Efficacy of Bacillus subtilis V26 as a biological control agent against Rhizoctonia solani on potato

The aim of this study is to evaluate the efficacy of the strain Bacillus subtilis V26, a local isolate from the Tunisian soil, to control potato black scurf caused by Rhizoctonia solani. The in vitro antifungal activity of V26 significantly inhibited R. solani growth compared to the untreated control. Microscopic observations revealed that V26 caused considerable morphological deformations of the fungal hyphae such as vacuolation, protoplast leakage and mycelia crack. The most effective control was achieved when strain V26 was applied 24h prior to inoculation (protective activity) in potato slices. The antagonistic bacterium V26 induced significant suppression of root canker and black scurf tuber colonization compared to untreated controls with a decrease in incidence disease of 63% and 81%, respectively, and promoted plant growth under greenhouse conditions on potato plants. Therefore, B. subtilis V26 has a great potential to be commercialized as a biocontrol agent against R. solani on potato crops.

Heterologous expression and secretion of an antifungal Bacillus subtilis chitosanase (CSNV26) in Escherichia coli

The aims of the study were the production improvement, the purification, the characterization and the activity investigation of chitosanase CSNV26 of Bacillus subtilis (V26). The gene csnV26 encoding for this protein was amplified and cloned in the pBAD vector then expressed in Escherichia coli (Top10). The SDS-PAGE and zymogram analysis of the recombinant protein showed that it has two active forms sized 27 and 31 kDa, corresponding to the protein with and without signal peptide. This protein has the particularity of being secreted by Top10-pBAD-csnV26 with a high yield of 6.2 g/l. The HPLC purification of CSNV26 from supernatant confirmed the presence of the two sizes. The investigation of the CSNV26 thermostability showed that the pure protein is highly stable keeping 68 % of its activity after 30-min treatment at 100 °C, contrarily to the protein present within the supernatant of E. coli and B. subtilis (V26). The molecular dynamics study of the predicted structure of protein in both forms showed that the presence of the peptide signal in the form of 31 kDa gave it a remarkable thermal stability. The antifungal activity of CSNV26 was evidenced on Rhizopus nigricans and Rhizopus oryzae. Indeed, it has provoked an alteration and embrittlement of their hyphae with onset of protoplast.

Use of two bacteria for biological control of bayoud disease caused by Fusarium oxysporum in date palm (Phoenix dactylifera L) seedlings

The Bayoud, caused by Fusarium oxysporum f. sp. albedinis (Foa), is the most destructive disease of date palm (Phoenix dactylifera L) in Morocco and Algeria, with no effective control strategy yet available. In this work, two bacteria, Bacillus amyloliquefaciens strain Ag1 (Ag) and Burkholderia cepacia strain Cs5 (Cs), were examined for their potential to control this disease. Both bacterial strains inhibited both growth and sporulation of Foa. They released compounds into the culture medium, which resulted into cytological changes in Foa's mycelial structure. When Jihel-date palm plantlets, a susceptible cultivar, were induced with these bacteria, the size of the necrosis zone, which reflected the spreading of the pathogen, was reduced by more than 70%, as compared with uninduced controls. To further investigate the mechanisms of such disease reduction, phenolic compounds and peroxidase activity were assessed. One month after inoculation, date palm defense reactions against Foa were different depending on the bacterium used, B. cepacia led to higher accumulation of constitutive caffeoylshikimic acid isomers while B. amyloliquefaciens triggered the induction of new phenolic compounds identified as hydroxycinnamic acid derivatives. Peroxidase activity has also been stimulated significantly and varied with the bacterial strain used and with Foa inoculation. These results add to the promising field of investigation in controlling Bayoud disease.

Biological control of Botrytis cinerea using the antagonistic and endophytic Burkholderia cepacia Cs5 for vine plantlet protection

Antifungal activity of the Burkholderia cepacia Cs5 was tested in vitro and in vivo for the control of Botrytis cinerea . Bacterial biomass was significantly improved by the amendment of ZnSO4, Mo7(NH4)6O24, and mannitol to the NBY medium; consequently, the amount of the secreted fungicides was increased. The quantification of B. cinerea inhibition, in liquid and solid conditions, showed an important sensitivity of this fungus to the strain Cs5 fungicides. Microscopic monitoring impact of these fungicides on mycelium structure showed an important increase in their diameter and ramifications in the presence of 0.75% supernatant. For the in vivo application of the strain Cs5, Vitis vinifera plantlets were inoculated with a Cs5 bacterial suspension, then with B. cinerea spores. The plantlets protection was total and durable when these two inoculations were made 3 weeks apart, which is the time for the endophytic bacterium to colonize the plantlets up to the top leaves. This protection is due to Cs5 antagonism and the elicitation of the plantlets self-defense via the root overgrowth.