Salicylic acid and ROS signaling modulate hypocotyl elongation in darkness via NPR1 and EX1

During early seedling growth in darkness, germinating seeds must balance growth and defense to ensure a successful transition from a heterotrophic to a photoautotrophic state. However, the molecular mechanisms regulating this process in etiolated seedlings remain poorly understood. Here, we investigate the role of salicylic acid (SA), a key defense hormone, in controlling hypocotyl elongation under dark conditions. SA inhibited hypocotyl growth in a dose-dependent manner, with a more substantial effect in the Arabidopsis npr1 mutant, revealing NPR1’s role in maintaining growth under SA-accumulating conditions. In etiolated npr1 seedlings, auxin-responsive genes, including multiple SMALL AUXIN UP-REGULATED RNA ( SAUR ) and EXPANSIN genes, were down-regulated. Overexpression of SAUR19 or SAUR63 fully rescued SA-induced growth inhibition. We also identified EXECUTER1 (EX1), a mediator of singlet oxygen ( 1 O 2 ) signaling, as a contributor to this process. SA-driven lipid peroxidation increased 1 O 2 levels in darkness, triggering EX1-mediated retrograde signaling that represses auxin-related genes.