| For the better adaptation to environment and resistance to adverse circumstance, the plant has formed many abilities to adapt external environment in the long-term evolution process. The phototropism is in the case, which enables plant to bend towards light so that they are able to obtain optimum light. The discovery of blue light receptors phototropin (PHTO1,PHOT2), which provided the new ways for us to conduct further studies on the mechanism of phototropism induced by blue light. Now research suggested that PHTO1 and PHOT2 interacted with its downstream signal protein NPH3 and RPT2, regulated the activation and localization of the auxin-efflux and -influx transporters, AUX1 and PIN1, which resulted in asymmetric distribution of the auxin between the illuminated and shaded side of the hypocotyls, and induces phototropic bending. But we are not completely clear how blue light signal regulates the phototropism.In this study, phot1( a mutant of blue light receptor) was used as experimental materials, through mutagenesis by 0.3% EMS and handled with 100μmol m-2s-1 unilateral blue light for 12 hs to screen M2 generation of seedlings on the large scale. Now 3 potential mutants were obtained which were named as sph1 (slow phototropic hypocotyl 1), lph1(littery phototropic hypocotyl 1), dph1(deletion phototropic hypocotyl 1) respectively.The findings demonstrated that compared with the phot1 mutant, sph1 mutant phototropism was slower than that of phot1 under the irritation of 100μmol m-2s-1 unilateral blue light. In addition, the sph1 lost the negative gravitropism under the25μmol m-2s-1red light, and this phenomenon restored to normal under 1μmol m-2s-1 blue light. sph1 and Ler were hybridized, segregation ratio statistics of backcross between F1 and F2 showed that sph1 was recessive single gene mutation.We located the gene on MSF3 BAC by map-based cloning. Every gene's sequence of this genomic region was examined. 3482bp point mutation occurred At2G18790.1 was found to be mutated in sphl by sequence analysis. Compared with wild type, sphl contained an G-to- A change at 3482bp in the gene DNA. Protein sequence analysis indicated that GCC was mutated to ACC, and 1096 lactamine mutated to threonine. So we assumed that the point mutations resulted in protein sequence change, which directly leads to the slow phototropic bending. Using phot1 mutant as the control materials, sph1 mutant was conducted a series of identification, the difference in their growth and development was observed. The results showed that there existed the remarkable difference between sph1 mutant and phot1 mutant. sph1 mutant was similar to phot1 mutant during nourishing development, the leaves were narrow and long, petioles were longer, appearing the counter clockwise screw arrangement, the stems are long, simultaneously the stomata was open, the density of stomata was low, water was losing slowly, the temperature on the surface of leaves is high, however there was no obvious difference between sph1 mutant and phot1 mutant in the fruit capacity. Its reason possibly is that the sph1 mutant can regulate the shade avoidance syndrome(SAS)by many ways such as the long petiole anti-clockwise reverse-acting spiral arrangement and the relatively long stem. Based on the light, each leaf blade of sph1 mutant adjusts its position to obtain the luminous energy to the utmost, to conduct photosynthesis, accumulate energy, and eventually it has no influence on its fruit capacity.As for dph1and iph1 mutants, after they were hybridized with Ler, segregation ratio statistics of backcross between F1 and F2 showed that dph1 and iph1 were recessive single gene mutants as well. At present, we has roughly located dph1 at the fifth chromosome base end, while located iph1 in the middle of the fourth chromosome in Arabidopsis thaliana.Above results suggest that SPH1,DPH1,IPH1 probably involved in strong BL-induced phototropic bending in Arabidopsis etiolated seedlings. And the exact mechanisms will be tested by complementary experiment further.
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