Long-Term Irrigation Affects the Dynamics and Activity of the Wheat Rhizosphere Microbiome.

Dmitri V Mavrodi,Sasha Tetu,Mingming Yang,James Parejko,David M Weller,Olga V Mavrodi,Ian T Paulsen,Linda S Thomashow,Liam D H Elbourne,Robert F Bonsall

doi:10.3389/fpls.2018.00345

Abstract

The Inland Pacific Northwest (IPNW) encompasses 1. 6 million cropland hectares and is a major wheat-producing area in the western United States. The climate throughout the region is semi-arid, making the availability of water a significant challenge for IPNW agriculture. Much attention has been given to uncovering the effects of water stress on the physiology of wheat and the dynamics of its soilborne diseases. In contrast, the impact of soil moisture on the establishment and activity of microbial communities in the rhizosphere of dryland wheat remains poorly understood. We addressed this gap by conducting a three-year field study involving wheat grown in adjacent irrigated and dryland (rainfed) plots established in Lind, Washington State. We used deep amplicon sequencing of the V4 region of the 16S rRNA to characterize the responses of the wheat rhizosphere microbiome to overhead irrigation. We also characterized the population dynamics and activity of indigenous Phz+ rhizobacteria that produce the antibiotic phenazine-1-carboxylic acid (PCA) and contribute to the natural suppression of soilborne pathogens of wheat. Results of the study revealed that irrigation affected the Phz+ rhizobacteria adversely, which was evident from the significantly reduced plant colonization frequency, population size and levels of PCA in the field. The observed differences between irrigated and dryland plots were reproducible and amplified over the course of the study, thus identifying soil moisture as a critical abiotic factor that influences the dynamics, and activity of indigenous Phz+ communities. The three seasons of irrigation had a slight effect on the overall diversity within the rhizosphere microbiome but led to significant differences in the relative abundances of specific OTUs. In particular, irrigation differentially affected multiple groups of Bacteroidetes and Proteobacteria, including taxa with known plant growth-promoting activity. Analysis of environmental variables revealed that the separation between irrigated and dryland treatments was due to changes in the water potential (Ψm) and pH. In contrast, the temporal changes in the composition of the rhizosphere microbiome correlated with temperature and precipitation. In summary, our long-term study provides insights into how the availability of water in a semi-arid agroecosystem shapes the belowground wheat microbiome.

Highlights

Most wheat (Triticum aestivum L.) in the Inland Pacific Northwest (IPNW) of the U.S.A. is grown throughout the Columbia Plateau, an area encompassing more than 62,000 square kilometers that comprises the largest contiguous cropping system in the western United States
A field survey we conducted in 2008–2009 revealed the presence of large indigenous populations of beneficial phenazineproducing (Phz+) Pseudomonas spp. in the rhizosphere of wheat grown across 22,000 km2 of arid parts of central Washington and northeastern Oregon (Mavrodi et al, 2012a; Parejko et al, 2013)
We tested our hypothesis by examining the effect of irrigation on the seasonal dynamics of Phz+ pseudomonads and levels of phenazine-1-carboxylic acid in the rhizosphere of field-grown spring wheat

Summary

Introduction

Most wheat (Triticum aestivum L.) in the Inland Pacific Northwest (IPNW) of the U.S.A. is grown throughout the Columbia Plateau, an area encompassing more than 62,000 square kilometers that comprises the largest contiguous cropping system in the western United States. Indigenous populations of PCA-producing (Phz+) rhizobacteria from Columbia Plateau soils are diverse and are part of the Pseudomonas fluorescens species complex They include at least 31 genotypes that are closely related to Pseudomonas synxantha, P. orientalis, and the provisional species P. aridus, and P. cerealis (Parejko et al, 2013). Our surveys of commercial wheat fields from throughout the Columbia Plateau revealed that the frequency of root systems colonized by Phz+ bacteria is inversely correlated with annual precipitation and irrigation (Mavrodi et al, 2012a,b) This observation prompted us to hypothesize that soil moisture strongly affects the rhizosphere microbiome of dryland wheat and plays a key role in the establishment and proliferation of indigenous phenazine-producing pseudomonads

Methods

Results

Conclusion