Mapping QTLs for milling yield and grain characteristics in a tropical japonica long grain cross

Percent milling yield is an economically important trait of commercial rice because it largely determines the price that farmers receive for their crop. Analyzing 22 trait variables including milling yield, grain dimensions, chemistry and appearance, we identified 43 quantitative trait loci (QTLs) in a long grain japonica by long grain japonica cross. We report one QTL explaining 20% of the variation in brown rice recovery; two QTLs explaining 14% and 13% of the variation in milled rice recovery; and one QTL explaining 14% of the variation in head rice (HR) recovery. QTLs for the proportion of pre-broken brown rice kernels, seed density, amylose content, and kernel whiteness and chalkiness were found in the same region as the HR QTL. QTLs explaining up to 54% of the variation in grain shape measurements were identified and mapped to areas independent from those identified for milling yield. Analyses of grain appearance traits identified two QTLs for chalk in brown rice and one in head rice, and a QTL explaining up to 33% of the variance in green kernel area. Our results confirm previous findings on the multigenically complex nature of milling yield.     

QTL mapping of grain quality traits in rice

Grain quality improvement is one of the most important goals in a rice breeding program. An indica variety with small grain size was crossed to a japonica variety with large grain size to construct a set of recombinant inbred lines (RILs) which was used to identify quantitative trait loci (QTLs) controlling eight grain quality traits. Based on a linkage map of 185 SSR markers, a total of 16 QTLs were mapped on six chromosomes. A pleiotropic main effect QTL (M-QTL) flanked by RM3204 and RM16 on chromosome 3 influences the grain length (GL), length width ratio (LWR) and head rice ratio (HRR), explaining the phenotypic variation of 46.0, 36.1 and 29.7%, respectively. A total of 18 epistatic QTLs were identified for all the traits except MRR, distributed on all the chromosomes except chromosome 10. Two M-QTLs for GL and one M-QTL for GW were involved in epistatic QTL. No significant interaction between M-QTL or epistatic QTL and environment was detected except AC having significant M-QTL by environment interaction with minor effect. GL and LWR have a significant negative relation with HRR which might make it difficult to develop long grain with higher HRR in the rice breeding practice.     

Molecular genetic mapping of quantitative trait loci for milling quality in rice (Oryza sativa L.)

Identification and mapping of genomic regions controlling quantitative trait loci (QTLs) was undertaken to determine the genomic regions associated with milling traits in rice to facilitate breeding of new rice varieties with high milling quality. The recombinant inbred (RI) population used was derived from cross of a japonica variety, ‘Asominori’, with an indica variety, ‘IR24’ through 289 RFLP markers. Three milling traits, namely, brown rice percentage (BRP), milled rice percentage (MRP), and milled head rice percentage (MHP), which are the main indicators of milling quality in rice, were estimated for each RI line and their parental varieties. Continuous distributions and transgressive segregations of three milling traits were observed in the RI population, showing that the three traits were quantitatively inherited. Two QTLs (qBRP-9 and qBRP-10) for BRP were identified and mapped to chromosomes 9 and 10, and explained 7.2 and 21.3% of the total phenotype variation, respectively. Two QTLs (qMRP-11 and qMRP-12) governing MRP were detected and mapped to chromosomes 11 and 12, accounted for 12.2 and 7.7% of total phenotype variation, respectively. In addition, three QTLs (qMHP-1, qMHP-3 and qMHP-5) controlling MHP were observed and mapped to chromosomes 1, 3 and 5, and explained 16.0, 22.1 and 8.7% of the total phenotype variation, respectively. Among them, five QTLs (qBRP-9, qBRP-10, qMRP-11, qMHP-3 and qMHP-5) from japonica parent, Asominori, and two QTLs (qMRP-12, qMHP-1) from indica IR24 can improve milling quality in rice. The results and the tightly linked molecular markers that flank the QTL will be useful in breeding for improvement of milling quality in rice.     

Influence of high daytime temperature during the grain filling stage on fissure formation in rice

High daytime temperatures during the grain filling stage in rice have negative impacts on milling quality traits. In this study, we used growth chambers to evaluate the influence of high daytime temperature (33 °C) during grain filling, together with grain moisture content at harvest (26%, 18% and 15%), on grain fissure formation. Varietal susceptibility to fissure formation was also evaluated by exposing grains to high temperature at different grain filling stages (milky, dough, maturing). Two fissure resistant varieties: Cypress (long-grain) and Reiziq (medium-grain) and susceptible varieties: YC53-00-7 (long-grain) and Baru (medium-grain) were compared. The average head rice yield (HRY) of Cypress declined from 62.7% at 25 °C to 53.5% at 33 °C, while Reiziq declined from 56.2% (25 °C) to 47.4% (33 °C). Both were significantly higher than the HRY of YC53-00-7 (39.2% and 24.9%) and Baru (39.3% and 31.7%) at 25 °C and 33 °C, respectively. When grains were drier at harvest (15% cf. 26%) there was a greater reduction in HRY. When the four varieties were exposed to high temperature, the highest average reduction of HRY was recorded at 21 days after heading. It is important to choose an optimal sowing date to avoid coincidence of the final grainfilling stage with high temperatures, in order to minimize milling quality losses.     

Association mapping of grain quality and flowering time in elite japonica rice germplasm

Grain quality traits play an important role in the economic prosperity of commercial rice markets. The objective of our research was to identify candidate molecular markers associated with three grain quality and flowering traits in a collection of elite rice japonica inbred lines evaluated in five U.S. states. Candidate marker effects were associated with the traits mapped within regions reported from previous QTL analyses while several new allelic interactions were also detected. Common markers for each trait were observed across two or more locations, and two-way interactions unique to a single location were also found. Significant genotype × location interactions were detected while broad-sense heritability estimates were low for all characters. All but one selected marker effect was associated with a reduction in apparent amylose content. A reduction in heading date at three locations was observed with one marker as a main effect or as a component of two-way interactions that mapped ∼ 5 cM from the Hd3a flowering locus. The majority of selected effects for head rice were associated with modest to substantial increases in value. Marker loci and their interactions identified in this study highlighted targeted regions for future association studies and marker-assisted breeding efforts of grain quality traits.     

稻米粒形和垩白度的QTL定位和上位性分析

 利用由181个家系组成的Lemont/特青籼粳交重组自交系群体,以及由161个RFLP、SSR标记和3个形态标记构建的全长为1916.5 cM、覆盖水稻基因组12 条染色体的连锁图,采用线性模型的复合区间作图方法（QTLMapper V10）,对粒长、粒宽、长宽比和垩白度等4个稻米品质性状的数量性状座位（QTL）进行了分析。在水稻的所有12 条染色体上共定位到7个加性主效QTL和19对上位性QTL,其中控制粒长、粒宽、长宽比的主效QTL各2个,控制垩白度的QTL 1个,分别解释12.8%、40.0%、26.0%和42.1%的表型变异;共检测到6对影响垩白度、6对影响粒长、7对影响长宽比的上位性QTL,分别解释52.2%、31.3%和38.2% 的表型变异。结果表明,上位性QTL和主效QTL一样在稻米粒形和垩白度的遗传中起着重要的作用。     

利用染色体片段置换系群体定位和分析水稻粒重和粒型QTL

[目的]挖掘水稻粒重和粒型相关性状QTL,对于解析水稻籽粒遗传机理具有重要作用.[方法]本研究以籼稻9311为受体、粳稻日本晴为供体构建的染色体片段置换系(Chromosome Segment Substitution Lines,CSSLs)群体为材料,在4个环境下对控制稻谷与糙米的粒重和粒型QTL进行了定位分析.[...     

Identification and validation of a novel major QTL for harvest index in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Background

Harvest index (HI) in rice is defined as the ratio of grain yield (GY) to biomass (BM). Although it has been demonstrated that HI is significantly related to yield and is considered as one of the most important traits in high-yielding rice breeding, HI-based high-yielding rice breeding is difficult due to its polygenic nature and insufficient knowledge on the genetic basis of HI. Therefore, searching for rice varieties with high HI and mapping genes associated with high HI can facilitate marker-assisted breeding for high HI in rice.

Results

Yuexiangzhan, a popular indica cultivar with good reputation of high HI was crossed with Shengbasimiao, an indica cultivar with lower HI to develop a recombinant inbred line population, and QTL mapping for HI and its component traits was conducted. In total, five QTLs for HI, three QTLs for GY, and six QTLs for BM were detected in two-year experiments. Among the three GY QTLs, one co-located with the HI QTL on chromosome 8, while the other two co-located with the two tightly-linked BM QTLs on chromosome 3. The co-located QTLs in each of the chromosomal regions produced additive effects in the same direction. Particularly, the HI QTL on chromosome 8, qHI-8, could be detected across two years and explained 42.8% and 44.5% of the phenotypic variation, respectively. The existence of qHI-8 was confirmed by the evaluation of the near isogenic lines derived from a residual heterozygous line, and this QTL was delimitated to a 1070 kb interval by substitution mapping.

Conclusion

In the present study, the detected GY QTLs overlapped with both HI QTL and BM QTL, suggesting a positive relationship between GY and HI or BM, respectively. With an understanding of the genetic basis for grain yield, harvest index and biomass, it is possible to achieve higher yield through enhancing HI and BM by pyramiding the favorable alleles for the two traits via marker-assisted selection (MAS). As qHI-8 has a large phenotypic effect on HI and expresses stably in different environments, it provides a promising target for further genetic characterization of HI and MAS of high HI in rice breeding.

     

Mapping of QTLs for Germination Characteristics under Non-stress and Drought Stress in Rice

Identification of genetic factors controlling traits associated with seed germination under drought stress conditions, leads to identification and development of drought tolerant varieties. Present study by using a population of F2：, derived from a cross between a drought tolerant variety, Gharib （indica） and a drought sensitive variety, Sepidroud （indica）, is to identify and compare QTLs associated with germination traits under drought stress and non-stress conditions. Through QTL analysis, using composite interval mapping, regarding traits such as germination rate （GR）, germination percentage （GP）, radicle length （RL）, plumule length （PL）, coleorhiza length （COL） and coleoptile length （CL）, totally 13 QTLs were detected under pole drought stress （-8 MPa poly ethylene glycol 6000） and 9 QTLs under non-stress conditions. Of the QTLs identified under non-stress conditions, QTLs associated with COL （qCOL-5） and GR （qGR-1） explained 21.28% and 19.73% of the total phenotypic variations, respectively Under drought stress conditions, QTLs associated with COL （qCOL-3） and PL （qPL-5） explained 18.34% and 18.22% of the total phenotypic variations, respectively. A few drought-tolerance-related QTLs identified in previous studies are near the QTLs detected in this study, and several QTLs in this study are novel alleles. The major QTLs like qGR-1, qGP-4, qRL-12 and qCL-4 identified in both conditions for traits GR, GP, RL and CL, respectively, should be considered as the important and stable trait-controlling QTLs in rice seed germination. Those major or minor QTLs could be used to significantly improve drought tolerance by marker-assisted selection in rice.     

QTL analysis of panicle morphology response to irrigation regime in aerobic rice culture

In water-efficient rice production, grain yield is often constrained by panicle size. The objective of this study was to genetically dissect the response of panicle morphology to irrigation regimes in aerobic rice culture. We grew ‘Akihikari’ (a lowland japonica cultivar) × ‘IRAT109’ (an upland japonica cultivar) backcross inbred lines in aerobic soils with full or limited irrigation for 2 years, and examined 4 panicle traits—number of florets per panicle (FPP), number of primary branches per panicle (BPP), number of florets per primary branch (FPB), and frequency of pre-flowering floret abortion (%FA)—and grain yield. QTLs for BPP were detected in both the irrigation regimes but QTLs for FPB and %FA were detected mostly only in either of the irrigation regimes. The QTL for FPP on chromosome 2 (RM3421–RM213) coincided with that for yield under full irrigation, showing that this QTL is related to sink capacity and yield potential in aerobic rice culture. On the other hand, the QTL for FPB on chromosome 1 (RM3148–RM243) coincided with that for yield under limited irrigation, when water deficit was moderate. The QTL for root axis length at vegetative stage, previously identified in the same mapping population, was located near this region. This study unravelled the complicated genetic control on panicle morphology in aerobic rice culture, and suggested the positive roles of the dehydration avoidance mechanism by vigorous root growth on panicle size and yield under dry soil conditions.     

Localization,validation and characterization of plant-type QTLs on chromosomes 4 and 6 in rice (Oryza sativa L.)

Improvement of rice (Oryza sativa L.) plant type is a major breeding objective. This study aimed to precisely localize and characterize key genomic regions for plant type using near-isogenic individuals. Selfing of partially heterozygous F₅ recombinant inbred (RI) individuals [parental varieties Milyang 23 (M23) and Akihikari (AK)] developed heterogeneous inbred families (HIFs) composed of 108 and 93 F₇ progenies, which segregated at molecular marker loci on the long arms of chromosomes 4 and 6, respectively. The progeny lines (F₈) were evaluated for traits composing plant type in Los Baños, Philippines, to locate quantitative trait loci (QTLs) using interval mapping and to evaluate the effects of the QTL region by phenotypic comparison between the genotypes. QTLs for the traits were detected in 17 cM across XNpb12 on chromosome 6. The M23 homozygote for the QTL region resulted in a >7% increase and decrease in plant length and tiller number at heading, respectively, relative to the AK homozygote. Consequently, culm length (CL) and traits determining flag-leaf and panicle sizes increased 5–56% by the M23 homozygote, together with a 15% decrease in panicle number. For a QTL region detected in 6 cM across XNpb235 on chromosome 4, the AK homozygote had similar effects on these traits, except CL. The directions and magnitudes of their effects agreed with those previously estimated in the RI line population, thus increasing confidence in primary QTL analyses for plant type. Analyzing the HIFs validated and characterized the two QTL regions greatly involved in determining varietal plant type from an early growth stage to maturity, providing information useful for empirical and marker-assisted breeding towards rice improvement.     

Fortification of rice with folic acid using parboiling technique: Effect of parboiling conditions on nutrient uptake and physical characteristics of milled rice

Parboiling was used as a technique for folic acid fortification in brown rice. A range of parboiling conditions (i.e. 70 °C soaking for 1, 2 and 3 h with 0.15, 0.3, 0.6 and 1.2 g folic acid/300 g brown rice and milled for 0, 60 and 120 s) were studied to investigate the uptake of folic acid in the fortified rice. Approximately 20–300 μg folic acid/g was found in the fortified rice and an average of 90% of folic acid was retained in the washed and cooked fortified rice. The quality of the fortified rice was evaluated based on head rice yield (HRY), kernel dimensions and color. No significant change in HRY and grain dimensions were observed between raw and fortified rice, suggesting the economic value of rice will not be compromised with the fortification process and rice uniformity may likely be achieved after mixing with the untreated rice. Despite the inherent yellow color of folic acid, in comparison with parboiled rice, a significant difference in yellowness was exhibited only in rice fortified at 1.2 g folic acid/300 g brown, but not among those fortified at lower fortificant concentrations. The difference in lightness (ΔL^∗) (between fortified rice and parboiled rice) was reduced as soaking duration increased.     

不同生态地点下稻米外观品质性状的QTL定位分析

 利用两个已测序品种93 11和日本晴为亲本,采取单粒传法创建由190个家系组成的重组自交系群体,并构建了包含178个SSR、CAP和STS标记的遗传连锁图谱。采用复合区间作图方法,在3个不同生态地点(陵水、合肥和怀远)对垩白(垩白率、垩白大小、垩白度)、粒形(粒长、粒宽、长宽比)等6个外观品质的数量性状基因座（QTL)进行了定位分析。共定位到39个QTL,单个性状QTL数目在6~7个,说明垩白和粒形是多基因控制的数量性状。8个QTL可在2个以上地点检测到,其中,两个垩白度相关QTL qCD 1、qCD 3(贡献率分别为288%、321%)在3个地点同时检测到;11个QTL具有一因多效性,单个QTL位点控制的性状为2~6个,如第3染色体RM16-RM143区段控制垩白率、垩白大小、垩白度、粒长、粒宽和长宽比等6个性状。比较3个地点的检测结果,发现外观品质性状的QTL定位都受环境影响,但不同性状受影响的程度不同。长宽比和垩白度受环境影响较小,粒宽受环境影响较大。     

Upland rice grown in soil-filled chambers and exposed to contrasting water-deficit regimes: II. Mapping quantitative trait loci for root morphology and distribution

Root morphological characteristics are known to be important in the drought resistance of some rice (Oryza sativa L.) varieties. The identification of quantitative trait loci (QTLs) associated with root morphology and other drought resistance-related traits should help breeders produce more drought resistant varieties. Stability in the expression of root growth QTL across rooting environments is critical for their use in breeding programs. A greenhouse experiment in which a mapping population of 140 recombinant inbred lines and the parental varieties Bala and Azucena were grown in glass-sided soil chambers and evaluated for root growth and water uptake was conducted. In each of 2 years, two treatments were used; an early water-deficit (WD0) in which seeds were sown into wet soil but received no more water, and a late water-deficit (WD49) in which the plants were watered for 49 days and then received no water for a week. The major differences between treatments and years in dry matter partitioning and root growth traits are reported elsewhere. Here, the identification of QTLs for root growth traits by composite interval mapping is described. At LOD>3.2, there were six QTLs for the weight of roots below 90 cm and maximum root length, 11 for root to shoot ratio, 12 for the number of roots past 100 cm, and 14 for root thickness. A total of 24 regions were identified as containing QTLs (these regions often contained several QTLs identified for different root traits). Some were revealed only in individual experiments and/or for individual traits, while others were common to different traits or experiments. Seven QTLs, on chromosomes 1, 2, 4, 7, 9 (two QTLs) and 11, where considered particularly noteworthy. The complex results are discussed in the context of previously reported QTLs for root growth in other populations, the interaction between QTL with the environment and the value of QTLs for breeding.     

Proteomic analysis of a disease-resistance-enhanced lesion mimic mutant spotted leaf 5 in rice

Background

Rice is a major source of dietary intake of arsenic (As) for the populations that consume rice as a staple food. Therefore, it is necessary to reduce the As concentration in rice to avoid the potential risk to human health. In this study, the genetic diversity in As accumulation and As speciation in rice grains was investigated using a world rice core collection (WRC) comprising 69 accessions grown over a 3-year period. Moreover, quantitative trait locus (QTL) analysis was conducted to identify QTLs controlling the dimethylarsinic acid (DMA) content of rice grains.

Results

There was a 3-fold difference in the grain As concentration of WRC. Concentrations of total-As, inorganic As, and DMA were significantly affected by genotype, year, and genotype-year interaction effects. Among the WRC accessions, Local Basmati and Tima (indica type) were identified as cultivars with the lowest stable total-As and inorganic As concentrations. Using an F₂ population derived from Padi Perak (a high-DMA accession) and Koshihikari (a low-DMA cultivar), we identified two QTLs on chromosome 6 (qDMAs6.1 and qDMAs6.2) and one QTL on chromosome 8 (qDMAs8) that were responsible for variations in the grain DMA concentration. Approximately 73% of total phenotypic variance in DMA was explained by the three QTLs.

Conclusions

Based on the results provided, one strategy for developing rice cultivars with a low level of toxic As would be to change the proportion of organic As on the basis of a low level of total As content.     

Mapping quantitative trait loci (QTLs) for seedling-vigor using recombinant inbred lines of rice (Oryza sativa L.)

Seedling-vigor is important for optimum stand establishment and increasing weed competitive ability in rice cropping systems. In the current study, three seedling-vigor-related traits, seed germination rate, seedling shoot length and dry matter weight, were investigated by the paper-roll tests with rice recombinant inbred lines derived from a cross between Lemont (japonica) and Teqing (indica). The phenotype data, together with a linkage map consisting of 198 marker loci, was used to conduct composite interval mapping by QTLMapper 1.0 to simultaneously map both main-effect and epistatic QTLs for seedling-vigor in rice. Totally, 13 putative main-effect QTLs and 19 pairs of epistatic loci with R² ≥ 5% were identified. Almost all of these QTLs or interactions individually explained only around 5–10% of the phenotypic variation. The majority (68%) of these main-effect and epistatic loci were clustered in seven chromosome regions, each spanning 12–28 cM (centi-Morgan) and containing three or more detectable loci. When detectable for the multiple seedling-vigor-related traits, either the main-effect QTLs or the epistatic interactions sharing the same map location had their additive or epistatic effects in the same direction, which agreed well with the positive correlations among the traits. The results demonstrated that seedling-vigor in rice could be controlled by many loci, most of which had small effects, but, relatively, epistasis as a genetic factor was much more important than main-effects of QTLs. Along with the results reported previously, this study revealed the extensive genetic diversity for seedling-vigor in rice. In addition, the QTL q^SV-7 on chromosome 7 was found to have the largest main-effects on multiple seedling-vigor-related traits and therefore could be used as a potential target to be genetically manipulated by marker-assisted selection in rice seedling-vigor breeding programs.     

QTL mapping for crude protein and protein fraction contents in rice (Oryza sativa L.)

Improvement of rice storage proteins is important in rice breeding for high nutritional quality. Seventy-one recombinant inbred lines (RIL) derived from a cross between japonica variety Asominori and indica variety IR24 were used to study the inheritance of crude protein and protein fraction contents in rice. A total of 16 QTL were identified and mapped on eight chromosomes. Several QTL affecting contents of different protein fractions were mapped in the same chromosomal region. In particular, two QTL with a significant contribution were identified to simultaneously affect prolamin and glutelin contents. One QTL denoted as qCP-12 affecting crude protein content (CP) was located in the same region as QTL qGLT-12 affecting glutelin content, in agreement with the positive correlation between glutelin level and protein content. QTL with larger genetic effects were further confirmed using two sets of chromosome segment substitution lines (CSSL), where Asominori and IR24 were used as the recurrent parents. By QTL comparative analysis, two QTL for CP, three for globulin content and one for prolamin content were located in the vicinity of CP QTL previously identified in polished rice. Application of these results in rice breeding is discussed.     

Identification of Rice QTLs for Important Agronomic Traits with Long-Kernel CSSL-Z741 and Three SSSLs

Rice kernel shape affects kernel quality (appearance) and yield (1000-kernel weight) and therefore is an important agronomic trait, but its inheritance is complicated. We identified a long-kernel rice chromosome segment substitution line (CSSL), Z741, derived from Nipponbare as a recipient and Xihui 18 as a donor parent. Z741 has six substitution segments distributed on rice chromosomes 3, 6, 7, 8 and 12 with an average replacement length of 5.82 Mb. Analysis of a secondary F₂ population from a cross between Nipponbare and Z741 identified 20 QTLs for important agronomic traits. The kernel length of Z741 is controlled by a major QTL (qKL3) and a minor QTL (qKL7). Candidate gene prediction and sequencing indicated that qKL3 may be an allele of OsPPKL1, which encodes a protein phosphatase implicated in brassinosteroid signaling, and qKL7 is an unreported QTL. Finally, we validated eight QTLs (qKL3, qKL7, qRLW3-1, qRLW7, qPH3-1, qKWT3, qKWT7 and qNPB6) using three selected single- segment substitution lines (SSSLs), S1, S2 and S3. Also, we detected five QTLs (qKL6, qKW3, qKW7, qKW6 and qRLW6) in S1, S2 and S3, which were not found in the Nipponbare/Z741 F₂ population. However, qNPB3, qNPB7 and qPL3 QTLs were not validated by the three SSSLs in 2019, suggesting that minor QTLs are susceptible to environmental factors. These results lay the foundation for studying the biodiversity of kernal length and molecular breeding of different kernel types.     

水稻第1染色体短臂粒长和粒宽QTL的精细定位

以第1染色体短臂RM1-RM3746和RM151-RM243区间内呈杂合、背景基本纯合的2个水稻剩余杂合体（RHL）衍生两个F6群体,将控制水稻粒长和粒宽的2个粒形QTL（qGL 1和qGW 1）定位于RM3746-RM243区间内。在此基础上,应用SSR标记检测,从其中1个群体中筛选到杂合区间分别为RM151-RM10404、RM10398-RM5359、RM10435-RM259和RM10381-RM243的4个单株,应用SSR标记进一步检测4套F2群体,从每套F2群体中分别筛选到母本珍汕97B和父本密阳46纯合型材料各10株,自交获得4套近等基因系材料并考查其粒长和粒宽。利用交迭重组染色体片段代换系分析法,将控制粒长和粒宽的QTL （qGL 1和qGW 1）界定于437.5 kb的RM10390-RM1344区间和392.9 kb的RM10376-RM10398区间,增效等位基因均来自母本珍汕97B,表明qGL 1和qGW 1是紧密连锁的不同座位。     

水稻粒形QTLs的研究进展

主要介绍了水稻粒形QTL的研究概况,包括水稻粒形性状的遗传特征,粒形QTL的研究进展,讨论了水稻粒形QTL定位与克隆在育种上的意义和前景。