The Calcium-Responsive Phospholipid-Binding BONZAI Proteins Control Global Osmotic Stress Responses in Plants Through Repression of Immune Signaling

SummaryHyperosmotic stress caused by drought and salinity is a significant environmental factor that limits plant growth and agricultural productivity. Plants respond to osmotic stress by activating Ca2+ signaling, accumulating the stress hormone abscisic acid (ABA), reprogramming gene expression and altering growth. Despite intensive efforts, no global regulators of all of these responses have been identified. Here, we show that the Ca2+-responsive phospholipid binding BONZAI proteins are critical for Ca2+ signaling, ABA accumulation, gene expression reprogramming and plant growth under osmotic stress by antagonizing plant immune responses mediated by intracellular immune receptor NLRs. Using a Ca2+-imaging-based forward genetic screen, we found that the Arabidopsis osmo1/bon1 mutant plants display a reduced cytosolic Ca2+ signal in response to hyperosmotic stress. The bon1/bon2/bon3 triple mutants are impaired in osmotic stress induction of gene regulation and ABA accumulation. Importantly, the bon mutants are hypersensitive to osmotic stress inhibition of plant growth. These defects were suppressed by mutations in the immunity regulator SNC1 or PAD4. Our findings suggest that that BON proteins function as global regulators of plant osmotic stress responses by repressing immune signaling.