Post-Translational Modification by Transglutaminase of Proteins Involved in Pear Self-Incompatibility

Gametophytic self-incompatibility (SI) is one of the mechanisms adopted by plants to prevent self-fertilisation, by blocking the growth of pollen tubes recognised as “self”. Among the known models of interaction between pistil-S and pollen S determinants, the S-RNase-based system has been described in the Rosaceae, Solanaceae and Plantaginaceae. Another model is that of Papaver rhoeas, in which the pistil S locus product is a small protein that interacts with incompatible pollen, triggering a Ca2+-dependent signalling pathway which results in cytoskelton modifications and finally leads to programmed cell death (PCD). In fact there is a specific relationship between the polymerization/depolymerization of actin and the onset of PCD of the pollen tube (Thomas et al., 2006; Wheeler et al., 2009). Aggregates of tubulin and punctuate aggregates of actin were also observed in Pyrus communis SI, suggesting a role for the cytoskeleton in Rosaceae SI (Di Sandro et al., 2007), and later this role was verified in vivo and in vitro (Del Duca et al., 2009, 2010). Liu et al. (2007) in Pyrus pyrifolia observed that stylar S-RNases modified the actin cytoskeleton. Recently the role of the cytoskeleton in Pyrinae SI has been reported: the S-RNase acts on depolymerization of actin and formation of high mass aggregates, DNA degradation, collapse of mitochondrial potential, leakage of cyt C, ROS disruption (Wang et al., 2010; Poulter et al., 2011). Thus the Papaver and Pyrinae models share common features of PCD.