Root exudates from grafted-root watermelon showed a certain contribution in inhibiting Fusarium oxysporum f. sp. niveum

Abstract

Grafting watermelon onto bottle gourd rootstock is commonly used method to generate resistance to Fusarium oxysporum f. sp. niveum (FON), but knowledge of the effect of the root exudates of grafted watermelon on this soil-borne pathogen in rhizosphere remains limited. To investigate the root exudate profiles of the own-root bottle gourd, grafted-root watermelon and own-root watermelon, recirculating hydroponic culture system was developed to continuously trap these root exudates. Both conidial germination and growth of FON were significantly decreased in the presence of root exudates from the grafted-root watermelon compared with the own-root watermelon. HPLC analysis revealed that the composition of the root exudates released by the grafted-root watermelon differed not only from the own-root watermelon but also from the bottle gourd rootstock plants. We identified salicylic acid in all 3 root exudates, chlorogenic acid and caffeic acid in root exudates from own-root bottle gourd and grafted-root watermelon but not own-root watermelon, and abundant cinnamic acid only in own-root watermelon root exudates. The chlorogenic and caffeic acid were candidates for potentiating the enhanced resistance of the grafted watermelon to FON, therefore we tested the effects of the two compounds on the conidial germination and growth of FON. Both phenolic acids inhibited FON conidial germination and growth in a dose-dependent manner, and FON was much more susceptible to chlorogenic acid than to caffeic acid. In conclusion, the key factor in attaining the resistance to Fusarium wilt is grafting on the non-host root stock, however, the root exudates profile also showed some contribution in inhibiting FON. These results will help to better clarify the disease resistance mechanisms of grafted-root watermelon based on plant-microbe communication and will guide the improvement of strategies against Fusarium-mediated wilt of watermelon plants.

Figure 1. Illustration of the re-circulating hydroponic culture system to continuously trap root exudates.

The system was modified according to Hao et al .

Figure 2. Effect of the root exudates collected from own-root bottle gourd, grafted-root watermelon and own-root watermelon on FON conidial germination.

Each treatment had three replications. Bars represent standard error for three replicates. Germination impact index = (The conidial germination number in treatment – Control)/Control. The different letters represent the significance between pairs of mean values at P≤0.05 according to LSD test.

Figure 3. Effect of the root exudates collected from own-root bottle gourd, grafted-root watermelon and own-root watermelon on FON growth.

Figure 4. Effect of the root exudates collected from own-root bottle gourd, grafted-root watermelon and own-root watermelon on FON growth.

Bars represent standard error for three replicates. Each treatment had three replications. Bars represent standard error for three replicates. Growth impact index = (The growth diameter in treatment – Control)/Control. The different letters represent the significance between pairs of mean values at P≤0.05 according to LSD test.

Figure 5. The chromatograms detected by HPLC both in standard chemicals and root exudates.

The peaks from left to right in the Standards represent the following standard compounds: 1, gallic acid; 2, coumaric acid; 3, β-hydroxybenzoic acid; 4,chlorogenic acid; 5, vanillic acid; 6, caffeic acid; 7, syringic acid; 8, ferulic acid; 9, benzoic acid; 10, salicylic acid; 11, cinnamic acid. The peaks identified by HPLC in the root exudates collected from own-root bottle gourd, grafted-root watermelon and own-root watermelon were showed by the corresponding numbers.

Figure 6. Effect of different concentrations of chlorogenic acid and caffieic acid on conidial germination of FON.

Values are the means of four independent tests with standards errors shown by vertical bars. Germination impact index = (The conidial germination number in treatment – Control)/Control.

Figure 7. Effect of different concentrations of chlorogenic acid and caffieic acid on FON growth.

Values are the means of four independent tests with standards errors shown by vertical bars. Germination impact index = (The conidial germination number in treatment – Control)/Control.