Pollen Tube Cytoskeleton, Transglutaminases and Self-Incompatibility

The cytoskeleton of pollen tubes is a network of single and polymer-forming proteins that are involved in many aspects of pollen germination and growth, from the asymmetrical distribution of membrane-bounded organelles to the deposition of cell wall material. In the self-incompatibility response, changes to both actin filaments and microtubules are likely to be triggered by specific proteins, resulting in either the de-polymerization of cytoskeleton filaments, or the formation of aberrant structures. Transglutaminases are enzymes widespread in all plant organs and cell compartments that catalyze the post-translational conjugation of polyamines to different protein targets. During self-incompatibility, the activity of TGase is enhanced leading to the formation of high molecular mass cross-linked products, including aggregates of tubulin and actin, as is also shown by the in vitro post-translational modification of actin and tubulin catalyzed by purified pollen TGase. In apple pollen, TGase was found as aggregates on the surface of pollen tubes germinated both in vitro and in the style, suggesting a role for TGase in the interaction between pollen tubes and the extracellular matrix during fertilization. We have investigated the distribution of TGase in pear pollen tubes and we determined that TGase is likely to be secreted by a mechanism involving both membrane dynamics and the cytoskeleton. Since actin filaments are perturbed during the self-incompatibility response, it is likely that the distribution and the activity of extracellular TGase is also affected, leading to the arrest of pollen tube growth.