How Do Plants Control Bacteria Inside Root Cells?

Jon Penterman, Ryan P. Abo, Nicole J. De Nisco, Markus F. F. Arnold, Renato Longhi, Matteo Zanda, and Graham C. Walker (2014) doi:10.1073/pnas.1400450111

Host plant peptides elicit a transcriptional response to control the Sinorhizobium meliloti cell cycle during symbiosis

In order for plants to thrive in their environment, many have evolved a win-win relationship with beneficial soil bacteria. Legumes, such as beans, soybeans, and peanuts, attract bacteria in the soil to ‘invade’ their roots. Once inside the root cells (nodules), the plant provides the bacteria nutrients in exchange for the microbe’s special skill; turning nitrogen gas into a useful form for the plant to use in various metabolic pathways. The ability of the bacterial cell to inhabit the interior of a plant cell without being degraded or dividing requires its conversion into a ‘bacteroid’. A burning question about this relationship is how the plant and bacterium form this beneficial coexistence. Researchers previously discovered that the plant cell releases a group of small amino acid peptides (NCR peptides) that were found to act like antibiotics at high concentrations but the affects at lower concentrations caused the bacteria to stop growing but continue to live. In this paper, researchers used a common bacterial species found in nodules, Sinorhizobium meliloti, that had synchronized cell cycles to identify the cellular changes in the presence of sublethal amounts of NCR peptide. They discovered that exposure to small amounts of NCR peptide caused the bacteria to stop dividing and significantly changed the genes expressed within the microbe. Interestingly, these small peptides also caused a stress response in the bacteria similar to responses by pathogens that researchers think is a way for the plant cell to signal the good bacteria of a pathogen invasion. Ultimately, these small peptides regulate which genes are active in the bacteria in order to establish the beneficial relationship.

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