MUTATIONS OF ARABIDOPSIS IN POTENTIAL TRANSDUCTION AND RESPONSE COMPONENTS OF THE PHOTOTROPIC SIGNALING PATHWAY

Four genetic loci were recently identified by mutations that affect ph ototropism in Arabidopsis thaliana (L.) Heyhn. seedlings. It was hypot hesized that one of these loci, NPH1, encodes the apoprotein for a pho totropic photoreceptor. All of the alleles at the other three mutant l oci (nph5 nph3, and nph4) contained wild-type levels of the putative N PH1 protein and exhibited normal blue-light-dependent phosphorylation of the NPH1 protein. This indicated that the NPH2, NPH3, and NPH4 prot eins likely function downstream of NPH1 photoactivation. We show here that, although the nph2, nph3, and nph4 mutants are all altered with r espect to their phototropic responses, only the nph4 mutants are also altered in their gravitropic responsiveness. Thus, NPH2 and NPH3 appea r to act as signal carriers in a phototropism-specific pathway, wherea s NPH4 is required for both phototropism and gravitropism and thus may function directly in the differential growth response. Despite their altered phototropic responses in blue and green light as etiolated see dlings, the nph2 and nph4 mutants exhibited less dramatic mutant pheno types as de-etiolated seedlings and when etiolated seedlings were irra diated with unilateral ultraviolet-A (UV-A) light. Examination of the phototropic responses of a mutant deficient in biologically active phy tochromes, hy1-100, indicated that phytochrome transformation by UV-A light mediates an increase in phototropic responsiveness, accounting f or the greater phototropic curvature of the nph2 and nph4 mutants to U V-A light than to blue light.