Conserved features of germination and polarized cell growth: a few insights from a pollen-fern spore comparison

BACKGROUND: The germination of both pollen and fern spores results in the emergence of a cell-pollen tube from pollen, rhizoid from spore-that grows in a polar fashion, primarily at its apical end. In both of these tip-growing cells, the delivery of secretory vesicles to the growing end is guided in part by a calcium gradient, with calcium entering at the tip where it is most highly concentrated. The similarities between the two systems extend beyond tip-focused calcium gradients to encompass signalling pathways and elements including calmodulin, nitric oxide, annexins and Rop-GTPases. SCOPE AND AIMS: This review is limited to those pathways and elements that function similarly in fern and pollen systems based on currently available evidence. The aim is to illustrate the common mechanisms by which tip growth occurs, facilitate further investigations into this area, and examine the implications for the evolutionarily conserved control of tip growth. CONCLUSIONS: The interplay of calcium, nitric oxide and other effectors in both pollen and fern spores suggests certain signalling pathways became important regulators of germination and growth early in the evolution of land plants. Both large- and small-scale comparative genomic methods have shown to be promising in their ability to find new and relevant comparisons for further research. Cross-species comparisons may serve to speed up this process by highlighting both basic pathways and system-specific deviations.