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.
Highlights
Watermelon (Citrullus lanatus (Thunb.) Matsum and Nakai) is a widely cultivated fruit that is consumed globally
The highest germination impact index was obtained by the addition of root exudates from own-root watermelon plants into the 2% water agar, while the lowest was obtained by the addition of root exudates from own-root bottle gourd plants
These results indicate that the root exudates from own-root watermelon plants could more effectively stimulate FON conidial germination than that from the own-root bottle gourd plants
Summary
Watermelon (Citrullus lanatus (Thunb.) Matsum and Nakai) is a widely cultivated fruit that is consumed globally. FON is considered the most important soil-borne facultative parasite, leads to economically important losses of watermelon, and limits production in many areas of the world. Among the methods used in controlling the soil-borne disease, grafting onto resistant rootstocks has been highly effective and is a routine technique in continuous cropping systems in many countries [7]. In China, approximately 20% of the watermelon crop has been grafted for avoidance of soil pathogens [7]. Previous studies on grafting watermelon onto rootstock have been mostly concerned with the survival rate, the product quality and the effect of disease resistance [9,10,11,12].
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