Phototropism: a family business

Abstract

Blue light regulates several developmental processes in plants, including de-etiolation, tropisms and entrainment of the circadian clock. Phototropic responses involve the familiar bending of seedling hypocotyls toward blue light, but also the relocation movements of chloroplasts within the plant cell to optimize light capture. These relocation responses, which are reminiscent of the phototactic behavior of unicellular organisms, are sophisticated in Arabidopsis. Under low levels of blue light, the chloroplasts migrate toward the brightest spots within the cell (accumulation response), whereas under high-intensity illumination, the chloroplasts ‘hide’ themselves near the side cell walls, thereby avoiding exposure to potentially harmful light levels (avoidance response). Winslow Briggs and co-workers had demonstrated that hypocotyl bending responses toward low irradiances of blue light are mediated by a photoreceptor that they called nph1 (for non-phototropic hypocotyl), or phototropin. This receptor is a plasma-membrane-associated protein that contains a kinase domain located within its C-terminus. The N-terminal region contains two LOV motifs that bind the chromophore, flavin mononucleotide (FMN). Until now, the photosensors involved in other phototropic responses, such as hypocotyl phototropism in response to high irradiances and chloroplast relocation remained unknown.Two recent papers shed new light into the regulation of tropisms by blue wavelengths. The take-home message? nph1 is not alone – it is a member of a family of flavin-based photoreceptors. Takatoshui Kagawa et al.1xArabidopsis npl1: a phototropin homolog controlling the chloroplast high-light avoidance response. Kagawa, T. et al. Science. 2001; 291: 2138–2141CrossRef | PubMed | Scopus (398)See all References1 devised an ingenious method to isolate mutants deficient in the chloroplast high-light-avoidance response. They covered leaves of mutagenized Arabidopsis plants using black plates with open slits, and exposed them to strong light. In leaves of wild-type plants, the area under the slits became pale-green; by contrast, in a few mutants, the exposed area remained green, indicating an impaired chloroplast-avoidance response. The mutation sites of these mutants, initially termed cav1 mutants (for chloroplast avoidance), were located within the recently described NPL1 gene (NPH1 Like), which encodes a protein similar in sequence to phototropin. In a related study, Tatsuya Sakai et al.2xArabidopsis nph1 and npl1: blue light receptors that mediate both phototropism and chloroplast relocation. Sakai, T. et al. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 6969–6974CrossRef | PubMed | Scopus (424)See all References2 showed that npl1, like nph1, noncovalently binds FMN within the LOV domains and functions as a light receptor kinase. Sakai et al. expanded the analysis of npl1 function using nph1 npl1 double mutants. These mutants showed impaired hypocotyl phototropism under both low- and high-intensity blue light, and lacked chloroplast relocation responses to low- and high-intensity blue light.The phototropin family of photoreceptors appears to be ubiquitous throughout the plant kingdom 3xThe phototropin family of photoreceptors. Briggs, W.R. et al. Plant Cell. 2001; 13: 933–997See all References3, and it is intriguing that the members of this family seem to have specialized in the control of movement. Movement of organelles within the cell probably requires interactions between phototropin signaling elements and the cytoskeleton. By contrast, tropic movements at the whole-organ level are based on coordinated responses of a population of cells. It will be interesting to learn which mechanisms allow the members of the phototropin family to control the tropism business in environments that appear to be so radically different in terms of scale and complexity. As a first step to facilitate communication, phototropin connoisseurs have recently agreed to use a common terminology, and the photoreceptors nph1 and nph2 have been redesignated as phot1 and phot2, respectively 3xThe phototropin family of photoreceptors. Briggs, W.R. et al. Plant Cell. 2001; 13: 933–997See all References3.