Both HY5 and HYH are necessary regulators for low temperature-induced anthocyanin accumulation in Arabidopsis seedlings.

The roles of two bZIP transcription factors LONG HYPOCOTYL 5 (HY5) and HY5 HOMOLOG (HYH) in inducing anthocyanin accumulation during low temperature treatment were studied in Arabidopsis. In all seedlings tested, low temperature significantly induced anthocyanin accumulation only in the presence of light. In the absence of HY5 or HYH, the low temperature-induced anthocyanin accumulation was significantly impaired compared to that of the wild type. Moreover, in the double mutant hy5hyh, no significant anthocyanin accumulation was induced by low temperature even in light, suggesting that the low temperature-induced anthocyanin accumulation was mediated by HY5/HYH. Through the RT-PCR assay, expressions of several "early" genes in anthocyanin biosynthetic pathway, chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), were up-regulated by low temperature in a manner that is at most partially dependent on HY5/HYH, whereas dihydroflavanol reductase (DFR), a "late" gene, was found to be up-regulated in a manner that was almost fully dependent on HY5/HYH. Thus, up-regulation of DFR in a HY5/HYH-dependent manner may address the question of why low temperature-induced anthocyanin accumulation relies upon light. In addition, we found that HY5/HYH expression was enhanced by low temperature in wild type Col-0, implying that low temperature induces anthocyanin accumulation, at least in part, through enhancing HY5/HYH protein levels. Collectively, our data suggest that HY5 and HYH are two necessary regulators that play a pivotal role during low temperature-induced anthocyanin accumulation in Arabidopsis seedlings.