Shoot apical meristem: the co-evolution of structure and of cell proliferation genetic control

Shoot apical meristem (SAM) produces the initial cells for all the tissues in the plant shoot system. Land plant evolution was accompanied by changes in the SAM cell arrangement, i.e. by the enrichment of the number of apical initial cells, by the initial cell shape evolution, by differentiation of separate functional zones of SAM. The increasing complexity of SAM is correlated with reorganization of the whole plasmodesmata net, as well as with free secondary plasmodesmata development. In different taxa the interaction of some homologues of CLAVATA1, 2, 3 and WUSHEL genes regulates the cell number in shoot and root apical meristems by the control of cellular division rate. The evolutionary trend from Gymnosperms to Angiosperms was marked by the WOX-family gene function specialization. We observe both spatial (different WOX-gene expression in shoot and root apical meristems) and temporal (WOX-gene expression on different stages of ontogenesis) specialization. Gene family CLE is widely spread in seed and spore plants and contains the CLAVATA 3 homologues. The functions of this gene family are not restricted to the meristem proliferation control. The receptor complexes, composed of CLV1-CLV2, CLV2-RPK2, are widely distributed among spore and seed plants, whereas CLV2-SOL2/CRN complex appeared to be the evolutionary innovation of Angiosperms. Thus Angiosperms evolved the most complex regulatory network in SAM development control. Any dysfunction in this net leads to stem fasciation. This phenomenon is well-documented for Angiosperms and Gymnosperms, but is not observed in spore plants until now.