SummaryThe Arabidopsis Thi2.1 thionin gene was cloned and sequenced. The promoter was fused to the uidA gene and stably transformed into Arabidopsis to study its regulation. GUS expression levels correlated with the steady-state levels of Thi2.1 mRNA, thus demonstrating that the promoter is sufficient for the regulation of the Thi2.1 gene. The sensitivity of the Thi2.1 gene to methyl jasmonate was found to be developmentally determined. Systemic and local expression could be induced by wounding and inoculation with Fusarium oxysporum f sp. matthiolae. A deletion analysis of the promoter identified a fragment of 325 bp upstream of the start codon, which appears to contain all the elements necessary for the regulation of the Thi2.1 gene. These results support the view that thionins are defence proteins, and indicate the possibility that resistance of Arabidopsis plants to necrotrophic fungal pathogens is mediated through the octadecanoid pathway.

In seedlings of Arabidopsis thaliana the thionin gene Thi2.1 is inducible by methyl jasmonate, wounding, silver nitrate, coronatine, and sorbitol. We have used a biochemical and genetic approach to test the signal transduction of these different inducers. Both exogenously applied jasmonates and jasmonates produced endogenously upon stress induction, lead to GUS expression in a Thi2.1 promoter-uidA transgenic line. No GUS expression was observed in a coi1 mutant background which lacks jasmonate perception whereas methyl jasmonate and coronatine but not the other inducers were able to overcome the block in jasmonic acid production in a fad3-2 fad7-2 fad8 mutant background. Our results show conclusively that all these inducers regulate Thi2-1 gene expression via the octadecanoid pathway.z 1998 Federation of European Biochemical Societies.

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The Arabidopsis thaliana Thi2.1 gene is inducible by necrotrophic fungi through a signal transduction pathway different from that for pathogenesis-related (PR) proteins. We have identified three ecotypes that are susceptible (Col-2, Ler, and Ws) and two ecotypes that are resistant (Mt-0 and Uk-4) to spray inoculation with Fusarium oxysporum f. sp. matthiolae. The Thi2.1 transcript level after infection correlates with resistance, being 5 to 10 times higher in the resistant than in the susceptible ecotypes. The beta-glucuronidase (GUS) expression of a Thi2.1-promoter-uidA fusion (with a promoter derived from Col-2) is on the average almost 10 times higher in the Uk-4 background than in the Col-2 background. This confirms the results obtained by Northern (RNA) blots and indicates that Uk-4, and probably other resistant ecotypes too, might have a more sensitive recognition system for F. oxysporum f. sp. matthiolae or might have a signal transduction system that gives a higher amplification of the original recognition signal. Our results suggest a role of the Thi2.1 gene in resistance against F. oxysporum f. sp. matthiolae and perhaps other necrotrophic fungi.

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