Dynamic QTL Mapping Associated With Water Logging Tolerance At Maize Seedling And Development Of Near Isogenic Lines Containing Specific QTL

Waterlogging (excess water in the root zone) and flooding (surface ponding) greatly reduce crop yields when it is poorly drained or when rain or irrigation is excessive. Soil waterlogging is a severe abiotic stress for most crops that suppresses yield, whereas rice and other wetland crops usually thrive. Waterlogging tolerance is a complex quantitative trait controlled by many genes that may interact with each other and complicated by confounding factors such as time and depth of flooding, temperature, development stage and soil type, and so on.The objective of this study was to identify the QTL associated with water logging tolerance for several seedling traits in maize by conditional phenotypic values of different time periods to water logging. At the same time, based on preliminary studies results of QTL mapping associated with water logging tolerance during the seeding stage, three major QTL were located on chromosome 4,7 and 9 respectively. Near isogenic lines containing target regions were developed using the simple sequence repeat markers flanking target regions.Phenotypic data were evaluated in 3,6 and 9 d after the initiation of water logging stress and normal moisture conditions using a segregating F2:3 families of 247 lines derived from a cross between two maize genotypes HZ32 (water logging-tolerant) and K12 (water logging-sensitive). Conditional phenotypic values were obtained by the mixed model approach for the conditional analysis of quantitative traits.QTL associated with the conditional phenotypic values of plant height, root length, shoot dry weight, root dry weight, total dry weight and water logging tolerance coefficient were identified via inclusive composite interval mapping (ICIM) under water logging stress and normal moisture conditions. At the same time, near isogenic lines of three major QTL associated with water logging tolerance for several seedling traits in maize were constructed through the foreground selection for the target QTL, the background selection for the genome of the recurrent parent in three backcross populations (BC1F1, BC2F1 and BC3F1). The main results are as follows:1. A new genetic map constructed by 223 SSR and 3 ESTmarkers spanned 1779.162 cM in length across maize genome, with an average interval of 7.87 cM between adjacent markers. The linear order of markers in the linkage map was in good agreement with IBM 2008 Neighbors. 2. QTL determining plant height, root length, shoot dry weight, root dry weight, total dry weight and their water logging tolerance coefficient under water logging stress and normal moisture conditions, thirty-four, sixty and eighty-eight QTL were detected in 3d, 6|3d and 9|6d, respectively. The effects of each QTL were in general small, most of them below 10%, with the biggest one explaining 37.5% of the phenotype variation in bin 2.05. Several QTL clusters determining water logging tolerance for several seedling traits and their water logging tolerance coefficients each mapped on chromosomes 1,2,3,4 and 7. These QTL were detected consistently by multiple traits in three experiments (time), which indicated that they may continually active in all periods to water logging stress.3. Few QTL were detected in both water logging stress and normal moisture conditions, most QTL for water logging tolerance coefficient were different from those for traits under the two water treatments, and most QTL were different detected in 3d, 6|3d and 9|6d, respectively, indicating the genetic basis of maize responded to water logging might be depended on treatment and time, and gene controlling water logging tolerance might be different.4. Near isogenic lines development by consecutive backcrossing through the foreground selection for the target QTL, the background selection for the genome of the recurrent parent, the targeted QTL of three near isogenic lines were obtained in the K12 background on chromosome 4,7 and 9, according to the previous QTL mapping.Conditional QTL mapping method could map and estimate the net effect of water logging related gene expression in different periods of time to water logging. Our results could reveal the inheritance of water logging tolerance for several seedling traits responded to water logging of different time periods. Using foreground and background selection, I have begun to transfer three major QTL for water logging tolerance from HZ32 to K12. Advance in high-throughputs DNA sequencing, Our NILs might be a powerful material to isolate and cloning the water logging tolerance related genes by map based cloning and other functional genomics methods. It is evident that a better understanding of mechanisms for water logging tolerance in maize will lead to improved production in water logging condition. Improving water logging tolerance at maize seedling stage using molecular breeding in combination with traditional breeding in field could significantly enhance maize breeding for water logging tolerance in future.