Caracterização da microbiota rizosférica do porta-enxerto citrumelo ‘Swingle’ em campo sob condições de estresse hídrico

Abstract

Despite Brazil is the world's largest orange producer, climate change, such as rising temperatures and drought, has caused significant losses for citrus production. Citrus cultivation is predominantly carried out through grafting, with the rootstock playing a fundamental role in water retention capacity and resistance to biotic and abiotic stresses such as other characteristics. Citrumelo 'Swingle' is the most widely used rootstock for citrus plants in Brazil, but its drought resistance is only moderate, and in recent years, it has struggled under adverse growth condition such as drought as consequence of clime change. During observations in commercial orchards, our research group identified sweet orange ("Valencia") trees grafted onto Citrumelo 'Swingle' that, despite being under the same environmental conditions and free from apparent diseases, exhibited contrasting drought stress symptoms. Based on this observation, we hypothesized that the composition of the rhizosphere bacterial community differs between plants with varying responses to water deficit and that the moderate stress phenotype may be associated with the presence of plant growth-promoting bacteria (PGPB). This study aimed to compare rhizobacterial populations in sweet orange plants subjected to different levels of drought stress and to select, through sequencing and physiological characterization, bacterial isolates with potential plant-beneficial traits. To test the hypothesis, root and rhizosphere soil samples were collected from plants with contrasting drought stress symptoms growing in the northwest of São Paulo State. The bacterial isolates were identified by sequencing the 16S rRNA gene and classified at the genus level through bioinformatics analyses. For the metataxonomic analysis, total rhizosphere DNA was sequenced, allowing the characterization of microbial diversity associated with each drought stress phenotype. The metataxonomic analysis revealed significant differences in the overall composition of bacterial communities, including variations in the relative abundance of different phyla and genera. Additionally, beta diversity indicated that rhizosphere communities differ between drought stress phenotypes, suggesting that microbiota may be linked to plant drought responses. Some isolates were characterized in vitro for their ability to solubilize phosphorus, produce siderophores and auxins, and mitigate drought stress through ACC deaminase production, biofilm formation, and in medium growth under water-restricted conditions. Isolates from samples with the moderate stress phenotype stood out in these tests, reinforcing the hypothesis that lower susceptibility to water deficit may be associated with microorganisms possessing plant growth-promoting traits. Furthermore, germination and greenhouse inoculation tests showed that isolates associated with the moderate stress phenotype also performed better. The most promising isolates were selected to form a synthetic community, which is currently being tested in vivo to evaluate its effect on drought stress mitigation. The results of this study demonstrate that the rhizosphere community indeed differs between plants experiencing different levels of drought stress and that this difference may be related to the presence of PGPB, with potential applications in crops under drought conditions.