Inhibition of pollen tube growth is controlled by the inhibition of a novel extracellular transglutaminase activity: is transglutaminase involved in the subsequent pollen self-incompatibility PCD events?

Plant sexual reproduction involves the interaction between male and female gametophytes, which still remains an enigma, and requires the Ca2+-dependent growth of pollen tube (male gametophyte). We report the detection of the first extracellular plant TGase and demonstration that its extracellular protein crosslinking activity is essential for pollen germination. Transglutaminases (TGase or TG) are Ca2+-dependent widespread enzymes able to post-translationally modify proteins by forming bridges between protein-bound glutaminyl residues and -lysines or polyamines (1). TGases are also involved in apoptosis: for instance, crosslinking by TG2 facilitates the phagocytic clearance of dead cells (1). The research on plant TGases lags much more behind that in mammalian cells, however TGase is also involved in flower PCD (2). In our model, extracellular inhibition of transamidating activity either by a monoclonal anti-TGase antibody or irreversible site directed inhibitors blocked pollen tube growth. TGase, immunolocalised on the pollen tube surface, was also capable of crosslinking labelled amines and the H6-Xpr-GFP substrate into extracellular proteins in immediate contact with the pollen tube. Contribution of TGase-mediated crosslinking in the pollen tube wall formation was confirmed also by the immunodetection of (-glutamyl)lysine crosslinks. These findings suggest that the enzyme functions in the attachment of the pollen tube to the female style and in the stabilisation of the pollen tube wall. Plant cell wall has been described as a cellular compartment important in cell communication. A well-documented case of this sort is the rejection of self-pollen in the self-incompatiblility (SI) reaction. SI-specific events involved in a PCD signalling pathway are subsequent to inhibition of pollen tube growth. As the extracellular matrix (ECM) is a key regulator of gametophyte communication, ongoing work is focusing on the putative involvement of the extracellular pollen TGase in the SI events that follows the inhibition of incompatible pollen tube growth. 1. Lorand L and Graham R M. (2003). Transglutaminases: crosslinking enzymes with pleiotropic functions. Nature Reviews Molecular Cell Biology Vol. 4, February 2003. 2. Serafini-Fracassini D, Del Duca S, Monti F, Poli F, Sacchetti G, Bregoli AM, Biondi S, Della Mea M. (2002). Transglutaminase activity during senescence and programmed cell death in the corolla of tobacco (Nicotiana tabacum) flowers.Cell Death Differ.: 9(3):309-21.