This study tested the hypothesis that cocoa monoculture (MS) and cocoa-açai agroforestry systems (AFS) may influence the microbial community structure and populations of plant growth-promoting bacteria (PGPR). Accordingly, the aim was to analyze the microbial community structure and PGPR populations in different agroecosystems in the Brazilian Amazon. To achieve this, the rhizosphere microbial community of cocoa and açai plants in both Amazonian seasons (dry and rainy) was analyzed using culture-dependent (PGPR screening) and -independent methods [PCR-DGGE based on rrs, alp, nifH gene, and intergenic region (ITS) of fungi]. Concerning PGPR screening, out of 48 isolated bacterial strains, 25% were capable of siderophore production, 29% of mineralized organic phosphate, 8% of inorganic phosphate solubilization, and 4% of indole acetic acid production. Moreover, 17% of isolates could inhibit the growth of various phytopathogenic fungi. Statistical analyses of DGGE fingerprints (p < 0.05) showed that bacterial and fungal community structures in the rhizosphere were influenced by the seasons, supporting the results of the physicochemical analysis of the environment. Furthermore, as hypothesized, microbial communities differed statistically when comparing the MS and AFS. These findings provide important insights into the influence of climate and cultivation systems on soil microbial communities to guide the development of sustainable agricultural practices.

Euterpe oleracea Mart., also known for its fruit açaí, is a palm native to the Amazon region. The state of Pará, Brazil, accounts for over 90% of açaí production. Demand for the fruit in national and international markets is increasing; however, climate change and diseases such as anthracnose, caused by the fungus Colletotrichum sp., lead to decreased production. To meet demand, measures such as expanding cultivation in upland areas are often adopted, requiring substantial economic investments, particularly in irrigation. Therefore, the aim of this study was to evaluate the potential of açaí rhizobacteria in promoting plant growth (PGPR). Rhizospheric soil samples from floodplain and upland açaí plantations were collected during rainy and dry seasons. Bacterial strains were isolated using the serial dilution method, and subsequent assays evaluated their ability to promote plant growth. Soil analyses indicated that the sampling period influenced the physicochemical properties of both areas, with increases observed during winter for most soil components like organic matter and C/N ratio. A total of 177 bacterial strains were isolated from rhizospheres of açaí trees cultivated in floodplain and upland areas across dry and rainy seasons. Among these strains, 24% produced IAA, 18% synthesized ACC deaminase, 11% mineralized organic phosphate, and 9% solubilized inorganic phosphate, among other characteristics. Interestingly, 88% inhibited the growth of phytopathogenic fungi of the genera Curvularia and Colletotrichum. Analysis under simulated water stress using Polyethylene Glycol 6000 revealed that 23% of the strains exhibited tolerance. Two strains were identified as Bacillus proteolyticus (PP218346) and Priestia aryabhattai (PP218347). Inoculation with these strains increased the speed and percentage of açaí seed germination. When inoculated in consortium, 85% of seeds germinated under severe stress, compared to only 10% in the control treatment. Therefore, these bacteria show potential for use as biofertilizers, enhancing the initial development of açaí plants and contributing to sustainable agricultural practices.

The hypothesis that cultivating açaí in agroforestry systems (AFS) can recruit beneficial microorganisms to its rhizosphere was tested in this study. For this purpose, rhizospheric soil samples were collected from an AFS area. The colony-forming unit (CFU) counts of the soil samples were 3.5 × 10⁶ CFU/g, with no statistically significant differences between the plants (p < 0.05). Regarding PGPR characteristics, of the 44 isolated strains, 18% produced siderophores, 9% mineralized organic phosphate, 15% solubilized inorganic phosphate, and 7% produced indole-3-acetic acid (IAA) and antimicrobial substances. Strains AP4-03, AP1-33, and AP2-36 were affiliated with the genus Bacillus sp. and produced IAA at 1.45, 1.35, and 2.02 µg/mL, respectively. Furthermore, these strains were able to inhibit the growth of the fungus Pestalotiopsis by 69%, 67%, and 71%, respectively. Regarding the antifungal activity of bacterial extracts, inhibition zones of 23 mm (AP-03), 20 mm (AP-33), and 18 mm (AP-36), with 96% and 92% inhibition at 50 mg/mL (AP4-03 and AP1-33) and 100% inhibition at 41 mg/mL (AP2-36), were observed. Considering seedling germination, açaí palms inoculated with the strain AP1-33 statistically differed from the controls in terms of root length and hypocotyl length. Furthermore, treatments inoculated with the strain AP2-36 or all strains in the consortium differed when only the hypocotyl length was compared to the control. Thus, the analyzed strains showed potential to improve the initial development of açaí plants.