A basic helix-loop-helix protein (GhFP1) promotes fibre elongation of cotton (Gossypium hirsutum) by modulating brassinosteroid biosynthesis and signalling

Basic helix-loop-helix (bHLH) proteins are involved in transcriptional networks controlling a number of biological processes in plants. However, little information is known on the roles of bHLH proteins in cotton fibre development so far. Here, we show that a cotton bHLH protein (GhFP1) positively regulates fibre elongation. GhFP1 transgenic cotton and Arabidopsis plants were generated to study how GhFP1 regulates fibre cell elongation. Fibre length of the transgenic cotton overexpressing GhFP1 was significantly longer than that of wild-type, whereas suppression of GhFP1 expression hindered fibre elongation. Furthermore, overexpression of GhFP1 in Arabidopsis promoted trichome development. Expression of the brassinosteroid (BR)-related genes was markedly upregulated in fibres of GhFP1 overexpression cotton, but downregulated in GhFP1-silenced fibres. BR content in the transgenic fibres was significantly altered, relative to that in wild-type. Moreover, GhFP1 protein could directly bind to the promoters of GhDWF4 and GhCPD to activate expression of these BR-related genes. Therefore, our data suggest that GhFP1 as a positive regulator participates in controlling fibre elongation by activating BR biosynthesis and signalling. Additionally, homodimerisation of GhFP1 may be essential for its function, and interaction between GhFP1 and other cotton bHLH proteins may interfere with its DNA-binding activity.