The plant's microbiome: fair-weather friend?

Sanna Sevanto, a physicist-turned-plant physiologist, was September’s Science on Tap speaker who shared her latest findings and research into the use of microorganisms to guide plant evolution.

Q: So, what was the most surprising thing you discovered about microbiome influence on plant growth?

A: The short answer is that some of the simplest, most seemingly logical methods used to promote drought tolerance in plants may not really be all that helpful.

One of the most common ways folks try to improve plant drought tolerance is to try to find microbes — bacteria and fungi — that stimulate root growth or plant growth in general. On the surface, this seems an entirely reasonable plan as stronger, larger root systems and plants should be better able to withstand greater challenges, right?

Interestingly, while researchers at Los Alamos National Laboratory confirmed that stimulating root growth does indeed increase moderate drought resistance for plants grown in a lab, they also discovered that this method of stimulating root growth also limits carbon allocation to above ground biomass if photosynthetic capacity is not stimulated as well.

What this means is that these microbially influenced plants, with their larger roots that may help them access deeper soil layers with more moisture, may actually suffer more than non-microbially influenced plants if a drought is severe enough that there is no water even in the deeper layers of soil. Essentially, a plant’s ability to find more moisture results in its need to have more moisture, and they wilt faster than their counterparts in the face of extreme drought.

So, then one might ask why plants without larger root systems actually seem to fare better during periods of intense drought. Surprisingly, what Lab researchers have found is that plants with more extensive and healthier root microbiomes actually tend to have smaller root systems because the microbiome collects the resources for the plant. This means the plant doesn’t need to spend as much energy growing a big a root system and can, instead, allocate more of that energy to aboveground growth (as long as feeding the microbiome requires less carbon that growing roots).

So, our most surprising — but completely understandable — discovery is that if we try to improve plant drought tolerance by encouraging bigger root systems and better soil water retention capacity, we might succeed in improving plant tolerance of moderate drought, but we actually reduce odds of that plant surviving severe drought.

For more information on Sanna’s research, please visit the Atmosphere, Climate and Ecosystem Science (ACES) webpage.