SPA1: A New Genetic Locus Involved in Phytochrome A–Specific Signal Transduction

Abstract

To identify mutants potentially defective in signaling intermediates specific to phytochrome A (phyA), we screened for extragenic mutations that suppress the morphological phenotype exhibited by a weak phyA mutant ( phyA-105 ) of Arabidopsis. A new recessive mutant, designated spa1 (for suppressor of phyA-105 ), was isolated and mapped to the bottom of chromosome 2. spa1 phyA-105 double mutants exhibit restoration of several responses to limiting fluence rates of continuous far-red light that are absent in the parental phyA-105 mutant, such as deetiolation, anthocyanin accumulation, and a far-red light-induced inability of seedlings to green upon subsequent transfer to continuous white light. spa1 mutations do not cause a phenotype in darkness, indicating that the suppression phenotype is light dependent. Enhanced photoresponsiveness was observed in spa1 seedlings in a wild-type PHYA background as well as in the mutant phyA-105 background but not in a mutant phyA null background. These results indicate that phyA is necessary in a non-allele-specific fashion for the expression of the spa1 mutant phenotype and that phyB to phyE are not sufficient for this effect. Taken together, the data suggest that spa1 mutations specifically amplify phyA signaling and therefore that the SPA1 locus encodes a component that acts negatively early in the phyA-specific signaling pathway.

The Negatively Acting Factors EID1 and SPA1 Have Distinct Functions in Phytochrome A-Specific Light Signaling

“…The analyzed spa1 allele carries a stop codon close to the N terminus of the protein that is localized in front of the strongest alleles analyzed by Hoecker et al (1998Hoecker et al ( , 1999. Furthermore, the two additional spa1 alleles isolated in the WS background during our screenings exhibited the same alterations under all analyzed light conditions (data not shown).…”

“…Finally, Lhcb1-transcript accumulation is even reduced in eid1-5 under VLF conditions. SPA1 also alters the sensitivity of HIR (Hoecker et al, 1998; this paper), but its influence is only weak compared with the strong eid1-3 and the weak eid1-5 alleles. The different influences on HIR obtained for spa1 and eid1 became most obvious regarding the action spectra for hypocotyl elongation in far-red light.…”

“…The spa1 mutant was originally screened as a suppressor of the weak phyA-105 allele under continuous far-red light (Hoecker et al, 1998). To test whether eid1 can also suppress the phyA-105 phenotype, eid1 phyA-105 double mutants were isolated, and hypocotyl elongation was analyzed under continuous farred light.…”

“…Compared with the large number of mutants with reduced phyA responses, only two mutants, spa1 and eid1, showed enhanced phyA-specific light responses (Hoecker et al, 1998;Bü che et al, 2000;Dieterle et al, 2001). Both mutants accumulate normal levels of phyA, and phyA degradation in the mutants was not altered.…”

“…They exhibited hypersensitivity in all analyzed HIR under continuous far-red light, and both showed an increased sensitivity under weak continuous red light (Hoecker et al, 1998;Bü che et al, 2000). However, it remained unclear whether these factors are functionally redundant and whether they are involved in independent or identical signal transduction chains.…”