Compatible bacterial mixture, tolerant to desiccation, improves maize plant growth

Abstract

Plant growth-promoting rhizobacteria (PGPR) increase plant growth and crop productivity. The inoculation of plants with a bacterial mixture (consortium) apparently provides greater benefits to plant growth than inoculation with a single bacterial strain. In the present work, a bacterial consortium was formulated containing four compatible and desiccation-tolerant strains with potential as PGPR. The formulation had one moderately (Pseudomonas putida KT2440) and three highly desiccation-tolerant (Sphingomonas sp. OF178, Azospirillum brasilense Sp7 and Acinetobacter sp. EMM02) strains. The four bacterial strains were able to adhere to seeds and colonize the rhizosphere of plants when applied in both mono-inoculation and multi-inoculation treatments, showing that they can also coexist without antagonistic effects in association with plants. The effects of the bacterial consortium on the growth of blue maize were evaluated. Seeds inoculated with either individual bacterial strains or the bacterial consortium were subjected to two experimental conditions before sowing: normal hydration or desiccation. In general, inoculation with the bacterial consortium increased the shoot and root dry weight, plant height and plant diameter compared to the non-inoculated control or mono-inoculation treatments. The bacterial consortium formulated in this work had greater benefits for blue maize plants even when the inoculated seeds underwent desiccation stress before germination, making this formulation attractive for future field applications.

Fig 1. Bacterial survival ratio (BSR) response to desiccation stress using soils as support matrices for desiccation.

A) A. brasilense Sp7, B) P. putida KT2440, C) Acinetobacter sp. EMM02, D) Sphingomonas sp. OF178. Green lines indicate desiccation using loamy sand for support, red lines indicates desiccation using sand for support, blue lines indicate bacterial desiccation without soil (control). Each point in each graph represents the mean of five independent determinations and the respective standard deviation. Values with identical letters in each graphic are not significantly different at p ≤ 0.05 based on Student’s t or Tukey’s test.

Fig 2. Effect of bacterial inoculation on the growth of maize 45 days after sowing (DAS) under greenhouse conditions.

A) Shoot dry weight, B) Root dry weight, C) Plant height, D) Plant diameter. WDS indicates experiment with seeds not subjected to desiccation stress before germination (Exp. 1). DS indicates experiment with seeds subjected to desiccation stress before germination (Exp. 2), E) Plants from germinated seeds inoculated with the bacterial consortium (WDS), F) Plants from germinated seeds inoculated with the bacterial consortium (DS). Bar color key: sky blue, non-inoculated control; orange, A. brasilense Sp7; gray, P. putida KT2440; yellow, Acinetobacter sp. EMM02; dark blue, Sphingomonas sp. OF178; green/bacterial consortium. Each value represents the media of the data for 35 independent plants with the respective standard deviation. Identical letters in each bar group indicate that the values were not significantly different at P ≤ 0.05 based on Student’s t or Tukey’s test.

Fig 3. Bacterial survival ratio (BSR) of bacteria associated with blue maize seeds subjected to 18 days of desiccation.

Fig 3A, BSR determination for seeds inoculated with single bacterial strains. Fig 3B, BSR determination for seeds inoculated with the bacterial consortium. Purple squares represent Sphingomonas sp. OF178, green triangles represent Acinetobacter sp. EMM02, red rectangles represent A. brasilense Sp7, and blue diamonds represent P. putida KT2440.