HBI1 Mediates the Trade-off between Growth and Immunity through Its Impact on Apoplastic ROS Homeostasis.

Plants continuously need to adapt to their environment and prioritize either growth or defense responses to secure survival and reproduction. Trade-offs between growth and defense are often attributed to the allocation of energy for growth to adaptation responses. Still, the exact mechanisms underlying growth and defense trade-offs are poorly understood. Here, we demonstrate that the growth-related transcription factor HOMOLOG OF BEE2 INTERACTING WITH IBH 1 (HBI1) regulates apoplastic reactive oxygen species (ROS) homeostasis by differentially controlling the expression of NADPH oxidases (NOXs) and peroxidases (POXs). The HBI1 target genes RESPIRATORY BURST OXIDASE HOMOLOG A (RbohA) and RbohC have contrasting effects on the regulation of cell size. In addition, the HBI1-controlled NOXs and POXs oppositely regulate susceptibility toward Pseudomonas syringae. Our findings reveal that the incompatibility between growth and defense programs can be attributed to the way apoplastic ROS homeostasis is modulated during both processes.