Brassinosteroids modulate autophagy through phosphorylation of RAPTOR1B by the GSK3-like kinase BIN2 in Arabidopsis

Abstract

Macroautophagy/autophagy is a conserved recycling process that maintains cellular homeostasis during environmental stress. Autophagy is negatively regulated by TARGET OF RAPAMYCIN (TOR), a nutrient-regulated protein kinase that in plants is activated by several phytohormones, leading to increased growth. However, the detailed molecular mechanisms by which TOR integrates autophagy and hormone-signaling are poorly understood. Here, we show that TOR modulates brassinosteroid (BR)-regulated plant growth and stress-response pathways. Active TOR was required for full BR-induced growth in Arabidopsis thaliana. Autophagy was constitutively up-regulated upon blocking BR biosynthesis or signaling, and down-regulated by increasing the activity of the BR pathway. BRASSINOSTEROID-INSENSITIVE 2 (BIN2) kinase, a GSK3-like kinase functioning as a negative regulator in BR signaling, directly phosphorylated Regulatory-Associated Protein of TOR 1B (RAPTOR1B), a substrate-recruiting subunit in the TOR complex, at a conserved serine residue within a typical BIN2 phosphorylation motif. Mutation of RAPTOR1B serine 916 to alanine, to block phosphorylation by BIN2, repressed autophagy and increased phosphorylation of the TOR substrate autophagy-related protein 13a (ATG13a). By contrast, this mutation had only a limited effect on growth. We present a model in which RAPTOR1B is phosphorylated and inhibited by BIN2 when BRs are absent, activating the autophagy pathway. When BRs signal and inhibit BIN2, RAPTOR1B is thus less inhibited by BIN2 phosphorylation. This leads to increased TOR activity and ATG13a phosphorylation, and decreased autophagy activity. Our studies define a new mechanism by which coordination between BR and TOR signaling pathways helps to maintain the balance between plant growth and stress responses.

Regulatory network of GSK3-like kinases and their role in plant stress response

“…Phosphorylation of RAPTOR1B by BIN2 thus activates the autophagy pathway. In the presence of BR, the inhibition effect of BIN2 on RAPTOR1B is attenuated, and results in increased TOR activity and ATG13a phosphorylation, and decreased autophagy activity (Liao et al, 2022). These results suggest that BIN2 functions as a key hub in the crosstalk between TOR and BR signaling pathways.…”

Interactions between autophagy and phytohormone signaling pathways in plants

“…In the presence of BR, when BIN2 is inhibited, RAPTOR1B Ser916 phosphorylation is decreased, which promotes phosphorylation of ATG13a by TOR and thus represses autophagy. This suggests a model in which when BR is absent, active BIN2 can suppress the TOR complex through phosphorylation of RAPTOR1B to induce autophagy [84]. These studies provide valuable resources and mechanistic insights into the interaction between BR signaling and TOR‐dependent autophagy.…”