Effect of Inoculation Level on the Impact of the PGPR Azospirillum lipoferum CRT1 on Selected Microbial Functional Groups in the Rhizosphere of Field Maize.

Sébastien Renoud,Danis Abrouk,Claire Prigent-Combaret,Yvan Moënne-Loccoz,Laurent Legendre,Florence Wisniewski-Dyé,Daniel Muller

doi:10.3390/microorganisms10020325

Abstract

The impact of inoculated plant growth-promoting rhizobacteria (PGPR) on its host physiology and nutrition depends on inoculum level. Whether the impact of the inoculated PGPR on the indigenous rhizosphere microbiota also varies with the PGPR inoculum level is unclear. Here, we tested this issue using the PGPR Azospirillum lipoferum CRT1—maize model system, where the initial seed inoculation is known to enhance maize growth and germination, and impacts the maize rhizomicrobiota, including microbial functional groups modulating plant growth. A. lipoferum CRT1 was added to the seeds at standard (105–6 cells.seed−1) or reduced (104–5 cells.seed−1) inoculation levels, in three fields. The effect of the two PGPR formulations was assessed on maize growth and on the nifH (nitrogen fixation), acdS (ACC deaminase activity) and phlD (2,4-diacetylphloroglucinol production) microbial functional groups. The size of the three functional groups was monitored by qPCR at the six-leaf stage and the flowering stage, and the diversity of the nifH and acdS functional groups (as well as the bacterial community) were estimated by MiSeq metabarcoding at the six-leaf stage. The results showed that the benefits of the reduced inoculant formulation were significant in two out of three fields, but different (often lower) than those of the standard formulation. The effects of formulations on the size of the three functional groups differed, and depended on field site and functional group. The reduced formulation had an impact on the diversity of nifH and acdS groups at one site, whereas the standard formulation had an impact at the two other sites. Inoculation significantly impacted the total bacterial community in the three fields, but only with the reduced formulation. In conclusion, the reduced inoculant formulation impacted the indigenous rhizosphere microbiota differently, but not less efficiently, than the standard formulation.

Highlights

The inoculation of plants with beneficial microorganisms such as Plant-GrowthPromoting Rhizobacteria (PGPR) to enhance crop productivity is an over one-hundred-yearold farming technique [1,2]
Quantitative PCR analysis of A. lipoferum CRT1 DNA on maize rhizosphere soil samples at the six-leaf and flowering stages did not yield any signal, indicating that the inoculant A. lipoferum CRT1 was under the detection threshold in the three field trials, irrespective of the formulation
At site L, seed inoculation with A. lipoferum CRT1 resulted in significantly higher shoot biomass (+26% with F1 vs. +29% with F2), leaf width (+10% with F1 only) or leaf length

Summary

Introduction

The inoculation of plants with beneficial microorganisms such as Plant-GrowthPromoting Rhizobacteria (PGPR) to enhance crop productivity is an over one-hundred-yearold farming technique [1,2]. Microorganisms 2022, 10, 325 and Africa, inoculated seeds are sown at a large scale, with millions of hectares of Fabaceae (e.g., soybean or bean) and Poaceae (e.g., maize, sorghum or wheat) inoculated by PGPR belonging mainly to the genera Bacillus, Paenibacillus, Pseudomonas or Azospirillum [2,9,10,11,12]. In the context of ecological awareness in agronomy, biofertilisation with microorganisms constitutes an attractive alternative to chemical inputs, which are polluting and decrease soil fertility [13,14,15,16]. Various studies reported that PGPR inoculant functioning under field conditions is impacted by soil type and plant genotype, as well as the PGPR strains used [2,9,10,17,18,19,20,21]

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