Sugar inhibits brassinosteroid signaling by enhancing BIN2 phosphorylation of BZR1

Author summary Genetic studies of the brassinosteroid (BR) deficient mutants revealed its essential role in seedling development in the dark, but subsequent studies showed no significant difference in BR level between seedlings grown under light and darkness. We recently observed that light does affect BR levels in Arabidopsis, but in a sugar dependent manner. In the dark, sugar increases BR level as well as BR sensitivity by stabilizing the steroid response factor BZR1 through the Target of Rapamycin (TOR) signaling pathway. However, the BR level is decreased by sugar under light and by darkness on sugar-free medium. These observations raised the question of how the combinations of light and sugar modulate BR signaling. We addressed this question using genetic physiological analyses and found interestingly that sugar inhibits brassinosteroid response in light-grown plants by stabilizing the glycogen synthase kinase 3 homolog BIN2 and attenuating the dephosphorylation of BZR1, but independently of TOR. Our results indicate that sugar acts through distinct pathways to promote and inhibit BR signaling in dark and light conditions. Our work illustrates an intricate three-way crosstalk whereby the combination of light and sugar signals modulate the brassinosteroid signaling pathway to optimize growth according to both environmental and metabolic conditions. Sugar, light, and hormones are major signals regulating plant growth and development, however, the interactions among these signals are not fully understood at the molecular level. Recent studies showed that sugar promotes hypocotyl elongation by activating the brassinosteroid (BR) signaling pathway after shifting Arabidopsis seedlings from light to extended darkness. Here, we show that sugar inhibits BR signaling in Arabidopsis seedlings grown under light. BR induction of hypocotyl elongation in seedlings grown under light is inhibited by increasing concentration of sucrose. The sugar inhibition of BR response is correlated with decreased effect of BR on the dephosphorylation of BZR1, the master transcription factor of the BR signaling pathway. This sugar effect is independent of the sugar sensors Hexokinase 1 (HXK1) and Target of Rapamycin (TOR), but requires the GSK3-like kinase Brassinosteroid-Insensitive 2 (BIN2), which is stabilized by sugar. Our study uncovers an inhibitory effect of sugar on BR signaling in plants grown under light, in contrast to its promotive effect in the dark. Such light-dependent sugar-BR crosstalk apparently contributes to optimal growth responses to photosynthate availability according to light-dark conditions.

Automated summary

The research found that sugar promotes BR signaling through the TOR pathway in dark conditions, while it inhibits BR signaling through the BIN2 kinase in light conditions. This suggests that the combination of light and sugar can regulate BR signaling through different pathways to optimize plant growth.