Characterisation of both Hoodia gordonii and the associating wilt causing pathogen Fusarium oxysporum

[摘要] English: Hoodia gordonii has been used by the San people for centuries as an appetitesuppressant while they were on long hunting trips. These succulents are globally knownas an important component in diet supplements and products which assist in weight-loss.Together with other plants these contribute towards a multimillion US dollar market, asmany pharmaceutical and nutritional companies have made significant financialinvestments in the research and development for people who suffer from weightproblems and obesity. In South Africa, H. gordonii is classified as being endangered,because so many pharmaceutical companies cashed-in on the dietary characteristics ofthese succulents. The plant is found in the South Western parts of Southern Africamainly in the Karoo, Kalahari and Namib deserts. Plants were also grown in cultivatednurseries under favourable conditions for commercial use. However, diseases havedramatically hampered production in nurseries with almost total loss of crop. Althoughnot many diseases have been documented on this plant, Fusarium wilt, a devastatingfungal disease of H. gordonii, caused by F. oxysporum had been identified. This diseaseis not only responsible for economic losses, but also contaminates the soils with spores,which remain dormant until the next season as inoculum. This study concentrated on themorphological characterisation and molecular identification of the Fusarium wilt causingpathogen present in four areas (Kakamas, Klein Pella, Pofadder and Prieska) as well asthe morphological evaluation of the host. Fusarium oxysporum was identified as thecausal agent of Fusarium wilt on H.gordonii plants. The AFLP analysis and DNAsequences resulted in two distinct groups. Those that clustered in the AFLP cluster Bwere also grouped in a TEF cluster B, however some isolates from AFLP cluster A alsogrouped together with the TEF cluster B. The low genetic variation revealed by the AFLPanalysis indicated that differences amongst the pathogen isolates occur, but the DNAsequences confirmed that these isolates share a common ancestor. DNA sequencinganalysis was used to place 44 South African F. oxysporum isolates into the phylogeneticgroups as described by O'Donnell and associates. South African F. oxysporum isolatesclustered into two groups. The observed genetic variation amongst individual isolateswas lower than the genetic variation between out-group isolates. Two clusters wereidentified; within each cluster isolates had a relatively high frequency of clones. Theseclones confirm that these pathogen isolates share similar allele frequencies. Results fromsequencing data showed that the isolates fall within the FOSC, however there was noisolates identified that show 100% similarities when compared with all three genessequences with recorded genetic sequences of F. oxysporum isolates in other parts of the world. Therefore, based on the current taxonomic system, of host specificity thefungus only infects H. gordonii, so the study has proved that a possible new formaespecialis has been identified. New preventative measures must be applied to the hostwhen planting in nurseries. Knowledge gained from analyses of the geneticfingerprinting, DNA sequencing of these isolates and the mporphological evaluation ofthe host might assist with the development of effective control strategies, i.e. resistancebreeding against Fusarium wilt. This will provide an incentive to potential farmers to plantH. gordonii, thus improving production of this succulent for pharmaceutical companiesand nurseries in South Africa.

[发布机构] University of the Free State