Shaping the rhizosphere : can in planta rhizobiome selection for phosphate solubilizing bacteria outperform classical in vitro selection methods?

Phosphorus is, next to nitrogen, one of the key nutrients for plant growth and development. As the majority of P in soils is bound to recalcitrant organophosphates, adsorbed on clay minerals or fixed in metal complexes, plants are often suffering of P-deficiency. The rising ecological awareness on the excessive use of chemical (P-)fertilizers led to the quest for highly efficient, beneficial phosphate solubilizing microorganisms. It was at the previous Biostimulants World Congress (4th BWC, 2019) that we highlighted pitfalls of the commonly used in vitro isolation and selection procedures, in which a major concern was the possible lack of rhizosphere competence of the bacterial isolates or consortia. As an alternative, we presented an in planta enrichment platform for the successful selection of rhizosphere competent, phosphate solubilizing bacteria, in which plants were fully relieved of P-stress. In this follow-up research, a metabarcoding analysis was performed to assess the bacterial composition of the sequentially enriched consortia. Trends in plant P-content were similar to trends in abundance of Proteobacteria, Verrucomicrobia and Actinobacteria. Additionally, a total of 250 bacterial isolates were selected in vitro from the enriched consortia, evaluated for their phosphate solubilizing traits (acid production, siderophore production, growth on selective media), their phosphate solubilizing capacity in liquid medium and their root colonizing ability (motility assay). Metabarcoding analysis and in vitro experiments were compared side by side to evaluate the added value of the in planta enrichment platform over the classical in vitro selection method