Underground Microbes Help Plants Survive, Reduce Stress Even in Poor Lighting Conditions

Everyone knows that terrestrial plants are firmly anchored in the earth through their roots, creating their food from the sunlight above and the nutrition below. However, a new study shows how one aspect of their nutrition steps up when the other starts to fail.

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How Plants Sustain Themselves in Varying Conditions

Photosynthesis, as a National Geographic feature explains, allows plants to utilize the energy from the sunlight and create chemical energy in chloroplasts, or the powerhouse of the plant cell. This makes the amount of and quality of light received by the plant, through its light-absorbing pigments like chlorophyll, factors in plant health.

On the other hand, the roots of the plant absorb moisture and nutrients from the soil. Additionally, the roots also host communities of bacteria and filamentous eukaryotes like fungi and other oomycetes. Underground, these are the factors that support plant growth and sustenance.

However, the relationship between the two aspects or how terrestrial plants utilize their underground resources to make up for aboveground stress remains largely unexplored.

With a new study from the Department of Plant-Microbe Interactions at the Max Planck Institute for Plant Breeding Research (MPIPZ), researchers hope to provide insight on how plants respond to stress due to subpar light quality and quantity through a process known as bidirectional root-shoot signaling. They present their findings in the article "A microbiota-root-shoot circuit favours Arabidopsis growth over defence under suboptimal light," published in the online portal for the Nature Plants journal.

Observing Thale Cress, from Above and Underground

Researchers compared growth conditions for Arabidopsis thaliana, better known as thale cress. An article in the Encyclopedia of Genetics identifies the thale cress as a "powerful model" used by scientists in observing plant behavior, more specifically its growth and development processes.

One set was grown in the absence of underground microbes, while another was grown in soil that had 183 types of bacteria, 24 fungi, and 7 oomycetes. Researchers discovered that the microbe communities helped provide for the plants placed in low-light conditions. Then, researchers tested the plants' ability to resist pathogens by inoculating them, revealing that the terrestrial plants that harbored various underground microbes had better resistance to aboveground leaf pathogens.

It was very likely that the presence of microbes in the roots of terrestrial plants promotes growth and resistance to invading pathogens.

Then, researchers varied the light conditions. They observed that the thale cress grew under sub-optima lighting conditions at the cost of defense, leaving the plants more susceptible to the leaf pathogens upon inoculation. This suggests that under sub-optimal lighting conditions, the underground microbes tend to focus more toward growth over the defense.

To further test this hypothesis, researchers screened different mutants of the terrestrial plant to identify which of these variants would fail to grow under low lighting conditions. Consistent with the hypothesis, mutants that failed were actually better at resisting pathogens.

Another factor the researchers inquired into was whether the composition of the underground microbes also affected the tendencies of the thale cress to focus more on microbe-drive growth instead of defense. They found 67 strains of bacteria that were likely to affect plant growth rescue under sub-optimal lighting. After additional testing, researchers concluded that A. thaliana wild-type plants were colonized with the 67 strains invested in growth while those that did not invest in defense instead.