SMAX1 integrates KAI2 and ethylene signaling to regulate Arabidopsis hypocotyl growth in light

Plant development is regulated by a complex interplay of multiple signaling pathways driven by small signaling molecules, including plant hormones. A signaling pathway activated by karrikins (KARs), a class of compounds found in smoke that putatively mimic an endogenous plant signaling molecule, "KAI2 ligand" (KL), controls seedling photomorphogenesis in Arabidopsis thaliana. We recently demonstrated that in roots of Lotus japonicus and A. thaliana, the gaseous plant hormone ethylene promotes accumulation of a transcriptional regulator SMAX1 that negatively regulates KAR/KL response and is targeted for degradation by the 26S proteasome after KAR/KL perception. This ethylene-inducible accumulation of SMAX1 suggests convergence of KAR/KL- and ethylene-driven signaling pathways at SMAX1. In this study, we investigated the effects of ethylene signaling on SMAX1-regulated hypocotyl growth in A. thaliana. Chemical inhibition of ethylene biosynthesis reduced hypocotyl elongation in a SMAX1-dependent manner that was independent of KAR/KL signaling. We propose a model in which SMAX1 mediates developmental processes by integrating multiple plant hormone-driven signaling pathways.