Karrikin-sensing protein KAI2 is a new player in regulating root growth patterns

Summary Roots form highly complex systems varying in growth direction and branching pattern to forage for nutrients efficiently. Here mutations in the KAI2 (KARRIKIN INSENSITIVE) α/β-fold hydrolase and the MAX2 (MORE AXILLARY GROWTH 2) F-box leucine-rich protein, which together perceive karrikins (smoke-derived butenolides), caused alteration in root growth direction (root skewing and waving) of Arabidopsis thaliana . This exaggerated root skewing was independent of endogenous strigolactone perception by the D14 α/β-fold hydrolase and MAX2. Thus KAI2/MAX2’s regulation of root growth may be through perception of endogenous KAI2-ligands, which have yet to be identified. Degradation targets of the KAI2/MAX2 complex, SMAX1 (SUPPRESSOR OF MAX2-1) and SMXL6,7,8 (SUPPRESSOR OF MAX2-1-LIKE) are also involved in the regulation of root skewing. Genetic data reveal a new potential target for degradation, as mutation in the SKS3 (SKU5 similar) but not the SKU5/SKS17 root plasma membrane glycoprotein suppresses the exaggerated root skewing induced by the lack of MAX2. In Arabidopsis thaliana therefore, the KAI2 karrikin-sensing protein acts to limit root skewing, and we propose a mechanism involving root radial expansion as the mutant’s gravitropic and mechano-sensing responses remained largely unaffected.