These bacteria perform a trick that could keep plants healthy
To stay healthy, plants balance the energy they put into growing with the amount they use to defend against harmful bacteria. The mechanisms behind this equilibrium have largely remained mysterious.
Now, engineers at Princeton have found an answer in an unexpected place: the harmless, or sometimes beneficial, bacteria that cluster around plants’ roots.
In an article published Dec. 24 in the journal Cell Reports, researchers showed that some types of soil bacteria can influence a plant’s balance of growth and defense. The bacteria produce an enzyme that can lower a plant’s immune activity and allow its roots to grow longer than they would otherwise.
“This is trying to get at a really big biological question where there are not good answers — about how microbiomes interface with host immune systems,” said senior study author Jonathan Conway, an assistant professor of chemical and biological engineering. “It’s a small step in the direction of trying to understand how microbes live on hosts — either plants or humans or other animals — all the time and don’t activate our immune responses constantly.”
To search for immune-balancing bacteria, Conway’s team turned to plants that were engineered to have heightened immune responses to a protein that makes up the threadlike appendages, called flagella, that allow bacteria to swim. The protein that makes up flagella, called flagellin, is a potent trigger of immune responses in hosts from plants to humans.
The researchers grew seedlings of Arabidopsis — a small plant in the mustard family that’s commonly used in plant research — from a line that was engineered to produce high levels of flagellin-sensing immune receptor in its roots. When grown on plates containing the piece of flagellin that activates this receptor, the seedlings’ roots are short and stubby, since their energy is directed toward immunity more than growth.
The experiment involved growing the seedlings on plates with flagellin as well as with 165 different bacterial species isolated from the roots of soil-grown Arabidopsis. 68 of these isolates, or 41%, suppressed the stunted growth response by tamping down the plants’ immunity and allowing their roots to grow longer.
One of the bacterial species that allowed the roots to grow the best was Dyella japonica. Previous work had shown that that this species’ immune-modulating activity was dependent on a bacterial secretion system — a protein complex that can move substances out of bacterial cells and into the environment, including inside plant cells or the spaces between plant cells.
A scan of D. japonica’s genome revealed a gene encoding a secreted enzyme called a subtilase, with the potential ability to chop flagellin into small pieces and prevent it from activating the immune response.
The team used both genetic and biochemical methods to demonstrate that the subtilase enzyme was indeed capable of degrading the specific segment of flagellin that triggers the immune response. The degradation was sufficient to tamp down the immune response and allow for increased growth in Arabidopsis seedlings.
The researchers ran into some snags when trying to purify the subtilase enzyme, said Samuel Eastman, a co-first author of the paper and a postdoctoral research associate in Conway’s lab. Obtaining pure protein is essential for definitively demonstrating an enzyme’s function in a test tube.
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