Interactions within a network of phytochrome, cryptochrome and UV-B phototransduction pathways regulate chalcone synthase gene expression in Arabidopsis leaf tissue

The Arabidopsis gene encoding the key flavonoid biosynthesis enzyme chalcon e synthase (CHS) is regulated by several environmental and endogenous stimu li. Here we dissect the network of light signalling pathways that control C HS expression in mature leaves using cryptochrome (cry) and phytochrome (ph y) deficient mutants. The UV-A/blue light induction of CHS is mediated prin cipally by cry1, but neither cry1 nor cry2 is involved in UV-B induction or in the UV-A and blue light signalling pathways that interact synergistical ly with the UV-B pathway to enhance CHS expression. Moreover, these synergi stic responses do not require phyA or phyB. Phytochrome is a positive regul ator of the cry1 inductive pathway, mediating distinct potentiation and coa ction effects. A red light pretreatment enhances subsequent cry1-mediated C HS induction. This potentiation is unaltered in phyA and phyB mutants but m uch reduced in a phyA phyB double mutant, indicating that it requires princ ipally phyA or phyB. In contrast, the cry1-mediated induction of CHS,withou t pretreatment, is much reduced in phyB but not phyA mutants, indicating co action between cry1 and phyB. Further experiments with phy-deficient mutant s demonstrate that phyB is a negative regulator of the UV-B inductive pathw ay. We further show that phyB acts upstream of the points of interaction of the UV-A and blue synergism pathways with the UV-B pathway. We propose tha t phyB functions to balance flux through the cry1 and UV-B signalling pathw ays.