Bacterial blight resistance in cotton: genetic basis and molecular mapping

Bacterial blight (BB, caused by Xanthomonas citri pv. malvacearum , Xcm) is a worldwide disease of cotton ( Gossypium spp.). The disease has been effectively controlled through the use of BB resistant cultivars and planting of acid-delinted seed. However, a resurgence of BB has been noted in the US in recent years due to the spread of Xcm race 18 and growing of susceptible transgenic cultivars, which calls for a renewed effort to develop new BB resistant cultivars. However, there has been a paucity of information in genetics and breeding for BB resistance since the 1990s due to the lack of research efforts. This review was prepared to fill this void with an objective to provide detailed results from past qualitative and quantitative genetic studies on BB resistance, including genetic designs and specific germplasm used for conducting research. More than 20 major resistance B genes ( B 1 to B 8 , B 9K , B 9L , B 10K , B 10L , B 11 , B 12 , B In , B n , B s , and more than 4 unnamed genes), with at least two polygene complexes ( B Sm and B Dm ), have been identified. One B gene may be resistant to a single or multiple Xcm races, and pyramiding of several B genes can enhance resistance to a single or multiple Xcm races. Allelic relationships among some of the genes are currently unknown. Quantitative genetics has been employed to estimate heritability, gene effects, additive and dominance variances, and effective number of genes for BB resistance. The studies suggest that the additive effect and additive variance play a predominant role in BB resistance, while the dominant effect and variance play a reduced role in resistance. Heritability estimates are moderate to high depending on environmental errors, and 1–2 effective numbers of genes have been estimated, consistent with Mendelian genetic studies. Studies in molecular mapping of several BB resistance genes ( B 2 , B 3 , b 6 , and B 12 ) have been conducted with the focus on B 12 as it is resistant to races 1 through 19. Portable DNA markers have been developed and used in marker-assisted selection for BB resistance. Finally, areas where there is a lack of information and controversies are identified and assessed. This review provides an updated comprehensive account of the genetic basis for BB resistance in cotton.