Cakile maritima is a typical halophyte of the Mediterranean coasts. In addition to its ecological and industrial properties, C. maritima has antiscorbutic, diuretic and purgative roles in folk remedies. This plant is infected by different fungal species, mainly belonging to Alternaria genus. Two-hundred Alternaria strains were collected from four different pedo-climatic areas in Tunisia, from C. maritima fresh plant tissues showing symptoms of Alternaria infection. Phylogenetic analyses of 79 representative Alternaria strains, were carried out using multi-locus gene sequencing. All the strains clustered in the Alternaria Section: 47 strains had high homology with A. alternata reference strain, 13 grouped with A. arborescens reference strain, 12 grouped with A. mali reference strain, and seven strains were not well defined with A. mali as their closest species. In vitro production of tenuazonic acid (TA), alternariol (AOH), alternariol-monomethyl ether (AME), and altenuene (ALT) was evaluated. Approx. 68% of strains simultaneously produced AOH, AME and TA. Only two A. alternata and one A. mali strains were ALT producing. Pathogenicity tests on leaves of C. maritima were carried out with 41 representative strains. Alternaria arborescens showed the greatest pathogenicity compared to A. alternata and A. mali, although no statistically significant differences in pathogenicity were observed. This is the first study on Tunisian populations of Alternaria species isolated from the extremophile C. maritima.

Cakile maritima is a halophytic plant model that is well known by its ability to tolerate high salt concentrations. Salinity was reported to improve the tolerance of halophytes to several abiotic stresses; however, the involvement of salt in the tolerance to biotic stress is still scant. In the present work, the effect of salt on C. maritima responses towards the pathogenic Alternaria alternata was investigated. For that, C. maritima seeds were germinated for four weeks. Plants were then divided into four groups: i) Plants irrigated with salt (200mM NaCl); ii) Plants infested by fungus; iii) Plants irrigated with salt and infested by fungus and finally control plants (0mM NaCl, without inoculation). Our results showed that upon salt stress or fungal attack, plants reduced biomass production, hydration status and photosynthetic performance which were associated with a decrease in the gas exchange and chlorophyll fluorescence parameters, with a more pronounced effect upon fungal attack. However, under combined stress, a significant increase of these parameters was noticed, with a level close to that of control. Concerning nutrient contents, K, Zn, Fe, Cu and Mg decreased in the C. maritima leaves exposed to both stresses applied individually. In contrast, all these nutrients were increased in plants grown under combined stress. Taken together, we can conclude that plants grown under combined stresses had better growth rate and physiological performance compared to all other treated plants, and that salt may be the key in improving the C. maritima ability to tolerate fungal attack.