Bell-like homeodomain selectively regulates the high-irradiance response of phytochrome A

STANELONI , R.J.; RODRIGUEZ BATTILLER, M. J.; LEGISA, D; SCARPIN, M.R.; AGALOU, A.; CERDÁN, P.D.; MEIJER, A. H.; OUWERKERK, P.B.F.; CASAL J.J.

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA

NATL ACAD SCIENCES

Año: 2009

0027-8424

Plant responses mediated by phytochrome A display a first phasesaturated by transient light signals and a second phase requiringsustained excitation with far-red light (FR). These discrete outcomes,respectively so-called very-low-fluence response (VLFR) andhigh-irradiance response (HIR), are appropriate in different environmentaland developmental contexts but the mechanisms thatregulate the switch remain unexplored. Promoter analysis of alight-responsive target gene revealed a motif necessary for HIR butnot for VLFR. This motif is required for binding of the Bell-likehomeodomain 1 (BLH1) to the promoter in in vitro and in yeast1-hybrid experiments. Promoter substitutions that increased BLH1binding also enhanced HIR. blh1 mutants showed reduced responsesto continuous FR and to deep canopy shadelight, but theyretained normal responses to pulsed FR or red light and unfilteredsunlight. BLH1 enhanced BLH1 expression and its promotion by FR.We conclude that BLH1 specifically regulates HIR and not VLFR ofphytochrome A.