Study On The Mechanism Of PIFs Involved In Sugar Regulating Plant Growth And Metabolism

The phytochrome interacting factors (PIFs) are basic helix-loop-helix transcript factors intracting with phytochrome and have been proposed to act negatively on phytochrome signal transduction to regulate plant growth and development in light. Wild types Col-0 and pif mutants were used here to study the mechanism of PIFs involved in sugar regulating the growth and metabolism of Arabidopsis seedlings. The result show PIFs are positively necessary transcript factors in regulating sucrose-induced hypocotyl elongation in darkness, here, we show that the PIF1,3,4,5 together play a central role in sucrose-induced hypocotyl elongation. When seedlings grown in light were transferred to darkness, exogenously applied sucrose significantly induced hypocotyl elongation in wild type Col-0, but this effect was impaired in all tested pif mutants, especially in the quadruple mutant piflpi13pif4pif5 (pifq). For example, after sucrose treatment 2 days in darkness, the hypocotyl enlongation increased by 0.324 cm in Col-0, but 0.099 cm in pifq. Subsequent experiments showed that sucrose under various light/dark (L/D) cycle conditions still markedly induced hypocotyl elongation in Col-0, but exhibited little effects in pifq. For example, under 18 h L/6 h D condition, the hypocotyl length increased from 0.157 cm to 0.227 cm with sucrose treatment, whereas under 6 h L/18 h D, it increased from 0.207 cm to 0.422 cm. However, in pifq, regardless of sucrose treatment, all seedlings had the similar hypocotyl lengths.Phytohormone gibberellins (GAs) have been proven to be required for sucrose-induced hypocotyl elongation, but application of GA3 (a form of active GAs) was not able to rescue the impairment observed in pifq, suggesting that impairment of sucrose-induced hypocotyl elongation in pifq is not from the reduced endogenous GAs. Interestingly, through RT-PCR assay, we found that sucrose up-regulated the transcript levels of PIF 1,3,4,5 in darkness. Furthermore, this effect was dependent on the presence of GAs. Additionally, under continuous light condition, sucrose markedly inhibited the hypocotyl elongation in Col-0 but not in pifq, whereas exogenous GA3 could recover the repression in Col-0 but only showed slight effect in pifq. These results collectively indicate that PIFs together with GAs control the effect of sucrose on hypocotyl elongation in Arabidopsis seedlings. In addition, the role of PIFs on anthocyanin accumulation under continuous red light was studied. The result show more anthocyanin accumulated in pif mutants (pif1-1, pif3-3,pif4-2, pif5-2, pifq) as compared to wild type Col-0. However the anthocyanin accumulation inpaired or even abolished in the pif overexpression mutants (pif3ox, pif5ox). Theses results indicate PIFs negatively regulate anthocyanin accumulation in red light. Furthermore, through the qPCR assay, it was found that expression levels of several genes in anthocyanin biosynthetic pathway, such as CHS, CHI, F3H, F3’H,DFR, LDOXwere significantly higher in pif mutants when compared to that in Col-0. For the genes encoding sevel transcript factors related to anthocyanin accumulation, such as PAP1 and TT8, the same results were obtained. In our subsequent experiments, anthocyanin accumulation in red light was found to be negatively regulated by GAs, and the effect of pif mutants on anthocyanin accumulation could be abrogated by exogenously appled GA3. Taken together, these data imply that GAs signaling is possibly involved in PIFs-regulated anthocyanin accumulation under red light condition.Overall, these primary results indicate that PIFs may represent a node of crosstalk between sugar and light signaling in regulating plant growth and metabolism. But for completely understanding how PIFs participate in this process more work should be done in our coming study.