Dynamic QTL Mapping Of Seed Vigor And Physiological Mechanism Of Seed Tolerance To Desiccation During The Development Of Rice Seeds

Rice(Oryza.sativa L.)is one of the most important crops in the world.Seed quality is an important factor for the production of rice.Meanwhile,seed vigor is an important characteristic of seed quality.It's important to develop new rice cultivars with high vigor for rice production and the long-term development of seed industry in China.In this study,the QTL mapping for seed vigor during seed developmental stage,and the physiological mechanism of desiccation tolerance were conducted in rice.The results are as follows:1.The rice of Jiucaiqing(Oryza sativa ssp.japonica)and IR26(Oryza sativa ssp.indica)and their 150 RIL(F2:10)population were used to determine the characteristics of seed vigor,including germination potential(GP),germination rate(GR),germination index(GI)and time to 50%of germination(T50),at 4(early),5(middle)and 6(late)weeks after heading,and the dynamic QTL analysis was conducted in this study.The results showed that Jiucaiqing had higher seed vigor at the early stage than IR26,while IR26 had higher seed vigor at the middle and late stages.There was significant difference of seed vigor among RILs.A total of 24 additive QTLs for seed vigor,including 16,4 and 8 QTLs for the early,middle and late stages,respectively,and 5,7,6 and 6 QTLs for GP,GR,GI and T50 were identified using single-maturity phenotypic.Of them,two QTLs qGP4.1 and qT503.2 were simultaneously identified at both stages,and one QTL qGI7 was simultaneously identified at three stages.Meanwhile,a total of 9 epistatic QTLs were identified in joint analysis of three-maturity phenotypic values.Among them,each three epistatic QTLs were identified for GR,GI and T50 respectively.The additive,epistatic and QTL×development interaction effects regulated the dry maturity developmental process to improve seed vigor in rice.The phenotypic variation explained by each additive,epistatic QTL and QTL×development interaction ranged from 5.86%to 40.67%,4.64%to 11.28%and 0.01%to 1.17%,respectively.In 2013,the additive QTLs identified in 2012 were verified using bulked segregant analysis(BSA).A total of 32 simple sequence repeat(SSR)markers associated with seed vigor were detected,including 11,10 and 13 SSR markers for the early,middle and late stages,respectively.Of them,two SSR markers RM3555 and RM3362 were simultaneously identified at both stages.A total of 20 additive QTLs were stably expressed in two years which might play important role(s)in the establishment of seed vigor in rice.By comparing chromosomal positions of these stably expressed additive QTLs with those previously identified,the regions of 15 QTLs for seed vigor are likely to coincide with QTLs for grain size,low temperature germinability and seed dormancy,while 5 additive QTLs might represent novel genes.According to phenotype and QTL mapping,four RILs were selected and three cross combinations of seed vigor for the development of RIL populations were predicted;19 elite alleles could be pyramided by each combination.These results showed that the identified QTLs will be applicable to improve rice seed vigor by marker assisted selection.2.Two rice varieties Jiucaiqing and Wuyunjing 7(Oryza sativa L.ssp.japonica)were used to investigate the dynamic changes of desiccation tolerance,including GP,GR,GI and seedling percentage(SP)produced by dried seeds,during 18 to 42 days after heading(DAH).Meanwhile,the grain weight(GW),moisture content(MC)and electrical conductivity(EC)during seed development and the changes in malonyldialdehyde(MDA)level,hydrogen peroxide(H2O2)content and the activity of superoxide dismutase(SOD),catalase(CAT)and glutathione reductase(GR)were conducted.Results showed that desiccation tolerance gradually increased with seed development and reached the highest level at 38 DAH.The MC and EC gradually declined while GW increased with seed development.The MDA and H2O2 content in fresh,dry and imbibed seed and SOD,CAT and GR activity in fresh and imbibed seed rapidly decreased during the early developmental stage(18 to 30 DAH),and then stably changed with further development(30 to 42 DAH).Regression analysis indicated that H2O2 content in fresh seed and SOD activity in imbibed seed play more important role(s)in desiccation tolerance of rice.The H2O2 content in fresh seed and SOD activity in imbibed seed negatively and positively correlated with seed germination,respectively,which are good parameters for assessing the desiccation tolerance of rice.Seed desiccation tolerance of rice could be predicted by the regression equations using the physiological traits.3.Different concentrations of diquat,paraquat and ethephon were applied to hybrid rice Yanliangyou 88(Oryza sativa ssp.indica)at 4 weeks after heading and to pure rice Wuyunjing 7(Oryza sativa ssp.japonica)at 6 weeks after heading.The effects of treatment on the MC,GW,GR,GI and SP were conducted in 2013.Results showed that the reductions in MC were significant two and four days after twelve chemical applications.Chemical applications had no adverse effects on GW,GR and GI,but there were negative effects on the SP.Desiccation effects increased with increase in the period after application,while the effect of ethephon combined with diquat or paraquat on desiccation was limited compared to that of diquat or paraquat alone in a short period after application.Compared to the control,chemical applications reduced the MC by from 0 to 6.4%,the GR by from 0 to 3.3%and the SP by from 13.3 to 100%.The 120 g/ha application with the best performance in 2013 was verified in 2014;after application,the reduction of MC was significant compared to the control,while no significant changes on GW,GR and GI.Among the treatments,diquat applied at 120 g/ha resulted in effective desiccation with fewer negative effects on grain weight and seed germination in 2013 and 2014.Therefore,diquat may have potential as a pre-harvest chemical desiccation treatment for rice.These results may provide a basis for developing and implementing protocols for large scale field trials.In this study,five novel QTLs for the establishement of seed vigor were identified during seed developmental stages in rice,and the physiological mechanism of desiccation tolerance was revealed.Meanwhile,the diquat was identified as a potential pre-harvest chemical desiccant for rice.These results might be applicable to improve seed vigor of rice using the methods of genetic breeding and seed production in future.