Epidermal cell differentiation in cotton mediated by the homeodomain leucine zipper gene, GhHD‐1

Abstract

Summary: Gossypium hirsutum L. (cotton) fibres are specialized trichomes a few centimetres in length that grow from the seed coat. Few genes directly involved in the differentiation of these epidermal cells have been identified. These include GhMYB25‐like and GhMYB25, two related MYB transcription factors that regulate fibre cell initiation and expansion. We have also identified a putative homeodomain leucine zipper (HD‐ZIP) transcription factor, GhHD‐1, expressed in trichomes and early fibres that might play a role in cotton fibre initiation. Here, we characterize GhHD‐1 homoeologues from tetraploid G. hirsutum and show, using reporter constructs and quantitative real‐time PCR (qRT‐PCR), that they are expressed predominantly in epidermal tissues during early fibre development, and in other tissues bearing epidermal trichomes. Silencing of GhHD‐1 reduced trichome formation and delayed the timing of fibre initiation. Constitutive overexpression of GhHD‐1 increased the number of fibres initiating on the seed, but did not affect leaf trichomes. Expression of GhHD‐1 in cotton silenced for different fibre MYBs suggest that in ovules it acts downstream of GhMYB25‐like, but is unaffected in GhMYB25‐ or GhMYB109‐silenced plants. Microarray analysis of silencing and overexpression lines of GhHD‐1 indicated that it potentially regulates the levels of ethylene and reactive oxidation species (ROS) through a WRKY transcription factor and calcium‐signalling pathway genes to activate downstream genes necessary for cell expansion and elongation.