Biodiversité bactérienne du sol rhizosphérique de Capsicum annuum

Abstract

This study was conducted to estimate the bacterial biodiversity of rhizospheric soil and to characterize the activities of plant growth-promotimg bacteria (PGPB) associated to the roots of chili pepper (Capsicum annuum). Rhizospheric and non-rhizospheric soil samples and the roots of three industrial chili pepper genotypes were collected from an agricultural land in the Wilaya of Guelma. Compared to bulk soil, the rhizosphere of the three genotypes has different soil texture; low pH and low carbonate content. Bacterial growth is most abundant around the roots of the three genotypes with different rhizosphere/soil ratios. Analysis of genetic diversity through T-RFLP showed that bacterial diversity increased significantly only in the rhizosphere of Biskra genotype. 515 bacterial isolates were collected, 301 isolates from the three rhizospheres and 214 isolates from bulk soil. The analysis of PGPB specific activities revealed that the production of indole compounds was stimulated in the rhizosphere of the three cultivars; however, the antiphytopathogenic activity was stimulated only by one genotype. The bacterial isolation from the roots of the three cultivars yielded 83 endophytes. Amplification by BOX-PCR revealed high genetic diversity of the isolates. Endophytes have shown high plant growth promoting abilities in vitro. All isolates produce auxins via tryptophan-dependent synthetic pathways. GC-MS analysis shows that indole acetic acid is predominantly produced by IPyA pathway. Endophytes have important ability to produce siderophores but with a less chelating capacity than the rhizospheric microbiota. The use of UHPLC-MS/MS Q Exactive detect 23 different iron chelators in 21 isolates. The production of long-chain acyl-homoserine lactones (AHL) is detected in seven isolates. However, inhibition of AHL auto-induction is observed in 22 endophytes, among of which 10 and 15 isolates inhibited auto-induction in C. violaceum CV026 and A. tumefaciens NT1 respectively. Antifungal activity analysis revealed that 25 isolates could inhibit F. oxysporum growth. In addition, 79 % of endophytes have the capacity to solubilize inorganic phosphate. The isolates KEK36, KEK1, FEK22, FEO15 and BEK8 were selected for in vivo study effect. 16S rDNA amplification classified the isolate FEO15 as a member of the Streptomyces genus with high similarity to the species S. caeruleatus. The isolates FEK22, BEK8, KEK36 and KEK1, which belong to the genus Pseudomonas, are phylogenetically similar to the species: P. moorei, P. japonica, P. granadensis and P. brassicacearum respectively. In vivo, these five strains increased significantly the growth parameters for the three genotypes. FEO15 and FEK22 isolates were selected to study the impact on yield after five months in culture, with separate and combined inoculations. Our results show that the consortium is more efficient than separate inoculation. Karam genotype shows 199 % and 377 % improvement in fruit dry weight after the treatment by FEO15 isolate and the consortium respectively. In addition, an increase of 58 % and 138 % is reported after Al-Fares genotype inoculation with FEO15 isolate and the mixture respectively. As a conclusion from this study, the consortium of FEO15 and FEK22 isolates showed promising results concerning their use as biostimulaters. However, further studies are required to complete the formulation and field application steps.