Distribution of Fusarium oxysporum f. sp. niveum and the expression of the defense-related genes in inoculated resistant and susceptible watermelon plants

• Main Authors: Ching-Chi Hsu, 許靖祺
• Other Authors: Pi-Fang Linda Chang
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/03389684692580642797

碩士 === 國立中興大學 === 植物病理學系 === 93 === Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (FON), is one of the limiting factors on watermelon production. The molecular and histological mechanisms regarding to the Fusarium wilt resistance in watermelon, however, remain unclear so far. In this study, a Fusarium wilt resistant line (JSB), which was derived from mutation of susceptible Sugar Baby population in the watermelon fields, was used to investigate the resistance mechanism of watermelon to FON. The resistant and susceptible watermelon plants were grown in FON-infested soil and the pathogen was isolated from infected tissues at the 9th day post inoculation (dpi) by plating on PCNB selective media to compare the pathogen distribution patterns in resistant and susceptible plants. Results indicated that FON could be recovered from 88% of the roots in both watermelon lines, and from 54% and 66% of the stem segments in resistant and susceptible watermelon plants, respectively. Furthermore, plants inoculated for 9 and 35 days were also used for paraffin and free hand sectioning, respectively, to observe the pathogen distribution, and the ratio of pathogen colonization in xylem vessels at 35 dpi was calculated. According to the tissue sections, the xylems of roots (9 and 35 dpi) and lower stems (the portions close to soil surface, 35 dpi) in resistant watermelon plants were also colonized by FON as in the susceptible ones; nevertheless, no colonization within higher stems (the portions close to cotyledons, 35 dpi) was observed in the resistant plants. The ratios of FON colonization in xylem vessels of roots, lower and higher stems were much lower in resistant plants than in susceptible ones (35 dpi). In addition, even though the susceptible plants grown in infested soil were all dead by 35 dpi, the remaining resistant ones still survived thereafter. Therefore, the remaining resistant plants grown in infested soil (81 dpi) were also used to analyze pathogen distribution patterns by plating the tissues on PCNB selective media. Results showed that pathogens could be isolated from roots and shoot bases; whereas stems and tissue parts above cotyledons were free of pathogens. These phenomena suggest that the resistance in JSB may result in reducing FON colonization in the vascular systems of the host. Moreover, in order to understand whether the watermelon acidic class III chitinase (WM-Chi) or phenylalanine ammonia lyase (PAL) is related to the resistance mechanism against Fusarium wilt, the gene expression of these two proteins was analyzed using reverse transcription-polymerase chain reaction (RT-PCR). The expression of PAL was induced by FON in both resistant and susceptible plants at 3 and 6 dpi. Its expression maintained to 9 dpi in resistant plants but was down-regulated in susceptible ones. On the other hand, there was no difference in the expression level of WM-Chi in resistant and susceptible plants even though its expression was also induced by FON in both lines. The results revealed that PAL, but not WM-Chi, may involve in resistance of watermelon to Fusarium wilt pathogen.