A study of the interaction between alfalfa (Medicago sativa) and one of its fungal pathogens, Colletotrichum trifolii

Plant pathogens such as Collelotrichum trifolii have devastating effects on crop species. A rapid and effective activation of plant defenses is necessary for the establishment of resistance to the pathogen. The induction of several defense responses in alfalfa (Medicago sativa ) after infection by a virulent and avirulent race of C. trifolii was examined. A phytoalexin response was differentially induced in resistant and susceptible leaves. Free medicarpin and conjugates of medicarpin (glycosylated medicarpin) and formononetin (formononetin-7- O-glucoside) started to accumulate 24 and 48 hours after inoculation in resistant leaves, respectively, as opposed to 48 and 72 hours after inoculation in susceptible leaves. A more rapid induction of the phytoalexin response in resistant leaves compared to susceptible leaves was further demonstrated by the earlier accumulation of transcripts of genes involved in the biosynthesis of medicarpin, including PAL, CHR and IFR, 4 hours after inoculation.; The alfalfa/C. trifolii interaction follows a gene-for-gene interaction for race-cultivar specificity where in general, products of host resistance genes providing resistance to the fungus are interacting with direct or indirect products of avirulence genes (specific-elicitors) from the pathogen. The differential response for phytoalexin accumulation in alfalfa after C. trifolii infection, along with the nature of the interaction suggests the involvement of race-specific elicitors; however it was not possible to demonstrate their existence.; Glucanases are lytic enzymes that hydrolyse β-1,3-glucans present in high amounts in fungal cell walls, and are produced by the plant after pathogen attack. Glucanases can directly affect fungi by digesting their cell walls, or indirectly by releasing elicitors that will activate plant defenses. The second potential role was investigated by constitutively expressing the aglu1 cDNA that encodes an alfalfa glucanase normally present in alfalfa leaves only after fungal infection, and study the effect of aglu1 expression on the phytoalexin response during infection by C. trifolii. The constitutive expression of the aglu1 transgene did not increase the phytoalexin response, therefore suggesting that aglu1 glucanase may not be involved in releasing elicitors from the fungal cell walls, at least for the fungus used in this study, C. trifolii.; Genetic engineering has proven to be a powerful tool in plant disease control. Transgenic alfalfa plants transformed previously with a glucanase and/or a rice chitinase under the constitutive control of the CaMV 35S promoter were analyzed for their level of expression of glucanase and chitinase activities. No difference in chitinase activity could be detected. Differences in glucanase activity could be detected in plants transformed with the glucanase cDNA although the transgene was no longer constitutively expressed, suggesting that some other mechanisms increasing the glucanase activity level were occurring along with silencing of the transgene. Trangenic plants re-transformed with the aglu1 cDNA were evaluated for resistance against C. trifolii . Overexpressing the aglu1 transgene in alfalfa did not reduce the symptoms following pathogen attack. However, transgenic plants expressing the transgene produced other glucanase isoforms that may play a role in plant defense perhaps against other fungi. This needs to be further investigated.