The extensin multigene family responds differentially to superoxide or hydrogen peroxide in tomato cell cultures

Changes in extensin gene expression were examined in cultured tomato cells following treatments leading to the production of activated oxygen species. Digitonin, a steroid glycoalkaloid compound, has been shown to trigger a rapid and transient production of superoxide anion, O2-*. 6 h after application of 50 or 100 microM of digitonin, the accumulation of four extensin transcripts (1.5, 2.6, 4.0 and 6.1 kb) was observed. Superoxide dismutase strongly inhibited the digitonin-mediated response, suggesting a key role of O2-* in the signalling cascade. Furthermore, cells treated with enzymatically produced O2-* generated by xanthine oxidase (0.015 U/ml) gave a similar extensin response and again, SOD exerted a strong inhibitory effect on the response. On the other hand, H2O2 (2 mM) or the enzymatic H2O2 generator, glucose oxidase (0.34 U/ml), elicited the accumulation of only three of the four transcripts (1.5, 2.6 and 4.0 kb), indicating that the corresponding genes could be regulated either by H2O2 or O2-* but that the gene encoding the 6.1 kb transcript was exclusively expressed in response to O2-*. Finally, it was shown that lipid peroxidation, which was only induced when cells were exposed to H2O2, did not participate in the AOS-mediated gene expression for extensin. It can be concluded from these results that tomato cells are able to discriminate H2O2 from O2-* and they probably sense the latter by the specific oxidation of an extracellular component.