A non‐DNA‐binding activity for the ATHB4 transcription factor in the control of vegetation proximity

Abstract

In plants, perception of vegetation proximity by phytochrome photoreceptors activates a transcriptional network that implements a set of responses to adapt to plant competition, including elongation of stems or hypocotyls. In Arabidopsis thaliana, the homeodomain-leucine zipper (HD-Zip) transcription factor ARABIDOPSIS THALIANA HOMEOBOX 4 (ATHB4) regulates this and other responses, such as leaf polarity. To better understand the shade regulatory transcriptional network, we have carried out structure-function analyses of ATHB4 by overexpressing a series of truncated and mutated forms and analyzing three different responses: hypocotyl response to shade, transcriptional activity and leaf polarity. Our results indicated that ATHB4 has two physically separated molecular activities: that performed by HD-Zip, which is involved in binding to DNA-regulatory elements, and that performed by the ETHYLENE-RESPONSIVE ELEMENT BINDING FACTOR-associated amphiphilic repression (EAR)-containing N-terminal region, which is involved in protein-protein interaction. Whereas both activities are required to regulate leaf polarity, DNA-binding activity is not required for the regulation of the seedling responses to plant proximity, which indicates that ATHB4 works as a transcriptional cofactor in the regulation of this response. These findings suggest that transcription factors might employ alternative mechanisms of action to regulate different developmental processes.

Light affects tissue patterning of the hypocotyl in the shade-avoidance response

“…These HD-ZIPII transcription factors have an Nterminal EAR motif used to form transcriptional repressor complexes with TOPLESS/TOP-LESS-related co-repressors [15]. In fact, ATHB4 acts as a transcriptional repressor on targets such as the YUCCA5 gene encoding an auxin biosynthetic enzyme [16]. The complex regulation of YUCCA5 supports the idea that HD-ZIPIIs impede auxin production to dampen PIF function and avoid excessive auxin-dependent growth.…”

Progress of Research on the Regulatory Pathway of the Plant Shade-Avoidance Syndrome

“…PHYB, PHYD, and PHYE are involved in the regulation of ATHB2 by low R:FR light and ATHB2 is recognized by PIF5 in vivo (Hornitschek et al, 2012). Under a shade environment, inactivation of PHYB increases the stability of PIF proteins, which induces transcriptional expression of ATHB2 and ATHB4 (Hornitschek et al, 2012: Gallemí et al, 2017. Subsequently, the elongation reaction of plants is further promoted.…”

Adaptation to plant shade relies on rebalancing the transcriptional activity of the PIF-HFR1 regulatory module

“…Cardamine hirsuta (Oxford ecotype, Ox, Ch WT ) plants have been described before 19,25,31,33 . Plants were grown in the greenhouse under long-day photoperiods (16 h light and 8 h dark) to produce seeds, as described 14,50,54 .…”

“…To detect and quantify transgenic AtHFR1 and ChHFR1, proteins were extracted from ~50 mg of 7-day old seedlings (grown as indicated) or from 50-75 mg of agroinfiltrated N. benthamiana leaves. Plant material was frozen in liquid nitrogen, ground to powder and total proteins were extracted using an SDS-containing extraction buffer (1.5 µL per mg of fresh weight), as described 14 .…”

“…Indeed, pif7 mutant responds poorly to low R:FR compared to the pif4 pif5 double or pif1 pif3 pif4 pif5 quadruple (pifq) mutants 10,11 . PhyB-mediated shade signaling involves other transcriptional regulators, such as LONG HYPOCOTYL IN FR 1 (HFR1), PHYTOCHROME RAPIDLY REGULATED 1 (PAR1), BIM1, ATHB4 or BBX factors, that either promote or inhibit shade-induced hypocotyl elongation [12][13][14][15][16][17][18] . HFR1, a member of the bHLH family, is structurally related to PIFs but lacks the phyB-and DNA-binding ability that PIFs possess 19,20 .…”