Priming Watermelon Resistance by Activating Physiological Response and Defense Gene Expression to Alleviate Fusarium Wilt in Wheat-Watermelon Intercropping

Wheat intercropping in watermelon could provide relief from the occurrence of Fusarium wilting of watermelon, a severe soil-borne disease caused by the fungus <i>Fusarium oxysporum</i> f. sp. <i>niveum</i> (FON). The current study aims to investigate the effect of root exudates from three wheat cultivars and one watermelon cultivar on the growth of FON and the responses of Fusarium wilt in watermelon to intercropping with wheat. The results revealed the contrasting effects of root exudates on the mycelial growth of FON; the wheat root exudates inhibited the mycelial growth of FON, and watermelon root exudates promoted the mycelial growth of FON. Watermelon plants suffered less Fusarium wilt in the intercropping system than in the monocropping system. Wheat intercropping reduced the incidence of Fusarium wilt in watermelon, and this effect was associated with the role of wheat root exudates that inhibited the growth of FON. Malondialdehyde (MDA) contents decreased in the intercropping system compared with the monocropping system after FON inoculation. The catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), and polyphenol oxidase (PPO) activities, and total phenolics and flavonoid contents in the roots of watermelon in the intercropping system were significantly higher than those in the monocropping system. Real-time PCR analysis showed that <i>ClCAT</i>, <i>ClSOD</i>, <i>ClAPX</i>, and <i>ClPPO</i> defensive enzymes and <i>ClPDF2.1</i> and <i>ClPDF2.4</i> defensin-like gene expression were significantly induced during the early stage after FON inoculation in the intercropping system compared to the monocropping system, while peroxidases did not show a significant response to FON infection. It is suggested that intercropping with wheat alleviates Fusarium wilt of watermelon by reducing the population of FON in rhizospheric soil and activating physiological responses and defense gene expression to protect watermelon from FON infection and improve the resistance of watermelon to FON in the intercropping system.