JXB Advance Access originally published online on August 30, 2005
Journal of Experimental Botany 2005 56(420):2745-2753; doi:10.1093/jxb/eri267
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RESEARCH PAPER |
Identification and physiological analyses of a locus for rice yield potential across the genetic background
National Institute of Agrobiological Sciences, Kannondai 2-1-2, Tsukuba, Ibaraki 305-8602, Japan
* To whom correspondence should be addressed. Fax: +81 29 8388347. E-mail: kenshi{at}nias.affrc.go.jp
Received 13 June 2005; Accepted 18 July 2005
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Abstract |
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A new locus responsible for increased yield potential across the genetic background in rice (Oryza sativa L.) was identified and evaluated. Quantitative trait loci (QTLs) were analysed for the ratio of filled grains, a yield component, in backcrossed inbred lines of a japonica ‘Nipponbare’xindica ‘Kasalath’ cross for 3 years. Only one QTL (rg5), with a positive Kasalath allele, was detected across environments (years). The physiological functions of rg5 were clarified in a near-isogenic line (NILrg5) with a Kasalath chromosome segment containing rg5 in a Nipponbare genetic background. In NILrg5, carbohydrate storage capacity before heading or sink activity in the first or last stages of the reproductive phase was significantly higher than in Nipponbare (control). The ratio of filled grains and yield per plant were significantly higher in NILrg5 than in Nipponbare, by 5% (P <0.01) and 15% (P <0.05), respectively. These results suggest that rg5 improves carbohydrate storage capacity and keeps sink activity higher in the reproductive stage, and consequently increases yield potential. Greater capacity to accumulate carbohydrate is the main target for increasing rice yield potential; therefore, rg5 might function under other genetic backgrounds. Substitution of the rice cv. Kasalath chromosome segment containing rg5 gave higher yield potential in the top premium rice cv. Koshihikari. These results suggest that rg5 might be able to affect yield under different genetic backgrounds, and physiological analyses of the targeted locus might reveal these effects.
Key words: Quantitative trait loci (QTL), yield
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Sink or source capacity broadly determines yield potential in cereals. In rice (Oryza sativa L.), yield potential has four components (1000-grain weight, panicle number per plant, grain number per panicle, and ratio of filled grains). Total grain number, as calculated by the product of grain number per panicle and panicle number per plant, is used as an index of sink size (Horton, 2000
). To improve yield potential, sink size has been increased by rice breeding programmes (Yang et al., 2002), but the remaining problem is the poor ratio of filled grains (Yuan, 1998). This ratio is directly connected to the quality of rice and is very important, not only for increasing production but also to increase marketability (Matsushima et al., 1966).
The ratio of filled grains is strongly affected by environmental factors via source in addition to sink capacity (Matsushima et al., 1966). For example, in northern Japan in 1993, a chilly summer markedly reduced source capacity and dramatically damaged the ratio of filled grains and the yield (Shimada et al., 1995). On the other hand, 1000-grain weight is nearly completely governed by genetic factors and is barely dependent on environmental factors. To reduce the risk of annual changes in yield, the source capacity of cereals must be stabilized.
In rice, the source capacity is estimated as the total quantity of carbohydrate available from both resources by photosynthesis after heading and accumulation (mainly in the stems) before heading. Under normal condition, the proportions attributable to both are estimated to be 70% and 30%, respectively (Cook and Yoshida, 1972). Photosynthesis depends on environmental factors, whereas the accumulation of carbohydrate occurs by integration over a long period and is barely affected by environmental factors. Researchers have tried various genetic or gene engineering methods to increase photosynthetic ability (Mann, 1999). Unfortunately, however, it has not yet been reported that photosynthetic ability can be upgraded to improve yield potential (Dunwell, 2000; Horton, 2000).
Improvement in carbohydrate storage capacity before heading is proposed to contribute to higher yield in rice (Dingkuhn et al., 1991; Kumura, 1995). In addition, when photosynthesis is limited by adverse conditions such as low temperature after heading, the carbohydrate accumulated before heading increases in importance in grain filling (Gallagher et al., 1976). However, no clear target has been set for improvement of this capacity, because the mechanism of carbohydrate storage in rice is complex. Before heading, the stem accumulates carbohydrate as a sink organ. After heading, the stem changes its role to that of a source organ, and carbohydrate is exported into the panicles (Yoshida, 1972). Perez et al. (1971) suggested that the function of the stem is controlled by complex mechanisms that involve various genes.
Combining physiology and quantitative genetics is valuable in physiology and functional genomics (Hirel et al., 2001; Limami et al., 2002; Ishimaru et al., 2004). Allelic variations have been developed within rice, and the use of near-isogenic lines (NILs) is an effective method of characterizing quantitative trait loci (QTLs) in detail (Ishimaru, 2003; Kashiwagi and Ishimaru, 2004). In a previous study, a locus determining 1000-grain weight (tgw6) was identified and it was found that tgw6 improved the capacity to accumulate carbohydrate and, consequently, increased yield potential by as much as 15% compared with a control (rice cv. Nipponbare) (Ishimaru, 2003). QTLs for grain number per panicle have been mapped on rice chromosomes (Ishimaru et al., 2001c). Many other researchers have reported on QTLs for the ratio of filled grains in rice (Lin et al., 1996; Lu et al., 1996; Xiao et al., 1996; Cui et al., 2003). However, the physiological functions of these QTLs have not been clarified.
By selection with DNA markers, the results of QTL analyses in breeding can be used; this method is called marker-assisted selection (MAS). A common approach in MAS is to map QTLs in a small sample of progeny from a cross, choose a marker linked to the targeted QTL, and apply MAS in a larger set of progeny from the same cross. This has been done for agronomic traits in barley (Hordeum vulgare; Zhu et al., 1999) and for blight resistance in chickpea (Cicer arietinum; Millan et al., 2003) and rice (Davierwala et al., 2001). However, many studies have shown that the effects of QTLs are not detected in other genetic backgrounds (McKendry et al., 1996; Toojinda et al., 1998), and this phenomenon is a major obstacle to the efficient use of MAS in breeding. Factors determining the most important traits in agriculture (i.e. yield, plant height) have been studied in the fields of physiology and biology; therefore, physiological analyses of the function of the locus might suggest its effect under another genetic background.
With the ultimate aim of understanding the mechanism determining rice yield, an attempt was made to identify a locus responsible for the ratio of filled grains and to elucidate its physiological function. In addition, it has been demonstrated that the physiological analyses of a locus could predict its effect under a different genetic background.
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Materials and methods |
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Plant materials for QTL analyses
Ninety-eight BC1F8 lines (hereafter referred to as backcross inbred lines (BILs)) were developed from a backcross of rice (Oryza sativa) cultivars Nipponbare/Kasalath//Nipponbare by the single-seed descent method at the National Institute of Agrobiological Sciences, Japan. Seeds of these BILs and their two parental lines were sown at the beginning of May 1996, 1997, and 1998 in a greenhouse. The seedlings were transplanted at the beginning of June and were grown under natural conditions at Tsukuba (latitude 36° N) with three replications of 10 plants in a randomized complete design. The ratio of filled grains was defined as the number of grains that sank to the bottom of a beaker filled with salt solution with a specific gravity of 1.06, as a percentage of the total number of grains (Yamamoto et al., 1991
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Environmental conditions
In 1997, the duration of sunshine was about 50% higher in June and 33% lower in September than in 1996, but similar in the other 2 months to values in 1996 (Fig. 1). The mean temperatures in 1998 were similar to those in 1996 except in September (when it was higher), although the duration of sunshine was half that in 1996. Sunshine duration in 2000 was almost the same as in 1996 and the mean temperature was higher by 2 °C from July to September than in 1996. In 2003, the sunshine duration was about 29% in July and 67% in August of the 1996 values, and the mean temperature was lower by 3.5 °C in July and higher by 2 °C in September than in 1996. (Source: Japan Meteorological Agency; http://www.jma.go.jp/JMA_HP/jma/indexe.html.)
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QTL analysis, and selection and growth conditions of NILs
Chromosome locations of putative QTLs were determined by single-point analysis by the General Linear Model procedure of QGENE version 3.06 (Nelson, 1997
) according to the method of Ishimaru et al. (2001c). A probability level (P) of 0.01 was used as the threshold to detect significant differences in mean values of two genotypic classes, homozygous for Nipponbare and homozygous for Kasalath alleles (Ishimaru et al., 2001c). To represent a QTL on the map, the chromosome region corresponding to log(LOD) >LODmax–1 was selected with a LOD–1 interval method (Ishimaru et al., 2001b). Using marker-assisted selection, NILrg5 was selected, which carried a chromosomal segment from Kasalath for a QTL for the ratio of filled grains (rg5). The positions of QTLs for grain number per panicle have been reported by our group (Ishimaru et al., 2001c). A NIL was also selected that carried chromosome segments from Kasalath, including a QTL for grain number per panicle, on chromosome 8 in the genetic background of Nipponbare (tentative named NILgn8). At this QTL, Kasalath had the positive allele. These NILs and Nipponbare (control) were grown under natural conditions at Tsukuba in 2000, with three replications of 20 plants in a randomized complete design.
Measurements of carbohydrate contents in the canopy
Two days before heading, five canopies in Nipponbare or each NIL were sampled and then dried at 80 °C for 2 d. Samples of approximately 50 mg dry weight were powdered in liquid nitrogen with a mortar and pestle and extracted twice with 80% (v/v) ethanol at 80 °C. Each sample was centrifuged at 12 000 g for 10 min. The pellets were boiled in distilled water for 2 h, then digested with amyloglucosidase for 15 min at 55 °C. Starch contents were measured enzymatically, as described by Ishimaru et al. (2001a), and all enzymes used in these procedures were obtained from Boehringer Mannheim GmbH (Mannheim, Germany). Senescence of flag leaves was observed by changes in chlorophyll content per area. For chlorophyll measurements, one or two fresh leaf discs (0.28 cm2) were taken from each flag leaf and soaked in 1 ml of 96% (v/v) ethanol. Chlorophyll content was measured by the method of Wintermans and De Mots (1965).
Photosynthetic activity
Rates of photosynthetic CO2 assimilation were measured with a portable gas-exchange system (LI-6400; Li-Cor Inc., Lincoln, Nebraska, USA). Measurements were made on intact flag leaf blades between 11.00 h and noon, 2 d after heading. Light was provided by an LED source (red/blue, 6400-02 LED source; Li-Cor Inc.). For the measurement of photosynthetic CO2 assimilation rates, the photon flux density was 1200 µmol photons m–2 s–1, leaf temperature was 25 °C, and the reference CO2 concentration was 350 µl l–1 according to the method of Ono et al. (1999). After heading, the chlorophyll content of the flag leaves was measured with the above-mentioned method.
Sink activity in the panicle after heading
The sink activity of the panicle in the reproductive stage was evaluated as the increase in dry weight of the panicle per unit of dry weight present per unit of time, according to Usuda et al. (1999). That is, it was estimated from the equation:
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Selection of a chromosome segment substitution line (CSSL) with rg5 and growth conditions
Koshihikari, a japonica top premium rice, is cultivated in 40% of the paddy fields in Japan and in various other countries (i.e. Taiwan, USA, Australia, Vietnam, and China) and commands a high price (official statistics by the Ministry of Agriculture, Forestry and Fisheries of Japan: http://www.tdb.maff.go.jp/toukei/a02stopframeset). CSSLs were developed from backcrosses of KoshihikarixKasalath. The genotype of each line was determined by using 130 restriction fragment-length polymorphism markers distributed along the 12 rice chromosomes (http://www.rgrc.dna.affrc.go.jp/ine39.html). CSSLrg5 substituted by a chromosomal segment from Kasalath underlying rg5 was selected from among them by genetic data, and this line and Koshihikari were sown at the beginning of May 2003 in a greenhouse. The seedlings were transplanted at the beginning of June and were grown under natural conditions at Tsukuba with three replications of 10 plants in a randomized complete design. The ratio of filled grains was measured by the method mentioned above.
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Results |
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Phenotypic variation in BILs
Transgressive segregants were observed in each year, and showed continuous variation in the ratio of filled grains (Fig. 2A). The ratio of filled grains in BILs varied with the year: the ratios were highest in 1998 when the duration of sunshine was lower than those in the other two years. The average ratio in cv. Nipponbare over the 3 years was 89.6±4.3%, about 1.2 times that in cv. Kasalath (73.1±10.1%). The coefficients of determination (r2) in the ratio of filled grains were 0.39 between 1996 and 1997, 0.48 between 1997 and 1998, and 0.29 between 1996 and 1998 (P <0.01).
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QTL analysis of the ratio of filled grains over three years
Putative QTLs for the ratio of filled grains were localized on a rice genetic map under three different environments (years). The positions of QTLs for the ratio of filled grains varied each year, with the exception of one QTL that was detected on chromosome 5 (tentatively named rg5) and had the same nearest marker throughout the three years (Fig. 2B; Table 1). Individual QTLs explained between 8.3% and 19.8% of the total phenotypic variation (R2). Kasalath had a positive allele across environments at rg5.
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On chromosome 5, peak LOD scores occurred at the same locus in all three years; rg5 had the highest LOD score near marker R2558 (Fig. 3A). From among a series of NILs developed by the National Institute of Agrobiological Sciences, Japan (http://www.rgrc.dna.affrc.go.jp/index.html.en), a NIL was selected that carried a Kasalath chromosomal segment containing rg5 under a Nipponbare genetic background (NILrg5). QTLs for grain number per panicle have been reported in the same plant materials; among them, only one QTL, on chromosome 8 of Kasalath (tentatively named gn8), has a positive effect (Ishimaru et al., 2001a
). Therefore a NIL that carried a Kasalath chromosomal segment containing gn8 under a Nipponbare background (NILgn8) was selected (Fig. 3B). These NILs were used for further biochemical and physiological analyses to identify loci that increase yield in Nipponbare, a high-yielding cultivar grown in Japan.
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Physiological and morphological characters and contents of accumulated carbohydrate
Flag leaf area was significantly smaller in NILrg5 than in Nipponbare, but the photosynthetic rates in the flag leaves were the same among plants (Table 2). Before heading, the contents of accumulated carbohydrate (starch, sucrose, and total carbohydrate) were measured in the canopy. In NILrg5, the starch content was significantly higher than in Nipponbare (at 165%). The sucrose or total carbohydrate content was also significantly higher. There were no significant differences in the content of starch, sucrose, or total carbohydrate between NILgn8 and Nipponbare. There was no difference in plant height (the length from the ground to the top of ear), heading date, or the progression of senescence between NILrg5 and Nipponbare, as indicated by the rate of decrease in chlorophyll content of the flag leaves (data not shown). The photosynthetic rate, flag leaf area, and starch content in the canopy of Nipponbare have been reported previously (Ishimaru, 2003
).
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Sink activity in the panicle
At 10 d after heading, sink activity in the panicle of NILrg5 was 2.3 times that in Nipponbare and 3.2 times that in NILgn8 (Fig. 4). There was no difference in sink activity among plants during the period from 20–30 d after heading. In the last period of the reproductive stage (at 40 d after heading), sink activity in NILrg5 was significantly higher than that in Nipponbare or NILgn8 (P <0.01). Nipponbare and NILgn8 showed the same tendency in sink activity throughout the reproductive stage. The ratio of the number of caryopses on the first and second rachis-branches of the panicle was the same in Nipponbare and NILrg5 (data not shown).
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Yield and yield components in NILs
The ratio of filled grains in NILrg5 was significantly higher than in Nipponbare (P <0.01) (Table 3). Compared with Nipponbare, the 1000-grain weight and yield per plant in NILrg5 were 8% higher (P <0.01) and 15% higher (P <0.05), respectively. The grain number per panicle and the panicle number per plant in NILrg5 were the same as in Nipponbare. In NILgn8, the grain number per panicle was 1.4 times that in Nipponbare, but the ratio of filled grains was significantly lower (P <0.01). There was no difference in sink size, calculated as the product of panicle number per plant and grain number per panicle, between NILrg5 and Nipponbare (data not shown).
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Effects of rg5 under a different genetic background
A chromosome segment substitution line that carried the Kasalath chromosome segment containing rg5 (CSSLrg5) in a Koshihikari genetic background was selected; this was developed by the National Institute of Agrobiological Sciences, Japan (http://www.rgrc.dna.affrc.go.jp/ine39.html). The ratio of filled grains in CSSLrg5 was 30% higher than that in Koshihikari (P <0.01; Fig. 5). There were no differences between Koshihikari and CSSLrg5 in the yield components related to sink size (panicle number per plant or grain number per panicle) (data not shown). The content of accumulated carbohydrate before heading was significantly higher in CSSLrg5 than in Koshihikari (data not shown).
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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A new locus was identified for yield potential across environments (years) and genetic background. QTL expression is sensitive to environmental conditions (Sari-Gorla et al., 1997
), and the ratio of filled grains (the main target in rice breeding) depends on environmental factors (Matsushima et al., 1966). It is important to identify a QTL with a stable effect across environments, not only in breeding but also in analyses of its physiological function. QTLs for the ratio of filled grains were analysed under different environmental conditions for three years.
The ratios of filled grains were highest in 1998, while the sunshine duration was lower than in the other two years (Figs 1, 2A). Similarly, Sagawa et al. (1999) reported the highest ratio in 1998 throughout three years with a rice cultivar (Akitakomachi). The ratio of filled grains is determined by the balance between total grain number and source capacity. This high ratio in 1998 could be explained by the lower total grain number per plant that had been affected by the unsuitable environments in the early stages (Sagawa et al., 1999).
A total of seven QTLs for the ratio of filled grains were detected, but all except one varied each year (Fig. 2B; Table 1). Only a QTL on the middle of chromosome 5 (tentatively named rg5) was detected in each of the three years (Table 1), and at the rg5 peak, the LOD scores for the three years overlapped (Fig. 3A). At rg5, the Kasalath allele had a positive effect on the ratio of filled grains throughout each of the three years (Table 1). These results suggest that the same locus (rg5) might function across environments and be the main target for improving the ratio of filled grains in rice. A NIL carrying a chromosome segment from Kasalath containing rg5 (NILrg5) or a QTL that included grain number per panicle on chromosome 8 (tentatively named NILgn8; Ishimaru et al., 2001c) were used for later physiological analyses to clarify their function.
Compared with Nipponbare, a high-yielding cultivar in Japan (Saitoh et al., 1993), the ratio of filled grains, 1000-grain weight, and yield per plant were significantly higher in NILrg5, at 105%, 108%, and 115%, respectively (Table 3). These results indicate the presence of rg5 and its effect on yield potential. Sink size, as defined by panicle number per plant and grain number per panicle, were the same in Nipponbare and NILrg5. It is generally accepted that, under the same sink capacity, source capacity is closely associated with yield potential. Thus, the higher yield potential of NILrg5 might be due to superiority in source capacity compared with Nipponbare.
Source capacity in rice is determined by the total sum of photosynthate after heading and accumulation before heading (Cook and Yoshida, 1972). At the ripened stage, the flag leaf contributes most to carbohydrate production by photosynthesis (Cook and Evans, 1983). Photosynthetic ability is determined by the rate per leaf area and by the total leaf area (Mann, 1999). The photosynthetic rate of the flag leaves in NILrg5 was the same as in Nipponbare, whereas the flag leaf area was significantly smaller in NILrg5 than in Nipponbare (Table 2). These results suggest that the flag leaf's ability to act as a source after heading was lower in NILrg5 than in Nipponbare. Just before heading in NILrg5, the content of accumulated starch in the canopy was 1.6 times that in Nipponbare. At the end of the ripening stage (40 d after heading), the content of carbohydrate was almost zero in all plants (data not shown). The carbohydrate storage capacity largely determined the higher yield potential in a japonicaxindica F1 hybrid than in common cultivars (Song et al., 1990). In NILrg5, the higher capacity of carbohydrate accumulation could make up for the narrow flag leaf area and improve thr total source capacity and, consequently, the ratio of filled grains.
A NIL that carried a Kasalath chromosomal segment corresponding to tgw6 in a Nipponbare genetic background was analysed to clarify the physiological function of this locus; tgw6 improved the carbohydrate storage capacity and consequently increased yield potential in NILtgw6. The content of starch in NILrg5 was 1.4 times that in NILtgw6 (as calculated from previous results; Ishimaru, 2003). However, yield per plant was the same in NILrg5 and NILtgw6. The photosynthetic ability per area was the same in both NILs, but leaf area was smaller by 24% in NILrg5 than in NILtgw6. These results suggest that the lower photosynthetic ability resulting from the smaller leaf area might be the main reason why NILrg5 could not take full advantage of its higher capacity to accumulate carbohydrate before heading to maximize yield. As a next step, using the progeny of a cross between NILrg5 and Nipponbare, an attempt will be made to analyse the linkage relationship between rg5 and flag leaf area. If these traits can be segregated, rg5 could contribute greatly to yield potential.
Sink activity is a physiological restraint that includes multiple factors and key enzymes involved in carbohydrate utilization and storage (Usuda et al., 1999; Wang et al., 1993). The timing of growth in the caryopses depends on the position of the branches in rice (Mohapatra and Sahu, 1992). From the early stage of grain filling, the ‘superior’ caryopses on the primary rachis-branches start to elongate, but those on the secondary (‘inferior’) positions do so later. Because caryopses compete with each other for carbohydrate translocated from the source, the ratio of filled grains is genetically higher among superior caryopses than among inferior ones (Shimotsubo and Nakayama, 1974). Sink activity in NILrg5 was significantly higher than in Nipponbare or NILgn8 in the first or last stages of the reproductive phase (Fig. 4). There was no difference between Nipponbare and NILrg5 in the morphological characters of the panicle (data not shown). These results suggest that the higher sink activity in NILrg5 in the last stage might contribute to an increase in the ratio of filled grains in ‘inferior’ caryopses; this, in addition to an increase in the ratio in ‘superior’ ones in the early stage, would consequently improve the total ratio. Umemoto et al. (1994) has suggested that low activity of plural starch synthase might be related to a low content of amylose in the inferior caryopsis. The locus increasing the ratio of filled grains (rg5) might have plural functions linked to a few genes. If this is the case, a huge amount of effort to isolate the gene(s) underlying rg5 would not be required; instead, rg5 might be suitable for MAS to improve yield potential.
There was no difference between NILgn8 and Nipponbare in the traits related to source capacity (Table 2). In NILgn8, grain number per panicle (as an indicator of sink size) was 1.4 times that in Nipponbare, but the ratio of filled grains in NILgn8 was significantly lower (by 16%) than in Nipponbare (P <0.01). The increase in sink size by gn8 did not contribute to yield potential; this is similar to the results obtained in indicaxjaponica lines with large grain numbers per panicle (Yang et al., 2002). These results show that, regardless of source capacity, unilateral improvement in sink size could not induce higher yield potential, at least in Nipponbare. Widstrom et al. (2003) have proposed a new method of pyramiding QTLs by crossing among NILs, each with a QTL for enhancing interesting traits. To overcome barriers to rice yield, the introduction of both rg5 and gn8 might contribute to breeding by this method.
These results suggest that rg5 might increase source capacity through greater carbohydrate accumulation before heading and consequently improve yield potential. It has been proposed that this capacity is the key contributor to higher yield potential in rice (Dingkuhn et al., 1991; Kumura, 1995; Ishimaru, 2003) and so rg5 might have an effect under different genetic backgrounds. Kumura (1995) proposed that a higher carbohydrate storage capacity before heading could improve yield potential in rice. A high positive correlation has been detected between carbohydrate accumulation and yield among many modern indica and japonica cultivars and F1 hybrids (Ishikawa et al., 1993). These results suggest that rg5 might increase carbohydrate storage before heading; thus it is hypothesized that rg5 might function in different genetic lines. To elucidate this point, a CSSL line containing rg5 from Kasalath under a Koshihikari genetic background (CSSLrg5) was selected and analysed. The sink size (grain number per plant) was the same in CSSLrg5 and Koshihikari (data not shown), but the ratio of filled grains in CSSLrg5 was significantly higher (by 30%) than in Koshihikari (Fig. 5). The content of accumulated carbohydrate before heading was significantly higher in CSSLrg5 than in Koshihikari (data not shown). These results showed that rg5 could function in another genetic background, i.e. in Koshihikari, which is similar to Nipponbare, and further increase yield potential.
As mentioned in the Introduction, the favourable effects of many QTLs have not been detected in other genetic backgrounds. MAS can be adapted to progeny only from the same cross used and its adaptive flexibility is uncertain in other genetic backgrounds. This is the main reason for the reduced efficiency of MAS in breeding. The close-relation score between Koshihikari and Nipponbare is 0.2188 (http://www.pgcdna.co.jp/). Koshihikari and Nipponbare might have an allele from the same ancestor associated with the ratio of filled grains. The function of rg5 in increasing the capacity to accumulate carbohydrate before heading is consistent with the broad aim of improving yield potential across rice cultivars (Kumura, 1995). It is proposed that a physiological analysis of the targeted locus will be able to clarify the possibility of introducing it into MAS and using it to increase the efficiency of breeding.
In 2003, environmental factors were remarkably unsuitable before heading (July and August) (Fig. 1). The accumulation of carbohydrate occurs by integration over a long period before heading and is barely affected by environmental factors (Gallagher et al., 1976). Therefore, rg5 might have worked to increase the ratio of filled grains in CSSLrg5 under unsuitable environmental conditions in 2003. The environmental factors were very different between the years 2000 and 2004 (Fig. 1) and rg5 increased the ratio of filled grains in both years across rice cultivars. These results suggest that rg5 might function across environmental factors.
In conclusion, a locus increasing yield potential in rice was identified. The results in this study suggest that rg5 might increase carbohydrate storage capacity. In a previous work (Ishimaru, 2003) our group had already found a locus for 1000-grain weight that had a similar function. It is interesting to note that the loci identified as increasing yield potential (tgw6 and rg5) have similar functions, i.e. to increase the capacity to accumulate carbohydrate before heading. These results are strong evidence that increasing the capacity to accumulate carbohydrate would improve yield potential in rice. Furthermore, rg5 can function similarly in other genetic backgrounds and improve yield potential in the premium rice Koshihikari under adverse conditions. These results indicate that rg5 might be very useful in breeding for improvement of rice yield potential.
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Acknowledgements |
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We thank the Rice Genome Project of the National Institute of Agrobiological Sciences, Japan and the Rice Genome Resource Center for donation of rice materials. We also thank Dr H Sasaki of Tokyo University for his kind suggestions. This work was supported by the Program for Promotion of Basic Research Activities for Innovative Biosciences (PROBRAIN).
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