HOMEODOMAIN PROTEIN 1 is required for jasmonate‐mediated glandular trichome initiation in Artemisia annua

Summary Glandular trichomes are generally considered biofactories that produce valuable chemicals. Increasing glandular trichome density is a very suitable way to improve the productivity of these valuable metabolites, but little is known about the regulation of glandular trichome formation. Phytohormone jasmonate ( JA ) promotes glandular trichome initiation in various plants, but its mechanism is also unknown. By searching transcription factors regulated by JA in Artemisia annua , we identified a novel homeodomain‐leucine zipper transcription factor, HOMEODOMAIN PROTEIN 1 (Aa HD 1), which positively controls both glandular and nonglandular trichome initiations. Overexpression of Aa HD 1 in A. annua significantly increased glandular trichome density without harming plant growth. Consequently, the artemisinin content was improved. Aa HD 1 interacts with A. annua jasmonate ZIM‐domain 8 (Aa JAZ 8), which is a repressor of JA , thereby resulting in decreased transcriptional activity. Aa HD 1 knockdown lines show decreased sensitivity to JA on glandular trichome initiation, which indicates that Aa HD 1 plays an important role in JA ‐mediated glandular trichome initiation. We identified a new transcription factor that promotes A. annua glandular trichome initiation and revealed a novel molecular mechanism by which a homeodomain protein transduces JA signal to promote glandular trichome initiation. Our results also suggested a connection between glandular and nonglandular trichome formations.