Plant Growth-Promoting Bacteria Enterobacter sp. 15S Enhance P Mobilization and Uptake in Monocot and Dicot Plants

Introduction: Phosphorus is an essential macronutrient for plants and the use of P-solubilizing bacteria can enhance the availability of this element providing a promising alternative to chemical fertilizers. In this sense, this study was aimed at evaluating the effectiveness of Enterobacter sp. 15S, a putative diazotroph isolated from horticultural soil with ability to solubilize phosphate, in promoting the growth of cucumber and maize under hydroponic P-limiting conditions. Methods: Five-days-old seedlings (cucumber and maize) from surface sterilized seeds were transferred to plastic pots filled with 1.5 L of nutrient solution, either P-free or supplemented with 0.1 mM KH2PO4. After seven days of growth, both conditions received additional treatments to perform a complete 2 x 4 factorial design: inoculation with Enterobacter sp. 15S (106 cell mL-1); inoculation with Enterobacter sp. 15S (106 cell mL-1) supplemented with 24 mg Ca3(PO4)2; 24 mg Ca3(PO4)2 without bacterial inoculation and a control without Ca3(PO4)2 nor inoculation. Insoluble Ca3(PO4)2 was added inside a dialysis bag to avoid direct contact with the root surface. Plants were grown in a climatic chamber (24/19˚C, 14 h photoperiod) for 15 (maize) or 21 (cucumber) days and evaluated for the root biometric parameters, root exudates composition, ionomic profile and the expression of genes involved in P-acquisition (e.g. phosphate transporters). Results: The analysis of roots indicates that the Enterobacter sp. 15S was more effective in promoting the growth of cucumber than maize, regardless the P-availability condition. The ionomic profile indicated that inoculation with Enterobacter sp. 15S induced higher absorption and translocation of P in cucumber, when cucumber plants were grown in addition with Ca3(PO4)2. The releasing of root exudates (phenols and flavonoids) was greater in treatments without Ca3(PO4)2. qRT-PCR analyses showed a stronger up-regulation of some members of the PT1 gene family (Phosphate transporter genes which play important roles in Pi uptake and translocation) in the treatments without Ca3(PO4)2. Conclusions: Overall, the results suggest a differential interaction of Enterobacter sp. 15S with monocots and dicots, with a probable role of root exudates and plant species-specific genes in developing effective growth-promotion associations under P-limiting conditions. Funding agencies: Capes, CNPq, Fundação Araucária, UEL.