Genetic linkages between circadian clock-associated components and phytochrome-dependent red light signal transduction in Arabidopsis thaliana.

The current best candidates for Arabidopsis thaliana clock components are CCA1 (CIRCADIAN CLOCK-ASSOCIATED 1) and its homolog LHY (LATE ELONGATED HYPOCOTYL). In addition, five members of a small family, PSEUDO-RESPONSE REGULATORS (including PRR1, PRR3, PRR5, PRR7 and PRR9), are believed to be another type of clock component. The originally described member of PRRs is TOC1 (or PRR1) (TIMING OF CAB EXPRESSION 1). Interestingly, seedlings of A. thaliana carrying a certain lesion (i.e. loss-of-function or misexpression) of a given clock-associated gene commonly display a characteristic phenotype of light response during early photomorphogenesis. For instance, cca1 lhy double mutant seedlings show a shorter hypocotyl length than the wild type under a given fluence rate of red light (i.e. hypersensitivity to red light). In contrast, both toc1 single and prr7 prr5 double mutant seedlings with longer hypocotyls are hyposensitive under the same conditions. These phenotypes are indicative of linkage between the circadian clock and red light signal transduction mechanisms. Here this issue was addressed by conducting combinatorial genetic and epistasis analyses with a large number of mutants and transgenic lines carrying lesions in clock-associated genes, including a cca1 lhy toc1 triple mutant and a cca1 lhy prr7 prr5 quadruple mutant. Taking these results together, we propose a genetic model for clock-associated red light signaling, in which CCA1 and LHY function upstream of TOC1 (PRR1) in a negative manner, in turn, TOC1 (PRR1) serves as a positive regulator. PRR7 and PRR5 also act as positive regulators, but independently from TOC1 (PRR1). It is further suggested that these signaling pathways are coordinately integrated into the phytochrome-mediated red light signal transduction pathway, in which PIF3 (PHYTOCHROME-INTERACTING FACTOR 3) functions as a negative regulator immediately downstream of phyB.