Glycoprotein secretions and pollen tube kinetics in apple

The obturator bridges the downward transit of the pollen tube through style to ovary in many angiosperms. Examining pollen tube growth kinetics in the pistil in Malus × domestica and relating these to changes occurring on the obturator using histochemistry and immunocytochemistry in order to determine how the key access point of the obturator is mediated, Losada and Herrero show that glycoprotein secretion is required for a lightning pollen tube wall elongation on the surface of the obturator.

Glycoprotein distribution in the apple pistil during the progamic phase. Outlines of a median longitudinal section of an individual apple pistil at different developmental stages from anthesis to the time of fertilization 6 days after pollination. (A) JIM13 epitopes (green) are developmentally secreted to the apple stigma before pollination. (B) Pollen tube elongation in the stigma lasted for 2 d, and induced secretion of JIM8 epitopes (green) to the stigmatic surface, while JIM11 epitopes (extensins) were accumulating on the top area of the stylar transmitting tract (red). (C) Pollen tubes elongate along the apple pistil for a further 2 d, and both extensin and callose epitopes were accumulated in the transmitting tissue of the style in a basipetal fashion; concomitantly, JIM13 and JIM11 epitopes (green and red) were secreted towards the obturator surface. (D) Pollen tube elongation through the obturator triggered secretion of JIM8 epitopes, which vanished along with JIM13 and JIM11 epitopes following pollen tube passage.

This secretion is depleted following on from the passage of pollen tubes, which strongly suggests that glycoprotein secretion has a pivotal role in regulating pollen tube access to the ovule. The glycoproteins involved are present also in the apple stigma, which has the same developmental origin and points to a conserved evolutionary trend among flowering plants.