Arabidopsis ICX1 is a negative regulator of several pathways regulating flavonoid biosynthesis genes

Flavonoid biosynthesis gene expression is controlled by a range of endogenous and environmental signals. The Arabidopsis icx1 (increased chalcone synthase expression 1) mutant has elevated induction of CHS (CHALCONE SYNTHASE) and other flavonoid biosynthesis genes in response to several stimuli. We show that ICX1 is a negative regulator of the cryptochrome 1, phytochrome A, ultraviolet (UV)-B, low temperature, sucrose, and cytokinin induction of CHS expression and/or anthocyanin accumulation, demonstrating that these pathways are regulated either directly or indirectly by at least one common component. Expression analysis of CHS and other genes (LTP, CAB, and rbcS) indicates that ICX1 functions in both seedlings and mature leaf tissue and acts principally in the epidermis, consistent with the alterations in epidermal development seen in icx1. The mutant was unaltered in the synergistic interactions between UV-B, blue, and UV-A light that regulate CHS and we propose a model of action of ICX1 in these responses.