve breeding efficiency. The frequency of maintainers and restorers has been significantly increased after initiating specific maintainer and restorer breeding programs. Diverse parental lines with better grain quality and multiple resistance have been developed in China and at IRRI. Random mating of composite populations of maintainers and restorers developed at IRRI has helped to widen the genetic base of parental lines. Transferring restorer genes into tropical japonicas and incorporating wide compatibility genes into promising elite lines are considered essential to developing indica/japonica hybrids. Thermosensitive genic male sterile lines are being generated in indica, tropical japonica, and basmati genetic backgrounds. Anther culture and marker-aided selection can be deployed to expedite the parental line improvement program. This chapter discusses the limitations in current parental lines, progress made in improving them, and strategies envisaged to increase the efficiency of hybrid rice breeding.

Technological innovations to lower the costs of hybrid rice seed production

C.X. Mao, S.S. Virmani, and I. Kumar

The economic viability and adoption rate of hybrid rice technology depend on the level of hybrid rice seed yields in a country. Over the years (1976-94), mean seed yields in China have increased from 0.27 to 2.25 t ha-1, with a high of 7.39 t ha-1. At IRRI, hybrid rice seed yields increased from 0.15 t ha-1 in 1989 to 1.09 t ha-1 in 1994; the highest was 2.05 t ha-1. India has also recorded a similar improvement in hybrid seed yield. In 1991, the average seed yield was 0.50 t ha-1. In 1995, the maximum hybrid seed yield was 3.30 t ha-1. The same trend in increased seed yield in hybrid rice was also seen in the Philippines and Vietnam. This increase in seed yield can be attributed to improved seed production technology and increased familiarity with it, experience with parental lines, and the selection of better locations and seasons. This chapter presents some technological innovations made worldwide to increase hybrid seed yields and decrease the cost of hybrid seed production.

Diversifying the CMS system to improve the sustainability of hybrid rice technology

D.S. Brar, Y.G. Zhu, M.I. Ahmed, P.J. Jachuk, and S.S. Virmani

Cytoplasmic male sterility (CMS) resulting from nuclear-cytoplasm interaction has been commercially exploited for the production of F1 hybrid seed in a number of crops such as maize, sorghum, sunflower, and sugarbeet. More recently, major progress has been made in developing high-yielding hybrid rice varieties based on the CMS system, occupying over 18 million hectares in China. The first CMS line used to develop commercial F1 hybrids was developed in China in 1973 from a single male sterile plant (Oryza sativa f. spontanea) designated as wild abortive (WA). Since then, more than 20 CMS sources have been developed from various accessions of cultivated rice and wild species. A large number of CMS lines have been produced in China, at IRRI, and in several other countries from these cytosterility systems. But 95% of the total area planted to hybrid rice has a single CMS source, WA cytoplasm. This cytoplasmic uniformity of hybrid varieties could result in disease epidemics, such as the outbreak of Southern corn blight due to Helminthosporium maydis on U.S. maize hybrids carrying T-type cytoplasm. To overcome the danger of genetic vulnerability of hybrid rice to diseases and insects, we urgently need to diversify the cytoplasm of the male sterility sources. Conventional backcrossing and somatic cell hybridization are possible avenues to achieve diversification using cultivated and wild species as CMS donors. Two major problems are (1) the lack of efficient techniques for characterizing CMS sources and (2) nonavailability of effective restorers, particularly when the cytoplasmic donors (wild species) and recipient parents are distantly related. Under such situations, the donor species itself should be explored as a possible restorer. CMS sources are distinguished from each other on the basis of the fertility of crosses of CMS lines with restorers.

These procedures are laborious and fertility is affected by environment. Advances in molecular biology offer potential to develop precise, reliable, and quick molecular techniques to characterize different CMS sources. Fertile and male sterile lines including different sources of CMS can be distinguished from each other using molecular diagnostic probes based on the restriction endonuclease fragment pattern of mtDNA, including the nature of mitochondrial protein products and ultrastructure of the mitochondrial genome. But the role of these elements in producing cytosterility is not well understood. Protoplast fusion offers promise to produce cybrids and new alloplasmic male sterile lines. Moreover, this technology can further enhance efficiency through the quick transfer of CMS from one source to other elite breeding lines. Otherwise, this requires 5-6 generations of conventional backcrossing. Both two-line (photoperiod-sensitive and thermosensitive genic male sterility) and one-line (apomixis) systems offer a good alternative to the current three-line (CMS) system for producing hybrid seed. But until these systems become available for commercial production of hybrid seed, CMS continues to be the best strategy and thus needs diversification for the sustainability of hybrid rice technology.

Managing vulnerability of hybrid rice to biotic stresses in China and India

A.P.K. Reddy, K. Krishnaiah, Z.T. Zhang, and Y.Shen

Hybrid rice is grown mainly along the middle and lower reaches of the Yangtze River. It is also grown in southwest and southern China. At least three diseases and five insect pests that cause economic losses are known to occur widely. Compared with conventional varieties, rice hybrids that are highly responsive to fertilizer are more prone to pest damage. Rice yield losses also occur in India because of several diseases and insect pests in conventional varieties. Most of the currently released rice hybrids in India are susceptible to the major diseases and insect pests. These hybrids are now grown in relatively less pest-prone areas. Rice hybrids from China and India have been screened to identify resistance. Several parental lines and hybrids showed resistance to some diseases and pests. This chapter discusses pest problems that could affect hybrid rice production in the future.

Physiology-based crop management for yield maximization of hybrid rice

S. Peng, J. Yang, F.V. Garcia, R.C. Laza, R.M. Visperas, A.L. Sanico, A.Q. Chavez, and S.S. Virmani

High grain yield of hybrid rice is attributed to high vegetative biomass production, high leaf area, large panicles, and, in some cases, high tillering capacity. Crop management strategies based on physiological characteristics of temperate hybrid rice have been developed to maximize yield in China. Early growth of temperate hybrids is promoted by proper seedbed management, plant spacing, and nutrient supply. Unproductive tillers are controlled by proper timing and rate of N application and by mid-season drainage or deep irrigation. Nitrogen topdressing based on leaf N status is practiced to regulate canopy size and architecture. Nutrient and water management even during late grain filling and delayed harvest is important for improving the grain filling of temperate hybrid rice.

Hybrid rice has recently been released for commercial cultivation in the tropics. Crop management practices used for conventional rice are also commonly used in tropical hybrid rice. Knowledge is limited on management strategies to maximize yield of tropical hybrid rice. The methods developed to maximize yield of temperate hybrid rice cannot be adopted for tropical hybrid rice. In 1992, IRRI began research on crop management of hybrid rice, especially N management for tropical hybrid rice based on physiological characteristics. Applying N at the basal and mid-tillering stages improved biomass production and sink size for both the tropical hybrid and the inbred cultivar. As sink size increased, however, the grain filling percentage of the inbred cultivar declined more than that of the tropical hybrid. Therefore, the tropical hybrid responded more to N application at the basal and mid-tillering stages than the inbred cultivar. Application of N at flowering increased leaf N concentration, Rubisco content, and photosynthetic rate in flag leaves. The grain filling percentage and yield of tropical hybrid rice were improved significantly by late-season N application. The inbred cultivar did not respond to N application at flowering in terms of grain yield. Rectangular planting and increased hill density improved sink size and grain yield only in hybrid rice. These results suggest that crop management strategies should be developed based on the physiological characteristics of tropical hybrids to fully express their yield potential.

Improving grain quality of hybrid rice: challenges, strategies, and achievements

S.S. Virmani and F.U. Zaman

A major challenge in indica rice hybrid breeding is to ensure that the heterotic rice hybrids possess grain quality that is at least comparable, if not superior, to that of inbred check varieties grown by farmers. For this, a close linkage with an inbred rice breeding program is important to provide continuous access to newly developed elite inbred lines possessing desired grain quality characteristics. Most IRRI-bred cytoplasmic male sterile and restorer lines have grain quality similar to that of the check varieties. Therefore, grain quality in derived hybrids should be acceptable to farmers. To breed basmati rice hybrids, both parents need to possess basmati grain quality. Preliminary results from India and IRRI indicate the possibility of developing heterotic basmati rice hybrids. Deploying the thermosensitive genic male sterility system and anther culture could expedite the breeding of basmati rice hybrids.

Molecular and genetic approaches to understanding and engineering apomixis: Arabidopsis as a powerful tool

U. Grossniklaus, J.M. Moore, and W.B. Gagliano

Introducing apomixis into sexual crops will have far-reaching implications for seed production and crop improvement. Apomixis will allow the immediate fixation of any desired genotype and its clonal propagation through seeds. The manipulation of apomixis through biotechnology will only be possible through an interdisciplinary and multifaceted approach to investigating the regulatory mechanisms underlying plant reproduction. Apomictic and sexual reproduction are intimately related to each other, and the engineering of apomixis will require a detailed understanding of the genetic basis and the molecular mechanisms that control megasporogenesis, megagametogenesis, fertilization, and seed development. Arabidopsis thaliana, a member of the Brassicaceae, is particularly suitable for molecular and genetic analyses. Here we report on novel approaches that allow the identification and rapid molecular isolation of genes that control female reproduction in this model system. We describe genetic screens aimed at the isolation of mutants that display certain characteristics of apomixis and discuss their potential for engineering apomixis in agriculturally important crops.

Molecular strategies for hybrid rice: male sterility and apomixis

R.A. Jefferson and S. Nugroho

There is an urgent need for germplasm enhancement in rice to improve yield, quality, and adaptability to diverse environments. One promising approach is to exploit heterosis and develop high-yielding rice hybrids. Current hybrid technology is based on three-line systems involving a limited number of CMS sources. Efforts are being made to exploit the two-line system using TGMS and PGMS sources. We review some existing or proposed molecular technologies, and outline the potential for developing a robust two-line hybrid system. We also indicate the strategies needed to generate autonomous apomixis for fixing different hybrid combinations and thus take the next steps toward a new type of highly diversified and decentralized plant breeding. In particular, this chapter focuses on two areas of research under way at CAMBIA.

Developing and using novel sources of male sterility

K.K. Narayanan

In most grain crops, the exploitation of heterosis hinges on the availability of a good male sterility system. In the tropics, cytoplasmic uniformity, caused mainly by the exclusive and extensive use of the wild abortive (WA) type of CMS source, makes hybrid rice potentially vulnerable to the ravages of pests and diseases. To diversify CMS sources and solve problems such as incomplete panicle exsertion, plant cell and molecular manipulations are attempted. The earliest study involved the transfer of tissue-specific expression of the "toxin" gene that disrupted normal pollen development. The antisense RNA technique has been widely used to inhibit the expression of a locus, like that of male sterility. Genetic rearrangements in plant mitochondria have been associated with CMS. These techniques have been used extensively in rice and tobacco. The development of new CMS sources through either wide hybridization or protoplast fusion would also require the identification of good fertility restorers. This chapter outlines attempts to design and construct transposon traps to isolate fertility restorer genes.

Mapping and molecular marker-based genetic analysis for efficient hybrid rice breeding

Q. Zhang and N. Huang

Work in mapping and molecular marker-based genetic analysis of fertility-related genes currently employed in hybrid rice breeding is reviewed. This includes genes for fertility restoration of wild abortive (WA) cytoplasmic male sterility (CMS), photoperiod-sensitive genic male sterility (PGMS), and thermosensitive genic male sterility (TGMS), and genes for wide compatibility. Studies showed that fertility restoration of WA CMS is controlled by at least two independent dominant genes (RF-1 and RF-3). The chromosomal locations of these two genes need to be resolved further. Single-locus inheritance has been observed for all TGMS mutants. The TGMS genes of two independently derived mutants were mapped to two different chromosomes. Studies demonstrated a relatively simple mode of inheritance of PGMS. But the chromosomal locations of the PGMS genes were complicated. A single wide compatibility gene having a large effect on the fertility of indica/japonica F1 hybrids has been confirmed in a range of indica/japonica crosses based on the results from studies on inheritance and mapping. Another locus modifying hybrid fertility was also detected in the presence of the wide compatibility gene. Future efforts should be directed to reconciling the discrepancies between the results of mapping and inheritance studies in some of the systems and identifying markers tightly linked to the genes of interest to facilitate marker-aided selection.

Diversification of cytoplasmic male sterility systems through somatic cell hybridization

N.W. Blackhall, H. Akagi, T. Fujimura, J.P. Jotham, M.R. Davey, J.B. Power, and E.C. Cocking

Cytoplasmic male sterility (CMS) occurs widely in higher plants and is due to interactions between nuclear and cytoplasmic (mitochondrial) genome transcripts. Most hybrid rice currently grown is based on wild abortive cytoplasm. Using sexual crossing, CMS sources from closely related A genome Oryzae have been transferred into the nuclear background of cultivated rice. Somatic hybridization offers the ability to speed up the process of hybrid production and to produce novel nuclear-cytoplasmic combinations not possible by sexual crossing. In Japan, asymmetric protoplast fusion has been employed for the transfer of CMS to a wide range of japonica cultivars. The aim of our work is to develop, via somatic hybridization, alloplasmic lines between a range of cultivated rice and Oryza species other than A genome ones. Cell suspension cultures of O. australiensis, O. granulata, and O. latifolia have begun. Japanese studies have already produced two novel lines based on BT-type CMS and have demonstrated that somatic hybridization has considerable potential for transferring a well-established source of CMS into a broad range of cultivars. At Nottingham, a more speculative approach is being employed, which will produce more diverse CMS lines for use in rice breeding for the next millennium.

Use of anther culture in hybrid rice breeding

D.Y. Zhu, Z.X. Sun, X.G. Pan, X.H. Ding, X.H. Shen, Y. Wan, H. Pan, J.H. Yin, M.S. Alejar, L.B. Torrizo, and S.K. Datta

This paper outlines improvements in techniques of anther culture in indica rice, including the optimal stage for inoculation of pollen, media, and physical and chemical pretreatments. It analyzes the genetic stability and diversity of pollen progenies and the feasibility of using anther culture in hybrid rice breeding on the basis of results from many studies. It summarizes advances and achievements in hybrid rice breeding via anther culture, including purification and development of parental lines for hybrid rice. The paper also discusses the tendency of anther culture research and prospects for its application in hybrid rice breeding.

Apomixis in crop improvement: traditional and molecular approaches

W. Hanna, D. Roche, and P. Ozias-Akins

Apomixis is a method of reproduction that allows a chromosomally unreduced egg cell to develop into an embryo without fertilization by a sperm. It is controlled by qualitative genetics and found mainly in polyploid species in the tertiary gene pools. The apomictic mechanism is present in a number of fruit and nut crop species and in the wild relatives of some important agronomic crops. The main advantages of apomixis are that it would simplify hybrid seed production, provide the opportunity to develop and fix the genotype of unique and superior hybrids, and make hybrid production possible in crops without good male sterility systems. Apomixis can be identified by progeny tests, test crosses, cytological methods, and molecular methods. Progress is being made in transferring this mechanism from wild to cultivated species by the backcrossing method. Molecular methods are being used to develop molecular markers linked to the gene(s) controlling apomixis and to map it(them). Ultimately, the gene controlling apomixis will have its greatest value if it can be cloned and made to express itself stably when inserted into an alien genome. The gene controlling apomixis could have a major impact on food, feed, and fiber production around the world.

Current status of research on apomixis in rice and prospects for its use in heterosis breeding

G.S. Khush, D.S. Brar, and J. Bennett

Apomixis is being explored as a new frontier to exploit hybrid vigor and to develop true-breeding hybrid rice varieties. Apomixis is common in grasses and in several polyploid plant species and is controlled by one or a few genes. Among the major cereals, maize, wheat, and pearl millet have apomictic relatives. But there is no clear evidence of apomixis in rice. A-genome diploid wild relatives have been examined based on studies of crosses with dominant marker stocks. Reports on cyto-embryological studies also lack genetic evidence for the occurrence of apomixis in rice. We proposed three strategies to develop apomictic rice: (1) screening germplasm of tetraploid wild species as a source of apomixis and transferring the apomictic trait to rice cultivars, (2) inducing apomictic mutants in rice through mutagenesis, and (3) developing apomictic rice using molecular approaches. We have screened 108 accessions of tetraploid Oryza species for apospory (multiple embryo sac development) and 86 accessions for diplospory (based on callose detection), including five related genera. But none of the accessions showed any evidence of apomixis. We have also undertaken the second approach to induce apomictic mutants. Mutagenized populations derived from treating seeds and fertilized egg cells with gamma rays, ethyl methane sulphonate, and N-methyl-N-nitrosourea are being screened.

We have selected a dominant purple leaf mutant of rice for identification of the apomictic mutants following mutagenesis. IRRI is collaborating with advanced laboratories to develop apomictic rice through molecular approaches. Molecular markers linked to the apomictic mode of reproduction have been identified in progenies of maize (Tripsacum) and in crosses of sexual and apomictic wild species of Pennisetum. Cloning of the gene(s) for apomixis is under way from apomictic plant species such as Tripsacum, Pennisetum, Brachiaria, and Cenchrus. Once such genes become available, they will be introduced into elite breeding lines of rice using transformation technology. We are also exploring the possibility to identify dividing nucellar cells capable of forming adventitious embryos in a transgenic rice line (HSK-1). Apomixis will increase the efficiency of heterosis breeding in producing many true-breeding hybrids compared with those produced by using three-line or two-line hybrid breeding systems. The availability of a large number of hybrids will increase genetic diversity and reduce genetic vulnerability. Moreover, the possible vulnerability to pests and diseases because of narrow cytoplasmic male sterility sources will also be eliminated. The development of apomictic rice would enable resource-poor farmers in developing countries to adopt high-yielding hybrid rice technology. This would lead to an increase in area planted to hybrid rice, resulting in higher productivity and production.

Hybrid rice technology in India: current status and future outlook

E.A. Siddiq, M. Ilyas Ahmed, B.C. Viraktamath, M. Rangaswamy, R. Vijay Kumar, B. Vidyachandra, F.U. Zaman, and S.D. Chatterje

The commercial success of hybrid rice in China has clearly demonstrated its potential to meet the increased demand for rice. Efforts to develop and use this technology in India began in the 1970s and were systematized and intensified in December 1989 with the launch of a mission-oriented project. Within seven years, more than 12 hybrids from the public and private sectors were made available for commercial cultivation. During the 1996 wet season, more than 60,000 ha were planted to hybrid rice in India. Some more promising hybrids with better grain quality, resistance to pests and diseases, and a higher magnitude of heterosis are in the final stages of evaluation. Cytoplasmic male sterile lines with diversified sources of sterility-inducing cytoplasm have been developed. Research on two-line heterosis breeding and the development of intersubspecific indica x tropical japonica hybrids has begun and the results attained appear promising. A seed production technology has been developed and its adaptability demonstrated on a large scale. Seed yields of 1.0-1.5 t ha-1 can be obtained. During the 1996 dry season, 1,300 t of F1 seed were produced by private- and public-sector seed agencies. Crop production and protection practices for the successful cultivation of hybrids in the target areas are being standardized. Future research and development strategies for hybrid rice technology are briefly discussed. Prospects for the large-scale adoption of this technology in India appear to be bright.

Hybrid rice research and development in Vietnam

N.T. Hoan, N.N. Kinh, B.B. Bong, N.T. Tram, T.D. Qui, and N.V. Bo

The commercial success of hybrid rice in China has encouraged Vietnam to introduce and develop this technology to increase rice yields in the country, especially in irrigated areas of the Red River Delta. Some Chinese rice hybrids were introduced and found to yield significantly higher than local check varieties. F1 seeds of these hybrids were imported from China and distributed to rice farmers in the Red River Delta covering an area of 86,000 ha in 1996. Difficulties faced with this strategy included total dependence on China for hybrid seeds, the high cost of seeds, and susceptibility of hybrids to brown planthopper and bacterial blight. Collaboration with IRRI has been strengthened to overcome these difficulties. Several IRRI hybrids and parental lines have been introduced, some of which have been found to be suitable in both the Red and Mekong River deltas of Vietnam. Seed production technology developed in China and at IRRI has also been adapted. This chapter discusses results of adaptive research on hybrid rice technology introduced from China and IRRI.

Hybrid rice in the Philippines: progress and prospects

J.C. de Leon, E.D. Redoña, I.A. dela Cruz, M.F. Ablaza, F.M. Malabanan, R.J. Lara, and S.R. Obien

In June 1994, the National Seed Industry Council approved the commercial release of the promising hybrid IR64616H. Registered as PSB Rc26H and named Magat hybrid, IR64616H became the first hybrid rice variety in the Philippines. An upcoming hybrid, IR68284H, showed standard heterosis of 16.4% across seasons (dry and wet) and 26.9% during the dry season in the National Cooperative Trials for rice. More heterotic hybrids from PhilRice, IRRI, and Cargill are now being evaluated in test nurseries. Hybrid rice research at PhilRice has been strengthened through international collaboration. In the Cagayan Valley region, farmers were trained in on-farm F1 hybrid seed production. In 1994, the National Rice Seed Production Network facilitated the nationwide dissemination of new hybrid rice varieties. This chapter discusses the challenges facing hybrid rice breeding research and technology development in the Philippines.

Developing hybrid rice technology in Malaysia

H.P. Guok, S. Azlan, and K.H. Ku Yahaya

The Malaysian Agricultural Research and Development Institute (MARDI) began hybrid rice research in the country in 1984. Local cytoplasmic male sterile (CMS) lines MH805A, MH813A, MH821A, and MH841A were developed through a backcrossing program. More than 100 new CMS lines are presently in the BC2 and BC3 generations. To date, more than 130 restorer lines have been identified for producing hybrid seed. More than 530 F1 hybrids have been evaluated in yield trails from 1991-92 to 1995-96 in the main season for identifying heterotic hybrids. At Bumbong Lima in the 1995-96 main season, IR62829A/IR46R significantly outyielded MR84 by 26%. During the same season, MH841-IA/MR167 significantly outyielded MR167 by 24% at Bertam. Efforts are under way to produce large quantities of seeds of male sterile lines and hybrids to extend heterotic hybrid rice cultivation in farmers' fields.

Developing intersubspecific hybrid rice in the DPR of Korea

Lee Ul Byong and Mun Jong Won

The commercial cultivation of hybrid rice is currently restricted to areas south of 350N latitude in the DPR of Korea. This suggests that prevalent unfavorable conditions in high-latitude areas limit the exploitation of hybrid rice. Yield of an inbred variety in the DPR of Korea is high. Yield of hybrids must be 30% more than that of an inbred variety to use hybrid rice for commercial production in the country. Yield of hybrid rice seed produced via natural outcrossing must also be more than 1.5-2.0 t ha-1. To achieve these targets, development of intersubspecific hybrid rice began in 1983. Research has focused on solving problems such as nuclear sterility, lodging caused by transgressive heterosis for height, cold damage, poor grain quality, faster senescence in the F1 generation, and low seed yield. Results are reported in this chapter.

Research and development for hybrid rice technology in Egypt

A.O. Bastawisi, I.R. Aidy, H.F. El-Mowafy, and M.A. Maximos

Research and development for hybrid rice in Egypt passed through two main stages: the evaluation of hybrids for heterosis, and the transfer of cytoplasmic male sterility and restoring ability to Egyptian rice. Starting in 1986, materials evaluated, which included hybrid varieties provided by the International Rice Research Institute and hybrid seed companies, were either comparable or inferior to local high-yielding varieties. Only in 1995 did some hybrids show standard heterosis of 5% to 16%. The transfer of the cytoplasmic male sterility (CMS) factor/s and restoring-ability gene(s) to Egyptian rice began in 1995. Using 15 CMS lines received from IRRI, local varieties, and elite lines, 132 test crosses were made. Another 49 crosses were made to transfer the restoring-ability gene(s) to Egyptian varieties. Six thermosensitive genic male sterile lines were sown on two different dates to evaluate percentage sterility in Upper Egypt (New Valley). There the average temperature is above 30 0C during the reproductive stage. This two-line method for producing hybrid rice, if successful, might be used. Although the productivity in inbred rice is very high, some hybrids may produce still higher yields in Egypt.

Research on hybrid rice technology in the United States

D.J. Mackill and J.N. Rutger

The high cost of seed production as well as poor grain quality have hindered the commercial application of hybrid rice in the United States. Development of U.S.-adapted rice hybrids is carried out in the private sector. Research by USDA-ARS has focused on characterizing germplasm for genetic diversity and developing genetic mechanisms of hybrid seed production. U.S. cultivars have been classified via RAPD and AFLP markers. The maximum genetic diversity of U.S. cultivars occurs between long-grain (tropical japonica) and short/medium-grain (temperate japonica) cultivars. In the relatively cool environment of California, hybrids between temperate and tropical japonicas may be appropriate. In the southern U.S., where indica cultivars are adapted, indica-japonica hybrids may be feasible. Cytoplasmic male sterility, restorer genes, and wide compatibility are being transferred into California cultivars so that the potential for various hybrid combinations can be evaluated. Photoperiod-sensitive genetic male sterile (PGMS) mutants are being sought in U.S. cultivars. In addition, a program is being planned to transfer apomixis from Pennisetum species into rice by molecular techniques.

Developing hybrid rice in Brazil: methodology, highlights, and prospects

E.P. Guimarães, V. dos A. Cutrim, and J.A. Mendonça

EMBRAPA-CNPAF began to explore the prospects for and problems of hybrid rice in 1984 via two areas: the introduction of allogamic traits to develop A and B lines and reciprocal recurrent selection. The methodological steps involve identifying maintainer and restorer lines, introducing allogamic traits into maintainer lines, and transferring the cytoplasmic genetic male sterility system to derived F4 maintainer lines. Development of line 046I with allogamic traits and good agronomic behavior and several hybrids is the most significant result. Future plans are directed toward developing an economically viable methodology to produce hybrid seeds, locating partners in the private sector, identifying new cytoplasm sources to produce genetically diverse A lines, and evaluating the economic need to use the allogamic traits.

Research and development for hybrid rice technology in Colombia

D. Muñoz, P. Gutiérrez, and E. Corredor

Rice is the most important crop after coffee in Colombia and it is the second source of protein after meat. Area planted to rice is 300,000 ha. In 1995, the average productivity of the crop was 5.5 t ha-1 in irrigated lands and 4.6 t ha-1 in rainfed uplands. The development of 14 semidwarf high-yielding varieties has been pivotal to improved rice production during the past two decades. But average yields have reached a plateau. Rice hybrids offer an opportunity to break through the yield ceilings of semidwarf inbred varieties. At Saldaña (Tolima) in 1995, the highest-yielding hybrid was IR58025A/Oryzica Yacú-9. It produced an average yield of 7.1 t ha-1, 16% higher than that of Oryzica Yacú-9, a commercial variety released in 1994. Of the 10 hybrids evaluated at Saldaña, seven showed less white leaf virus disease (hoja blanca virus) than their parents. In these tests, 82% of the hybrids recorded more grains panicle-1 than their restorer parents. The results suggest that by using the female parent IR58025A, white center is increased in grains of the hybrids. Fortunately, the level of white center is still acceptable to rice millers and consumers. Currently, the hybrid rice program in Colombia has two major objectives: (1) to breed heterotic hybrids with a 15-20% yield advantage, good grain appearance, and good cooking quality, and (2) to ease the hybrid seed production process.

Research and development for hybrid rice technology in Sri Lanka

S.W. Abeysekera, M.P. Dhanapala, and D.S. de Z. Abeysiriwardena

Sri Lanka has reached a stage where further expansion in rice area is not possible. With a per capita consumption of about 100 kg yr-1 and a limited annual rice land area of about 830,000 ha, Sri Lanka must raise its present yield level of 3.4 t ha-1 to 4.5 t ha-1 within the next 5 yr to achieve self-sufficiency in rice. The low and stagnating yields of semidwarf inbred varieties presently cultivated in Sri Lanka limit the scope for increasing production. Sri Lanka's Rice Research and Development Institute began a research program on hybrid rice in collaboration with IRRI in 1980. The program concentrated mostly on evaluating promising genetic materials received from IRRI and other countries for adaptability in the target environments of Sri Lanka. Attempts were also made to identify cytoplasmic male sterile (CMS) lines suitable for Sri Lanka and transfer the CMS character from IRRI-developed CMS lines to promising Sri Lankan lines. During the past two seasons, nine CMS lines possessing wild abortive cytoplasm were tested for field performance and promising lines were selected. Pollen sterility in the selected CMS lines ranged from 92% to 99.5%. The heterotic combinations identified included 48 from CMS/IRRI restorer test crosses and five heterotic combinations selected from test crosses made between IRRI CMS lines and SL elite lines. Of the 32 test crosses made during the minor dry (yala) season of 1996, two, IR58025A/Ld 355 and IR68887A/Bg 2039, were found to have high pollen sterility. They are being backcrossed with repetitive male parents to develop locally adapted CMS lines.

Hybrid rice breeding in Japan

R. Ikeda

Motivation for breeding and producing hybrid rice has been weak during the past decade because of a decline in the area under rice cultivation in Japan. Despite this discouraging situation, some basic research has been conducted on hybrid rice. Simultaneously, the private sector has continued to invest in hybrid rice breeding in conjunction with breeders in China, India, and other countries. Marketing of a small amount of hybrid rice seed has been announced recently by a private company. This chapter briefly outlines ongoing work in hybrid rice breeding in Japan.

Hybrid rice in Thailand

S. Amornsilpa

Encouraged by experiences in China and at IRRI, rice breeders in Thailand have been exploring the prospects of hybrid rice technology for increasing yield potential in rice. IRRI-bred rice hybrids have not shown a consistent yield advantage over local checks although the grain quality of some hybrids was comparable with that of the checks. This chapter outlines strategies for developing hybrid rice technology in Thailand.

Research and development for hybrid rice technology in Indonesia

B. Suprihatno and D. Satoto

Hybrid rice research in Indonesia began in 1983, with the initial objective to explore the prospects and problems of using this technology. During the past 15 yr, although the yield advantage of hybrids over inbred rice has been established, no hybrid has been released and recommended for cultivation, primarily because of the nonavailability of a commercially usable cytoplasmic male sterile line. During the past few years, government support for the program has been reduced. Currently, some promising F1 hybrids are in the pipeline and might qualify for release by 1998. Hybrid rice seed production still faces some problems, especially in the synchronization of flowering of parental lines. Seasonal variation within a location is believed to be one of the factors affecting the flowering behavior of parental lines. Therefore, identification of suitable locations having more stable weather conditions is needed.

Recommendations of the 3rd International Symposium on Hybrid Rice

The global requirement for rice by 2020 is expected to be around 800 million t compared with current production of 520 million t. With shrinking resources-particularly arable land area, irrigation water, and energy-the only option left is to increase production. Increasing rice production by 300 million t during the next 25 yr is a challenging task. Of the possible genetic approaches to meet this challenge, hybrid rice technology is an immediate option because it has been a proven technology over the past two decades in China and now has commercial prospects in India. The theme of the 3rd International Symposium on Hybrid Rice was "enhancement and sustenance of hybrid rice technology." The symposium was cosponsored by the Indian Council of Agricultural Research (ICAR), the United Nations Development Programme (UNDP), and the International Rice Research Institute (IRRI). About 150 Indian delegates and 50 international delegates from 20 countries addressed various aspects and issues for improving this technology and making it available outside of China. These issues were discussed in six sessions (current scenario, increasing breeding efficiency and enhancing yield heterosis, sustainability of hybrid rice technology, tissue culture and molecular approaches in heterosis breeding, toward true-breeding hybrids, and status of development and adoption of hybrid rice technology in various countries) and at a meeting of the International Task Force on Hybrid Rice. During the sessions, the following major recommendations emerged.

Introduction: IRRI's role and vision for hybrid rice

Hybrid rice technology in India: problems and prospects

Prospects for hybrid rice in tropical Asia

Hybrid rice breeding in China

Hybrid rice research and development in the tropics

Ushering in an era of hybrid rice in India

Using tropical japonica germplasm to enhance heterosis in rice

The wide compatibility system: current knowledge of its genetics and use for enhanced yield heterosis

Breeding and characterizing indica PGMS and TGMS lines in China

Advances in two-line hybrid rice breeding

Improving parental lines to increase efficiency of hybrid rice breeding: some new approaches

Technological innovations to lower the costs of hybrid rice seed production