Figure 3.

Main signaling molecules involved in the communication between pollen tube and transmitting tract during its growth in this tissue, towards the ovule. This image aims to focus mainly on the transmitting-tract-secreted signals, rather than on the signals generated by the pollen tube, well described in many reviews. TTS (transmitting-tissue-specific) is shown at the surface of the TT cells either still intact or being deglycosylated (scissors) by the growing PT. The glycosylated TTS is decreased after the arrival of the PTs in the TT, thereby establishing a gradient of increasing TTS glycosylation from the top to the bottom of the style. AGPs (arabinogalactan proteins) are also present in the TT cells and ECM and are thought to play essential roles in PT growth, by some signaling pathway which remains to be unraveled. AGPs are represented by the branched glycans acting as Ca²⁺ reservoirs (green dots), which are probably involved in Ca²⁺ signaling pathways in the PT. LTP (lipid transfer protein) represents the SCA, stylar cysteine-rich adhesion molecule, secreted by the TT cells, forming an adhesive matrix with pectins between the PT and the TT surface. Chemocyanin is also shown, which has a chemotropic guidance effect on PTs by acting together with SCA. VGD1 (VANGUARD1) is a pectin methylesterase (PME) from the PT, acting on pectin demethyesterification and leading to TT cell wall loosening to facilitate PT growth, possibly by coordinated action with endopolygalacturonases. A GABA (γ-amino butyric acid) gradient created by male POP2 (POLLEN ON PISTIL 2), a GABA transaminase, is thought to provide cues for PT growth along the TT. Besides this, GABA may modulate Ca²⁺ channels from the PT, influencing the dynamic extracellular and intracellular Ca²⁺ gradient essential for PT growth along the pistil tissues. Also involved in Ca²⁺ gradient changes is D-serine from the pistil tissues, which induces Ca²⁺ permeable GLRs (glutamate receptor-like channels) at the PT tip. Created with BioRender.com(accessed on 25 February 2021).