An L1 box binding protein, GbML1, interacts with GbMYB25 to control cotton fibre development

Abstract

Transcription factors play key roles in plant development through their interaction with cis-elements and/or other transcription factors. A HD-Zip IV family transcription factor, Gossypium barbadense Meristem Layer 1 (GbML1) has been identified and characterized here. GbML1 specifically bound to the L1 box and the promoters of GbML1 and GbRDL1. GbML1 physically interacted with a key regulator of cotton fibre development, GbMYB25. Truncated and point mutation assays indicated the START–SAD domain was required for the binding to the C terminal domain (CTD) of GbMYB25. GbML1 overexpression in Arabidopsis increased the number of trichomes on stems and leaves and increased the accumulation of anthocyanin in leaves. Taken together, the L1 box binding protein, GbML1 was identified as the first partner for GbMYB25 and the role of START domain was discovered to be a protein binding domain in plants. Our findings will help the improvement of cotton fibre production and the understanding of the key role of HD-Zip family and MYB family in plants.

The Hairless Stem Phenotype of Cotton (Gossypium barbadense) Is Linked to aCopia-Like Retrotransposon Insertion in aHomeodomain-Leucine ZipperGene (HD1)

“…In G. barbadense, GbML1 (GbHD1) could bind to its own promoter region, leading to positive-feedback regulation of its own gene expression (Zhang et al 2010). Our gene expression analysis partially confirmed this observation: the insertion in At-HD1 caused significant reduction in the expression of both alleles of HD1.…”

“…A total of 185 clones from 18 plants of 15 G. barbadense varieties, including six primitive cultivated forms, was sequenced (Table S3). Sequencing results confirmed the identity of the HD1 gene, which consists of 10 exons and 9 introns, by the presence of basic HD domains in the predicted protein (Zhang et al 2010) (Figure 2 and Figure 3). The A genome gene amplicon is 3105 bp long, while the D genome amplicon is 3104 bp long, showing 97.6% identity with 16 predicted amino acid changes.…”

“…Recently, two cotton homologs of an Arabidopsis positive trichome formation regulator (AtML1), GhHD1 and GbML1, were shown to be associated with cotton stem trichome formation (Zhang et al 2010;Walford et al 2012). GhHD1, which belongs to the HD-ZIP IV family, plays a vital role in regulating trichomes and fiber initiation as a member of an important protein complex, together with several key MYBMIXTA-like transcription factors (Zhang et al 2010;Walford et al 2012;Bedon et al 2014).…”

Allele-specific effects ofPDF2on floral morphology inArabidopsis thaliana

“…4 While the mammalian START domain proteins bind lipids or sterols, 5 the START domain of a cotton HD-ZIP IV protein has been shown to interact with a MYB transcription factor, suggesting formation of a protein complex that specifies target genes. 6 The START-adjacent (SAD) domain follows the START domain in HD-ZIP IV proteins 4 but its function remains unknown. Molecular and genetic studies of the HD-ZIP IV family in Arabidopsis thaliana, which consists of 16 genes, have revealed some of their roles in plant development: GLABRA2 (GL2) is required for trichome and root hair differentiation, 7,8 ANTHOCYANINLESS2 (ANL2) for anthocyanin accumulation in the epidermis, 9 and HOMEODOMAIN GLABROUS11 (HDG11, pronounced "hedge"11) and HDG2 for normal trichome development.…”

“…1B), 11 does not represent a mere loss-of-function mutation but rather has some additional effect on stamen and petal development. Since the HD-ZLZ domains are sufficient for HD-ZIP IV proteins to form dimers, 6 truncated Pdf2 proteins in pdf2-1, pdf2-2, and pdf2-3 alleles (Fig. 1A) could form homo -or hetero-dimers by their retained HD-ZLZ domain, if expressed, and interfere with the binding of normal HD-ZIP IV proteins to target L1-box sequences.…”