Biocontrol of Rhizoctonia solani damping-off and promotion of tomato plant growth by endophytic actinomycetes isolated from native plants of Algerian Sahara

Thirty-four endophytic actinomycetes were isolated from the roots of native plants of the Algerian Sahara. Morphological and chemical studies showed that twenty-nine isolates belonged to the Streptomycesgenus and five were non-Streptomyces. All isolates were screened for their in vitro antifungal activityagainst Rhizoctonia solani. The six that had the greatest pathogen inhibitory capacities were subsequentlytested for their in vivo biocontrol potential on R. solani damping-off in sterilized and non-sterilized soils,and for their plant-growth promoting activities on tomato seedlings. In both soils, coating tomato seedswith antagonistic isolates significantly reduced (P < 0.05) the severity of damping-off of tomato seedlings.Among the isolates tested, the strains CA-2 and AA-2 exhibited the same disease incidence reduction asthioperoxydicarbonic diamide, tetramethylthiram (TMTD) and no significant differences (P < 0.05) wereobserved. Furthermore, they resulted in a significant increase in the seedling fresh weight, the seedling length and the root length of the seed-treated seedlings compared to the control. The taxonomic positionbased on 16S rDNA sequence analysis and phylogenetic studies indicated that the strains CA-2 AA-2were related to Streptomyces mutabilis NBRC 12800ᵀ(100% of similarity) and Streptomyces cyaneofuscatus JCM 4364ᵀ(100% of similarity), respectively

The Saharan isolate Saccharothrix algeriensis NRRL B-24137 induces systemic resistance in Arabidopsis thaliana seedlings against Botrytis cinerea

Background and aim Saccharothrix algeriensis NRRL B-24137, isolated from a Saharan soil, has been described as a potential biocontrol agent against Botrytis cinerea and other phytopathogens. However, the plant protection mechanisms involved still need to be described. The aim of this study was to determine this protection phenomenon as well as parts of the mechanisms involved, using Arabidopsis thaliana seedlings and B. cinerea. Methods The bacterial colonization process was evaluated on A. thaliana seedlings using fluorescence in situ hybridization. Protection of A. thaliana seedlings inoculated with NRRL B-24137 against B. cinerea was then evaluated. Parts of the mechanisms involved in the systemic protection against B. cinerea were evaluated using known mutants of genes involved in jasmonate (JA)/ethylene (ET)/salicylic acid (SA) signaling. Other Arabidopsis mutants, AtrhbohD-3, AtrhbohF-3, and ups1-1 were also screened to determine other parts of the mechanisms involved. Results The results showed that the strain NRRL B-24137 colonized, epi- and endophytically, the roots of Arabidopsis seedlings but the strain was not a systemic colonizer during the time of the experiment. The strain NRRL B-24137 also reduced B. cinerea symptoms and the protection was linked to known mechanisms of induced systemic resistance (ISR; JA/ET signaling), as well as to functionality of AtrbohF oxidase and of UPS1. Crosstalk between ET/JA and SA signaling could also be involved. Conclusions The isolate NRRL B-24137, after colonizing the root systems of A. thaliana, induces an ISR against B. cinerea, which is JA/ET dependent, but could also require SA crosstalk and protection could also require NAPDH oxidases and UPS1 functionalities