Brassinosteroid signaling affects secondary cell wall deposition in cotton fibers

- Transgenic cotton plants with altered expression of the brassinosteroid receptor BRI1 were developed.
- Plants with increased BRI1 expression produced cotton fibers with thicker secondary cell walls.
- Fibers from plants with reduced BRI1 expression had thinner cell walls and reduced micronaire.
- Alteration of BRI1 expression correlated directly with cellulose synthase gene expression.

Differentiation of cotton fibers involves sequential cell elongation and secondary cell wall deposition as they develop from fiber initials to highly elongated and thickened trichomes on the seed integument. Phytohormones appear to play important regulatory roles in the development of these economically important cells. Previous pharmacological experiments indicate that brassinosteroid (BR), along with auxin and gibberellins, promote cotton fiber elongation. To further evaluate the role of the BR signaling pathway in cotton fiber development, transgenic cotton plants that express transgenes designed to either over-express or suppress the expression of the BR receptor BRI1 were created. Analysis of the cotton fibers from these plants indicated that alteration of BRI1 expression had little effect on fiber length. However, over-expression of BRI1 led to increased fiber cellulose deposition while suppression of BRI1 expression strongly inhibited secondary cell wall development, resulting in fibers with reduced maturity. These alterations in fiber cell wall development corresponded with changes in cellulose synthase gene expression, indicating that secondary wall deposition is affected by BR-dependent modulation of cellulose synthase gene expression. These results indicate that BR signaling promotes the maturation of cotton fibers through the deposition of cellulose into the secondary cell wall.