Deciphering composition and function of the root microbiome of a legume plant

Kyle Hartman,Jean-Claude Walser,Valexia Roussely-Provent,Marcel Ga Van Der Heijden,Klaus Schlaeppi

doi:10.1186/s40168-016-0220-z

Kyle Hartman, Jean-Claude Walser + Show 3 more

Open Access

https://doi.org/10.1186/s40168-016-0220-z

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Abstract

BackgroundDiverse assemblages of microbes colonize plant roots and collectively function as a microbiome. Earlier work has characterized the root microbiomes of numerous plant species, but little information is available for legumes despite their key role in numerous ecosystems including agricultural systems. Legumes form a root nodule symbiosis with nitrogen-fixing Rhizobia bacteria and thereby account for large, natural nitrogen inputs into soils. Here, we describe the root bacteria microbiome of the legume Trifolium pratense combining culture-dependent and independent methods. For a functional understanding of individual microbiome members and their impact on plant growth, we began to inoculate root microbiome members alone or in combination to Trifolium roots.ResultsAt a whole-root scale, Rhizobia bacteria accounted for ~70% of the root microbiome. Other enriched members included bacteria from the genera Pantoea, Sphingomonas, Novosphingobium, and Pelomonas. We built a reference stock of 200 bacteria isolates, and we found that they corresponded to ~20% of the abundant root microbiome members. We developed a microcosm system to conduct simplified microbiota inoculation experiments with plants. We observed that while an abundant root microbiome member reduced plant growth when inoculated alone, this negative effect was alleviated if this Flavobacterium was co-inoculated with other root microbiome members.ConclusionsThe Trifolium root microbiome was dominated by nutrient-providing Rhizobia bacteria and enriched for bacteria from genera that may provide disease protection. First microbiota inoculation experiments indicated that individual community members can have plant growth compromising activities without being apparently pathogenic, and a more diverse root community can alleviate plant growth compromising activities of its individual members. A trait-based characterization of the reference stock bacteria will permit future microbiota manipulation experiments to decipher overall microbiome functioning and elucidate the biological mechanisms and interactions driving the observed effects. The presented reductionist experimental approach offers countless opportunities for future systematic and functional examinations of the plant root microbiome.

Highlights

Diverse assemblages of microbes colonize plant roots and collectively function as a microbiome
Composition of the Trifolium root microbiome The 16S amplicon sequencing of 24 Trifolium root samples and 15 soil samples from climate chamber and natural site growth experiments (Fig. 1, Table 1, Additional file 1: Figure S2,) yielded 9,923,925 high-quality, nonchimeric sequences across all samples, with a median of 153,072 sequences per sample (Additional file 2)
We confirmed in the Trifolium root microbiome the typical patterns that are often observed in microbial ecology

Summary

Introduction

Diverse assemblages of microbes colonize plant roots and collectively function as a microbiome. Earlier work has characterized the root microbiomes of numerous plant species, but little information is available for legumes despite their key role in numerous ecosystems including agricultural systems. Legumes form a root nodule symbiosis with nitrogen-fixing Rhizobia bacteria and thereby account for large, natural nitrogen inputs into soils. We describe the root bacteria microbiome of the legume Trifolium pratense combining culturedependent and independent methods. For a functional understanding of individual microbiome members and their impact on plant growth, we began to inoculate root microbiome members alone or in combination to Trifolium roots. Recent studies have highlighted the root bacteria microbiome of several plant species, including Arabidopsis [3, 4] and a number. Trifolium’s association with rhizobia suggests its microbiome may differ from non-legumes in that rhizobia are expected to be highly abundant [14]

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