Phytochrome interacting factors (PIFs) are essential regulators for sucrose-induced hypocotyl elongation in Arabidopsis

Phytochrome interacting factors (PIFs) are members of a subfamily of basic helix-loop-helix transcript factors and have been proposed to act as positive regulators of hypocotyl elongation under normal condition. Here, we show that 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 pif1pif3pif4pif5 (pifq). Subsequent experiments showed that under various light/dark (L/D) cycle conditions sucrose still markedly induced hypocotyl elongation in Col-0, but exhibited little effects in pifq. Phytohormone gibberellins (GAs) have been proven to be required for sucrose-induced hypocotyl elongation, but application of GA3 (an active form of GAs) was not able to rescue the impairment observed in pifq, suggesting that impairment of sucrose-induced hypocotyl elongation in pifq is not due to the reduced endogenous GAs. Interestingly, through RT-PCR assay, we found that sucrose up-regulated the transcript level of PIF1, 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.