Fungus among us
What is this?
Network-forming fungi spread stress among connected trees. How? By stealing sugars and making them more vulnerable to drought.
Why does this matter?
- Because we suspect that plants use most stored sugars to remain hydrated during droughts. But we have no proof!
- Because we don't know how fungal networks may influence the capacity of forests to thrive under future droughts.
- Because finding that stressed plants also stress their neighbors through their shared fungi would be super cool!
A simple video summarizing this project
How did you study this?
We planted pairs of pine seedlings in pots and inoculated the soil with fungi to create a network. We put one of the seedlings in the dark to stop sugar production through photosynthesis. This forced the poor seedling to use its stored sugars. Then, we tracked the movement of sugars through the plants and their fungal network using unique molecules of carbon dioxide called isotopes. This allowed us to see whether carbon-based sugars would stay in the plant, travel to the fungi near them, reach the fungi of their neighbors, or even enter into the neighboring plants.
We also measured if plants consumed their stored sugars in response to either being in the dark, or because they were connected through fungi to plants in the dark. Lastly, we studied whether consuming stored sugars impacted several physiological processes that are important under drought.
We measured processes like the capacity of plants to move and supply water to organs that need it. Also, the capacity of plants to retain water in their bodies and not lose it towards the dry air, how turgid or "perky" their bodies stayed, and much more. Importantly, we did all of this while keeping the plants well-watered. Why would we keep them watered if we were interested in drought, you say? Because it is impossible to find how changes in stored sugars affect water-related physiological processes. This is because drought -which affects water and sugars on its own- is messing around with our variables! By keeping drought out of the equation, we could be sure that any changes in the supply or retention of water, or the perkiness of the plants were due to changes in stored sugars.
So what did you find?
We found many things! We found that plants in the dark depleted their storage of sugars. This was no surprise as we blocked their light so that they could not make sugars through photosynthesis. What did surprise us was to find that their neighboring plants had also depleted their storage of sugars. These plants were well lit, so how was this possible? The culprit was the fungi! This only happened in pots where one tree was shaded. This means that, if stressed trees within the network -the ones in the dark- run out of sugars to give, fungi will take them from healthy neighboring trees!
We saw that plants were perfectly capable of supplying water as needed despite having less stored sugars. However, we found that depleting stored sugars made trees less capable of keeping that water and staying hydrated. In our experiment, stored sugars were depleted in two ways. Some plants because they remained in the dark. Others, their neighbors, because the fungal network took their sugars to cover for what the darkened trees were not providing. Regardless of the cause, plants that depleted their stored sugars had a lower capacity to retain water. Turns out that plants use most of their stored sugars to retain water as if they were millions of tiny sponges. They do this through a process called osmoregulation.
As such, plants that consume their sugar reserves can’t retain as much water and dry faster than plants allowed to keep their sugars and use them like sponges. We actually saw this happening! Despite being well-watered, plants that depleted their stored sugars had drier stems and their leaves wilted as if they were experiencing drought! This was really surprising because our pines are not supposed to wilt unless it experiences more drought. Yet, our pines were wilting without even experiencing drought. This suggested that we had somehow modified the plants' tolerance to drought. However, the point at which a tree wilts, a trait related to drought tolerance, is not known to be this variable nor tightly linked to stored sugars. Just in case, we decided to check if plants that depleted their stored sugars were losing turgidity -what we commonly call wilting- with less drought. We also checked if we had indeed modified the wilting point, the point at which turgor reaches 0.
YES! We modified the wilting point (horizontal grey line), and A LOT! Now everything made sense. Trees store sugars to among other things, use them as tiny sponges to hold onto water and remain hydrated. The more sugars they have stored, the better they are at staying hydrated. In turn, this allows them to withstand more intense droughts without wilting. These findings have big implications! Plants use stored sugars for many processes other than retaining water. Most of these processes require consuming these sugars to obtain energy, but consuming them means destroying the invaluable sponges that retain water under drought. Hence, trees may face a hard choice under drought, hold onto the water and be hungry or eat and dry out. Each one has its risks.
And the big point is...?
Sometimes trees might not even be able to choose between keeping their sugar pantry full or depleting it. In our experiment, the fungal networks took the sugars they needed from the trees and made them less tolerant to drought. This brings up some critical questions that need answers.
Will forests be more vulnerable to droughts due to the costs of maintaining fungal networks? These networks are virtually everywhere in temperate forests! On the other hand, it is possible that our fungal networks temporarily forced healthy neighboring trees providing the sugars that stressed darkened trees couldn't afford to give. This may have relieved darkened trees from a lethal burden. In nature, fungal networks connect numerous trees. Could fungi help forests by temporarily reducing and spreading the costs for struggling trees among their many neighbors?
As it often happens in science, we answered a big unknown, but many more have appeared as a consequence. The never-ending quest to unravel the secrets of nature continues. Or, how I like to put it, there is always mushroom for more.
The actual paper for the nerds: