An extracellular transglutaminase is required for apple pollen tube growth

An extracellular form of the Ca2+-dependent protein-cross-linking enzyme transglutaminase (TGase) was demonstrated to be involved in the apical growth of Malus domestica pollen tube. Apple pollen transglutaminase and its substrates were co-localized within aggregates on the pollen tube surface, as determined by indirect immuno-fluorescence staining and the in situ cross-linking of fluorescently labeled substrates. Transglutaminase-specific inhibitors and an anti-TGase monoclonal antibody blocked pollen tube growth, whereas incorporation of a recombinant fluorescent mammalian TGase substrate (histidine-tagged green fluorescent protein: His6-Xpr-GFP) into the growing tube wall enhanced tube length and germination, consistent with a role of TGase as modulator of cell wall building and strengthening. The secreted pollen TGase catalyzed the cross-linking of both polyamines (PAs) into proteins (released by the pollen tube) and His6-Xpr-GFP into endogenous or exogenously added substrates. A similar distribution of TGase activity was observed in planta on pollen tubes germinating inside the style, consistent with a possible additional role for TGase in the interaction between the pollen tube and the style during fertilization