SRL1: a new locus specific to the phyB-signaling pathway in Arabidopsis

As part of an effort to isolate new Arabidopsis mutants specifically defect ive in responsiveness to red light, we identified srl1 (short hypocotyl in red light) by screening an EMS-mutagenized M-2 population derived from a ph ytochrome B (phyB)-overexpressor line (ABO). The srl1 mutant shows enhanced responsiveness to continuous red but not far-red light, in both wild-type and ABO backgrounds, consistent with involvement in the phyB-signaling path way but not that of phyA. The hypersensitive phenotype of srl1 is not due t o overexpression of endogenous phyA or phyB, and the locus maps to the cent er of chromosome 2, distinct from any other known photomorphogenic mutants. srl1 seedlings display enhancement of several phyB-mediated responses, inc luding shorter hypocotyls, more expanded cotyledons, shorter petioles and m odestly higher levels of CAB gene expression under red light than the wild type. Double mutant analyses show that the hypersensitive phenotype of srl1 is completely phyB-dependent. The data suggest, therefore, that SRL1 may e ncode a negatively acting component specific to the phyB-signaling pathway.