Influence of fungal exudates of Alternaria dauci on carrot partial resistance

Alternaria leaf blight, caused by the necrotrophic fungus Alternaria dauci, is the most damaging foliar disease in carrot production. Fungicide use has variable efficiency, and presents economic and ecological costs. Popular partially resistant cultivars exist, but their resistance level is still unsatisfactory. A better knowledge on carrot A. dauci resistance mechanisms may help to produce more resistant cultivars faster. More generally, partial resistance mechanisms in plants are still poorly understood. In order to better understand partial resistance to leaf blight in carrot, we implemented an a priori approach, investigating the role of different resistance mechanisms in the carrot-A. dauci interaction. It has been suggested that Alternaria dauci produces toxins, such as zinniol. The work presented here is centered on the role played by these toxins in the plant-fungus interaction, and more especially plant resistance. Plant cell suspensions from several genotypes were treated with raw fungal extracts. A good correlation was found between whole plant resistance to the fungus and metabolic activity of the cell suspensions after 48 h of exposure. Similar results were obtained using different techniques and different exposure times. Additionally, it was found that the toxicity of fungal extracts was due to secreted apolar compounds, which did not include zinniol. Zinniol is only produced when the fungus is grown in anoxic conditions. Nevertheless, fungal growth condition and zinniol content do not impact extract toxicity. Moreover, zinniol was not toxic to plant cells at physiological concentration.