| Waterlogging stress, negatively affect plant growth, biomass production, and potential yield. This stress affects agriculture in many countries, causing multi-million dollar losses for farmers and the agriculture industry and contributing to poverty worldwide. Incidence of waterlogging stress is increasing due to the effects of climate change. The development of waterlogging-tolerant varieties with high yield potential should be one of the main goals of the maize breeding program. Our study aimed to remap the quantitative trait loci (QTLs) associated with waterlogging tolerance related traits, including plant height (PH), root length (RL), shoot fresh weight (SFW), root fresh weight (RFW), root dry weight (RDW), shoot dry weight (SDW), total dry weight (TDW) and their waterlogging tolerance coefficient (WTC) during maize seedling stage by using advanced backcross QTL (AB-QTL) analysis approach in a mixed linear model and inclusive composite interval mapping method under waterlogging and control conditions. On the other hand, based on earlier studies results of candidate genes detected associated with waterlogging tolerance at the seeding stage, functional markers co-localized with major waterlogging tolerant QTLs were developed using the simple sequence repeat markers flanking target regions. A266BC2F2population derived from a cross between a waterlogging-tolerant’HZ32’and a susceptible ’K12’ was used. Results summary are as follows:1. Normality tests performed on the seven waterlogging-related traits, showed that the frequency distributions of the BC2F2:3families were normal over two seasons except of SDW and RDW under waterlogging stress. Both skew and kurtosis values of each trait were<1.0, suggesting that these traits were quantitative genetic effects.2. The broad sense heritability’s (h2) of the various phenotypic traits were relatively high across the two pot experiments, ranging from0.62for the WTC of shoot dry weight and total dry weight to0.83for root fresh weight under waterlogging stress. The high h2implied that most of the phenotypic variance for each trait was genetic and could be effectively improved by selective breeding programs.3. Correlation coefficients between pairs of all seven seedling traits were calculated under waterlogged, normal conditions and their WTC in the two seasons separately. Highly significant correlations were observed in both Experiments, for root dry weight, root fresh weight, shoot dry weight and their WTC. While, a weak relationship was observed between root length and root dry weight.4. A new linkage map constructed for the BC2F2population, consisting of three ESTs, nine miRNA and155polymorphic SSR markers, spanned1797.6cM in length across a maize genome, with an average distance of10.8cM between adjacent markers. The linear order of markers on the linkage map was a good agreement with IBM2008Neighbors.5. A total of fifty-three and sixty-eight QTLs were detected in EXP.1and EXP.2, separately. Of these,56and34putative QTLs were detected under waterlogging treatment and control conditions, respectively.31putative QTLs were detected for waterlogging treatment coefficient. These QTLs were distributed over all10chromosomes, and had LOD scores ranging from2.58to14.74, explaining3.77to31.44%phenotypic variation in the individual traits. Out of which, thirty major QTLs individually accounted for more than15%of the phenotypic variation; they were governed traits associated with RL, PH, SDW, RDW, TDW and RFW were mapped in the different genomic region on chromosomes1,2,3,4,6,7and9.6. In order to provide additional evidence regarding the role of corresponding genes in waterlogging tolerance, mapping of Expressed Sequence Tags (EST) markers and micro RNAs were conducted. Ten candidate genes were observed to co-localize with the identified QTLs on chromosomes1,2,4,6,7and9, and may important candidate genes for waterlogging tolerance.The study results reveal that the former major QTL remapped by AB-QTL, could be selected in backcross population for fine mapping of waterlogging tolerance. The co-localization of major QTL and developmental markers indicated that some miRNA may be contributed to waterlogging tolerance pathways and directly response for waterlogging related traits. Although the functions of these genes need to be confirmed deeply through transgenic and association analysis, the strategy used in the current study is a good starting point for the discovery and mapping of waterlogging-responsive genes. The results may provide new insight into the molecular basis of the waterlogging response of seedlings stage and useful markers for MAS. |
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