Summary: MYB transcription factors have been implicated in regulation of the development of ovule epidermal cells into the elongated seed fibres of cotton. An R2R3 MYB, GhMYB25‐like, identified from its reduced expression in a fibreless mutant of cotton (Xu142 fl), is here shown to play a key role in the very early stages of fibre cell differentiation. A GhMYB25‐like promoter–GUS construct was expressed predominantly in the epidermal layers of cotton ovules before anthesis (−3 days post‐anthesis, dpa), increasing in expression in 0‐dpa ovules, primarily in those epidermal cells expanding into fibres, and then in elongating fibres at +3 dpa, declining thereafter. This was consistent with GhMYB25‐like transcript abundance during fibre development. RNA interference suppression of GhMYB25‐like resulted in cotton plants with fibreless seeds, but normal trichomes elsewhere, phenocopying the Xu142 fl mutant. Like Xu142 fl these plants had reduced expression of the fibre‐expressed MYBs, GhMYB25 and GhMYB109, indicating that GhMYB25‐like is upstream from those MYBs. This hierarchy was supported by the absence of any change in transcript level of GhMYB25‐like in GhMYB25‐ and GhMYB109‐silenced transgenic lines. Transgenic cotton with an additional copy of the native gene had elevated expression of GhMYB25‐like in ovules, but no obvious increase in fibre initials, suggesting that there may be other factors that interact with GhMYB25‐like to differentiate epidermal cells into fibre cells.

Vernalization, the promotion of flowering by a prolonged period of low temperature, results in repression of the floral repressor FLOWERING LOCUS C (FLC) and in early flowering. This repression bears the hallmark of an epigenetic event: the low expression state is maintained over many cell division cycles, but expression is derepressed in progeny. We show that the two stages of the response of FLC to vernalization, the repression of FLC and the maintenance of the repression during growth at normal temperatures after vernalization, are mediated through different regions of the FLC gene. Both promoter and intragenic regions are required for the responses. We also identify a 75-bp region in the FLC promoter that, in addition to intragenic sequences, is required for expression in nonvernalized plants.

Two genetic loci control the vernalization response in winter cereals; VRN1, which encodes an AP1-like MADS-box transcription factor, and VRN2, which has been mapped to a chromosome region containing ZCCT zinc finger transcription factor genes. We examined whether daylength regulates expression of HvVRN1 and HvVRN2. In a vernalization-responsive winter barley (Hordeum vulgare), expression of HvVRN1 is regulated by vernalization and by development, but not by daylength. Daylength affected HvVRN1 expression in only one of six vernalization-insensitive spring barleys examined and so cannot be a general feature of regulation of this gene. In contrast, daylength is the major determinant of expression levels of two ZCCT genes found at the barley VRN2 locus, HvZCCTa and HvZCCTb. In winter barley, high levels of HvZCCTa and HvZCCTb expression were detected only when plants were grown in long days. During vernalization in long-day conditions, HvVRN1 is induced and expression of HvZCCTb is repressed. During vernalization under short days, induction of HvVRN1 occurs without changes in HvZCCTa and HvZCCTb expression. Analysis of HvZCCTa and HvZCCTb expression levels in a doubled haploid population segregating for different vernalization and daylength requirements showed that HvVRN1 genotype determines HvZCCTa and HvZCCTb expression levels. We conclude that the vernalization response is mediated through HvVRN1, whereas HvZCCTa and HvZCCTb respond to daylength cues to repress flowering under long days in nonvernalized plants.

Analysis of the functions of Short Vegetative Phase (SVP)-like MADS-box genes in barley (Hordeum vulgare) indicated a role in determining meristem identity. Three SVP-like genes are expressed in vegetative tissues of barley: Barley MADS1 (BM1), BM10, and Vegetative to Reproductive Transition gene 2. These genes are induced by cold but are repressed during floral development. Ectopic expression of BM1 inhibited spike development and caused floral reversion in barley, with florets at the base of the spike replaced by tillers. Head emergence was delayed in plants that ectopically express BM1, primarily by delayed development after the floral transition, but expression levels of the barley VRN1 gene (HvVRN1) were not affected. Ectopic expression of BM10 inhibited spike development and caused partial floral reversion, where florets at the base of the spike were replaced by inflorescence-like structures, but did not affect heading date. Floral reversion occurred more frequently when BM1 and BM10 ectopic expression lines were grown in short-day conditions. BM1 and BM10 also inhibited floral development and caused floral reversion when expressed in Arabidopsis (Arabidopsis thaliana). We conclude that SVP-like genes function to suppress floral meristem identity in winter cereals.

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.