LAUSR

Although the division of the pericycle cells initiates both the lateral root development and the root-derived callus formation, these developmental processes are affected differently in the strigolactone (SL) and karrikin (KAR)/KAI2 ligand (KL) signalling mutant, more axillary growth 2 (max2). Whereas max2 produces more lateral roots than the wild type, it is defective in the regeneration of shoots from root explants. We suggest that the decreased shoot regeneration of max2 originates from a delayed callus primordium formation, yielding less callus material to regenerate shoots. Indeed, when incubated on callus-inducing medium, the pericycle cell division was reduced in max2 and the early gene expression varied when compared to the wild type, as determined by a transcriptomics analysis. Furthermore, the expression of the LATERAL ORGAN BOUNDARIES DOMAIN genes and of callus induction genes was modified in correlation with the max2 phenotype, suggesting a role for MAX2 in the regulation of the interplay between cytokinin, auxin, and light signalling in callus initiation. Additionally, we found that the in vitro shoot regeneration phenotype of max2 might be caused by a defect in KAI2, rather than D14, signalling. Nevertheless, the shoot regeneration assays revealed that also the SL biosynthesis mutants max3 and max4 play a minor role.