Fig. 9.

Ligand and target specificity in D14/KAI2 signalling. Possible models of D14/SMXL signalling in different land plant groups, based on (1) identified D14/KAI2 and SMXL proteins in each taxon, (2) presence or absence of MAX2-interaction residues in D14/KAI2 proteins and (3) predicted shape of ligand binding pockets in D14/KAI2 proteins [29]. In charophyte algae, there may be a KAI2-MAX2 signalling module, but there are no apparent SMXL proteins, and the function of this complex is not clear. In hornworts and liverworts, there is likely to be canonical KAI2 signalling, but there does not seem to be any canonical SL signalling, and SLs may act only as mycorrhizal signals. In mosses, mycorrhizal associations have been lost, and SL may have been independently recruited as a hormonal or ‘quorum sensing’ signal. This may plausibly have occurred by neo-functionalization of DDK and SMXL proteins, but either way is MAX2 independent. In both lycophytes and monilophytes, the status of SL signalling, and whether there is any connection with DDK proteins, is largely unclear. In monilophytes, DDK proteins probably act independently of MAX2, and it is unclear with SMXL proteins interact with the KAI2-MAX2 module. In gymnosperms, canonical D14-mediated SL signalling is present, but both SL and KL probably target the same SMXL protein for degradation. In angiosperms, SL and KL target separate SMXL proteins for degradation. It is unclear whether the downstream targets of the pathways are still shared, or have also separated