Further Development 7.14: UN 2 (Growth of the Pollen Tube Requires Calcium)

Fertilization: Beginning a New Organism

Calcium has long been known to play an essential role in pollen tube growth (Brewbaker and Kwack 1963). The pollen tube grows only at its tip, where open calcium channels are concentrated and Ca2+ accumulates (Jaffe et al. 1975; Steinhorst and Kudla 2013). There is direct evidence that pollen tube growth in the field poppy is regulated by a slow-moving wave of Ca2+ controlled by the same IP3 signaling pathway that we detailed in animal fertilization (Franklin-Tong et al. 1996). This Ca2+ influx occurs both at the tip of the pollen tube and on the sides, and altered Ca2+ influx is observed when the pollen tube is self-incompatible with the style (Franklin-Tong et al. 2002).

Two A. thaliana genes, POP2 and POP3, have been identified that specifically guide pollen tubes to the ovule, with no other apparent function in the plant (Wilhelmi and Preuss 1996, 1999). These genes function in both the pollen and the carpel, where POP2 is involved in GABA metabolism. In POP2 mutants, there is an altered gradient of GABA in the carpel, the pollen tubes do not find the micropyles, and sterility results (Palanivelu et al. 2003).

References

Brewbaker, J. L. and B. H. Kwack. 1963. The essential role of calcium ion in pollen germination and pollen tube growth. Am. J. Bot. 50: 859–865.

Franklin-Tong, V. E., B. K. Drobak, A. C. Allan, P. A. C. Watkins, and A. J. Trewavas. 1996. Growth of pollen tubes in Papaver rhoeas is regulated by a slow-moving calcium wave propagated by inositol 1,4,5-trisphosphate. Plant Cell 8: 1305–1321.

Franklin-Tong, V. E., T. L. Holdaway-Clarke, K. R. Straatman, J. G. Kunkel, and P. K. Hepler. 2002. Involvement of extracellular calcium influx in the self-incompatibility response of Papaver rhoeas. Plant J. 29: 333–345.

Jaffe, L. A., M. H. Weisenseel, and L. F. Jaffe. 1975. Calcium accumulation within the growing tips of pollen tubes. J. Cell Biol. 67: 488–492.

Palanivelu, R., L. Brass, A. F. Edlund and D. Preuss. 2003. Pollen tube growth and guidance is regulated by POP2, an Arabidopsis gene that controls GABA levels. Cell 114: 47-59.

Steinhorst, L. and J. Kudla. 2013. Calcium - a central regulator of pollen germination and tube growth. Biochim Biophys Acta 1833: 1573-1581.

Wilhelmi, L. K. and D. Preuss. 1996. Self-sterility in Arabidopsis due to defective pollen tube guidance. Science 274: 1535–1537.

Wilhelmi, L. K. and D. Preuss. 1999. The mating game: Pollination and fertilization in flowering plants. Curr. Opin. Plant Biol. 2: 18–22.

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