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 <i>Arabidopsis npr1</i> mutant, revealing NPR1's role in maintaining growth under SA-accumulating conditions. In etiolated <i>npr1</i> seedlings, auxin-responsive genes, including multiple <i>SMALL AUXIN UP-REGULATED RNA</i> (<i>SAUR</i>) and <i>EXPANSIN</i> genes, were down-regulated. Overexpression of <i>SAUR19</i> or <i>SAUR63</i> fully rescued SA-induced growth inhibition. We also identified EXECUTER1 (EX1), a mediator of singlet oxygen (¹O₂) signaling, as a contributor to this process. SA-driven lipid peroxidation increased ¹O₂ levels in darkness, triggering EX1-mediated retrograde signaling that represses auxin-related genes.