<scp>OsCK2</scp> Phosphorylates <scp>OsSIZ1</scp> to Regulate Phosphate Starvation Response and Defence Response in Rice

Phosphorus is a crucial nutrient for the normal growth and development of rice. Enhancing the efficiency of phosphorus acquisition in plants is vital for optimising soil phosphorus utilisation. The SUMO E3 ligase OsSIZ1 is an important factor for regulating plant phosphorus acquisition and starvation response in rice. However, how its activity responds to Pi-supplied conditions remains unknown. In this study, we discovered that the mutations in OsCK2β3, which encodes a regulatory subunit of CK2, influenced the Pi starvation response. The global level of SUMOylation was found to decrease in Osck2β3 mutants, mirroring the effects observed under Pi-deficiency treatment. Additionally, we identified that OsCK2 can interact with and phosphorylate OsSIZ1, thereby impacting its stability. Transcriptome analysis with Ossiz1 and Osck2β3 mutants revealed that the 'OsCK2-OsSIZ1' module primarily regulates defence responses by influencing the expression of genes associated with the biosynthesis of diterpenoid phytoalexins and pathogenesis-related genes, in addition to impacting the response to phosphate starvation. Notably, the overexpression of a variant of OsSIZ1, in which the phosphorylation sites targeted by OsCK2 were mutated, enhanced phosphorus acquisition and defence responses while preserving normal growth. Consequently, we not only showed how OsSIZ1 activity responds to Pi starvation but also created a variant of OsSIZ1 that improves phosphorus acquisition efficiency and enhances resistance against multiple pathogens, all without inducing growth defects.