Three root hair defective genes, GhRHD3-1, GhRHD4-1, and GhRSL4-1, regulate fiber cell elongation in cotton

Cotton fibers, single epidermal cells, are most prevalent natural product in textile industries. Root hairs, similar to cotton fibers, are also single cell trichomes. Root hair defective gene (RHD) has been found to greatly modulate root hair development. However, the function of RHD remains unexplored in cotton. We identified 19 RHDs in Gossypium hirsutum. GhRHD3-1, GhRHD4-1, and GhRSL4-1 were highly expressed in fiber elongation stage and promoter-driven GUS of these three genes were mainly expressed in root hair. GhRHD3-1, GhRHD4-1, and GhRSL4-1 localized to the endoplasmic reticulum, cytomembrane, and nucleus, respectively. Overexpression of these three genes each in Arabidopsis increased root length. Silencing these three genes each in cotton reduced plant height and fiber length. Gene Ontology (GO) analysis showed that cell wall, photosynthesis, and oxidoreductase activity related processes were enriched in down-regulated genes after silencing GhRHD3-1, GhRHD4-1, and GhRSL4-1, respectively. Transcripts of the genes identified from enriched terms were also reduced after silencing GhRHD3-1, GhRHD4-1, and GhRSL4-1. Our results suggest that GhRHD3-1, GhRHD4-1, and GhRSL4-1 promote fiber elongation through regulating cell wall, photosynthesis, and oxidoreductase activity pathways, respectively. This study deepens our understanding of the GhRHDs in cotton and will help us breed better cotton varieties in future.