REDI 1s Necessary for Phytochrome B-Mediated Red Light-Specific Signal Transduction in Arabidopsis

Seedlings of a transgenic Arabidopsis line (ABO) that overexpresses phytochrome B (phyB) display enhanced deetiolation specifically in red light. To identify genetic loci necessaty for phytochrome signal transduction in red light, we chemically mutagenized ABO seeds and screened M2 seedlings for revertants of the enhanced deetiolation response. One recessive, red light-specific extragenic revertant, designated redl, was isolated. The mutant phenotype was expressed in the original A60 background as well as in the nontransgenic Nossen (No-O) progenitor background. redl is also deficient in severa1 other aspects of red light-induced responses known to be mediated by phyB, such as inhibition of petiole elongation and the shade avoidance response. redl was mapped to the bottom of chromosome 4 at a position distinct from all known photoreceptor loci. Together with complementation analysis, the data show that redl is a nove1 photomorphogenic mutant. The evidence suggests that redl represents a putative phytochrome signal transduction mutant potentially specific to the phy6 pathway.