Conceptually similar to how bacteria can be either helpful or harmful to humans (certain strains of Escherichia coli can synthesize vitamin K2, but bubonic plague is caused by Yersinia pestis), bacteria can also be either helpful or harmful to plants.
In addition to the well-known synthesis of ammonia from nitrogen, some bacteria can help plants withstand adverse salt or drought conditions. Others increase nutrient uptake from the soil, reducing the need for synthetic fertilizer.
Such bacteria are of great interest for coaxing enhanced productivity and resilience from agricultural and biofuel crops. Relevant to such endeavors is the research of Chanyarat Paungfoo-Lonheinne (University of Queensland, Australia) and coworkers.
These scientists have shown that plants can digest non-pathogenic microbes for food. In addition to sustainable agriculture needs, this discovery suggests that plants might prey on some of the microbes drawn to nutrients excreted by plant roots, a surprising "man bites dog" story of the plant world.
The scientists studied the uptake and digestion of two non-pathogenic (safe for plants) microbes, Escherichia coli (a bacteria) and saccharomyces cerevisiae (a fungus), by two plants, arabidopsis thaliana (thale cress) and solanum lycopersicum (tomato). Their plants were grown in water (not soil), meaning that the intended bacteria and fungus were the only two microbes present in the plants' growth medium.
In some of the experiments, microbes were enhanced with nitrogen-15, to enable the scientists to determine the origin of the nitrogen in their plants (i.e. from the microbes or not). The microbes were carefully added to plant roots and grown for typically either 4 hours (thale cress) or 3 days (tomato), followed by observation via fluorescence or electron microscopy.
Microbial death in plant roots.
The scientists observed microbes within live plant roots within 4 (thale cress) or 12 (tomato) hours. This uptake was at least somewhat specific.
In other words, the plant roots aren't taking up everything they find. Silica microparticles, roughly similar in size to the microbes, were not taken up by either plant.
Most of the fungal cells taken up by tomato plants were dead within one week, and were reduced to microbial debris after two weeks. In contrast, the tomato plants were still healthy.
The microbes were not observed in the plant leaves. Thus, the microbes were confined within the plant roots and broken down.
Microbial entry into plant roots.
Given that plant cell walls are generally effective barriers to microbial entry, how do the microbes get in the plant roots in the first place? In the case of Escherichia coli, they enter the plant roots via a combination of enzymatic degradation of the cell wall, and possibly by physically trapping them near the root surface.
These entry results strongly suggest that the plants are controlling microbial entry. In contrast, pathogenic microbes attack plant roots to gain entry.
The plants seem to be in charge here. They are presumably expending their own energy to get at the microbes.
Microbes become plant food.
What advantage do the plants gain by chewing up microbes? The scientists found that the microbes become plant food.
After incubating tomato plants with bacteria enhanced with nitrogen-15 for one hour, followed by two weeks of growth, enhanced levels of nitrogen-15 were found in the leaves.
In other words, nitrogen from the bacteria found their way into plant leaves. The scientists found 35 micrograms of nitrogen-15 per gram of dry weight, and almost none in plants not incubated with bacteria.
The research described herein may point another way towards utilizing bacteria to enhance plant growth: direct consumption. Future research directions should focus on determining how often this happens under typical plant growth conditions, and how much of an energy contribution it makes to typical plant physiology.
On a practical note, it would also be extremely helpful to apply this knowledge to agriculture and biofuel production. Can microbes be designed to be "tasty" to useful plants, and/or can plants be engineered to chew up specific microbes? Positive results may be extremely beneficial for making optimal use of limited resources, and help build an environmentally-sustainable future.
for more information:
Paungfoo-Lonhienne, C., Rentsch, D., Robatzek, S., Webb, R. I., Sagulenko, E., Näsholm, T., Schmidt, S., & Lonhienne, T. G. A (2010). Turning the Table: Plants Consume Microbes as a Source of Nutrients PLoS ONE, 5 (7) DOI: 10.1371/journal.pone.0011915