Determination And Mechanism Of Disease Resistance Against Bacterial Wilt Caused By Ralstonia Solanacearum In Eucalyptus Spp.

In this study, in order to obtain a concise, accurate detection and determination technique, the disease of resistance of Eucalyptus spp. against bacterial wilt caused by Ralstonia solanacearum was determined using different inoculation methods on 18 eucalypt species and clones, which from different sources and culturing by different reproductive mothods. The diversity of endophytic bacteria in clones with different resistance was investigated by using 16S rDNA PCR-DGGE to explore the relationship between the resistant ability and bacterial diversity, which might provide new clues and ways to understand the resistance mechanism and prevent the bacteria wilt.1 The resistance ability of Eucalyptus spp. against bacterial wilt caused by Ralstonia solanacearum was determined by dipping inoculation of shoot cuttings and shoot tip inoculation using potted plantlets and tissue culture seedlings, as well as field disease survey of eucalypt plantation. The inhibition effects of root exudate and supernate of tissue triturate collected from different resistant clones of Eucalyptus against R. solanacearum were performed using the methods of liquid culture to explore the relationship of root exudate, tissue triturate and the levels of disease resistance. The results showed that the clones, bd1, bd2, E. camaldulensis and E. grandis×urophylla from Nanning were highly resistant to bacterial wilt; U6, E. urophylla from Nanning,Lei9 and Qin32-29 were the mid wilt-resistant clones; DH32-27, Qin9, Nanningguang9, Qin8, E. dunnii, Qin32-22, Lei2, E. grandis, E. urophylla and Qinguang9 were susceptible clones. The results also indicated that root exudates and the supernatant of triturated tissue had no direct inhibition to the growth of R. solanacearum. However, the proliferation number of R. solanacearum in root exudate and tissue triturate from wilt-susceptible clones was markedly higher than the ones from highly wilt-resistant clones, which was in accordance with the determination results of disease resistance using dipping inoculation and tip inoculation of shoot cuttings, indicating that the resistance ability of different clones could be manifested by the growth of R. solanacearum in root exudate and supernate of triturated tissue.2 In order to make a further exploration about the relationship of endophytic bacteria population structure and the levels of disease resistance, the combination of PCR-DGGE (denaturing gradient gel electrophoresis) technology and 16S rDNA gene library construction method were used to study the endophytic bacteria diversity among wilt resistant and susceptible Eucalyptus clones(Lei 2, Lei 9, bd2), the relationship of bacterial population and the disease resistance ability were explored by analysing the dominant and specific bacteria group. The results showed that there was no significant difference among resistant and susceptible clones in DGGE profile, which had 11 bands with same number and position of each Eucalyptus clone. Through cutting and sequencing the two dominant bands sequence the Genebank comparion indicated that they were both uncultured bacterial clones. As for 16S rDNA gene library construction, 18 clones were obtained by cloning PCR products directly, the sequencing analysis showed that all the clones were of the same uncultural bacteria, belonging to Cyanobacteria phylum, which was in conformity with DGGE method. The result indicated that Lei2, Lei9, bd2 had same endophytic bactieria variety, the uncultured bacterium clone was the dominant group, which preliminary demonstrated there was no necessary link between the bacterial population structure and the levels of disease resistance.