English abstract
In this study, endophytic and rhizospheric bacteria were isolated and characterized
from Moringa olifera and M. perigreina from Oman and their biocontrol potential
against Pythium aphanidermatum damping-off of cabbage (Brassica oleracea) was
tested under greenhouse conditions. A total of 12 endophytic and 27 rhizosphere
bacteria were isolated from Moringa spp. Among them, Bacillus pumilus MPE1
showed the maximum antagonistic activity against P. aphanidermatum in a dual
culture assay followed by Paenibacillus glucanolyticus MPE3 and Pseudomonas
indica strains MOR3 and MOR8. These bacterial isolates induced abundant
morphological abnormalities in the hyphae of P. aphanidermatum, observed under
scanning electron microscopy. In vitro cross-streak assay showed that these bacterial
isolates were compatible, except for P. indica MOR8 P. glucanolyticus MPE3.
These antagonists released volatile organic compounds that inhibited the growth of P.
aphanidermatum in an in vitro assay. These antagonistic bacteria released 2,4-
dimethylheptane and 4-methyloctane as the predominant volatile organic compounds.
Of the four antagonistic bacterial strains P. indica MOR8 was capable of forming
biofilm, which is considered a trait that improves the efficacy of rhizosphere
inoculants. The results of greenhouse experiments showed that soil treatment with B.
pumilus MPE1 showed the highest reduction (59%) in the incidence of P.
aphanidermatum damping-off in cabbage, evidencing its potential as a biocontrol
agent for the management of this disease.
The sensitivity of antagonistic bacterial strains from Moringa spp. to antimicrobial
compounds of kale (Brassica oleracea var. acephala) was evaluated. The methanolic
extract obtained from kale leaves was analyzed by thin-layer chromatography (TLC).
The observation of the TLC plate under UV light (254 nm) revealed the presence of
two prominent spots with retention factor values of 0.695 and 0.623. These
compounds were eluted from the preparative TLC silica plates and tested for in vitro
antimicrobial activities against endophytic and rhizospheric antagonistic bacterial
strains from Moringa spp. Of the two fractions, the one with Rf 0.695 showed
inhibitory activities against P. indica MOR3, P. indica MOR8 and Bacillus pumilus
MPE1. However, the same fraction did not show any antibacterial activity against the
endophytic bacterial isolate P. glucanolyticus MPE3. Gas chromatography-mass
spectrometry (GC-MS) analysis identified benzyl 2,3-anhydro--D-ribopyranoside,
undecane and 1,2-propanediol, 3-benzyloxy-1,2-diacetyl as major compounds in the
active fraction (Rf 0.695). The tolerance of the endophytic bacterial isolates P.
glucanolyticus MPE3 to the antibacterial compounds of kale might be due to their
better adaptability to the glucosinolate of host plant.
