Innovative Rhizosphere-Based Enrichment under P-Limitation Selects for Bacterial Isolates with High-Performance P-Solubilizing Traits

Abstract

The use of phosphate solubilizing bacteria (PSB) as inoculants for the rhizosphere is a well-known strategy to mitigate P-deficiency in plants. However, despite the multiple modes of action to render P available for plants, PSB often fail to deliver in the field as their selection is often based on a single P-solubilizing trait assessed <i>in vitro.</i> Anticipating these shortcomings, we screened 250 isolates originating from rhizosphere-based enriched consortia for the main <i>in vitro</i> P-solubilizing traits, and subsequently grouped the isolates through trait-based HCPC (hierarchical clustering on principal components). Representative isolates of each cluster were tested in an <i>in planta</i> experiment to compare their <i>in vitro</i> P-solubilizing traits with their <i>in planta</i> performance under conditions of P-deprivation. Our data convincingly show that bacterial consortia capable to mitigate P-deficiency <i>in planta</i> were enriched in bacterial isolates that had multiple P-solubilizing traits <i>in vitro</i> and that had the capacity to mitigate plant P-stress <i>in planta</i> under P-deprived conditions. Furthermore, although it was assumed that bacteria that looked promising <i>in vitro</i> would also have a positive effect <i>in planta</i>, our data show that this was not always the case. Opposite, lack of performance <i>in vitro</i> did not automatically result in a lack of performance <i>in planta</i>. These results corroborate the strength of the previously described <i>in planta</i>-based enrichment and selection technique for the isolation of highly efficient rhizosphere competent PSB. <b>IMPORTANCE</b> With the growing awareness on the ecological impact of chemical phosphate fertilizers, research concerning the use of phosphate solubilizing bacteria (PSB) as a sustainable alternative for, or addition to these fertilizers is of paramount importance. In previous research, we successfully implemented a plant-based enrichment technique for PSB, which simultaneously selected for the rhizosphere competence and phosphate solubilizing characteristics of bacterial suspensions. Current research follows up on our previous findings, whereas we screened 250 rhizobacteria for their P-solubilizing traits and were able to substantiate the results obtained from the enriched suspensions at a single-isolate level. With this research, we aim for a paradigm shift toward the plant-based selection of PSB, which is a more holistic approach compared to the plate-based methods. We emphasize the strength of the previously described plant-based enrichment and selection technique for the isolation of highly efficient and diverse PSB.