Practice and Thought on Developing Hybrid Rice for Super High Yield by Exploiting Inter-subspecific Heterosis

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1 Rice Science, 2005, 12(1): Practice and Thought on Developing Hybrid Rice for Super High Yield by Exploiting Inter-subspecific Heterosis LU Chuan-gen, ZOU Jiang-shi (Institute of Food Crops, Jiangsu Academy of Agricultural Sciences, Nanjing , China; rb8@jaas.ac.cn) Abstract: Since the breakthrough of grain yield owing to the development of dwarf rice and three-line system hybrid rice, rice breeding for high yield hardly had showed significant progress in the next successive two decades. It was considered that utilizing heterosis between subspecific varieties (Oryza sativa L.) would be an effective approach to increase yield further. During , an indica-japonica hybrid Yayou 2 yielded as high as 10.5 t/ha; however, it failed to be commercialized because of seed purity problem due to non-uniform emasculation by chemical agent in seed production, and sensitivity of seed setting in F 1 plants to environmental conditions. In the past decade, two inter-subspeific hybrids, Liangyoupeijiu (Peiai 64S/9311, javanica/indica) and Liangyou E32 (Peiai 64S/E32, javanica/ japonica); both of them exhibited grain yield higher than 10.5 t/ha, and were widely judged as the pioneers of super hybrid rice. Liangyoupeijiu has been successfully popularized over 4 million hectare in wide climatic areas, while Liangyou E32 made a yield record and offered a model of plant ideotype for super hybrid rice. It was considered that in combination with plant ideotype, active physiological functions, and wide-range adaptability to ecological conditions, exploitation of indica-japonica heterosis would be the key approach for super hybrid rice breeding. Key words: rice; breeding; intersubspecific hybrid; plant type; two-line approach Rice is a worldwide major source of food, especially in Asia where there is rapidly growing population and food shortage. Increasing rice yield will be extremely important as the population grows and arable land decreases in a highly populated country like China. Rice improvement for high yield in China could be divided into three phases, namely dwarf rice, hybrid rice and super rice breeding. In the early 1950s, rice varieties usually produced grain yields below 6 t/ha due to their high stature and weak resistance to nitrogen fertilizer. In 1956, a dwarf mutant was found in an indica variety Nantehao in Guangdong Province. Since then, Huang et al had initiated dwarf rice breeding in Guangdong and southern China. A series of indica semi-dwarf varieties such as Guangluai 4 and Guichao 2 were successfully bred and released. They displayed a yield potential of 7.5 t/ha or 20-30% higher than the traditional varieties, owing to the improved resistance to more fertilizer and lodging [1]. In addition, another semi-dwarf variety IR8, once honored as miracle rice, was released by IRRI in 1960s. It showed high grain yield and was widely popularized all over the world. The dwarf rice breeding was regarded as the highlight of Received: 25 July 2004; Accepted: 24 December 2004 so-called green revolution in rice breeding. Breakthrough for hybrid rice breeding, in fact, was made in China during Later, a series of indica hybrids such as Nanyou 2 and Shanyou 63 were released and widely planted covering half of rice area in China. Generally speaking, hybrid rice showed yield potential of 9 t/ha or 10-20% higher than indica or japonica pure lines. Since 1984, rice production in China has shown no significant improvement both in yield potential and breeding technique. In terms of national food security, super rice breeding project was established by China National Science and Technology Council, and it was headed by China National Hybrid Rice Research & Development Center (CNHRRDC) and China National Rice Research Institute (CNRRI), and cooperated by agricultural institutes and universities. The grain yield potential aimed at 10.5 t/ha for the first five-year plan ( ), while 12 t/ha for the second five-year plan ( ). Several super rice combinations such as Liangyoupeijiu and Xieyou 9308 had been bred. They exhibited grain yields over 10.5 t/ha or 15% higher than traditional hybrids [2]. The history of rice breeding in China can also be divided into three phases where breeding were characterized by exploitation of favorable genes,

2 2 Rice Science, Vol. 12, No. 1, 2005 heterosis and improved plant type, respectively. The significant increase in grain yield was brought by the dwarf rice improvement benefited from using a dwarf gene, sd1. The successful utilization of rice heterosis was relied on the discovery of a wild abortion gene (WA). In 1980s, a wide compatibility gene (WCG), S5-n, was discovered. It can overcome indica-japonica hybrid sterility, the main obstacle in utilization of inter-subspecific hybrids, and becomes an important favorable gene in super hybrid rice breeding by utilizing intersubspecific heterosis [3, 4]. Utilization of indica-indica heterosis was a fundamental principle of rice improvement during 1970s-1980s. In the past two decades, a stronger heterosis of inter-subspecific hybrid between indica and japonica varieties showed importance for super rice breeding. A wide cross between rice and other cereal crops would provide rice yield and other traits with tremendous heterosis. It has been hoped by rice scientists and expected to bring an incalculable breakthrough for rice improvement. Improving plant type is another important objective in high yield rice breeding. In the past half-century in China, rice varieties showed a great shift from high plant stature and drooping leaves to semi-dwarf and erect upper leaves. Rice improvement exhibited a transfer of paying attention from only to morphological traits gradually to both morphological and functional traits. The practices not only improved rice morphological traits to avoid lodging, but improved efficiency of receiving light, and leaf physiological functions such as photosynthetic rate. Practice on developing hybrid rice for super high yield by exploiting intersubspecific heterosis Exploiting indica-japonica heterosis with chemical emasculation during In 1987, an elite line, 02428, which was later awarded by the Ministry of Agriculture, China, was identified as WCG possessing source. This line was developed at Jiangsu Academy of Agricultural Sciences (JAAS), in a progeny population from F 1 between two M 1, derived from 60 Co-irradiated japonica varieties, Pangxiegu and Jibangdao [5]. For its wide compatibility, the line was later widely used as parent or donor of S5-n gene in intersubspecific hybrid rice breeding. The author s research group has attempted to use to develop intersubspecific hybrid rice with the aid of chemical agent. One of the hybrids, Yayou 2 showed a high yield of 10.5 t/ha and was ever planted over ha. However, it failed to be widely commercialized, because its seed purity was only 60-80% even at favorable seed production conditions due to non-uniform chemical emasculation. Furthermore, as a typical indica-japonica hybrid, it showed sensitivity to planting conditions, and its seed setting fluctuated drastically as temperature and other conditions changed adversely around flowering and milking stages [6]. Super hybrid rice breeding by two-line approach in the past decade After the failure of practice on using indicajaponica F 1 by chemical agent, the authors had adjusted the technical path. We considered that, using indica-javanica or japonica-javanica hybrid might avoid such sensitivity in typical indica-japonica F 1. On the other hand, a two-line system approach by using environmental-sensitive genic male sterile (EGMS) line showed high efficiency and was successfully being used in rice breeding. If an EGMS line possessed the WAG, it would be beneficial in intersubspecific hybrid rice breeding, for it could be crossed to both indica and japonica varieties. The safety of seed production could be secured by selecting a suitable climatic area, and by a so-called warm-water irrigation technique if the female was attacked by a cool weather during the sterilitysensitive stage [7]. By persistent practice, we successfully released two hybrids, Liangyoupeijiu and Liangyou E32. Liangyoupeijiu was developed with a cross of Peiai 64S/9311 (65002). The female parent Peiai 64S was a javanica EGMS line bred by CNHRRDC with its WCG from Padi, a variety from Indonesia, and temperature-sensitive character from an EGMS line, Nongken 58S, a mutant from a japonica variety Nongken 58. The male parent was an elite line developed from a released commercial indica variety, Yangdao 6 by JAAS [8, 9].

3 LU Chuan-gen, et al. Practice and Thought on Developing Hybrid Rice by Exploiting Inter-subspcific Heterosis 3 Liangyoupeijiu exhibited an improved plant type with erect uppermost three leaves, a higher light transmission rate and a lower light extinction coefficient in its middle to late growth phases in comparison with an indica hybrid Shanyou 63 which had been widely popularized in China and tropical areas [10]. For combination both of heterosis and plant ideotype, Liangyoupeijiu showed a high yield potential, fine grain quality, and strong resistance to rice bacterial leaf blight and blast diseases. The hybrid showed its growth period of 150±24 d, with leaves, and an averaged grain yield of 8.4±2.6 t/ha according to data of 19 testing sites in national regional yield trial (northern latitude ) of southern China in 1999 and 2000 [10]. During 1999 to 2000, in a national test for super hybrid rice, Liangyoupeijiu exhibited its average grain yield more than 10.5 t/ha in 38 sites, over 6.7 ha for each site in Hunan and Jiangsu Provinces. A grain yield as high as 15.3 t/ha was realised in Yunnan Province in The hybrid was evaluated to be good grain quality of classⅡ,and half of its indices reached the standard of classⅠ, by comparing the general investigation result from CNRRI with China Rice Quality Standard. As two-line system for seed production, the hybrid used to get its seed yield by 2-3 t/ha, and possess its purity of about 98% as well in suitable regions for seed production in the past six years. In terms of both high yield and good grain quality, Liangyoupeijiu is well known as the pioneer of Super Hybrid Rice, and rewarded the China National Prize on Technique Innovation, Dupont Innovation Prize, and First Class Prize of Science and Technology in Jiangsu Province. It hit the first headline of The top ten news of science and technology in China in The hybrid has been registered in six provinces and nationally registered as the first two-line hybrid, and had been popularized over 4 million ha during in wide climate areas from northern latitude of 0 to 35 in southern regions of China and southeastern Asia, e.g., Vietnam, Philippines and Indonesia. Liangyou E32 (Peiai 64S/E32, or 65396), bred by JAAS and CNHRRDC, is a two-line system hybrid using Peiai 64S as female parent and japonica line E32 as male parent. The hybrid had been planted from north latitude 18 to 35 and registered in Vietnam in In 1999, it yielded 17.1 t/ha in a 720 m 2 plot in Yongsheng County, Yunnan Province, China, which made a yield record of hybrid rice of the time, and touched the daily grain yield level of 100 kg/ha, which was one of the targets for super hybrid rice breeding. In terms of both hybridization and morphological improvement, the hybrid has been praised as a pioneer of Super Hybrid Rice and ideotype plant for super hybrid rice as proposed by Yuan [11]. Thought on rice breeding for super high yield Utilization of heterosis As rice F 1 hybrids showed heterosis in their yield potential and other traits, the hybrids from indica or japonica varieties have been tested and utilized in China since In the past decade, it has been extended to India, Vietnam and other tropical countries, and shown yield increase of 10-20% over indica or japonica pure lines. The heterosis of inter-subspecific hybrids between indica and japonica varieties could be higher as much as 30% over indica or japonica varieties in biomass, but the utilization of heterosis has been difficult because of partial sterility of panicles in F 1 hybrids, which is known to be the main barrier in the use of the pronounced heterosis. Spikelet sterility in indica-japonica hybrids has been attributed to an allelic interaction at locus S5 on chromosome 6. An allele S5-n was detected in some varieties, which is referred to as wide-compatibility variety (WCV), and was known to overcome the sterility in indica-japonica hybrids [3, 4]. It created a possibility to increase rice yield significantly by using indica-japonica heterosis. Since the detection of the S5-n allele, several indica-japonica hybrids with WCV as parents have been tested for their yield potential. Most of such hybrids showed normal spikelet fertility and a pronounced heterosis in grain yield. Although WCVs were used as parents, some of the hybrids exhibited an unstable seed setting rate ranging from 20 to 90% in varied environmental conditions especially at lower

4 4 Rice Science, Vol. 12, No. 1, 2005 temperatures. In general case, some indica-japonica hybrids could attain the seed setting level of 75-90%, but hardly over 90% while for their parents or inbred varieties, this seed setting level is common. For example, Yayou 2 (3037/02428), a typical indicajaponica hybrid, showed its seed setting rate of about 90% and displayed a significant heterosis in grain yield under a favorable condition. However, its seed setting rate could decline to 20% when daily average temperature below 25 occurred during temperaturesensitive stages. In addition, its seed setting rate was seldom over 90% even under favorable conditions. Thus, it was not desirable to apply these hybrid combinations in areas with north latitude over 34, where temperature is unstable during rice heading stage. In these areas, it was found that the female gametes possessed viability while the pollen may loss viability at lower temperature. It was also found that even under favorable conditions, the pollen fertility was lower than 90% by morphological check, and of those morphologically normal pollen, a portion of them still lacked functions for fertilization. Such trait was observed in many indica-japonica hybrids. It was assumed that, pollen abortion caused the reduction of pollen fertility and unstable fertilization, resulting in a lower and unstable seed set [6]. To develop indica-japonica hybrid rice with high and stable fertility, it is necessary to improve the parents for wider compatibility [12]. Neutral alleles S5-n, S7-n, S8-n, S9-n, S15-n and S16-n at the corresponding sterility loci, and pollen sterility-neutral genes, ga11-n and ga14-n have been detected in some varieties, such as Akihikari, Dular and Ketan Nangka [13, 14]. From a three-way cross, Akihikari//IR36/Dular, neutral genes at ga11 and six sterility loci, S5, S7, S8, S9, S15 and S16, were combined and elite lines were developed after successive progeny selection as reported in the author's previous paper. It was confirmed that some elite lines have carried seven or six neutral alleles at the sterility loci and male gamete abortion locus. By testing the performance of F 1 s, which used the breeding lines and various testers as parents, these lines actually increase seed set of their F 1 s through mitigated spikelet sterility by six sterility loci and gamete abortion by a gametophyte gene ga11. These lines could be used as parents or potential donors to increase width of compatibility of rice varieties for improving fertility in inter-subspecific hybrid rice breeding [15, 16]. Model for plant ideotype There are several models for rice plant ideotype. For example, a semi-dwarf and early-growth methodology was proposed by Huang et al, based on the dwarf rice breeding [1]. A model of large and erect panicle suggested by Yang et al was successfully practiced in northeastern China [17]. A new plant type emphasizing large panicle and less tillering capacity was proposed by IRRI. Recently, Yuan put forwarded a model for super hybrid, which was characterized by erect, narrow and thick uppermost three leaves with lasting function [18]. These models supported rice breeding with solid basis for both theory and practice. From the models mentioned above, the authors noticed that a rice plant ideotype may include two aspects, i.e., basic type and ecotype [7, 19]. The basic type contains common traits in all models of ideotype, such as a short and strong basal internode, erect uppermost leaves, while ecotype contains the traits corresponding to environmental conditions, such as plant height, leaf length and width [19]. Furthermore, we realized that a suitable plant type should be varying to correspond with growth stage. Thin leaves and loose plant type were beneficial for enlarging leaf area and rapid growth during early stage, while, thick and erect leaves increasing biomass and seed set by improving ecological and physiological traits of rice colony during middle and late growing stages [20]. According to the climatic and ecological conditions, along with the author's thinking and breeding practices on rice plant type, a model of ideotype for super hybrid rice (indica) in the lower reaches of Yangtze River Valley was proposed as follows: a length of cm and width of 2 cm for the top leaf, and cm and cm in length, respectively, for the second and third leaves from the top. In addition, having angle by 5, 10, 15, respectively, and a curvature of cm -1 for leaf face at heading stage; uppermost three leaves keeping their activities long as 70 d, which led to a LAI as 3 at full ripening stage; loose plant type by thin [Specific leaf weight (SLW)=2.5-3 mg/cm 2, dry weight] and

5 LU Chuan-gen, et al. Practice and Thought on Developing Hybrid Rice by Exploiting Inter-subspcific Heterosis 5 curve-slant leaves during early growing period, and compact plant type by thick (SLW=4-5 mg/cm 2 ) and erect leaves during middle and late growing periods; with a coefficient of light extinction as , which allowed to contain an optimal LAI high as 8-10 during middle growing period; plant height of cm, with 2-4 cm basal internode and a long uppermost internode occupying 45% of total stem length; cm panicle with 8-10 spikelets per centimeter and showing bend-type in ripening; a rich chlorophyll which led to a high net photosynthetic rate, and tolerance to light shading and photooxidation which was benefit of increasing the adaptability to varying light conditions; million panicles per hectare, spikelets per panicle, and seed setting rate of 85% [7, 19, 21]. Rice breeding based on ecological conditions The yield potential of a rice plant growing in an environment would be expressed according to the adaptability to the ecological conditions. Rice breeding should base on the ecological resources. The lower reaches plain of Yangtze River Valley belongs to a sub-region of The Central China Double and Single Rice Cropping Region in Chinese regionalization. Rice is commonly sowed there in May and harvested in October. The climatic and ecological conditions are as below: during May and first half in June, temperature, light and water are sufficient for rice sowing and growing; the second half in June and the first ten days in July used to be rainy and wet, with deficient light intensity and less daily temperature fluctuation; from the middle of July to the middle of August, always accompanied with high temperature and strong solar light; from the middle of August to the middle of September, used to be attacked twice or three times by typhoon accompanied with heavy rain; in September, having a suitable temperature and light for growing and milking; in October, with a suitable temperature, light and water for rice milking, except for occasional cool-dry weather [19]. According to such conditions, rice breeding for there should pay attention to tillering ability at early growing stage for ensuring adequate tillers under lower temperature and light intensity during June. At the middle growing stage, high chlorophyll content, which led to high net photosynthetic rate and tolerance to shading and photooxidation, is important to mitigate the damage caused by high temperature and intensive light during July and August. A plant height of cm is in favor of biomass accumulation and lodging tolerance when attacked by typhoon accompanied with heavy rain. Resistance to early senescence at cool weather is necessary as well for a long growth hybrid [19]. For a released hybrid, suitable planting areas, favorable and safe temperatures for flowering and fertilization, and suitable seasons of sowing and heading for various cropping systems could be determined according to its biological and ecological characteristics [22]. These data would be helpful for the hybrid to express its maximum yield potential. ACKNOWLEDGEMENTS The research was supported by China National High Technical Project (2003AA212040; 2002AA ). REFERENCES 1 Huang Y X, Lin Q S. Thinking and practice on rice breeding methods for high yield and good grain quality. Guangdong Agric Sci, 1994, (4): 1-6. (in Chinese) 2 Lu C G, Zou J S, Ikehashi H. Super hybrid rice breeding in China. Agric & Horticul, 2002, 77 (7): (in Japanese) 3 Ikehashi H, Araki H. Screening and genetic analysis of wide-compatibility in F 1 hybrid of distant crosses in rice (Oryza sativa L.). Tsukuba: Tropical Agriculture Research Center, Ikehashi H, Zou J S, Huhn P M, Maruyama K. Compatibility genes and indica-japonica heterosis in rice for temperate countries. In: Virmani S S. Hybrid Rice Technology. Manila: IRRI, Zou J S, Nie Y Q, Pan Q M, Fu C X. The tentative utilization of wide-compatibility strain in indica-japonica hybrid rice. Sci Agric Sin, 1989, 22 (1): (in Chinese with English abstract) 6 Lu C G. Survey and application of segregation-distortionneutral alleles to improve pollen fertility in indica-japonica hybrid rice breeding. Doctoral thesis. Kyoto: Kyoto University, Zou J S, Lu C G, Wang C L, Zong S Y, Zhao L, Sun Y H. Thinking and practice on breeding for two-line hybrid rice between subspecies with high yield and good quality. In:

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