Lignin is one of the major components of secondary cell wall (SCW) in plant vascular tissues, responsible for providing structural and mechanical support, and facilitating water and nutrient transport. Lignin is crucial to erect plant habit and helped colonizing terrestrial ecosystem. SCW and lignin biosynthesis is an outcome of complex regulatory network with R2R3-MYBs as part of Phenylpropanoid pathway being key regulators. We identified R2R3 transcription factors, AtMYB42 and AtMYB85 as paralogs arising out of Brassicaceae specific segmental duplication. In the present study, AtMYB42 and AtMYB85 were characterized to understand functional conservation and divergence and role in secondary cell wall and other developmental pathways. Constitutive overexpression of AtMYB42 and AtMYB85, and, artificial microRNAs (amiRNA) separately targeting AtMYB42 and AtMYB85 was used to generate reverse genetic mutants. Phenotypic traits at vegetative and reproductive stages, and levels of selected transcripts involved in lignin biosynthesis and phenylpropanoid pathway were evaluated. Both MYB42 and MYB85 were found to negatively regulate lignin biosynthesis, with overexpression resulting in significantly reduced lignin content in stem and silique tissues. Analysis of mutant lines revealed changes in stem thickness, vascular bundle characteristics, plant height, and branch numbers. Repressor lines exhibited increased lignin deposition and thicker interfascicular fibers, while overexpression lines showed reduced lignin content and vessel numbers. Comparative analysis of the phenotypic and molecular data suggests partially redundant function of the two paralogs. Based on our data, a hypothesis of a negative feedback loop for regulation of lignin levels mediated via AtMYB42, AtMYB85 and unidentified intermediates is proposed. Overexpression or gene silencing of AtMYB42 and AtMYB85 may lead to altered resource balance between the phenylpropanoid and flavonoid pathway, and trade-offs. Future studies thus must include comparative and comprehensive analysis of transcriptome and metabolome not only to validate the hypothesis, but also to study the balance between flavonoid and phenylpropanoid pathways. The study thus underscores the complexity of gene regulation in lignin biosynthesis and deposition as part of secondary cell wall, and provides insights into multiple roles of MYB transcription factors.

AtMYB42 and AtMYB85 Regulate Secondary Cell Wall Development and Lignin Level Possibly Through a Feedback Mechanism

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发表日期：

2025年8月18日

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Shobha Yadav,
Sandip Das

摘要

Lignin is one of the major components of secondary cell wall (SCW) in plant vascular tissues, responsible for providing structural and mechanical support, and facilitating water and nutrient transport. Lignin is crucial to erect plant habit and helped colonizing terrestrial ecosystem. SCW and lignin biosynthesis is an outcome of complex regulatory network with R2R3-MYBs as part of Phenylpropanoid pathway being key regulators. We identified R2R3 transcription factors, AtMYB42 and AtMYB85 as paralogs arising out of Brassicaceae specific segmental duplication. In the present study, AtMYB42 and AtMYB85 were characterized to understand functional conservation and divergence and role in secondary cell wall and other developmental pathways. Constitutive overexpression of AtMYB42 and AtMYB85, and, artificial microRNAs (amiRNA) separately targeting AtMYB42 and AtMYB85 was used to generate reverse genetic mutants. Phenotypic traits at vegetative and reproductive stages, and levels of selected transcripts involved in lignin biosynthesis and phenylpropanoid pathway were evaluated. Both MYB42 and MYB85 were found to negatively regulate lignin biosynthesis, with overexpression resulting in significantly reduced lignin content in stem and silique tissues. Analysis of mutant lines revealed changes in stem thickness, vascular bundle characteristics, plant height, and branch numbers. Repressor lines exhibited increased lignin deposition and thicker interfascicular fibers, while overexpression lines showed reduced lignin content and vessel numbers. Comparative analysis of the phenotypic and molecular data suggests partially redundant function of the two paralogs. Based on our data, a hypothesis of a negative feedback loop for regulation of lignin levels mediated via AtMYB42, AtMYB85 and unidentified intermediates is proposed. Overexpression or gene silencing of AtMYB42 and AtMYB85 may lead to altered resource balance between the phenylpropanoid and flavonoid pathway, and trade-offs. Future studies thus must include comparative and comprehensive analysis of transcriptome and metabolome not only to validate the hypothesis, but also to study the balance between flavonoid and phenylpropanoid pathways. The study thus underscores the complexity of gene regulation in lignin biosynthesis and deposition as part of secondary cell wall, and provides insights into multiple roles of MYB transcription factors.