Genetic identification of FIN2, a far red light-specific signaling component of Arabidopsis thaliana

Phytochrome A (PhyA) mediates most, if not all, various plant responses to far-red (FR) light. Here, we report a novel genetic mutation that impairs a variety of responses in the PhyA-signaling pathway of Arabidopsis thaliana . The mutation was isolated by screening seedlings that show reduced sensit ivity to continuous far-red (FRc) light irradiation, but not to continuous red (Rc) light irradiation. The mutation named fin2-1 is not allelic to a P HYA mutation. Furthermore, immunoblot analysis indicated that the amount of the phytochrome A apoprotein in the fin2-1 mutant was comparable to that i n wild type. Seedling of the fin2-1 mutant showed defects in hypocotyl grow th inhibition and apical hook and cotyledon opening in FRc tight but not in Re light. The results showed that the mutation occurred in a downstream si gnaling component potentially specific to PhyA. Other PhyA-mediated respons es such as FR-preconditioned blocking of greening, anthocyanin accumulation , reduction of gravitropic response, and expression of the CAB and CHS gene s were impaired by the fin2-1 mutation: the degree of the mutant effect on the responses was variable. However, FR light-mediated seed germination and photoperiodic flowering responses were not affected significantly in the m utant. These results showed that FIN2 defines an upstream branch point in t he PhyA signaling pathway.