PIF4-mediated regulation of H2O2 homeostasis controls Arabidopsis seedling thermomorphogenesis

Thermomorphogenesis under high ambient temperature involves extensive developmental changes, including hypocotyl elongation at the seedling stage, in Arabidopsis. Reactive Oxygen Species (ROS), particularly hydrogen peroxide (H2O2), are important signaling molecules, playing crucial roles in plant development and stress responses. While ROS-homeostasis is shown to be crucial for maintaining cellular functions and mediating various developmental responses, the involvement of ROS-homeostasis in the regulation of thermomorphogenic responses and the underlying genetic basis remains poorly understood. In this study, comprehensive transcriptomic analyses revealed strong induction of ROS homeostasis and signaling genes in Arabidopsis seedlings under high ambient temperature. Pharmacological and genetic experiments showed that maintaining H2O2 homeostasis is crucial for seedling thermomorphogenesis. We identified PHYTOCHROME INTERACTING FACTOR 4 (PIF4) as a key regulator of H2O2 homeostasis via direct transcriptional activation of CAT2 and CAT3 genes, which are involved in the regulation of H2O2 levels, to modulate hypocotyl elongation under high temperature. Genetic and biochemical experiments confirmed that CATs act downstream to PIF4 in the same signaling pathway to regulate high-temperature-responsive hypocotyl elongation. Interestingly, elevated H2O2 levels reduced PIF4 protein abundance under high temperature. Together, our findings establish a PIF4-CAT-H2O2 regulatory module, functioning alongside the canonical PIF4-Auxin module, that integrates to auxin signaling to fine-tune hypocotyl elongation under high temperature by maintaining H2O2 homeostasis.