Figure 7.

Model showing the possible mode of action of PME48 during pollen dehydration, imbibition, and germination. 1, PME48 may be secreted in the intine wall during the maturation of the pollen grain and start to act on the methylesterified form of the HG by random or block-wise actions. Methylesterified HG of the intine (black) is then transformed in weakly methylesterified HG (orange). The dry pollen grain contains mostly weakly esterified HG that could form pectate gel with calcium. 2, Imbibition of the pollen grain is enhanced by the presence of the more hydrophilic weakly methylesterified HG. 3, The source of calcium creating the calcium influx in the pollen grain may originate, at least partially, from the release of Ca²⁺ from the pectate gel, thus weakening the mechanical properties of the intine. 4, The intine may be degraded by PGases and/or PLs (a) and/or break under the turgor pressure (b). 5, These processes may promote the emergence of the pollen tube tip and allow the germination of the pollen grain. GalA, (1,4)-α-d-GalUA; pm, plasma membrane; Sp, sperm cells; Veg n, vegetative nucleus.