Pollen Transglutaminase in Pear Self Incompatibility and Relationships with S-RNases and S-Allele Variability

The pear self-incompatibility in the cv. Abbe Fetel (P. communis) has been investigated in order to identify the S-alleles and pollen putative determinant transglutaminase (TGase), an enzyme able to post-translationally form bridges among proteins. In the self-pollinated style (incompatible system), the activity of TGase is stimulated in comparison to styles pollinated with compatible pollen (compatible system), and high molecular mass cross-linked products are formed. High mass aggregates of tubulin and punctuate aggregates of actin were observed in the incompatible system, suggesting a role of cytoskeleton in SI. In vitro experiments with purified pollen TGase and purified actin and tubulin, also incubated with kinesin and myosin motor-proteins, show that the inhibition of tube growth in incompatible crossing might be mediated by a cytoskeleton abnormal reorganisation caused by cross-linked protein networks catalysed by TGase. A molecular approach to cloning pear TGase and S-RNases has been performed. The high homology between the pear and apple TGase sequences suggests that the established knowledge relating to the apple isoform can be transferred to pear. The authors have also considered the possible key role carried out by TGase in controlling the interaction between stylar RNases and pollen determinants of the SI mechanism. In contrast to the pollen determinant, S-RNases have been very well characterised in pear and several S-alleles have been identified. The update characterization of the European pear S-allele variability allows the determination of pear interfertility groups.