Gametophytic Pollen Tube Guidance: Attractant Peptides, Gametic Controls, and Receptors

Abstract

Pollen tube guidance in flowering plants is a unique and critical process for successful sexual reproduction. The pollen tube that grows from pollen, which is the male gametophyte, precisely navigates to the embryo sac, which is the female gametophyte, within the pistil. Recent advances have clarified the molecular framework of gametophytic pollen tube guidance. Multiple species-specific attractant peptides are secreted from synergid cells, the proper development and function of which are regulated by female gametes. Multiple receptor-like kinases on the pollen tube tip are involved in sensing species-specific attractant peptides. In this Update article, recent progress in our understanding of the mechanism of gametophytic pollen tube guidance is reviewed, including attraction by synergid cells, control of pollen tube guidance by female gametes, and directional growth of the pollen tube by directional cue sensing. Future directions in the study of pollen tube guidance also are discussed.

Figure 1.

Pollen tube guidance by the female gametophyte. A, Pollen tubes emerged from the pollen grain (PG) grow through the transmitting tract (TT) and emerge on the surface of the septum (ST) separating two ovary locules. Each pollen tube targets the ovule (OV) via the surface of the funiculus (FL) and the micropyle (MP) of the ovule. B, Two synergid cells (SYs) secrete many peptides, including attractant LUREs and ZmEA1, through the filiform apparatus (FA). The tip-growing pollen tube (PT) is activated by ovular arabinogalactan AMOR to respond to LURE attractant peptides in Torenia. The egg cell (EC) inhibits synergid cells from cell fate switching to the egg cell. CCG and CBP1 in the central cell (CC) control MYB98 expression in synergid cells and some Cys-rich peptide (CRP) expression in the central cell, the function of which remains unknown.

Figure 2.

Directional growth of the pollen tube toward LURE attractant peptide. The LURE1 attractant peptide of Arabidopsis is sensed by the receptor-like kinases MDIS1-MIK and PRK6 with Leu-rich repeats (LRRs; blue boxes) in their ectodomains. LURE1 enhances the dimerization of MDIS1-MIK receptors and the autophosphorylation and transphosphorylation (P) of kinase domains (green boxes) by MIK1. PRK6 interacts with PRK3, LIP1, and LIP2 in the LURE signaling cascade and with ROPGEFs, which might transduce LURE signal to ROPs that regulate the cytosolic calcium gradient, the dynamics of actin filaments (gray lines), and the exocytosis of secretory vesicles (SV) in pollen tube tip (PT tip) growth. The direct interaction of PRK6 with LURE1 has not been shown biochemically. The cyclic nucleotide-gated channel CNGC18 is a Ca²⁺-permeable channel essential for micropylar pollen tube guidance.