Strigolactone- and karrikin-independent SMXL proteins are central regulators of phloem formation

SMAX1 (SUPPRESSOR OF MAX2 1) was identified genetically as a suppressor of the enhanced seed dormancy phenotype of the max2 mutant, which is affected in both strigolactone (SL) and karrikin (KAR) responses (karrikins are smoke-derived germination promoters). SMAX1 is the founding member of an eight-member gene family in Arabidopsis, the SMAX1-LIKE family (SMXL); the encoded proteins show weak homology to HEAT SHOCK PROTEIN 101. Wallner et al. investigated the function of previously uncharacterized SMXL3, 4 and 5, through single mutants, double mutant combinations, and the triple mutant. The single mutants showed no abnormal phenotype, but all double mutants and the triple mutant showed reduced root growth. The genes are expressed in the protophloem, promote phloem development, and do not depend on SL or KAR signalling components. Curr. Biol. 10.1016/j.cub.2017.03.014