Phosphate solubilizing bacteria isolated from greenhouse ornamentals improve health and promote growth in French marigold 'Durango Yellow' grown in soilless substrate under phosphate limiting conditions.

Abstract

<h4>Background</h4>Phosphate solubilizing bacteria (PSB) can break down insoluble forms of phosphorus such as Ca₃(PO₄)₂. In several soil grown crops, PSB inoculation improved phosphorus nutrition. However, positive results are inconsistent and there is comparably little information about PSB efficacy in soilless culture systems. This study aimed to identify phosphate solubilizing bacteria (PSB) from a greenhouse rhizobacteria collection that could be used to improve floriculture crop growth and health under phosphorus limiting conditions.<h4>Results</h4>A bromophenol blue (pH indicator) based colorimetric assay was used to identify bacterial isolates that acidified the culture media. From the collection of 1044 bacteria, 35 isolates that reduced media pH were identified, and their phosphate solubilization capacity was quantified. The top 14 PSB were selected for whole-genome sequencing. The isolates belonged to genera Enterobacter (8), Pantoea (5), and Raoultella (1). In planta effects of strains C2B11 and C8D10 were evaluated in French marigolds (Tagetes patula) grown in a peat-based substrate (pH = 7). Phosphorus was supplemented weekly as insoluble Ca₃(PO₄)₂. C2B11 and C8D10 inoculated marigolds were greener than non-inoculated controls. Only C2B11 increased shoot digital biomass and bloom area.<h4>Conclusions</h4>Bromophenol blue proved useful to identify bacteria that reduced culture media pH, but not all identified isolates showed high phosphate solubilization. Pantoea formicae C8D10 and Pantoea trifolii C2B11 strains improved growth or health of marigolds grown in soilless substrate under phosphate limiting conditions. We showed the applicability of bromophenol blue for high-throughput PSB identification. C8D10 and C2B11 are phosphate solubilizing bacteria native to soilless substrate systems, and both strains have potential for use as biostimulants to support phosphorus management in ornamental greenhouse production.