Complexity of SMAX1 signaling during seedling establishment.

The SMAX1 proteins are gradually destabilized upon exposure to warm temperatures and the thermodynamic control of SMAX1 abundance balances the phyB action, thus thermosensitizing the PIF4-dependent hypocotyl morphogenesis. The photo-induced stabilization of SMAX1 represses the nuclear accumulation of DELLA proteins in hypocotyl cells, in which SMAX1 integrates light and KAR signals into the GA-DELLA signaling pathways. The D14-mediated SL signaling pathway and the KAI2-mediated KAR signaling pathway act together to enhance plant tolerance to environmental constraints. SMAX1 lies at the crosstalks of SL and KAR signals and serves as a molecular hub that links various growth hormone signaling cascades with seedling establishment. The protein structural and functional organization and presence of predicted IDRs in the SMAX1 protein suggest that SMAX1 could incorporate multiple cellular and growth hormone signals into optimizing seedling establishment. Karrikins (KARs) are small butenolide compounds identified in the smoke of burning vegetation. Along with the stimulating effects on seed germination, KARs also regulate seedling vigor and adaptive behaviors, such as seedling morphogenesis, root hair development, and stress acclimation. The pivotal KAR signaling repressor, SUPPRESSOR OF MAX2 1 (SMAX1), plays central roles in these developmental and morphogenic processes through an extensive signaling network that governs seedling responses to endogenous and environmental cues. Here, we summarize the versatile roles of SMAX1 reported in recent years and discuss how SMAX1 integrates multiple growth hormone signals into optimizing seedling establishment. We also discuss the evolutionary relevance of the SMAX1-mediated signaling pathways during the colonization of aqueous plants to terrestrial environments. [ABSTRACT FROM AUTHOR]