The phototropic response of seedlings is important for the rapid orientation of the recently germinated plant to light, essential for photosynthesis and the survival of the young plant. The Briggs lab has recently reported the isolation and characterization of eight mutants in Arabidopsis which are severely impaired or lack this directional growth response to blue light (Liscum and Briggs, 1995). These mutants fall into four genetic loci designated nph1, nph2, nph3, and nph4, where nph stands for non phototropic hypocotyl. One of these loci probably encodes a plasma protein that becomes rapidly phosphorylated in response to blue light irradiation, either in vivo or in vitro in the presence of ATP and Mg++ . This protein appears to be involved in the phototropic response. Physiological and biochemical evidence strongly suggest that it may be the photoreceptor, although this hypothesis awaits a definitive test. The second and third loci encode proteins that are likely downstream from the photoperception event, since although the plants fail to respond to unilateral light with a normal growth curvature, they show normal amounts of the putative photoreceptor protein and normal phosphorylation. The fourth locus encodes a protein essential both for phototropism and for gravitropism--the directional response of growing plant organs to gravity--and hence is likely still farther downstream from the putative photoreceptor.

Several members of the Briggs lab have recently been using a technique known as Amplified Fragment Length Polymorphism (AFLP) to generate large numbers of polymorphic DNA fragments that are genetically linked to a given nph locus. Two such fragments, each within 0.3 cM of the nph1 locus and on opposite flanking regions, were subcloned and used to isolate genomic DNA containing the wild-type NPH1 gene. The gene is currently being characterized with a view to establishing the relationship to the 120-kD phosphoprotein. Furthermore, isolation and characterization of the NPH1 gene will also open up approaches to a wide range of physiological and biochemical questions such as: What is the fate of the protein following phosphorylation? Why is there so little of it in light-grown compared to dark-grown seedlings? What is (are) its reaction partner(s) in the signal transduction chain? What is the direct and immediate physiological consequence to the plasma membrane of the phosphorylation event, and how is this change related to the subsequent growth change?

Winslow R. Briggs