Association Analysis For Photosynthetic Efficiency And Yield Components In Winter Bread Wheat(Triticum Aestivum L.) By SSR Markers And Candidate Gene Rubisco Activase

Wheat production needs to be increased to fulfill the growing demands because of the ever-growing population and changing diets. Furthermore, the potential of crops to cope with changes in environmental conditions and rising water shortages needs to be improved. Photosynthesis, being the basis of crop growth, biomass and yield production,is one of the primary physiological processes strongly affected by abiotic stresses, such as drought and heat. There is growing evidence that genetic variation for photosynthetic rates among available germplasm does exist and that recent yield progress associated with increased photosynthesis.To further understand the genetic control of photosynthesis traits in bread wheat, two approaches were adopted to investigate thephotosynthetic efficiency in a panel of 59 winter bread wheat genotypes. Firstly, association mapping was carried out to detect novel QTLs for the photosynthesisand yield components,and their drought tolerance indices in wheat grown under water-stressed and well-watered conditions in 2012-13 and 2013-14 crop growth seasons using 269 whole-genome SSR markers. Secondly, candidate gene association analyses were carried-out to investigate the effects of TaRca2-? expression on photosynthesis and yield traits through the expression levels of its 3 homoeologous copies i.e. TaRca2-?-4AL, TaRca2-?-4BS and TaRca2-?-4DS at heading(Z55), anthesis(Z67) and grain-filling(Z73) stages and their association with Pn at the corresponding growth stages, BMPP and GYPP in 59 winter bread wheat genotypes under field conditions in 2014-15 crop growth season.The candidate gene analyses were further elucidated through investigation of Rubisco activase activity and their co-relationship in the test panel. The main results were as following:1. Association mapping of SSR markers with photosynthesisand yield related traitsSignificant differences(P<0.01) were revealed by analysis of variance for all the photosynthesis and yield related traitsinvestigated among the 59 wheat genotypes under water-stressed and well-wateredconditions in both growth seasons. The phenotypic traits were affected by water stress, and significant variations were also observed among the genotypes. Significant and positive(P<0.05, 0.01) correlations were found between Pn with BMPP, TKW and GYPP under two water regimes as well as in both crop seasons. BMPP, TKW and GYPP were also significantly and positively correlated with each other in both moisture conditionsand both growth seasons. Significant and positive(P<0.05, 0.01) correlations were also found between the drought tolerance indicesand theirrespective traits under two water regimes in 2012-13 and 2013-14. Generally, genotypes with high STI and low SSI values performed better for the phenotypic traits in both growth seasons.Population structure analysis based on SSR marker data assigned the 59 genotypes to 3 subgroups(SG) with SG 1 consisted of 4 genotypes, SG 2 comprised of 25 genotypes and SG 3 contained 30 genotypes. A total of 36046 marker pairs were detected in the test genotypes. 1670 linked pairs were found, of which 119 were in LD at P<0.001 level. An LD decay distance of around 2.2 cM was estimated for whole genome, whereas it was around 2.2, 0.9 and 8.7 cM for A, B and D genomes, respectively.Association mapping analysis revealed large number of marker-trait associations for the photosynthesis and yield related traits under both water regimes and the tolerance indices in both seasons. However, the number of significant associations drastically reduced when subjected to Bonferroni correction(FPDR) test(05/269 = 0.00018). After applying FPDR test, single marker i.e. Xpsp3123-7D was associated with multiple traits across water regimes and the same marker also showed consistent association with all the traits in both crop seasons. Xgwm182-5D was stably associated with TKW under water-stressed condition in 2013-14 and 2013-14, whereas it was also stably associated with GYPP under well-watered conditions in both seasons. Another marker i.e. Xcfd33-6D showed consistent association with BMPP in 2012-13 and 2013-14 crop season under well-watered condition. Rest of the markers(12) which qualified Bonferroni's correction, were associated with single trait in single water regime. A total of 28 associations for drought tolerance indices(STI and SSI) of phenotypic traits proved to be true positive when subjected to bonferroni correction of which Xpsp3123-7D was consistently associated with both indices of the phenotypic traits in both seasons. Two markers, Xwmc707-4A and Xgwm182-5D, were stably associated with STI-BMPP and STITKW, respectively across seasons. The remaining significant MTAs were detected for single index and in single season.2. Association of TaRca2-?expression withRubisco activase activityand photosynthesis related traitsDifferent but similar expression patterns were observed for the three copies of TaRca2-? at the three growth stages with highest expression at grain-filling stage. TaRca2-?-4BSwas expressed at higher levels at the three stages(0.54, 0.63 and 0.76 at heading, anthesis and grain-filling stages, respectively than TaRca2-?-4AL and TaRca2-?-4DS. Significantly different patterns were observed for Rubisco activase enzyme activity over the three growth stages with highest at grain-filling(0.13) and the lowest at heading(0.05) stages. Regression analysis showed positive(P?0.01) linear association between TaRca2-?expression at heading, anthesis and grain filling stages and activase activity at the same growth stages, the association was comparatively more significant at grain-filling stage(R2 = 0.502).The 59 genotypes could be clustered into three groups as high(7 genotypes), medium(41 genotypes) and low(11 genotypes) expression based on the expression of the three copies of TaRca2-? at three growth stages. Significant variations(P<0.01) were observed among the three groups of bread wheat genotypes for Pn, BMPP and GYPP. Generally, the genotypes with higher TaRca2-? expression also showed higher values for the phenotypic traits.The expression of the three copies of TaRca2-? at heading, anthesis and grain-filling stages had positive correlations with Pn, BMPP and GYPP(P<0.01) with stronger association for TaRca2-?-4BSwith Pn and at grain-filling stage(R2 = 0.712). Enzyme activity was also positively associated with Pn, BMPP and GYPP, with strong associations at grain-filling stage(R2 = 0.493 for Pn, 0.304 for BMPP and 0.261 for GYPP).In summary, richgenetic variations were observed among genotypes for Pn, yield components, and their drought tolerance indices,association mapping revealed new loci associated with Pn, BMPP and GYPP under both water regimes and their drought tolerance indices. Differential expression analysis showed similar but different expression patterns for the three homoeologous copies of TaRca2-? at the three growth stages, and the genotypes that expressed at higher levels also performed better for the phenotypic traits.These results provide a potential basis for the manipulation and improvement of photosynthetic efficiency and yield in common wheat...