Cell wall reinforcement in watermelon shoot base related to its resistance to Fusarium wilt caused by Fusarium oxysporum f. sp niveum

Abstract

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is one of the limiting factors for watermelon production in Taiwan. In recent research, the phenylalanine ammonia lyase (PAL) gene expressed in the shoot base of the Fusarium wilt resistant line JSB was related to Fusarium wilt resistance. Phenylalanine ammonia lyase is the key regulatory enzyme in the phenylpropanoid metabolic pathway. The downstream products of phenolic compounds are considered to be involved in the complicated plant defence mechanisms. They could act as signal molecules, antimicrobial substances and/or structural barriers. To study the resistant mechanisms of Fusarium wilt, the resistant JSB line was examined for comparison of Fusarium oxysporum-watermelon interactions with the susceptible Grand Baby (GB) cultivar. Unlike infected GB, which was seriously colonized by Fusarium oxysporum in the whole plant, the pathogen was limited below the shoot base of inoculated JSB, suggesting that the shoot base of JSB may contribute to Fusarium resistance. The data indicated that a significant increase in PAL activity was found in shoot bases of the resistant JSB line at 3, 9, 12 and 15 days after inoculation (DAI). Shoot bases of resistant watermelons accumulated higher amounts of soluble and cell wall-bound phenolics at 3-9 DAI; the susceptible GB cultivar, however, only increased the cell wall-bound phenolics in shoot bases at 3 DAI. High lignin deposition in the cell walls of vascular bundles was observed in the shoot bases of JSB but not of GB seedlings at 6 and 9 DAI. In the roots and shoot bases of JSB seedlings at 6 DAI, peroxidase enzyme activity increased significantly. In summary, the results suggest that accumulation of cell wall-bound phenolics and increase of peroxidase activity in shoot bases of JSB seedlings during Fusarium oxysporum inoculation, together with the rapid deposition of lignin in the cell walls of vascular bundles, may have provided structural barriers in resistant JSB line to defend against Fusarium oxysporum invasion.