Fig. 1.

Schematic representations of multi-stage controlled pollen tube guidance and pre-zygotic barriers. Pollen tube guidance involves multistage controls (A–F). In pollen tube guidance, the pollen tube enters the stigma after pollen germination (A), and elongates the pollen tube through the stigma and style (B). The pollen tube penetrates the transmitting tract (C), grows on the inner surface of the ovule (D), and enters the micropyle toward the synergid cell (E). Finally, the pollen tube releases the sperm cells (F). Pre-zygotic barriers can also be a multi-stage mechanism (G–J). In the case of pollination with other species, pollen hydration and germination is inhibited on the stigma (G) and pollen tube elongation through stigma and style is prevented (H). In some inter-specific hybrids, these steps (G–H) may involve S-locus genes, which regulate self-incompatibility. The pollen tube is attracted by attractants, LURE peptide, in a species-preference manner (I). Pollen tubes entering the micropyle cannot burst to release the sperm cells, and overgrow inside the female gametophyte (J).