Involvement of pollen transglutaminase on cytoskeleton protein dynamics in the pear self incompatibility.

The pear self-incompatibility in the cv Abbé Fètel (P. communis) has been investigated in order to identify the S-alleles and pollen putative determinant transglutaminase (TGase), an enzyme able to post-translationally forming bridges among proteins. In the self-pollinated style (incompatible system), the activity of TGase is stimulated in comparison to style-pollinated with compatible pollen (compatible system), and high molecular mass cross-linked products are formed. During pollen growth, the cytoskeleton undergoes a dynamic reorganization. Microfilaments and motor proteins are responsible for the apical growth and transport of vesicles and organelles (Tirlapur et al., 1995). Microtubules, co-localized with microfilaments, are involved, with motor proteins (i.e., dynein, kinesin, myosin) in the migration of male germ unit and in cytoskeleton organization (Romagnoli et al., 2003). The cytoskeleton role in SI phenomena is scarcely known. Solely in incompatible papaver tubes, F-actin foci are formed due to a not characterized crosslinking mechanism, able to block tube elongation and causing tube PCD. In the pear self-incompatibility 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 clone pear TGase and S-RNases has been performed. The high homology between the pear and apple TGase sequences suggests that the good knowledge of the apple isoform can be transferred to pear. Contrarily to the pollen determinant, S- RNases have been very well characterised in pear and several S-alleles were identified. The authors have also approached the possible key role carried out by TGase in controlling the interaction between stylar RNases and pollen determinants of the SI mechanism. The update characterization of the European pear S-allele variability allows to set the pear interfertility groups.