Screening of the High-Rhizosphere Competent Limoniastrum monopetalum’ Culturable Endophyte Microbiota Allows the Recovery of Multifaceted and Versatile Biocontrol Agents

Abstract

Halophyte Limoniastrum monopetalum, an evergreen shrub inhabiting the Mediterranean region, has well-documented phytoremediation potential for metal removal from polluted sites. It is also considered to be a medicinal halophyte with potent activity against plant pathogens. Therefore, L. monopetalum may be a suitable candidate for isolating endophytic microbiota members that provide plant growth promotion (PGP) and resistance to abiotic stresses. Selected for biocontrol abilities, these endophytes may represent multifaceted and versatile biocontrol agents, combining pathogen biocontrol in addition to PGP and plant protection against abiotic stresses. In this study 117 root culturable bacterial endophytes, including Gram-positive (Bacillus and Brevibacillus), Gram-negative (Proteus, Providencia, Serratia, Pantoea, Klebsiella, Enterobacter and Pectobacterium) and actinomycete Nocardiopsis genera have been recovered from L. monopetalum. The collection exhibited high levels of biocontrol abilities against bacterial (Agrobacterium tumefaciens MAT2 and Pectobacterium carotovorum MAT3) and fungal (Alternaria alternata XSZJY-1, Rhizoctonia bataticola MAT1 and Fusarium oxysporum f. sp. radicis lycopersici FORL) pathogens. Several bacteria also showed PGP capacity and resistance to antibiotics and metals. A highly promising candidate Bacillus licheniformis LMRE 36 with high PGP, biocontrol, metal and antibiotic, resistance was subsequently tested in planta (potato and olive trees) for biocontrol of a collection of 14 highly damaging Fusarium species. LMRE 36 proved very effective against the collection in both species and against an emerging Fusarium sp. threatening olive trees culture in nurseries. These findings provide a demonstration of our pyramiding strategy. Our strategy was effective in combining desirable traits in biocontrol agents towards broad-spectrum resistance against pathogens and protection of crops from abiotic stresses. Stacking multiple desirable traits into a single biocontrol agent is achieved by first, careful selection of a host for endophytic microbiota recovery; second, stringent in vitro selection of candidates from the collection; and third, application of the selected biocontrol agents in planta experiments. That pyramiding strategy could be successfully used to mitigate effects of diverse biotic and abiotic stresses on plant growth and productivity. It is anticipated that the strategy will provide a new generation of biocontrol agents by targeting the microbiota of plants in hostile environments.

Complete Genome Sequence of Enterobacter roggenkampii ED5, a Nitrogen Fixing Plant Growth Promoting Endophytic Bacterium With Biocontrol and Stress Tolerance Properties, Isolated From Sugarcane Root

“…To check the disinfection method accomplishment, the former washing double distilled water was spread on the nutrient agar (NA) and potato dextrose agar (PDA) plates, kept at 30 ± 2 and 26 ± 2 • C in an incubator for 3-5 days. The results were utilized as a sterilization control, and no fungal and bacterial colonies were capable to develop on the plates (Slama et al, 2019). Six different media were chosen for the nitrogen-fixing endophytic bacteria isolation, i.e., Ashby's glucose, Ashby's mannitol, Burk medium, Jensen medium, NA, and Yeast mannitol agar.…”

The Role of Microbial Inoculants on Plant Protection, Growth Stimulation, and Crop Productivity of the Olive Tree (Olea europea L.)

“…Furthermore, two very promising candidates with biocontrol abilities are the endophytic Bacillus strains, B. licheniformis LMRE 36 [64] and B. velezensis OEE1 [65]. Bacillus licheniformis LMRE 36 strain inhibited Fusarium solani strains Fso6 and Fso7 progression up to 75% compared to the untreated plants suggesting that secondary metabolites from Bacillus strongly interfere with fungal pathogens [64]. B. velezensis OEE1 strain was more effective in reducing the disease severity of Fusarium Solani compared to commercial fungicides [65].…”

Fusarium wilt biocontrol and tomato growth stimulation, using endophytic bacteria naturally associated with Solanum sodomaeum and S. bonariense plants

“…Endophytes can promote growth directly by the production and/or the regulation of plant growth hormones, the nitrogen fixation, and the phosphate solubilization. Indirectly, bacterial endophytes promote plant growth by protecting the plant against pathogens (Aydi Ben Abdallah et al Ben Slama et al 2019). The inoculation of mung beans (Vigna radiata) with Streptomyces thermocarboxydus S3 showed a significant increase in fresh weight, root length, and total length in the presence of IAA production as also state by Lasudee et al (2018).…”

“…Antagonistic bacteria such as Bacillus, Burkholderia, Serratia, Acinetobacter, Pseudomonas, Enterobacter, and Stenotrophomonas were widely known by their chitinase, protease, and β-1,3glucanase activities (Berg et al 2005). Microorganisms with extracellular enzyme activities are not only helpful in organic matter decomposition and plant growth-promoting, but also play an important role in disease suppression by antagonizing soilborne pathogens and facilitating intracellular colonization and development of endophytism (Ben Slama et al 2019).…”

“…Only A. chroococcum S11 and S. maltophilia S28 were shown to be able to modify the filter paper color from yellow to light-reddish color which indicated their HCN production potential when grown on NA medium amended with glycine ( Table 2). The production of HCN plays an important role in disease suppression of crop plants (Ben Slama et al 2019). The registered antifungal activity displayed by S. maltophilia S28 and A. chroococcum S11 may due, in part, to the production of HCN.…”