Abstract
SMAX1-LIKE (SMXL) proteins, previously linked to strigolactone and karrikin signalling, play diverse and partially redundant roles in plant development. The divergent SMXL4 superclade-comprising SMXL3, SMXL4 and SMXL5-is not subject to strigolactone- or karrikin-dependent proteolysis. Although these proteins have been associated with phloem differentiation and primary root growth in Arabidopsis thaliana, their broader functions remain underexplored. In this present work, we used double mutants and a series of complementing lines to investigate the in vivo functions of SMXL3 and its prominent motifs. Loss of both SMXL3 and SMXL5 resulted in spontaneous adventitious root formation on the hypocotyl, a phenotype suppressed in etiolated seedlings and modulated by carotenoid-derived signals. In contrast, anchor root formation, maintenance of primary root tip dominance and vertical root growth orientation were redundantly controlled by all SMXL4 superclade members and required an intact EAR repression motif in SMXL3. Beyond the root system, SMXL3, SMXL4 and SMXL5 collectively influenced vegetative growth, while the combined activity of SMXL3 and SMXL4 limited higher-order inflorescence branching and promoted embryo development. Together, our findings reveal that SMXL4 superclade proteins act in a combinatorial manner to control developmental timing and root architecture through both EAR motif-dependent and -independent mechanisms, with root-associated phenotypes likely reflecting altered auxin redistribution or local auxin accumulation. Overall, SMXL proteins emerge as key components controlling plant development in a context-dependent manner.