The OsBZR1-OsSPX1/2 Module Fine-tunes Growth-Immunity Trade-off in Adaptation to Pi Availability in Rice

The growth-promoting hormone 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. The resource shortages, including phosphate (Pi) deficiency, potentially disrupt this growth-defense balance. It has been reported that OsSPX1 and OsSPX2 sense Pi concentration to interact with the Pi signal mediator OsPHR2, thus regulating Pi-starvation responses. Herein, we discovered that OsSPX1/2 interacted with OsBZR1 irrespective of Pi-sufficient or Pi-deficient conditions, inhibiting BR-responsive genes. When Pi is sufficient, OsSPX1/2 is captured by OsPHR2, allowing most of OsBZR1 to advance plant growth and maintain basal resistance. In response to Pi starvation, more OsSPX1/2 is released from OsPHR2 to inhibit OsBZR1 activity, resulting in retarded growth. Altogether, the OsBZR1-SPX1/2 module balances plant growth-immunity trade-off in response to Pi availability.