A possibile involvement of transglutaminase in the HR response.

The occurrence of glutamyl-polyamines and changes in activity and levels of transglutaminase (TGase, EC 2.3.2.13), the enzyme responsible for their synthesis, are reported during the progression of the hypersensitive reaction (HR) of resistant NN tobacco plants (Nicotiana tabacum L. cv. Samsun) to tobacco mosaic virus (TMV). Mature leaves of tobacco were collected over 0-72 h after inoculation with TMV or phosphate buffer (mock). In vivo synthesis of polyamine glutamyl-derivatives (glutamyl-PAs), catalyzed by TGase activity, was evaluated after supplying labelled putrescine (Pu, a physiological substrate of TGase) to leaves. Results show that, starting from 24 h, mono-(-glutamyl)-Pu and bis-(-glutamyl)-Sd were recovered in TMV-inoculated samples, but not in mock-inoculated ones; two days later, in the former, the amount of glutamyl-derivatives further increased. An in vitro radiometric assay showed that, in TMV-inoculated leaves, TGase activity increased from 24 h onwards relative to mock controls. An immunoblot analysis with AtPng1p polyclonal antibody detected a 72-kDa protein whose amount increased at 72 h in TMV inoculated leaves and in the lesion-enriched areas. A biotin-labelled cadaverine incorporation assay showed that TGase activity occurred in S1 (containing soluble proteins), S2 (proteins released by both cell walls and membranes) and S3 (membrane intrinsic proteins) fractions. In S3 fraction, where changes were the most relevant, TGase activity was enhanced in both mock and TMV-inoculated samples, but the stimulation persisted only in the latter case. These data could be interpreted in the light of a possible role of TGase activity and glutamyl-polyamines in the defence against a viral plant pathogen.