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The growth-promoting hormones brassinosteroids (BRs) and their key signaling component BZR1 play a vital role in balancing normal growth and defense reactions. Here, we discovered that BRs and OsBZR1 upregulated sakuranetin accumulation and conferred basal defense against Magnaporthe oryzae infection under normal conditions. Resource shortages, including phosphate (Pi) deficiency, potentially disrupt this growth-defense balance. OsSPX1 and OsSPX2 have been reported to sense Pi concentration and interact with the Pi signal mediator OsPHR2, thus regulating Pi starvation responses. In this study, we discovered that OsSPX1/2 interacts with OsBZR1 in both Pi-sufficient and Pi-deficient conditions, inhibiting BR-responsive genes. When Pi is sufficient, OsSPX1/2 is captured by OsPHR2, enabling most of OsBZR1 to promote plant growth and maintain basal resistance. In response to Pi starvation, more OsSPX1/2 is released from OsPHR2 to inhibit OsBZR1 activity, resulting in slower growth. Collectively, our study reveals that the OsBZR1-SPX1/2 module balances the plant growth-immunity trade-off in response to Pi availability.
He et al. (Thu,) studied this question.