Functional Analyses of Cotton (Gossypium hirsutum L.) Immature Fiber (im) Mutant Whose Phenotype Grown Under Normal Conditions Is Similar to Wild Type Cotton Grown Under Severe Stress

Cotton fiber wall thickness is an important property of determining values of cotton fibers. Cotton fiber wall is mainly composed of secondary cell wall consisting of almost pure cellulose. How fiber cell wall development affects the degree of fiber wall thickness is not well understood. An immature fiber (im) mutant and its near isogenic wild type, Texas Marker-1 (TM-1) differing in fiber wall thickness provide a unique way of studying structural properties of cellulose. The wild type TM-1 produces thick and mature fiber walls under normal environmental conditions, but generated thin and immature fiber walls under severe stress conditions. The im mutant produced thin and immature fiber walls under normal conditions. Transcriptome analysis also showed that the differentially expressed genes (DEGs) in the im mutant fibers grown under normal stress conditions were similar to those in wild type cotton fibers grown under severe stress conditions. The low degree of fiber cell wall thickness of the im mutant fibers is associated with dysregulation of the genes involved in stress responses and cellular respiration. Phenotyping analyses showed common levels in cellulose content, crystallinity index, crystal size, matrix polymer composition, fiber perimeter, or width between the isogenic fibers, whereas there were significant differences in their linear density and the lateral order between their cellulose microfibrils (CMFs). The results provide an insight of understanding how cellulose biosynthesis process can be affected by environments during cotton fiber development.