RESUMEN
Rhizobacteria from desert plants can alleviate biotic stress and suppress plant diseases, and consequently can enhance plant growth. Therefore, the current study was performed to isolate and identify Prosopis glandulosa-associating rhizobacteria based on their antagonistic activity against Fusarium species and plant growth-promoting properties. Three bacterial isolates were identified as Bacillus subtilis: LDA-1, LDA-2, and LDA-3. The molecular analysis suggests the biosynthesis of the bacteriocins subtilisin and subtilosin, as well as the lipopeptide iturin, by these strains. In addition, the antagonistic study by dual-culture assay showed a high efficacy of all B. subtilis strains against phytopathogenic fungi (Fusarium nygamai, F. equisseti, F. solani, F. solani ICADL1, and F. oxysporum ICADL2) with inhibition percentages ranging from 43.3 to 83.5% in comparison to the control. Moreover, atomic force microscopy (AFM) analysis showed significant differences in the cell wall topography of the F. solani ICADL1 among the treated mycelia and untreated control. As a result, these three B. subtilis strains were used as bioinoculants for cotton seedlings infected by F. solani ICADL1 in pot trials, and the results revealed that the bacterial inoculations as an individual or combined with F. solani ICADL1 significantly improved cotton root and stem length, lateral roots, indole acetic acid (IAA), and gibberellic acid (GA3) contents, as well as increased antioxidants, flavonoids, and phenols in comparison to those obtained from healthy and infected control plants. In conclusion, the three bacterial strains of B. subtilis (i.e., LDA-1, LDA-2, and LDA-3) are considered promising tools as biocontrol agents for F. solani and cotton growth promoters, and consequently can be used as bio-ertilizer in sustainable agriculture systems.
RESUMEN
Azospirillum brasilense is a rhizobacterium that provides beneficial effects on plants when they colonize roots. The formation of complex bacterial communities known as biofilms begins with the interaction of planktonic cells with surfaces in response to appropriate signals. Nitric oxide (NO) is a signaling molecule implicated in numerous processes in bacteria, including biofilm formation or dispersion, depending on genera and lifestyle. Azospirillum brasilense Sp245 produces NO by denitrification having a role in root growth promotion. We analyzed the role of endogenously produced NO on biofilm formation in A. brasilense Sp245 and in a periplasmic nitrate reductase mutant (napA::Tn5; Faj164) affected in NO production. Cells were statically grown in media with nitrate or ammonium as nitrogen sources and examined for biofilm formation using crystal violet and by confocal laser microscopy. Both strains formed biofilms, but the mutant produced less than half compared with the wild type in nitrate medium showing impaired nitrite production in this condition. NO measurements in biofilm confirmed lower values in the mutant strain. The addition of a NO donor showed that NO influences biofilm formation in a dose-dependent manner and reverses the mutant phenotype, indicating that Nap positively regulates the formation of biofilm in A. brasilense Sp245.
