Integration of Brassinosteroid Signal Transduction with the Transcription Network for Fiber Development and Drought Stress in Gossypium hirsutum LIntegration of Brassinosteroid Signal Transduction with the Transcription Network for Fiber Development and Drought Stress in Gossypium hirsutum L

Cotton is the bulkiest sink of natural textile fiber and a main oilseed crop, with extensive scientific, environmental, and socio-economic footsteps. Cotton fiber is a highly elongated seed hair with a secondary wall composed of almost pure cellulose. Development of cotton fibers is positively affected by a number of phytohormones including auxin, gibberellins and brassinosteroids (BRs) [1,2]. BRs consist of a family of phytosterols found throughout the plant kingdom [3]. In general, BR has effects similar to auxin. BR stimulates germination, shoot elongation, vascular development and pollen formation. BR inhibits root elongation and promotes secondary root initiation. BR accelerates cell elongation and affects cytoskeletal and cell wall structure. BR promotes the transverse orientation of microtubules in azuki bean [4], and the brassinolide (BL)-induced epicotyl growth in Cicer arietinum is accompanied by increased β-tubulin expression [5]. Among the first BR-responsive genes to be identified, BR-unregulated gene 1 (BRU1) from soybean [6] was identified as a xyloglucan endotransglycosylase gene (i.e. XTH). These enzymes are involved in cell wall loosening [6] and provide a direct link between BR and its effects on cell wall development. Microarray analysis of BR-up-regulated genes showed that many cell wall-related genes, including fatty acid elongase (At1g01120), expansin (EXP), arabinogalactan protein (AGP), acyl carrier protein (ACP) and a number of XTHs (U43488,), are up-regulated by BL in Arabidopsis [7,8]. Interestingly, the cotton counterparts of these genes were also reported to be preferentially expressed in cotton fibers [9].