A HY5 interacting B-Box protein, BBX25, together with BBX24 negatively regulate the expression of BBX22 to suppress seedling photomorphogenesis in Arabidopsis.

GANGAPPA SN; CROCCO, C; JOHANSSON H; DATTA; S; HETTIARACHCHI, CH; HOLM M; BOTTO, JF

PLANT CELL

AMER SOC PLANT BIOLOGISTS

Lugar: Rockville; Año: 2013 vol. 25 p. 1243 - 1257

1040-4651

ELONGATED HYPOCOTYL5 (HY5) is a bZIP transcription factor, central for regulation of seedling photomorphogenesis. Here, we identified a B-BOX (BBX) containing protein, BBX25/SALT TOLERANT HOMOLOG (STH) as novel interacting partner of HY5, which was previously found to interact physically with COP1 (CONSTITUTIVE PHOTOMORPHOGENIC1).  BBX25 physically interacts with HY5 both in vitro and in vivo. By physiological and genetic approaches, we showed that BBX25 is a negative regulator of seedling photomorphogenesis. BBX25 and its homolog BBX24 regulate de-etiolation processes and shade avoidance hypocotyl response in an additive manner. Moreover, genetic relationships of bbx25 and bbx24 with hy5 and cop1 revealed that BBX25 and BBX24 additively enhance COP1 and suppress HY5 functions. BBX25 protein accumulates in a light dependent manner, and undergoes COP1 mediated degradation in dark and light conditions. Furthermore, a protoplast co-transfection assay showed that BBX24 and BBX25 repress BBX22 expression by interfering with HY5 transcriptional activity. As HY5 binds to BBX22 promoter and promotes its expression, our results provide a novel and direct mechanism through which the expression of BBX22 is being regulated. We suggest that BBX25 and BBX24 function as transcriptional co-repressors, probably by forming inactive heterodimers with HY5, down-regulating BBX22 expression for the fine-tuning of light mediated seedling development.