Fusaric acid accelerates the senescence of leaf in banana when infected by Fusarium.

Fusarium oxysporum f.sp. cubense (FOC) is a causal agent of vascular wilt and leaf chlorosis of banana plants. Chloroses resulting from FOC occur first in the lowest leaves of banana seedlings and gradually progress upward. To investigate the responses of different leaf positions to FOC infection, hydroponic experiments with FOC inoculation were conducted in a greenhouse. Fusarium-infected seedlings exhibited a decrease in net photosynthesis rate, stomatal conductance, and transpiration rate of all leaves. The wilting process in Fusarium-infected seedlings varied with leaf position. Measurements of the maximum photochemical efficiency of photosystem II (F(V)/F(max) and visualization with transmission electron microscopy showed a positive correlation between chloroplast impairment and severity of disease symptoms. Furthermore, results of malondialdehyde content and relative membrane conductivity measurements demonstrated that the membrane system was damaged in infected leaves. Additionally, the activities of phenylalanine ammonia-lyase, peroxidase and polyphenol oxidase were increased and total soluble phenolic compounds were significantly accumulated in the leaves of infected plants. The structural and biochemical changes of infected plants was consistent with plant senescence. As the FOC was not detected in infected leaves, we proposed that the chloroplast and membrane could be damaged by fusaric acid produced by Fusarium. During the infection, fusaric acid was first accumulated in the lower leaves and water-soluble substances in the lower leaves could dramatically enhance fusaric acid production. Taken together, the senescence of infected banana plants was induced by Fusarium infection with fusaric acid production and the composition of different leaf positions largely contribute to the particular senescence process.

Detection of the dynamic response of cucumber leaves to fusaric acid using thermal imaging.

Fusarium wilt is a major disease that causes severe losses in crop yield. Fusaric acid (FA), a non-specific fungal toxin produced by many Fusarium species, can accelerate the wilting of many crops. Unraveling the role of FA in the wilt process can enrich the understanding of the mechanism of pathogenesis. To investigate the dynamic process of the cucumber's response to FA, we used digital infrared thermography (DIT) to detect leaf temperature during the alternation of light and dark conditions in greenhouse hydroponic experiments. During FA treatment, we found that the leaf temperature of cucumber plants increased when stomata closure was induced by FA. Under the alternation of light and dark, FA-treated plants had a higher leaf temperature in the light and a lower temperature in the dark, when compared to untreated plants. To confirm the uncontrolled water loss was from damaged leaf cells, as a result of FA treatment, and not from the stomata, an experiment was conducted using a split-root system in which spatially separated cucumber roots were each supplied 0 ppm or 100 ppm of FA. In the split-root system, the low temperature areas of the leaves in the dark had a higher FA concentration and more severe membrane injury than the high temperature areas, demonstrating that FA is primary xylem transported. We concluded that membrane injury caused by FA led to non-stomata water loss and, ultimately, to wilting. Combining the response of the leaves under the light and dark conditions with the DIT employed in the present study permitted noninvasive monitoring and direct visualization of wilting development.