SMXL

SMXL stands for Suppressor of MAX2-like, a family of plant proteins that play a central role in the signaling pathway of strigolactones, a class of plant hormones involved in development and response to environmental cues. In general, SMXL proteins act as repressors in SL signaling, and their activity is controlled by the MAX2 F-box protein as part of an SCF ubiquitin ligase complex. Perception of strigolactones leads to the targeted degradation of SMXL repressors, enabling downstream changes in growth and architecture.

The SMXL gene family is diversified across land plants. In Arabidopsis thaliana, several members have distinct functions: SMXL3, SMXL4, and SMXL5 are associated with phloem development and vascular patterning, while SMXL6, SMXL7, and SMXL8 are more closely tied to the canonical strigolactone signaling branch that regulates shoot branching. In rice, a related SMXL member named D53 acts as a repressor of SL signaling. Across species, SMXL homologs form multiple clades and display both conserved and lineage-specific roles in growth regulation.

Mechanistically, SMXL proteins are targets of MAX2-dependent ubiquitination. Upon strigolactone perception, SMXL repressors are ubiquitinated and degraded, which relieves repression of downstream transcriptional programs that control branching, root architecture, and other developmental processes. This degradation is a key step linking hormone perception to growth responses.

Functionally, SMXL genes influence plant architecture by modulating shoot branching and root system development, with additional roles in vascular differentiation. Because their activity affects yield-related traits in crops, SMXL pathways are of interest for breeding strategies aimed at optimizing plant form and resource allocation.