Phytosulfokine peptide optimizes plant growth and defense via glutamine synthetase GS2 phosphorylation in tomato

Phytosulfokine (PSK) is a plant pentapeptide hormone that fulfills a wide range of functions. Although PSK has frequently been reported to function in the inverse regulation of growth and defense in response to (hemi)biotrophic pathogens, the mechanisms involved remain largely unknown. Using the tomato ( Solanum lycopersicum ) and Pseudomonas syringae pv. tomato ( Pst ) DC3000 pathogen system, we present compelling evidence that the PSK receptor PSKR1 interacts with the calcium‐dependent protein kinase CPK28, which in turn phosphorylates the key enzyme of nitrogen assimilation glutamine synthetase GS2 at two sites (Serine‐334 and Serine‐360). GS2 phosphorylation at S334 specifically regulates plant defense, whereas S360 regulates growth, uncoupling the PSK‐induced effects on defense responses and growth regulation. The discovery of these sites will inform breeding strategies designed to optimize the growth‐defense balance in a compatible manner. Synopsis The peptide hormone phytosulfokine (PSK) acts to promote plant growth at the expense of the pathogen defense against (hemi)biotrophic pathogens. Here, we show that the PSK‐induced phosphorylation of glutamine synthetase GS2 at two serine residues regulates this balance. CPK28 interacts with the PSK receptor PSKR1 and is required for PSK‐PSKR1 signaling‐mediated growth‐defense responses in tomato. PSK promotes the CPK28‐mediated phosphorylation of glutamine synthetase GS2 at Ser334 and Ser360. GS2 phosphorylation at Ser334 specifically regulates plant defense, whereas Ser360 regulates growth. Graphical Abstract The calcium‐dependent protein kinase CPK28 phosphorylates glutamine synthetase GS2 at two distinct sites to regulate the phytosulfokine‐dependent balance between plant immune responses and growth.