Tiny Plant Made Smaller By Microbes

Research reveals that the whole microbiome community, including fungi and protists along with bacteria, reduces the growth rate and modifies the morphology of the aquatic plant duckweed (Lemna minor). The research by Mark Davidson Jewell and colleagues, published in AoB PLANTS, contrasts with previous findings that isolated bacteria can enhance duckweed growth. The results show the importance of considering the whole microbial community, or microbiome, when studying plants.

Gardeners with ponds may know duckweed as a problem. It’s a small aquatic plant with one to four leaves, each less than a centimetre long (a third of an inch) and a width half that. Attached is a root, maybe up to 2 centimetres long. It produces tiny flowers, barely a millimetre wide, but the main way it multiplies is through division. When it gets so big, it splits, effectively making a clone of itself.

This vegetative reproduction can rapidly make a small colony of duckweed plants a big problem. It could also make the plant extremely useful for many tasks, such as clearing up toxic chemicals or producing biofuels. This is one reason botanists are keen on discovering what boosts or hinders duckweed growth.

Previous studies have found there are beneficial bacteria that can boost duckweed growth, but there’s a lot more to the microbiome than just bacteria. What happens when you examine the whole microbiome? Jewell and colleagues took duckweed samples from eight spots around Montreal, as well as samples of the microbiome. They then removed the microbes from the duckweed. After a thorough process to remove the microbes, a single frond of duckweed was chosen from each site to grow into a new colony and make genetically identical batches of duckweed to test. Once they had the microbe-free duckweed batches, they could divide the samples. Some continued in sterile conditions, while others were reintroduced their microbiomes.

“Contrary to our expectation, the presence of the microbiome consistently decreased plant fitness, on average by 12%,” write Jewell and colleagues in their article. “Although several important plant-bacteria and plant-fungi mutualisms have been identified for L. minor, our results highlight the importance of pathogens, parasites, competitors and herbivores in the microbial assemblage.”

The botanists were puzzled by what would give such different results, but they think they have identified a key difference, the parts of the microbiome that are not bacteria. “Given the discrepancy between our results and most other similar studies isolating the bacterial microbiome, these negative interactions are likely driven by the eukaryotic component of L. minor’s microbiome. Our results are also in line with literature documenting the importance of fungal and bacterial pathogens (Rejmankova et al. 1986; Underwood and Baker 1991; Zhang et al. 2010; Ishizawa et al. 2017a, 2017b) and algal competition (van Moorsel 2022) on L. minor growth.”

The team found that the microbiome also affected the phenotype, the traits that the duckweed has. Duckweed exposed to microbes had smaller fronds and shorter roots. They write: “One possible explanation for this decrease in frond size is the presence of photosynthetic algae that decrease nutrient availability through direct competition with L. minor. However, if this were the main mechanism through which the microbiome modified L. minor phenotype, then it would result in increased root length, a ubiquitous plastic response to decreased nutrient availability. However, we found the opposite, i.e. shorter roots when the microbiome was present, as well as an increase in the shoot-to-root ratio measured as frond area:root length.”

The results demonstrate the importance of looking at the whole microbiome in the natural environment. The authors conclude: “Although the L. minor microbiome has been shown to include important beneficial associations with many bacteria, their influence seems to be shadowed by pathogenic, parasitic and competitive interactions in our system, which includes eukaryotic elements of the microbiome.”

READ THE ARTICLE
Jewell, M.D., van Moorsel, S.J. and Bell, G. (2023) “Presence of microbiome decreases fitness and modifies phenotype in the aquatic plant Lemna minor,” AoB PLANTS, 15(4), p. lad026. Available at: https://doi.org/10.1093/aobpla/plad026.

Cover image: Lemna minor. Image: Canva.