Sending Out an SOS! Plant-Plant interactions under drought stress

Plants communicate with each other in some surprising and subtle ways. They have an intricate web of "language", that scientists are still working to understand. For instance, when plants are under attack by herbivores, they release chemicals into the air known as, VOC's or volatile organic compounds. VOCs warn other plants; neighboring plant species who receive these signaling chemicals can then alter the chemicals they release to repel pests and even attract the pest predator! Plants can also alert each other to threatening pathogens, recognize closely related species, and even detect impending drought. This post will focus on plant-plant communication in drought and the implications these studies will have in a world facing increasing climate change.

We know that plants communicate with their root systems and even the mycorrhizae that inhabit root systems. These fungal networks can act as a sort of telephone line between plants. Studies suggest the roots can communicate to the plant under drought conditions and stimulate the closing of the stomata which allows for less water evaporation. Plants in turn release certain VOCs that communicate to neighboring plants that drought is impending. Plants also have bacteria that colonize the root systems and these bacteria can act as a vaccine for the plant to help prevent disease. There are still studies being conducted to learn just how big of a role these bacteria can play in plant-plant communication. One study conducted, highlighted the possibility that plants can even recruit new bacterial species to colonize the roots to suit their needs when under periods of food and drought stress. This particular study was conducted on dissected root systems of drought- sensitive pepper plants (Capsicum annuum) The plants were grown with different amounts of water. When a comparison was done on the structure and diversity of the bacterial communities in the rhizosphere, it was found that plants grown in the desert with very little water had larger populations of a plant-growth promoting (PGB) bacteria. These bacteria enhance photosynthesis and biomass synthesis by as much as 40% under drought stress! The mechanism for these PGBs has not yet been decided but the bacteria are known to relieve salt stress by reducing the production of ethylene in plants. Ethylene is produced in response to stresses.  So can these bacteria communicate plant to plant? As was mentioned before, these bacteria act as a vaccine for the plant. The heightened disease response can be passed on to the next generation of plants. In that particular study, the bacterial effects appear to last only for offspring plant’s life, but are not passed on to a third generation.

As we have seen, plants utilize a wide array of methods to communicate. But under drought conditions it seems the root systems are the main component in plant-plant communication. We said earlier, that the roots relay the drought stress to the plant which stimulates closing of the plant stomata and the plants release volatile organic compounds to "warn" neighboring plants of the drought stress, but do the root systems communicate with each other?

A study was done on 11 pea plants (Pisum sativum) to examine how root systems "talk" to each other in response to drought stress. The root systems of plants 6-11 were connected to their neighbor via tube systems. This allowed chemicals to move from one root system to the next without moving through soil. Plants 1-6 were not connected. They inflicted osmotic shock on plant 6 by applying mannitol, a natural sugar that is used to mimic drought stress in vascular plants. A control experiment was also done in which water was applied to plant 6 instead of mannitol. After a time period of 15 minutes, the width of the stomatal openings were measured. The stressed plant closed it's stomata as did it's nearest unstressed neighbor. This suggests that a warning signal had passed between the roots. After an hour all five other plants, even the most distant, closed their stomata. This indicates they too received the message, as if it ran along a phone line. In the set-up where the root contact was blocked, the stomata stayed open suggesting the messages were sent via the root systems! While scientists are still trying to learn exactly what chemical releases cause this reaction, the most likely culprit is abscisic acid which is often released in drought and osmotic stress.

In the last post, we talked about fungal networks and how research on them would be important for the agricultural world in reference to plant health and feeding an ever-growing population of humans. Along with climate change and increasing temperature more research is necessary on plant-plant root communications and their responses to drought conditions. If different gene expressions, chemical releases and even bacterial colonization can be selected for food plants it could be revolutionary!

References

 R. Marasco et al., “A drought resistance-promoting microbiome is selected by root system under desert farming,” PLOS ONE, 7(10): e48479, 2012.

T. Rudrappa et al., “Root-secreted malic acid recruits beneficial soil bacteria,” Plant Physiol, 148:1547-56, 2008.

V. Lakshmannan et al., “Microbe-associated molecular patterns (MAMPs)-triggered root responses mediate beneficial rhizobacterial recruitment in Arabidopsis,” Plant Physiol, 160:1642-61, 2012.

Moran, J.F., Becana, M., Iturbe-Ormaetxe, I. et al. Planta (1994) 194: 346. doi:10.1007/BF00197534

M. Gagliano et al., “Out of sight but not out of mind: alternative means of communication in plants,” PLOS ONE, 7:e37382, 2012.

Y.Y. Song et al., “Interplant communication of tomato plants through underground common mycelial networks,” PLOS ONE, 5:e13324, 2010.