| Light as a signal factor has become the research focus in the field of biology, though it was known as a kind of important energy substance. Light not only participates in the regulation of animal behavior in response to the complex environment, but also affects the growth and development of plants. The plant movement, including phototropism, chloroplast relocation and leaf expansion, which is an adaptive response that reorient organ growth toward or away from a directional light source. We focus on the blending of Arabidopsis hypocotyl towards unilateral light.Phototropins mainly mediate phototropic responses induced by blue light. Blue light receptor PHOT1 mediates Arabidopsis hypocotyl phototropism in response to low blue light. Interestingly, PHOT1 and PHOT2 display some kind of functional redundancy on regulating Arabidopsis hypocotyl phototropism induced by high blue light. To explore the detail mechanism of that, we chose phot1 as EMS mutagenesis material and obtained NPH3 as the downstream factor of PHOT2. After studying on the material with point mutant of NPH3, we utilized nph3-6 to do some research to throw light on the function of NPH3 protein.NPH3 interacted with PHOT1 and PHOT2 determined by yeast two-hybrid. nph3 mutant, consistenting with phot1 nph3, phot2 nph3 and phot1 phot2, showed no hypocotyl phototropism in response to both high and low unilateral blue light, indicating that NPH3 acts as a mutual factor downstream of the two phototropins. In addition, protein structure analysis showed that NPH3 had no transmembrance domain,though it patchily distributed on plasma membrane. We suppose that the special membrane localization of NPH3 associated with its function. Sure enough, when giving blue light irradiation at a high fluence rate,NPH3 proteins migrated along the pattern of ‘membrane-cytosol-membrane'.To explore the unusual distribution pattern of NPH3 induced by high blue light, we observed the distribution of NPH3 in different Arabidopsis mutants. NPH3 proteins were presented in the way of clustering distribution, which was not affect by PHOT1, PHOT2 or RPT2 proteins under dark background,although NPH3 interacted with PHOT1, PHOT2 and RPT2 proteins directly. However, the migration of NPH3 along the pattern of ‘membrane-cytosol-membrane' in response to high blue light was regulated by PHOT1, PHOT2 and RPT2. The results showed that blue light stimulus did not induce any significant alterations of NPH3 proteins in phot1 mutant. In contrast, the phot2 and rpt2 mutants showed exaggeratedblue light induced granulation of NPH3 proteins in cytosol, and the proteins lost the capacity of relocating on plasma membrane. In brief, phototropins and RPT2 regulate the subcellular localization of NPH3 proteins in response to high blue light, then transmit blue light signal.Furthermore, yeast two-hybrid showed that NPH3 interacted with PKSs proteins directly. Phenotypic analysis showed that Arabidopsis mutant pks1 pks2 pks4 as well as nph3 bend hardly to the unilateral blue light. We conceived that NPH3 protein could perceive the optical signal from phototropins, and then carried on some form of signal transduction with PKS family proteins. Considering the result that our previous studies showed direct interaction between PKS1 and PIN1, we did some experiments on exploring interrelationship. PIN1, PIN2, PIN3, PIN4 and PIN7, a kind of auxin efflux carrier identified, were found to interact with NPH3. According to all these results analysis, we guess that NPH3 and PKS family proteins may regulate auxin transporter PIN proteins in some way. It is a remarkable fact that PIN proteins which involved in the regulation of the asymmetrical distribution of auxin contribute to Arabidopsis hypocotyl phototropism. Therefore we infer that NPH3 proteins perceive blue light signal, then regulate the polar distribution of PIN proteins with PKS family proteins, modulating phototropic response. Of course, it is just a hypothesis, and we need further study. |
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