The molecular odyssey during pollination

Flowering plants have evolved complex molecular mechanisms that ensure successful pollination and seed set. Continuous cell-cell communication between the pollen (male) and cells of the pistil (female part of the flower) is a critical determinant of reproductive success. The stigma, a specialized female tissue of the pistil receives and facilitates the growth of desirable pollen and blocks less desirable pollen for fertilization. Stigma development and receptivity (ability to accept compatible pollen) is likely regulated through reactive oxygen species (ROS). A functionally redundant mitogen-activated protein kinase (MAPK) signaling cascade ensures stigma receptivity in Arabidopsis (Jamshed et al., 2020). Self-recognition and self-incompatibility (SI) are mechanisms that prevent self-fertilization and consequent inbreeding in flowering plants. The molecular basis and genetic analysis of SI system have uncovered a complex, receptor-ligand mediated, phosphorylation-dependent signaling pathway that coordinates the rejection of self-pollen. The members of mustard family utilize methylglyoxal, a metabolic by-product of glycolysis to bring about SI response (Sankaranarayanan et al., 2015; Kenney et al., 2020). Compatible pollination results in a rapid stigma senescence response (Sankaranarayanan et al., 2013). Following which, the pollen forms a pollen tube to transport the non-motile sperm cells to the ovules for double fertilization. Defensin- like peptide LUREs function as pollen tube attractants secreted by the synergid cells of the ovules (Okuta et al., 2009). In Torenia fournieri, an ovular competency factor AMOR glycan is required to perceive LUREs (Mizukami et al., 2016). 4-O-methyl-glucuronosyl galactose, the terminal sugar of AMOR glycan is essential and sufficient for the competency induction of pollen tubes (Jiao et al., 2017). Given that pollination is an important event in plant reproduction, understanding the molecular mechanisms contributing to successful pollination could result in strategies to improve crop yield.