The role of MYB21 in Arabidopsis: transcending flower-specific functions to vegetative tissue

Abstract Jasmonic acid (JA) and its derivatives are lipid-derived phytohormones that coordinate plant growth, development, and stress responses through the bioactive conjugate jasmonoyl-isoleucine (JA-Ile). Their role in reproductive development is well established, particularly in stamen maturation through the R2R3-MYB transcription factor MYB21, which has been considered largely flower-specific. Here, we reveal a previously unrecognized role of MYB21 in vegetative tissues of Arabidopsis thaliana . Although basal MYB21 transcript levels in leaves are extremely low and spatially restricted, wounding and exogenous hormone applications induced MYB21 transcription in a JA- and COI1-dependent manner. Transcriptional GUS reporter analyses showed localized MYB21 promoter activity in specialized epidermal cells, including trichomes, hydathodes, and in the vasculature at wound sites. Functional characterization using the myb21-5 mutant indicated roles in germination and vegetative growth, partially phenocopying JA-insensitive mutants despite unaltered JA biosynthesis and signaling. Transcriptome profiling further revealed changes in expression of genes involved in lignin biosynthesis, light-harvesting complex components, cytokinin pathways, and defense-related responses, consistent with reduced resistance of myb21-5 to insect herbivory and infection by Botrytis cinerea . Together, these findings identify MYB21 as a JA-responsive regulator of growth and defense in seedlings and leaves, extending its function beyond reproductive development. Highlight The transcription factor MYB21, previously considered flower-specific, is induced by jasmonate signaling in Arabidopsis leaves and seedlings, where it regulates growth and defense responses, thereby expanding its role beyond reproductive development.