Criblage des souches d’actinobactéries pour leurs pouvoirs PGPR

Abstract

Abiotic stress due to climate change and phytopathogens attacks lead to several damages in agricultural crops. Excessive use of chemicals in agriculture causes environmental pollution and loss of agricultural lands with fertile soils. Plant growth-promoting bacteria (PGPB) are an alternative strategy for sustainable agriculture. Among PGPBs, actinomycetes have recently gained increasing attention for their ability to alleviate plant stress and improve agricultural productivity. This study aimed to select actinomycetes strains with plant growth promoting traits, their plantroots association capacities, and their biostimulating effects without and with salt stress as well as the biocontrol power against several fungal and bacterial plant pathogens. The strains were studied for their phosphate solubilization capacity, indole-3-acetic acid (IAA), hydrocyanic acid (HCN), and ammonia (NH3) production, and several enzymatic activities. Bacteria-root associations were studied by scanning electron microscopy (SEM), and a greenhouse experiment was performed to assess the inoculation effects on Solanum lycopersiumL.. Likewise, these strains were tested for in vitro halotolerant ability by several PGP tests with different salt concentrations (i.e., 0, 0.25, 0.5, 0.75, 1, 1.25, and 1.5 M NaCl) and by a greenhouse experiment on Triticum durum. The presence of 1-aminocyclopropane-1-carboxylate deaminase activity was also studied. Moreover, the strains were screened for their in vitro biocontrol activity by soluble and volatile compounds production against the fungal strains Fusarium oxysporum f. sp. radicis-lycopersici (FORL) and Rhizoctonia solani (RHS). The post-interaction events fungus-PGPB were analyzed by scanning electron microscopy. The antibacterial activity was evaluated against Pseudomonassyringae, Pseudomonas corrugata, Pseudomonas syringaepv. actinidiae, and Pectobacteriumcarotovorum subsp. carotovorum. Strains showing good in vitro biological control activities were studied for their antifungal and antibacterial activity using the cell culture filtrate (CFS). The consortium of potent strain was applied on Solanum lycopersicum L. and Daucus carota L. for in planta biocontrol activity. Among sixty actinomycetes, fourteen strains exhibited plant growth promoting characteristics. These strains were able to solubilize the phosphate, produce ammonia, and showed several enzymatic activities at different rates. The scanning electron microscopy revealed a good in vitro plant root association and colonization abilities. The actinomycetes inoculation influenced positively the tomato growth parameters. These strains exhibited important in vitro PGP traits under different salt concentrations. Most strains (86%) had 1-aminocyclopropane-1-carboxylate deaminase activity with significant amounts of ketobutyric acid. In the greenhouse experiment, inoculation with actinomycetes strainsimproved the durum wheat morpho-biochemical parameters, which recorded a significantly higher content of chlorophyll and proline than those of uninoculated controls both under normal and stressed conditions. Strains H12 (Streptomyces albidoflavus) and H14 (Nocardiopsisaegyptica) showed good in vitro antifungal activity with diffusible and volatile compounds (inhibition up to 85%) and antibacterial activity (diameter of inhibition> 10 mm). SEM micrographs showed morphological deterioration of the fungal filaments and changes in their structures. The CFS of potent strains was also active in fungal and bacterial pathogens inhibition (minimum inhibitory concentration up to 0.2%). The consortium was also able to alleviate the infection symptoms and allow a normal growth of the infected plants compared to the control. The obtained results demonstrate the efficacy of halotolerant actinomycetes strains as a biofertilizer and biocontrol agent for plant diseases management and for the alleviation of negative effects of salt stress in plants.