Elicitors are molecules known to trigger plant defence responses against pathogens. In a search for new sources of eliciting compounds from marine algae, an extract was prepared from green algae, Ulva spp., and its elicitor activity was established on the model legume, Medicago truncatula . When infiltrated into plant tissues or sprayed onto the leaves, this extract induced the expression of the defencerelated marker gene PR10 without provoking necrosis. Spraying a solution at 500 m m m m g mL ----1 was sufficient to obtain maximum induction of PR10 after 2 d. Using a cDNA array enriched in genes potentially involved in plant defence, the expression of 152 genes was monitored after one or two consecutive treatments. A broad range of defence-related transcripts was found to be up-regulated, notably genes involved in the biosynthesis of phytoalexins, pathogenesisrelated proteins and cell wall proteins. In contrast, the expression of primary metabolism-related genes did not change significantly. Consistent with its effect on defence gene expression, it was found that prior treatment of M. truncatula with the Ulva extract protected the plants against subsequent infection by the pathogenic fungus Colletotrichum trifolii .

In this study, a new pathosystem was established using the model plant Medicago truncatula and Colletotrichum trifolii, the causal agent of anthracnose on Medicago sativa. Screening of a few M. truncatula lines identified Jemalong and F83005.5 as resistant and susceptible to Colletotrichum trifolii race 1, respectively. Symptom analysis and cytological studies indicated that resistance of Jemalong was associated with a hypersensitive response of the plant. The two selected lines were crossed, and inoculations with C. trifolii were performed on the resulting F1 and F2 progenies. Examination of the disease phenotypes indicated that resistance was dominant and was probably due to a major resistance gene. Molecular components of the resistance were analyzed through macroarray experiments. Expression profiling of 126 expressed sequence tags corresponding to 92 genes, which were selected for their putative functions in plant defense or signal transduction, were compared in Jemalong and F83005.5 lines. A strong correlation was observed between the number of up-regulated genes and the resistance phenotype. Large differences appeared at 48 h postinoculation; more than 40% of the tested genes were up-regulated in the Jemalong line compared with only 10% in the susceptible line. Interestingly, some nodulin genes were also induced in the resistant line upon inoculation with C. trifolii.