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Summary• Fusaric acid (FA) is a toxin produced by Fusarium species. Most studies on FA have reported toxic effects (for example, alteration of cell growth, mitochondrial activity and membrane permeability) at concentrations greater than 10 − 5 M . FA participates in fungal pathogenicity by decreasing plant cell viability. However, FA is also produced by nonpathogenic Fusarii , potential biocontrol agents of vascular wilt fusaria. The aim of this study was to determine whether FA, at nontoxic concentrations, could induce plant defence responses.• Nontoxic concentrations of FA were determined from cell-growth and O 2 -uptake measurements on suspensions of Arabidopsis thaliana cells. Ion flux variations were analysed from electrophysiological and pH measurements. H 2 O 2 and cytosolic calcium were quantified by luminescence techniques.• FA at nontoxic concentrations (i.e. below 10 − 6 M ) was able to induce the synthesis of phytoalexin, a classic delayed plant response to pathogen. FA could also induce rapid responses putatively involved in signal transduction, such as the production of reactive oxygen species, and an increase in cytosolic calcium and ion channel current modulations.• FA can thus act as an elicitor at nanomolar concentrations.

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Summary StatementMerzouga sand dunes were among the Moroccan extreme environments. This ecosystem is relatively unexplored and present habitat to the search for Actinobacteria with potential antimicrobial activities and biotechnological applications. Biology Open • Accepted manuscriptby guest on April 27, 2019 http://bio.biologists.org/ Downloaded from Abstract:The absence of new antibiotics is guiding more and more researchers to specific ecosystems.One hundred sixty-three Actinobacteria isolates were isolated from Merzouga sand and

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Bacterial canker of tomato, caused by Clavibacter michiganensis subsp. michiganensis, continues to be a problem for tomato growers in the Souss-Massa Draa valley, South of Morocco. Assuming that biological control is an alternative for the management of this disease, a total of 303 fluorescent pseudomonads strains isolated from roots and rhizospheric soil of tomato plants were in vitro tested against C. michiganensis subsp. michiganensis. Fluorescent pseudomonads strains which showed the highest antagonistic properties were thereafter investigated for their ability to colonize tomato roots. Our results showed that fluorescent pseudomonads are more represented in rhizospheric soils. However, the most efficient fluorescent pseudomonads isolates were found in the rhizoplane soil and the endorhizosphere. Among 42 spontaneous antibiotic resistant mutants obtained by treatment of the wild-type isolates with five antibiotics (rifampicine, nalidixic acid, ampicilline and chloramphenicol), 28 completely colonized the roots of all tomatoes seedlings used in this investigation. The 42 wild type isolates were then used for in vivo screening with the cotyledon test. Using this test, eight isolates from 42 tested induced a significant decrease of disease incidence and disease symptoms. The eight efficient isolates were then tested for their effectiveness in the protection of tomato plants in pots under greenhouse conditions. Results obtained showed that all tested isolates applied as seed and root treatments reduced significantly (P B 0.001) the incidence of bacterial canker.

Fusarium spp. are ubiquitous fungi found in soil worldwide as both pathogenic and nonpathogenic strains. The signals leading to disease or the absence of disease are poorly understood. We recently showed that fusaric acid (FA), a nonspecific toxin produced by most Fusarium spp., could elicit various plant defense responses at 100 nM without toxic effect. In this study, we checked for the effect of FA on root and root hairs, probable first site of contact between the fungi and the host. Large FA concentrations reduce root and root-hair growth and induce a rapid transient membrane hyperpolarization, followed by a large depolarization, due to the inhibition of H(+)-ATPase currents. Nanomolar concentrations of FA induced only an early transient membrane hyperpolarization of root hairs compatible with the induction of a signal transduction pathway. FA at 10(-7) M failed to induce salicylic acid- and jasmonic acid/ethylene-dependent defense-related genes but inhibited the germination of the angiosperm parasite Orobanche ramosa in contact of FA-pretreated Arabidopsis thaliana seedlings. These data suggest that FA at nontoxic concentrations could activate signal transduction components necessary for plant-defense responses that could contribute to biocontrol activity of Fusarium spp.

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