Abstract:To explore the defense mechanisms involved in disease resistance of grafted watermelon to Fusarium oxysporum,we compared the histological responses using microscope and expression of defence genes of grafted and self-rooted watermelon infected by F.oxysporum.The results showed that:(1)F.oxysporum caused a quick defense response by formatting gums and tylosis in infected xylem vessels,and the thickening of xylem parenchyma cell wall in roots of grafted plants.These histological changes appeared later in roots of self-rooted plants.Serious infection resulted in wall disintegration of xylem parenchyma cell,the diseased vascular finally became hollow and plants developed wilt symptoms,which did not observed in grafted watermelon.(2)Highly induced expression of defence genes were observed in grafted plants than that in self-rooted plants,among which,the transcription of CHI,APX and PPO genes in grafted watermelon accumulated along the F.oxysporum inoculation,while transcription of PAL significantly increased and then sharply declined,but still with high expression level than base level.Different from the expression manner in grafted watermelon,only the transcription of PPO gene was up-regulated after F.oxysporum inoculation,in self-rooted watermelon,whereas expression of CHI,APX and PAL genes increased first and then sharply declined,which is consistent with PAL gene expression pattern in grafted watermelon.These results indicating that the resistance mechanism of grafted plants to F.oxysporum infection is because of the quick response in histological level and the high expression of defence genes in transcription level,which finally protect the grafted plant from infection by F.oxysporum;and that the significantly responses of defence genes in the interaction between grafted plants and F.oxysporum might be one of the important molecular mechanisms involving the resistance of grafting plants.