Abstract
Plant growth and productivity depend on the interactions of the plant with the associated rhizosphere microbes. Rhizosphere protists play a significant role in this respect: considerable efforts have been made in the past to reveal the impact of protist-bacteria interactions on the remobilization of essential nutrients for plant uptake, or the grazing induced changes on plant-growth promoting bacteria and the root-architecture. However, the metabolic responses of plants to the presence of protists or to protist-bacteria interactions in the rhizosphere have not yet been analyzed. Here we studied in controlled laboratory experiments the impact of bacterivorous protists in the rhizosphere on maize plant growth parameters and the bacterial community composition. Beyond that we investigated the induction of plant biochemical responses by separately analyzing above- and below-ground metabolite profiles of maize plants incubated either with a soil bacterial inoculum or with a mixture of soil bacteria and bacterivorous protists. Significantly distinct leaf and root metabolite profiles were obtained from plants which grew in the presence of protists. These profiles showed decreased levels of a considerable number of metabolites typical for the plant stress reaction, such as polyols, a number of carbohydrates and metabolites connected to phenolic metabolism. We assume that this decrease in plant stress is connected to the grazing induced shifts in rhizosphere bacterial communities as shown by distinct T-RFLP community profiles. Protist grazing had a clear effect on the overall bacterial community composition, richness and evenness in our microcosms. Given the competition of plant resource allocation to either defense or growth, we propose that a reduction in plant stress levels caused directly or indirectly by protists may be an additional reason for corresponding positive effects on plant growth.
Highlights
The rhizosphere is a hotspot of microbial interactions (Bakker et al, 2013)
terminal restriction fragment length polymorphisms (T-RFLP) profiles of bacterial communities not exposed to protists were well separated in nonmetric multidimensional scaling (nMDS) plots from profiles of communities interacting with protists irrespective of the used restriction enzyme (Figure 2)
PERMANOVA analysis revealed that bacterial community composition was mainly explained by the presence of protists (Table 1); protists had a significant positive influence on diversity (Shannon Index), richness and evenness for the two enzymes AluI and HhaI
Summary
The rhizosphere is a hotspot of microbial interactions (Bakker et al, 2013). It is densely populated with members from all domains of life and characterized by myriads of interactions (Bonkowski et al, 2009; Raaijmakers et al, 2009; Jacoby et al, 2017). Plant roots are key drivers of this habitat by releasing low and high-molecular weight carbon compounds into the soil in order to lubricate their root tips or by losing exudates through leaky root tips (Farrar et al, 2003; Hartmann et al, 2009). This plant-derived carbon lifts the C-limitation in soil leading to rapid bacterial growth, higher activity and microbial community shifts (Paterson, 2003; Jones et al, 2009; Steinauer et al, 2016), Protists Change Maize Metabolome which in turn mobilizes nutrients from soil organic matter, in particular nitrogen. Enhanced root branching in turn fosters growth and activity of soil bacteria by the increased release of carbon rich photosynthates
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