Guard Cell Activity of PIF4 Represses Disease Resistance in Arabidopsis.

Phytochrome Interacting Factor 4 (PIF4) plays a central role in coordinating plant growth regulation by integrating multiple environmental cues. However, studies on whether and how PIF4 regulates plant immunity have inconsistent findings. In this study, we investigated the role of PIF4 in disease resistance against Pst DC3000 by characterizing its loss-of-function mutants using different inoculation strategies. Our findings reveal that pif4 mutants exhibit enhanced disease resistance with spray inoculation but not with infiltration inoculation compared to wild-type plants, and that mutants displayed more closed stomata apertures, indicating that PIF4 promotes stomatal opening. Importantly, expression of PIF4 by a guard-cell-specific promoter was sufficient to restore disease resistance to the wild-type level in the pif4 mutant. Additionally, PIF4 overexpression enhances disease symptom development independent of disease resistance and chlorophyll degradation, while the loss of PIF4 function leads to higher chlorophyll accumulation. Thus, our findings highlight a crucial function of PIF4 in regulating stomata-mediated disease resistance and chlorophyll accumulation, providing new insights into the connection of growth and defense in plants.