Plant root consumption of soil bacteria

Research in a recent article in New Phytologist, "How significant to plant N nutrition is the direct consumption of soil microbes by roots?," attempted to demonstrate the uptake of soil microbes by wheat roots by using nitrogen and carbon isotopic labeling. Conventional wisdom was that plants only take up the chemical ion forms of nitrogen found in soil, ammonium and nitrate. Research over the past decade first showed the the direct uptake of amino acids is widespread in plants, and becomes very important in ecosystems where chemical forms of nitrogen are relatively rare compared to organic, such as in the arctic tundra. More recent work has suggested that Arabidopsis and hakeas may receive nitrogen from intact bacteria, suggesting bacteria are taken up by roots.

In this article, Hill, Marsden, and Jones used a system balance approach to gain an understanding of how microbial nitrogen (N) and carbon (C) behave in soil with active wheat roots. They added microbes enriched with isotopes of nitrogen or carbon in separate experiments. The N and C isotopes were measured by mass spectrometry and scintillation counting, respectively. Basically, the presence of these isotopes within the roots showed that their source was the bacteria. The bacteria could have ruptured in the growing media, releasing amino acids that the roots could take up, or they could have been mineralized by other bacteria and so release chemical forms of nitrogen that were taken up. The authors claim the ratios of the N and C isotopes in the roots suggest complete bacteria entered the roots.

This figure summarizes an experiment consistent with wheat roots taking up bacteria. (created by RBN)

These results are consistent with plant roots taking up whole bacteria, but are inconclusive. Perhaps microscopy or genetic techniques could validate the presence of bacteria inside the plant roots. Still, this may not tell whether the roots actively took up the bacteria, or if the bacteria actively entered the roots, either as pathogens or endophytes. Last, whether the nitrogen entering the plant is used will require confirming its presence in plant metabolic processes.

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