S-Nitrosylation-mediated differential regulation of STOP1 and STAR1 coordinates external and internal aluminum resistance in Arabidopsis.

Aluminum (Al) toxicity severely limits crop productivity on acidic soils. In Arabidopsis, STOP1 is a key transcription factor that promotes external Al detoxification primarily by inducing ALMT1-mediated malate secretion, whereas the ATP-binding cassette transporter complex STAR1/ALS3 mediates internal detoxification through vacuolar sequestration of Al. However, the regulatory mechanism that coordinates these external and internal detoxification pathways remains poorly understood, particularly with respect to the role of nitric oxide (NO) signaling. In this study, using a forward genetic screen, we identified rae8/cnx2-3, an Al-sensitive mutant defective in molybdenum co-factor biosynthesis and impaired in NO production. Through genetic, biochemical, and physiological analyses, we discovered that NO regulates Al resistance through S-nitrosylation-mediated modulation of both STOP1 and STAR1. Mechanistically, NO promotes S-nitrosylation of STOP1 at Cys-8, which enhances its interaction with the F-box protein RAE1 and thereby accelerates STOP1 degradation. By contrast, S-nitrosylation of STAR1 at Cys-231 stabilizes the protein and promotes its accumulation. Consequently, in rae8/cnx2-3, excessive accumulation of STOP1 and reduced STAR1 abundance disrupt the balance between external and internal Al detoxification, ultimately increasing Al sensitivity. Together, our findings reveal a coordinated regulatory mechanism by which NO fine-tunes external and internal Al resistance pathways through targeted S-nitrosylation of STOP1 and STAR1, providing important insights into plant adaptation to acidic soils.