Medicago truncatula in Interaction with Fusarium and Rhizoctonia Phytopathogenic Fungi: Fungal Aggressiveness, Plant Response Biodiversity and Character Heritability Indices

Fusarium and Rhizoctonia genera are important pathogens of many field crops worldwide. They are constantly evolving and expanding their host range. Selecting resistant cultivars is an effective strategy to break their infection cycles. To this end, we screened a collection of Medicago truncatula accessions against Fusarium oxysporum, Fusarium solani, and Rhizoctonia solani strains isolated from different plant species. Despite the small collection, a biodiversity in the disease response of M. truncatula accessions ranging from resistant phenotypes to highly susceptible ones was observed. A17 showed relative resistance to all fungal strains with the lowest disease incidence and ratings while TN1.11 was among the susceptible accessions. As an initiation of the characterization of resistance mechanisms, the antioxidant enzymes' activities, at the early stages of infections, were compared between these contrasting accessions. Our results showed an increment of the antioxidant activities within A17 plants in leaves and roots. We also analyzed the responses of a population of recombinant inbred lines derived from the crossing of A17 and TN1.11 to the infection with the same fungal strains. The broad-sense heritability of measured traits ranged from 0.87 to 0.95, from 0.72 to 0.96, and from 0.14 to 0.85 under control, F. oxysporum, and R. solani conditions, respectively. This high estimated heritability underlines the importance of further molecular analysis of the observed resistance to identify selection markers that could be incorporated into a breeding program and thus improving soil-borne pathogens resistance in crops.

The alternative oxidase pathway is involved in optimizing photosynthesis in Medicago truncatula infected by Fusarium oxysporum and Rhizoctonia solani

Phytopathogen infection alters primary metabolism status and plant development. The alternative oxidase (AOX) has been hypothesized to increase under pathogen attack preventing reductions, thus optimizing photosynthesis and growth. In this study, two genotypes of Medicago truncatula, one relatively resistant (Jemalong A17) and one susceptible (TN1.11), were infected with Fusarium oxysporum and Rhizoctonia solani. The in vivo foliar respiratory activities of the cytochrome oxidase pathway (COP) and the alternative oxidase pathway (AOP) were measured using the oxygen isotope fractionation. Gas exchange and photosynthesis-related parameters were measured and calculated together with antioxidant enzymes activities and organic acids contents. Our results show that the in vivo activity of AOX (v_alt ) plays a role under fungal infection. When infected with R. solani, the increase of v_alt in A17 was concomitant to an increase in net assimilation, in mesophyll conductance, to an improvement in the maximum velocity of Rubisco carboxylation and to unchanged malate content. However, under F. oxysporum infection, the induced v_alt was accompanied by an enhancement in the antioxidant enzymes, superoxide dismutase (SOD; EC1.15.1.1), catalase (CAT; EC1.11.1.6) and guaiacol peroxidase (GPX; EC1.11.1.7), activities and to an unchanged tricarboxylic acid cycle intermediates. These results provide new insight into the role of the in vivo activity of AOX in coordinating primary metabolism interactions that, partly, modulate the relative resistance of M. truncatula to diseases caused by soil-borne pathogenic fungi.