Mapping of post-flowering drought resistance traits in grain sorghum: association between QTLs influencing premature senescence and maturity

The identification of genetic factors underlying the complex responses of p lants to drought stress provides a solid basis for improving drought resist ance. The stay-green character in sorghum (Sorghum bicolor L. Moench) is a post-flowering drought resistance trait, which makes plants resistant to pr emature senescence under drought stress during the grainfilling stage. The objective of this study was to identify quantitative trait loci (QTLs) that control premature senescence and maturity traits, and to investigate their association under post-flowering drought stress in grain sorghum. A geneti c linkage map was developed using a set of recombinant inbred lines (RILs) obtained from the cross B35 x Tx430, which were scored for 142 restriction fragment length polymorphism (RFLP) markers. The RILs and their parental li nes were evaluated for post-flowering drought resistance and maturity in fo ur environments. Simple interval mapping identified seven stay-green QTLs a nd two maturity QTLs. Three major stay-green QTLs (SGA, SGD and SGG) contri buted to 42% of the phenotypic variability (LOD 9.0) and four minor QTLs (S GB, SGI.1, SGI.2, and SGJ) significantly contributed to an additional 25% o f the phenotypic variability in stay-green ratings. One maturity QTL (DFB) alone contributed to 40% of the phenotypic variability (I,OD 10.0), while t he second QTL (DFG) significantly contributed to an additional 17% of the p henotypic variability (LOD 4.9). Composite interval mapping confirmed the a bove results with an additional analysis of the QTL x Environment interacti on. With heritability estimates of 0.72 for stay-green and 0.90 for maturit y, the identified QTLs explained about 90% and 63% of genetic variability f or stay-green and maturity traits, respectively. Although stay-green rating s were significantly correlated (r = 0.22, P less than or equal to 0.05) wi th maturity, six of the seven stay-green QTLs were independent of the QTLs influencing maturity. Similarly, one maturity QTL (DFB) was independent of the stay-green QTLs. One stay-green QTL (SCG), however, mapped in the vicin ity of a maturity QTL (DFG), and all markers in the vicinity of the indepen dent maturity QTL (DFB) were significantly (P less than or equal to 0.1) co rrelated with stay-green ratings, confounding the phenotyping of stay-green . The molecular genetic analysis of the QTLs influencing stay-green and mat urity, together with the association between these two inversely related tr aits, provides a basis for further study of the underlying physiological me chanisms and demonstrates the possibility of improving drought resistance i n plants by pyramiding the favorable QTLs.