ABSTRACT
Tropaeolum majus L. is a versatile edible plant that is widely explored due to its medicinal properties and as a key element in intercropping systems. Its growth could be improved by the use of biofertilizers that can enhance nutrient uptake by the plant or provide tolerance to different abiotic and biotic stresses. In a previous study, 101 endophytes isolated from T. majus roots showed more than three plant growth-promoting (PGP) features in vitro, such as phosphate mineralization/solubilization, production of siderophores, antimicrobial substances and indole-related compounds, and presence of the nifH gene. To provide sustainable alternatives for biofertilization, the genomes of two promising endophytes-CAPE95 and CAPE238-were sequenced to uncover metabolic pathways related to biofertilization. Greenhouse experiments were conducted with 216 seeds and 60 seedlings, half co-inoculated with the endophytes (treatment) and half inoculated with 1X PBS (control), and the impact of the co-inoculation on the plant's bacteriome was accessed through 16S rRNA gene metabarcoding. The strains CAPE95 and CAPE238 were taxonomically assigned as Bacillus thuringiensis and Paenibacillus polymyxa, respectively. Metabolic pathways related to the enhancement of nutrient availability (nitrogen fixation, sulfate-sulfur assimilation), biosynthesis of phytohormones (indole-3-acetic acid precursors) and antimicrobial substances (bacilysin, paenibacillin) were found in their genomes. The in vivo experiments showed that treated seeds exhibited faster germination, with a 20.3% higher germination index than the control on the eleventh day of the experiment. Additionally, treated seedlings showed significantly higher plant height and leaf diameters (p < 0.05). The bacterial community of the treated plants was significantly different from that of the control plants (p < 0.001) and showed a higher richness and diversity of species (Chao and Shannon indexes, p < 0.001). A higher relative abundance of potential synergistic PGP bacteria was also shown in the bacteriome of the treated plants, such as Lysinibacillus and Geobacter. For the first time, co-inoculation of B. thuringiensis and P. polymyxa was shown to have great potential for application as a biofertilizer to T. majus plants. The bacterial consortium used here could also be explored in other plant species in the future.
ABSTRACT
Although Tropaeolum majus (nasturtium) is an agriculturally and economically important plant, especially due to the presence of edible flowers and its medicinal properties, its microbiome is quite unexplored. Here, the structure of the total bacterial community associated with the rhizosphere, endosphere and bulk soil of T. majus was determined by 16S rRNA amplicon metagenomic sequencing. A decrease in diversity and richness from bulk soil to the rhizosphere and from the rhizosphere to the endosphere was observed in the alpha diversity analyses. The phylum Proteobacteria was the most dominant in the bacteriome of the three sites evaluated, whereas the genera Pseudomonas and Ralstonia showed a significantly higher relative abundance in the rhizosphere and endosphere communities, respectively. Plant growth-promoting bacteria (236 PGPB) were also isolated from the T. majus endosphere, and 76 strains belonging to 11 different genera, mostly Serratia, Raoultella and Klebsiella, showed positive results for at least four out of six plant growth-promoting tests performed. The selection of PGPB associated with T. majus can result in the development of a biofertilizer with activity against phytopathogens and capable of favoring the development of this important plant.
ABSTRACT
Sweet potato is a subsistence crop cultivated worldwide. Although it is generally considered tolerant to different diseases, it is quite susceptible to the fungus Plenodomus destruens that causes foot-rot disease. Plant growth-promoting bacteria associated with sweet potato remain poorly studied, but some Bacillus strains may have potential as biological control agents. Here, we evaluate the persistence of two bacterial strains-Bacillus safensis T052-76 and Bacillus velezensis T149-19-in pot experiments and assess their impact on indigenous bacterial and fungal communities associated with sweet potato. Numbers of cells of both strains introduced into pots remained stable in the rhizosphere of sweet potato over the 180-day experiment. Denaturing gradient gel electrophoresis based on the rrs gene encoding bacterial 16S rRNA and the fungal ribosomal internal transcribed spacer region showed that bands corresponding to the introduced strains were not detected in plant endosphere. PERMANOVA and non-metric multidimensional scaling statistical analyses showed that: (1) strain T052-76 altered the structure of the indigenous bacterial community (rhizosphere and soil) more than strain T149-19; (2) T052-76 slightly altered the structure of the indigenous fungal community (rhizosphere and soil) and (3) strain T149-19 did not disturb the fungal community. Our results demonstrate the stability of both Bacillus strains in the sweet potato rhizosphere and, apart from the influence of B. safensis T052-76 on the bacterial community, their limited impact on the microbial community associated with this important crop plant.
