A redox-regulated RCC1-like protein controls catalase activity in Arabidopsis.

Reactive oxygen species (ROS) regulate plant growth and stress responses. Catalases play a central role in detoxifying hydrogen peroxide, predominantly within peroxisomes, yet key aspects of catalase regulation remain incompletely understood. Using affinity purification of the UV-B photoreceptor UVR8 coupled with mass spectrometry, we identified CATALASE-INTERACTING RCC1-LIKE 1 (CAIR1), which interacts with all three Arabidopsis catalases and their chaperone NO CATALASE ACTIVITY 1. Loss of CAIR1 reduces catalase activity and causes oxidative stress sensitivity, impaired root growth, and alkaline sensitivity, resembling cat2 and nca1 mutants. CAIR1 promotes peroxisomal import and proper localization of CAT2, preventing CAT2 aggregation and maintaining its activity. CAIR1 undergoes reversible redox-dependent oligomerization that enhances catalase binding, whereas mutation of Cys-356 and Cys-545 compromises this interaction and fails to rescue the oxidative stress sensitivity of cair1 mutants. UV-B weakens CAIR1-catalase interactions and suppresses catalase activity, linking light signalling with redox homeostasis. These findings identify CAIR1 as a redox-responsive regulator of catalase localization and activity.