Physcomitrium patensSMXL homologs are PpMAX2-dependent negative regulators of growth

Abstract

AbstractSMXL proteins are a plant-specific clade of type I HSP100/Clp-ATPases.SMXLgenes are found in virtually all land plant genomes. However, they have mainly been studied in angiosperms. InArabidopsis thaliana, three SMXL functional subclades have been identified: SMAX1/SMXL2, SMXL345 and SMXL678. Out of these, two subclades ensure transduction of endogenous hormone signals: SMAX1/SMXL2 are involved in KAI2-ligand (KL) signaling, while SMXL678 are involved in strigolactone (SL) signaling. Many questions remain regarding the mode of action of these proteins, as well as their ancestral role. We addressed these questions by investigating the function of the fourSMXLgenes of the mossPhyscomitrium patens.We demonstrate that PpSMXL proteins are involved in the conserved ancestral MAX2-dependent KL signaling pathway and act as negative regulators of growth. However, PpSMXL proteins expressed inA. thalianaunexpectedly cannot replace SMAX1/SMXL2 function in KL signaling, whereas they can functionally replace SMXL4/5 and restore root growth. Therefore, the molecular function of SMXL could be conserved, but not their interaction network. Moreover, one PpSMXL clade positively regulates transduction of the SL signal inP. patens.So far, this function has only been reported herein in moss, where it represents a novel crosstalk between SL and KL signaling pathways.