Male sterility in A rabidopsis induced by overexpression of a MYC 5‐ SRDX chimeric repressor

Summary Jasmonate hormone ( JA ) plays critical roles in both plant defense and reproductive development. Arabidopsis thaliana plants deficient in JA ‐biosynthesis or ‐signaling are male‐sterile, with defects in stamen and pollen development. MYC 2, MYC 3 and MYC 4 are JAZ ‐interacting bHLH transcription factors that play a major role in controlling JA responses in vegetative tissue, but are not likely to play a role in reproductive tissue. We found that a closely related transcription factor, MYC 5 ( bHLH 28), was able to induce JAZ promoters that control some of the early JA ‐responsive genes in a Daucus carota (carrot) protoplast expression system. A G‐box sequence in the JAZ 2 promoter was necessary and sufficient for induction by MYC 5 (as it is for MYC 2, MYC 3 and MYC 4), and induction of JAZ genes was repressed by co‐expression of a stabilized, JAZ 1ΔJas repressor. Two allelic myc5 mutants exhibited no overt phenotype; however, transgenic lines expressing MYC 5 fused to an SRDX ( SUPERMAN repressive domain X) motif phenocopied mutants defective in JA signaling. In particular, MYC 5‐ SRDX plants were male‐sterile, with defects in stamen filament elongation, anther dehiscence and pollen viability. Importantly, expression of MYB 21 and other transcription factors required for stamen and pollen maturation was strongly reduced in stamens of MYC 5‐ SRDX plants relative to the wild type. Taken together, these results indicate that MYC 5, probably together with other, redundant transcription factors, may be activated by JA signaling to induce the expression of MYB 21 and components required for male fertility.