A wheat-specific proline-rich protein regulates grain weight through a ubiquitin-mediated delay of endosperm cellularization.

Grain weight is a key determinant of wheat yield, yet the developmental mechanisms that define this trait remain poorly understood. Here, we identify TaPGW1-A1, a wheat-specific proline-rich gene that regulates grain weight. TaPGW1-A1 is a recently evolved gene found only in wheat. It is highly expressed in early developing grains, and its overexpression delays endosperm cellularization and prolongs the syncytial phase, resulting in an 8% increase in field-based grain weight. TaPGW1-A1 protein directly interacts with the ubiquitin receptor TaDSK2a (Dominant Suppressor of KAR2 homolog) via its N-terminal signal peptide, and the protein-protein complex localizes to the cell periphery. Biochemical assays show that TaDSK2a binds both K48- and K63-linked polyubiquitin chains. Loss-of-function of TaDSK2a leads to reduced grain size, confirming its essential role in grain development. The E3 ligase TaRGLG2 inhibits the binding of TaPGW1-A1 to TaDSK2a. Together, these findings define a TaPGW1-A1-TaDSK2a-TaRGLG2 regulatory module that links developmental timing with ubiquitin-mediated protein turnover, revealing a previously unrecognized mechanism controlling endosperm cellularization and grain yield in wheat.