Plants eat Bacteria!

When Justus von Liebig discovered the elemental composition of plants and promoted the agricultural use of synthetic chemicals with “Chemistry in its application to agriculture and physiology” in 1840, he did not foresee the enormous impact this would have on agriculture. In the following years this discovery set the stage for the widespread production and mining of nitrogen and phosphorus based fertilizers. Buying a bag of superphosphate rather than maintaining livestock and carting manure freed time and enabled farmers to work on larger acreages.

Today, agricultural chemistry forms an enormous branch of chemistry. It further allowed the development of genetically modified plants for herbicide resistance, significantly modified large parts of rural landscapes and lead to agriculture being a major polluter of waterways and the second largest emitter of greenhouse gases through nitrous oxide emissions from nitrogen fertilizers (There are of course many reasons for agricultural greenhouse gas emissions, among them plowing and grain-fed lifestock, but also substitution of carbon based fertilizers.).

The experiments of Liebig and his conclusions were based on the incineration of plant material and analysing that the ashes mainly contained Nitrogen (N), Phosphorus (P) and Potassium (K). He concluded that these were the necessary elements to grow and feed a plant. Parts of his conclusions then lead to great changes in agricultural practice. Mined inorganic fertilizers (mainly lime and guano) have been used a long time before Liebig’s discovery, after his work the focus shifted from maintaining soil fertility to supplying plant food.

The application of chemical fertilizers brought great increases in agricultural productivity because plants were supplied with readily available nutrients, enabling them to grow faster. However, it also caused large destruction to formerly biologically diverse landscapes and often turned them into so called “agricultural deserts” (Among them: over-fertilization, erosion, heavy metal and radioactive element accumulation.)

Nowadays, we are still applying the old principles of Liebig, although alternatives exist, which do not impoverish the soil and pollute waterways and climate. Why is it that the world’s heaviest vegetables are grown with compost tea and not in mineral solution (John Evans, Compost Tea Advocate with worlds largest Broccoli)?

The question arises: is there more to growing food than just applying chemical fertilizer? In other terms: Do plants “prefer to eat” more than the mentioned water soluble nutrients?

I decided to interview Dr. Chanyarat Paungfoo-Lonhienne after reading her paper „Turning the table: Plants Consume Microbes as a Source of Nutrients”where it is outlined that certain plants took up bacteria through their roots and digested them in order to grow. Is this an opportunity for agriculture to provide a more wholesome “diet” for the plant and therefore grow healthier plants that need less fertilizer and pesticides?

Dr. Paungfoo-Lonhienne and her team at the University of Queensland in Brisbane research about the processes in the vicinity of roots of a variety of plants, where plants can attract a range of bacteria to their roots with sugars produced from photosynthesis. So far it was not known why. Dr. Paungfoo-Lonhienne discovered that these plants actively “ate” the attracted microbes and used them as a source of nutrients, a process that has not been observed before.

– a plant stem with marked bacterial Nitrogen-

Tell me about the research. What did the plants do?

Our research suggests that there are successive specific events in the process for microbe consumption by plants. Step 1: microbes bind to roots through interaction with molecules present at the surface of the roots. Step 2: Roots generate an extracellular cell-wall like structure to capture microbes at the root surface. Step 3: plant cells produce cell wall degrading and loosening enzymes to facilitate incorporation of microbes into root cells. Step 4: microbes are incorporated into root cortex cells by an unknown process. Step 5: microbes are digested within cortex cells. Step 6: metabolites resulting from microbe digestion are used as nutrients by the plant.

Is your group the first to discover the cell wall modifications in order to take up microbes?

Yes, we show that uptake of microbes by roots occurs through major structural modifications of root cells that are controlled by the plant. In contrast, entry of pathogenic and symbiotic microbes into root cells is controlled predominantly or partly by microbes.

Is that unusual behaviour for plants?

Considering that plants have sophisticated ways to acquire nutrients such as formation of nodules to house nitrogen fixing bacteria or digestion of insects that are trapped by carnivorous plant, I don’t think this is an unusual behaviour, especially in the view that plants release up to 30% of their photosynthetic output to attract and maintain microbial soil colonies.

Do you think textbooks, which state that plants are autotrophic (synthesize organic compounds to feed themselves) need to be rewritten?

I do encourage that. There is rising evidence from other research groups that plants can use organic [in addition to anorganic] molecules as nutrient sources.

You mentioned in the article that photosynthetic plankton is known to be mixotrophic, is the process the same as for higher plants?

Mixotrophy is regarded as an evolutionarily derived character, facilitating the combined modes of heterotrophic (consumer of autotrophs) and autotrophic (primary producer) nutrition. However, similar to other mixotrophs, plants can complement photosynthetic energy gain with essential elements derived from organic polymers and microbes. We propose that higher plants should be considered mixotrophs that consist of autotrophic shoots and heterotrophic roots.

Which plants show this behaviour and which did you use for your experiment?

Mixotrophy is considered an exception in higher plants and restricted to carnivorous species, Orchids and Ericaceae. We used plants such as Arabidopsis (Brassica family), tomato, and sugarcane etc. that do not belong to aforementioned groups.

So you used agriculturally significant plants for your research. Is it possible that all higher plants show mixotrophy?

It is indeed a real possibility.

What is necessary to continue your research?

It’s crucial to understand how organic molecules including microbes are acquired and assimilated by roots and how these processes are linked to root function and morphology, and root/shoot biomass repartition.

So in summary: Agriculturally significant plants use microbes actively to complement their diet. This has not been observed before and is contrary to popular opinion of plants as primary producers.

The groundbreaking paper by Paungfoo-Lonhienne and her team was published in 2010. She mentioned that it is very difficult to get funding from the Australian government for continuing her research.

The investigated process indicated the potential relevance of organic plants nutrients and could lead to a paradigm shift in agriculture. This might mean cultivating bacteria in living soil instead of cultivating concrete-like wasteland with high energy inputs. This paradigm shift could help to feed the planet in a natural and more sustainable way, especially in a low-energy future because it seems to be already feeding plants in a natural way. It will help to bring biodiversity into farming and nature back into the soil and landscapes.