SUPPRESSOR OF MAX2 1-LIKE 5 promotes secondary phloem formation during radial stem growth (Plant J)

Plant tissues continue to develop postembryonically through proliferation at the shoot and root apical meristem. In the vasculature, radial growth occurs through the proliferation of the undifferentiated cambial cells. Members of the SUPPRESSOR OF MAX2 1-LIKE (SMXL) proteins are known to be involved in primary phloem development in the root vasculature. In this paper, Wallner et al. identified the role of SMXL4 and SMXL5 in secondary phloem formation in Arabidopsis shoot vasculature. SMXL4 is expressed in the differentiated phloem cells and SMXL5 is expressed in the cambium as well as in the mature phloem cells. Individual loss of function alleles of smxl4 and smxl5 showed no changes in the secondary vascular development but the double mutant smxl4 smxl5 had increased cambium-derived tissue. Through loss-of-function mutants and grafting experiments, the authors identified the role of SMXL5 in secondary phloem development. Transcriptomic analysis of smxl4 smxl5 showed downregulation of phloem regulators and the upregulation of stress-responsive genes. Further analysis of phloem marker expression in the mutants indicated the role of SMXL5 in secondary phloem development in the Arabidopsis shoot. Thus SMXL5 has been identified as a regulator of phloem development and a potential key player involved in plant biomass formation. (Summary by Suresh Damodaran) Plant J.  10.1111/tpj.14670