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Functional analysis of the MYB75 transcription factor as a regulator of secondary cell wall formation in Arabidopsis Bhargava, Apurva

Abstract

Deposition of lignified secondary cell walls in plants is spatially and temporally regulated by transcription factors (TFs). MYB75 is a known regulator of the anthocyanin branch of the phenylpropanoid pathway in Arabidopsis. It was established in this work that this TF also regulates secondary cell wall formation in Arabidopsis. A loss-of-function mutation in MYB75 (myb75-1) results in increased cell wall thickness in xylary and interfascicular fibers within the inflorescence stem. Transcript profiles from the myb75-1 inflorescence stem revealed marked up-regulation in the expression of a suite of genes associated with lignin biosynthesis and cellulose deposition, as well as genes encoding cell wall-modifying proteins and genes involved in photosynthesis and carbon assimilation. These patterns suggest that MYB75 acts as a repressor of the lignin branch of the phenylpropanoid pathway in the Arabidopsis inflorescence stem as part of a broader role in integrating the metabolic flux through different pathways. Secondary cell wall-associated and potential transcription factors may interact together and form a multi-protein complex to regulate the formation of the secondary cell wall. I identified many positive interactions among the known and putative secondary cell wall formation-regulating TFs, using yeast two-hybrid assays. The KNAT7-MYB75 interaction detected in these assays was selected for detailed study, since both of these TF’s individually have been shown to regulate some aspects of secondary cell wall formation in Arabidopsis. The KNAT7-MYB75 interaction was confirmed in vivo in an Arabidopsis protoplast system, and the major domains involved in these protein-protein interactions were identified. This interaction may also be involved in regulation of secondary cell wall formation during Arabidopsis seed coat development. This was evident through phenotypic analysis of single and double loss-of-function mutants using molecular and histological analyses that revealed epidermal cell wall defects. However, MYB75, unlike KNAT7, appears to be a specific regulator of secondary cell wall formation in the seed coat, with no role in mucilage biosynthesis. I propose that a MYB75-KNAT7-involving protein complex could be participating in regulation of secondary cell wall biosynthesis in the developing seed coat, based on identified protein-protein interactions among MYB75, KNAT7 and other transcription factors known to influence seed coat properties.

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