When a functional pollen grain lands on the stigma of a compatible stigma, it germinates. A key event is water uptake by pollen from the stigma: pollen loses most of its water as it matures. Germination involves the development of a pollen tube (Figure 37.3). The pollen tube either traverses the tissue of the style (part of the carpel; see Figure 37.2) or, if the style is hollow, grows on the inner surface of the style until it reaches an ovule. The growth rate of the pollen tube varies greatly among species, but it can be as fast as 1 centimeter an hour.

Question

Pollen tube growth requires loosening of the cell wall (hydrolase enzymes), deposition of new cell wall material (synthesis of cellulose and matrix polysaccharides), expansion of the cell membrane and cell contents (involvement of the Golgi apparatus), and cell expansion (driven by osmosis from the vacuole).

The growth of the pollen tube is guided in part by a chemical signal in the form of a small protein produced by the synergids within the ovule. If one synergid is destroyed, the ovule still attracts pollen tubes, but destruction of both synergids renders the ovule unable to attract pollen tubes, and fertilization does not occur.

Genetic screens (see Figure 36.2) of mutants in Arabidopsis have revealed several genes whose expression in synergids is needed for guiding the pollen tube. There is evidence that a small molecule made by the embryo sac—γ-aminobutyric acid, or GABA—is an attractant for pollen tube growth. This would be an amazing coincidence, since this molecule is also a signal in the vertebrate nervous system.