Mechanisms Of Phytochrome D On Plant Growth And Development In Arabidopsis Thaliana

Photoreceptors are responsible for the regulation of the plants growth by perceiving different light signals such as light spectrum, intensity, circadian clock. Phytochromes are a family of photoreceptors, absorbing primarily in red and far-red, and play important roles in many aspects of plant development. In Arabidopsis thaliana, the phytochrome family consists of five members, designated phyA to phyE. Among them, phyA is the predominant phytochrome perceiving far-red light, and phyB, phyC, phyD and phyE are photoreceptors which are responsible for red-light-induced responses. To date, people have been paying more attention to phyA and phyB due to their predominant roles in modulating plant behaviors, roles of phyC, phyD and phyE in plant development have received little attention and studies on the three phys members are processed slowly. Generally, phyC, phyD and phyE were considered to be less important in regulating developmental processes such as photomorphogenesis, flowering, shade avoidance, and to function with phyA or phyB redundantly, but the point is becoming unsupported by more emerging evidences. Recent studies found that phyC performs a significant role in the modulation of blue light sensing by interacting with cry2; at cooler temperatures (<16℃) phyE displayed functional dominance in promoting seed germination, with phyB displaying an accessory role. So, we wonder whether phyD is involved in other developmental processes in Arabidopsis, and Whether there is difference from other phytochromes for phyD still remain unknown.So, in this paper we focus on the function and mechanism of phyD in regulating growth and development in Arabidopsis, and attempt to identify unique roles of phyD. Results as follows:1. In seedlings grown under different light such as blue, red and far-red light, the mRNA expression of PHYD gene and Myc-PHYD fusion protein abundance showed less difference and indicated that their expression are light stable under light conditions except that the lower expression of PHYD mRNA and the less abundance of protein in dark-grown seedlings.2. In the over-expression of PHYD::GUS seedlings grown under blue, red and far-red light, the expression of promoter of PHYD gene showed difference, and also showed difference in different organs of Arabidopsis in response to different light. There was almost no expression of PHYD promoter in all organs of dark-grown seedlings. The highest expression in cotyledon was examined when seedlings were irradiated with red light, and then in far-red light, in which PHYD promoter was more active than in blue light; In hypocotyls, the promoter expressed at the highest level in blue light, and then in far-red light whereaes red-light-induced expression was found only in the upper portion of hypocotyls; For the acvities of PHYD promoter in roots, only very low activities were measured in far-red and blue-light-grown seedlings.3. PhyB and phyD synergeticly inhibit the negative gravitropic response induced by red light in hypocotyls. Furthermore, results indicated that red-light-induced negative gravitropism in hypocotyl is regulated in a fluence rate-dependent manner.4. PhyD had negative roles in regulating elongation of primary root in Arabidopsis. Compared with WT and phyB mutant, over-expression of PHYD mutant exhibited inhibition of root elongation, while phyD mutant showed accelerated root elongation. In contrast, the loss of PHYB cause decreased elongation rate in roots. Results indicated the function and mechanism of phyD in the regulation of root are different from those of phyB.5. PHYD C-terminal is responsible for the nuclear translocation of phyD, DCT514-GFP constitutively localized in nuclear and formed nuclear bodies with or without the induction of red light. Analysis of subcellular localization on PHYB-GFP, PHYD-GFP, DNT650-GFP and DCT514-GFP indicated that both PHYB-GPF and PHYD-GFP moved into nuclear in light-independent manner, but different from PHYB-GFP, PHYD-GFP showed no nuclear bodies formed. Surprisingly, in dark or red light grown seedling, DC514T-GFP constitutively localized in nuclear and showed increased nuclear bodies formation under continue red light, exclude the disability of nuclear localization of DN650T-GPF for the absent of nuclear localization signal.In this study, phyD was first time recognized in the inhibition of red-light-induced negative gravitropism of Arabidopsis. Moreover, phyB and phyD synergeticly function in the developmental process. This study alos found the predominant role in the inhibition of primary root elongation in Arabidopsis. By the investigation of subcellular localization of PHYB-GFP, PHYD-GFP, DNT650-GFP and DCT514-GFP, this study indicated that PHYD-GFP translocated into nuclear in respond to red light but could not form nuclear bodies. But DCT514-GFP constitutively localized in nuclear and formed nuclear bodies with or without the induction of red light, which indicated the PHYD C-terminal is functional and is relative with the formation of nuclear body in the developmental processes of Arabidopsis.All of the results in this study suggest that there are similarities or differences in mechanisms of phyD and phyB in the growth and development of Arabidopsis thaliana, but futher studies are required for details.