In contrast to animals and lower plants such as mosses and
ferns, sperm cells of flowering plants (angiosperms) are
immobile and require transportation to the female gametes via
the vegetative pollen tube cell to achieve double fertilization.
The path of the pollen tube towards the female gametophyte
(embryo sac) has been intensively studied in many intra- and
interspecific crossing experiments with the aim of increasing
the gene pool of crop plants for greater yield, improved biotic
and abiotic stress resistance, and for introducing new agronomic
traits. Many attempts to hybridize different species or
genotypes failed due to the difficulty for the pollen tubes in
reaching the female gametophyte. Detailed studies showed that
these processes are controlled by various self-incompatible
(intraspecific) and cross-incompatible (interspecific)
hybridization mechanisms. Understanding the molecular mechanisms
of crossing barriers is therefore of great interest in plant
reproduction, evolution and breeding research. I