Abstract:
Having non-motile sperm, flowering plants depend on sequential male-female interaction events starting from pollen hydration on the stigma to sperm delivery to the female gametophyte inside an ovule fer fertilization. Multiple barriers need to be overcome to avert unwanted mates but facilitate desirable mates for eventual encounter and fusion of the male and female gametes. Upon landing on the stigma, the pollen grain hydrates and germinates a pollen tube to penetrate the pistil and transport sperm as its cytoplasmic cargo to the female gametophyte. The pollen tube burrows through the extracellular matrix (cell wall) of pistillate cells before exiting the transmitting tissue in the main pollen tube growth pathway to target ovules one at a time and penetrate one of the paired synergid cells at the entrance of the female gametophyte. Upon penetrating the female gametophyte, in an event referred to as “pollen tube reception”, the two encapsulated sperm cells are released allowing them access to the female gametes, the egg and the central cell, for fertilization to produce a seed. Multiple barriers have evolved in higher plants to prevent supernumerary pollen tubes from penetrating the same ovule and avoid polyspermy. Research in the past decade elucidated that at the core of these strategies is a molecular signaling triad comprised of FERONIA or FERONIA-related receptor kinase and a glycosylphospatidylinositol-anchored protein (LORELEI or LORELEI-like) functioning as a co-receptor pair, and its ligand, a peptide from the Rapid Alkalinization Factor (RALF) family. Each of these signaling triad bears distinct elaborations to fulfill its mission. I shall discuss these recent advances.