| Wild types Col-0, WS and copl-4, cop1-6, hy5-215, hy5-221, hy5-ks50, hyh,hy5hyh, pif1-1, pif3-1, pif4-2 mutants were used here to study the mechanism by which the cross-talks between light and sucrose or low temperature regulate the growth and metabolism of Arabidopsis sesdlings.In present study, a new function of sugars (including sucrose, glucose, and fructose) in prompting Arabidopsis hypocotyl elongation in the dark was observed. Further investigation demonstrated that the phytohormone GAs were involved in this process. It was additionally found that GAs could negatively regulate sugar-induced NR activity in light. We also showed that the interaction between light and low temperature exists. The transcription factors HY5 and HYH were necessary for low temperature-induced anthocyanin accumulation, and GAs play a negative role during this process. The following is the main results:1 In this study, the effects of sucrose on hypocotyl elongation of Arabidopsis seedlings in light and in dark were investigated. Sucrose suppressed the hypocotyl elongation of Arabidopsis seedlings in light, but stimulated the elongation in dark. Application of paclobutrazol (PAC, a gibberellin biosynthesis inhibitor) impaired the effects of sucrose on hypocotyl elongation, suggesting that endogenous GAs are required for sucrose-induced hypocotyl elongation in dark. Exogenous GA3 application was able to reverse the repression caused by PAC application, indicating that exogenous GA3 could substitute, at least partially, for endogenous GAs in sucrose-induced hypocotyl elongation. In addition, we found that GA 3-oxidase 1 (GA3oxl), encoding a key enzyme involved in endogenous bioactive GAs biosynthesis, was up-regulated by sucrose in dark, whereas GIBBERELLIN INSENSITIVE DWARF 1a (AtGID1a), encoding a GA receptor and playing an important role during GAs degradation to DELLA proteins (DELLAs, repressors of GA-induced plant growth), was down-regulated. These results imply that endogenous bioactive GA levels are expected to be enhanced, but the degradation of DELLAs was inhibited by sucrose in dark. Thus, our data suggest that the sucrose-induced hypocotyl elongation in dark does not result from GA-induced degradation of DELLAs. Taken together, it is concluded that sucrose could stimulate hypocotyl elongation of Arabidopsis seedlings in dark in a GA-dependent manner.2 In the second part of this study, the role of the phytohormone gibberellins (GAs) on regulating the nitrate reductase (NR) activity was tested in Arabidopsis seedlings. The NR activity of light-grown Col-0 seedlings was reduced by exogenous GA3 (an active form of GA), but enhanced by exogenous paclobutrazol (PAC, a gibberellin biosynthesis inhibitor), suggesting GAs negatively regulation of NR activity in light-grown seedlings. Light through both photosynthesis and phytochromes are known to influence the NR activity. When etiolated seedlings were transferred to white or red light, both exogenously applied GA3 and PAC were shown to function on NR activity only in the presence of sucrose, implying that GAs don't regulate light signaling-induced but only negatively regulate photoproducts-induced NR activity. NR is regulated by light mainly at two levels:transcript level and post-translational level. Our reverse transcription (RT)-PCR assays showed that GAs didn't affect NR genes NIA1 and NIA2 expressions. But the divalent cations (especially Mg2+) were found to be required for GAs negatively regulation of NR activity, in view of the importance of divalent cations during the process of post-translational regulation of NR activity, which implies that GAs are very likely to regulate NR activity at post-translational level. In the following dark-light shift analyses, GAs were found to accelerate dark-induced decrease, but retard light-induced increase of the NR activity. Furthermore, it was observed that GAs were involved in diurnal variation of NR activity. Collectively, these results appear to indicate that the phytochrome GAs play an important role during light-dark shift regulating NR activity in nature.3 In the third part of this study, the roles of two bZIP transcription factors HY5 and HYH in inducing anthocyanin accumulation under low temperature were studied. In wild type Arabidopsis, low temperature induces anthocyanin accumulation in the presence of light. Meanwhile, the transcript levels of HY5 and HYH was also elevated by low temperature. In hy5 and hyh single mutants, however, the low temperature-induced anthocyanin accumulation was significantly decreased when compared to that of wild type (WT). Moreover, in double mutant hy5hyh, no anthocyanin accumulation was induced by low temperature even in the light, suggesting that the anthocyanin accumulation induced by low temperature in light was mediated by HY5 and HYH. Through the RT-PCR assay, expressions of several early genes in anthocyanin biosynthetic pathway, such as chalcone synthase (CHS), were up-regulated by low temperature only partially depending on HY5/HYH, whereas dihydroflavanol reductase (DFR), a late gene, was found to be up-regulated almost fully depending on HY5/HYH. Hence, up-regulation of DFR in a HY5/HYH-dependent manner may represent the reason why low temperature-induced anthocyanin accumulation relies upon the presence of light. In addition, the reduction of endogenous GAs was found to contribute to low temperature-induced anthocyanin accumulation. Paclobutrazol (PAC, a GA biosynthesis inhibitor) treatment significantly enhanced anthocyanin accumulation in the presence of HY5/HYH. Also, low temperature treatment up-regulated the expression of GA 2-oxidase 1 (GA2ox1), encoding a enzyme deactivating bioactive GAs, in a HY5/HYH-dependent manner. These data imply that GAs are also involved in low temperature-induced anthocyanin accumulation depending on HY5 and HYH. But for further elucidating the detailed mechanism by which GAs participate in this process, more work should be done in the coming study. Taken together, HY5 and HYH play a central role during low temperature-induced anthocyanin accumulation.
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