PbSnRK1-Mediated Phosphorylation of PbTCP13 Regulates Glucose-Responsive Lignin Biosynthesis via PbMYB11 During Pear Stone Cell Formation.

Stone cell lignification is a key determinant of pear fruit texture and commercial quality, and this process has been reported to be tightly coupled to cellular glucose availability. However, how glucose dose-dependently targets transcription factor phosphorylation during lignin biosynthesis remains largely unknown. Using yeast two-hybrid library screening and phosphorylation assays, we found that PbSnRK1, a glucose-responsive kinase, interacts with and directly phosphorylates PbTCP13, thereby participating in the regulation of stone cell lignification. PbSnRK1 serves as an energy-status sensor linking glucose signalling to biphasic regulation of lignin metabolism in pear. Under low-glucose conditions, activated PbSnRK1 phosphorylates the transcription factor PbTCP13 at Ser265, targeting it for 26S proteasome-mediated degradation and thereby derepressing PbMYB11-PbCCR1 cascade associated with lignin biosynthesis. Under high-glucose conditions, suppressed PbSnRK1 kinase activity results in PbTCP13 protein accumulation, which restricts excessive lignification by both repressing PbCCR1 transcription and physically interacting with PbMYB11 to inhibit its transactivation capacity. This PbSnRK1-PbTCP13-PbMYB11 cascade, balanced by glucose concentration, acts as a molecular switch that activates stone cell formation and lignification at low-glucose levels during early fruit development, while restricting excessive lignification at high-glucose levels during later stages, providing new mechanistic insights into energy-coupled secondary wall metabolism in fleshy fruits.