Arabidopsis thaliana HomeoBox 1 (AtHB1), a Homedomain-Leucine Zipper I (HD-Zip I) transcription factor, is regulated by PHYTOCHROME-INTERACTING FACTOR 1 to promote hypocotyl elongation.

Arabidopsis thaliana HomeoBox 1 (AtHB1) is a homeodomain-leucine zipper transcription factor described as a transcriptional activator with unknown function. Its role in A. 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 was 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 wild type (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. β-glucuronidase (GUS) activity in Nicotiana benthamiana leaves cotransformed with PromAtHB1::GUS and 35S::PIF1 indicated that PIF1 induces AtHB1 expression. Hypocotyl lenght was measured in seedlings of athb1, pif1, or double athb1/pif1 mutants and PIF1 or AtHB1 overexpressors in WT, athb1 or pif1 backgrounds, both in short- or long-day. These analyses allowed us to determine that AtHB1 is a factor acting downstream of PIF1. 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.