Zinniol, a non-host selective phytotoxin commonly produced by fungi of the Alternaria genus, has been reported as the metabolite responsible for the phytotoxicity of the lipophilic fraction of A. tagetica. While both the lipophilic fraction and zinniol have been shown to produce necrosis on leaves of susceptible marigold (Tagetes erecta) plants, the true role of zinniol in the infectious process remains uncertain. Using marigold cell cultures as a model, we evaluated the effects of zinniol and the lipophilic fraction at the cellular level and showed that pure zinniol is not markedly phytotoxic at concentrations known to induce necrosis in leaves of T. erecta. Moreover, the effects of zinniol on cell membranes and DNA fragmentation are less intense than those caused by the lipophilic fraction. These results suggest that zinniol may not play a significant role in the A. tagetica-T. erecta interaction and, consequently, its classification as a non-host selective phytotoxin is questionable.
Alternaria tagetica, a fungus that causes early blight in marigold (Tagetes erecta), produces two groups of phytotoxic metabolites: one hydrophilic and the other lipophilic that show phytotoxic activity when tested by the leaf-spot assay in T. erecta. We evaluated the cellular effects of the phytotoxic culture filtrate of A. tagetica and the hydrophilic and lipophilic fractions, then determined whether the filtrate or the fractions differentially induced pathogenesis-related mechanisms in the plant. The culture filtrate and the phytotoxic fractions had adverse effects on cell viability, fresh mass, and the number of cells, and induced ROS accumulation, lipid peroxidation and DNA damage on T. erecta cell suspension cultures, and these effects are related to pathogenic mechanisms attributed to phytotoxins. However, although exposure of marigold cells to the phytotoxic culture filtrate of A. tagetica triggered programmed cell death, the hydrophilic and the lipophilic phytotoxic fractions induced death that was more related to a toxic effect leading to necrosis. This study presents a complementary perspective in the search for the roles of metabolites, including phytotoxins, produced by phytopathogenic fungi during plant infection.
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