The HD-Zip IV gene GaHOX1 from cotton is a functional homologue of the Arabidopsis GLABRA2

Most of the plant homeodomain-containing proteins play important roles in organ patterning and development, and Arabidopsis GLABRA2 (GL2), a member of the class IV homeodomain-leucine zipper (HD-ZIP) proteins, is a trichome and non-root hair cell regulator. Here we report the analysis of two cotton homeodomain-containing proteins, GaHOX1 and GaHOX2, isolated from the diploid cotton Gossypium arboreum . Both GaHOX1 and GaHOX2 belong to the class IV HD-ZIP family. When expressed under the control of the GL2 promoter, GaHOX1 rescued trichome development of an Arabidopsis glabrous mutant of gl2-2 (SALK_130213), whereas GaHOX2 did not. On the other hand, expression of GaHOX1 with a Cauliflower mosaic virus (CaMV) 35S promoter in the wild-type Arabidopsis plants suppressed the trichome development just as the GL2 ectopic expression. Expression analysis by Northern, RT-PCR and in situ hybridization indicated that GaHOX1 is predominantly expressed in cotton fiber cells at early developmental stages, consistent with its putative role in regulating cotton fiber development, while GaHOX2 is expressed in both fiber and other ovular tissues, including outer and inner integuments. Our results suggest that GaHOX1 is a functional homolog of GL2 in plant trichome development.     

The R3-MYB Gene GhCPC Negatively Regulates Cotton Fiber Elongation

Cotton (Gossypium spp.) fibers are single-cell trichomes that arise from the outer epidermal layer of seed coat. Here, we isolated a R3-MYB gene GhCPC, identified by cDNA microarray analysis. The only conserved R3 motif and different expression between TM-1 and fuzzless-lintless mutants suggested that it might be a negative regulator in fiber development. Transgenic evidence showed that GhCPC overexpression not only delayed fiber initiation but also led to significant decreases in fiber length. Interestingly, Yeast two-hybrid analysis revealed an interaction complex, in which GhCPC and GhTTG1/4 separately interacted with GhMYC1. In transgenic plants, Q-PCR analysis showed that GhHOX3 (GL2) and GhRDL1 were significantly down regulated in −1–5 DPA ovules and fibers. In addition, Yeast one-hybrid analysis demonstrated that GhMYC1 could bind to the E-box cis-elements and the promoter of GhHOX3. These results suggested that GhHOX3 (GL2) might be downstream gene of the regulatory complex. Also, overexpression of GhCPC in tobacco led to differential loss of pigmentation. Taken together, the results suggested that GhCPC might negatively regulate cotton fiber initiation and early elongation by a potential CPC-MYC1-TTG1/4 complex. Although the fibers were shorter in transgenic cotton lines than in the wild type, no significant difference was detected in stem or leaf trichomes, even in cotton mutants (five naked seed or fuzzless), suggesting that fiber and trichome development might be regulated by two sets of genes sharing a similar model.     

A myb gene required for leaf trichome differentiation in Arabidopsis is expressed in stipules

The GL1 gene is required for the initiation of differentiation of hair cells (trichomes) on the crucifer, Arabidopsis thaliana. This gene has been localized to a 4.5 kb DNA fragment by molecular complementation of gl1 mutants. DNA sequence analysis has shown that the protein encoded by GL1 contains a Myb DNA-binding motif. Southern analysis and subsequence analysis of isolated lambda clones has established that GL1 is a member of an extensive myb gene family in Arabidopsis. The putative GL1 promoter directs the expression of the GUS reporter gene in non-trichome-bearing structures that appear to be stipules. This pattern of expression suggests that GL1 may control the synthesis of a diffusible signal that activates the developmental pathway for trichome differentiation.     

Roles of the GLABROUS1 and TRANSPARENT TESTA GLABRA Genes in Arabidopsis Trichome Development

Arabidopsis trichomes are branched, single-celled epidermal hairs. These specialized cells provide a convenient model for investigating the specification of cell fate in plants. Two key genes regulating the initiation of trichome development are GLABROUS1 (GL1) and TRANSPARENT TESTA GLABRA (TTG). GL1 is a member of the myb gene family. The maize R gene, which can functionally complement the Arabidopsis ttg mutation, encodes a basic helix-loop-helix protein. We used constitutively expressed copies of the GL1 and R genes to test hypotheses about the roles of GL1 and TTG in trichome development. The results support the hypothesis that TTG and GL1 cooperate at the same point in the trichome developmental pathway. Furthermore, the constitutive expression of both GL1 and R in the same plant caused trichomes to develop on all shoot epidermal surfaces. Results were also obtained indicating that TTG plays an additional role in inhibiting neighboring cells from becoming trichomes.