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Suppressive Soils: New Paradigms for an Old Story?

Linda Thomashow: USDA-ARS

<div>Long known as biocontrol agents, phenazine-1-carboxylic acid (PCA)-producing pseudomonads are widely distributed across dryland cereal cropping systems of the Pacific Northwest, USA. We investigated the effect of soil moisture on the population levels, diversity, and composition of rhizosphere microbial communities, and the mechanisms driving accumulation of PCA producers in soils. A field study conducted for 3 consecutive years in adjacent irrigated and dryland plots at a long-term wheat monoculture site revealed the magnitude of changes in the wheat rhizosphere microbiome in response to soil moisture and temperature, and shed light on the impact of environmental factors on indigenous bacterial communities and phenazine production <em>in situ</em>. Related studies in mesocosms revealed that PCA biosynthesis genes were expressed under both dryland and irrigated conditions, that PCA persisted in soils with a half-life of 3 to 4 days, and that PCA-producing bacteria on dryland roots were associated with more robust biofilms than were colonies of an isogenic PCA nonproducer. PCA also influenced the turnover of biomass from <sup>15</sup>N-labeled bacteria, including the transfer of <sup>15</sup>N from bacteria to roots. Collectively, our results indicate that PCA production is an adaptation to water stress that promotes the formation and development of root-associated biofilms, potentially influencing the availability of bacterial N to crops and the formation of soil organic matter.</div>

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