AtHB1, an Arabidopsis thaliana HD-Zip I Transcription Factor, is regulated by Phytochrome-Interacting Factor 1 to promote hypocotyl elongation

CAPELLA M; RIBONE PA; ARCE AL; CHAN RL

NEW PHYTOLOGIST

WILEY-BLACKWELL PUBLISHING, INC

Año: 2015 vol. 207 p. 669 - 682

0028-646X

·       AtHB1 is a homeodomain-leucine zipper transcription factor described as a transcriptional activator with unknown function. Its role in Arabidopsis thaliana development was investigated. ·       AtHB1 expression was analyzed in transgenic plants bearing its promoter region fused to reporter genes. Knock-down mutant and overexpressor plant phenotypes were analyzed in different photoperiod regimes. ·       AtHB1 is mainly expressed in hypocotyls and roots and up-regulated in seedlings grown under a short-day photoperiod. AtHB1 knock-down mutants and overexpressors showed shorter and longer hypocotyls, respectively, than WT. AtHB1 transcript levels were lower in Phytochrome-Interacting Factor 1 (PIF1) mutants than in controls, suggesting that AtHB1 is regulated by PIF1 in hypocotyls. GUS activity in tobacco leaves co-transformed with PromAtHB1::GUS and 35S::PIF1 indicated that PIF1 induces AtHB1 expression. Hypocotyl length measurements in seedlings grown in short- or long-day athb1, pif1, or double athb1/pif1 mutants and PIF1 or AtHB1 overexpressors in WT, athb1 or pif1 backgrounds allowed us to determine that AtHB1 is a PIF1 downstream component. Finally, a transcriptome analysis of athb1 mutant hypocotyls revealed that AtHB1 regulates genes involved in cell wall composition and elongation. ·       The results suggest that AtHB1 acts downstream of PIF1 to promote hypocotyl elongation, especially in response to short-day photoperiods.