Soil Not Your Average Dirt!

Plant-Soil Interactions In Drought

When I think of soil, I think of it as the living entity that keeps all other beings alive; without healthy soil, there is no healthy plants and without plants there are no animals. To say that plant-soil interactions are important would be an understatement! Soil gives water and nutrients to plants, is the substrate to which plant roots anchor themselves and is host to more microbial viruses, bacteria, actinomycetes, fungi, and protozoa than can measured. Soil can be defined as a natural body solids (composed of minerals and organic matter), liquids, and gases that occur on the surface of the land, it occupies space, and is characterized by horizons or layers that can distinguishable from the initial material as a result of additions, losses, transfers and transformation of energy and matter or the ability to support rooted plants in a natural environment. Soil is the very heart of the earth, teeming with life giving abilities. In this post we will discuss how drought effects soil or how soil effects drought!

Drought is a natural phenomenon that we are facing more frequently with the increasing atmospheric temperature. While we cannot eradicate drought, we can mitigate the devastating effects it brings. Soil gives life to plants but in turn vegetation protects the soil and keeps it from crusting and helps the soil to maintain permeability so that rainfall can be absorbed. Drought severity is often exacerbated by the vulnerability of an area or a population. The demand for water, the age, and health of the population affected by the drought, and the efficiency of water supply and energy supply systems are all things that effect the severity of drought conditions. The effects of drought are more commonly pronounced in areas that have lost wetlands, (wetlands continually recharge aquifers), areas that are dependent on agriculture, have low existing food stocks or areas with undeveloped drought-response management.  

So now that we know a little about drought and a little about soil; let's talk about how they interact in grassland ecosystems. Grasslands possess about 12% of global SOM (soil organic matter). (3)Increasing duration and intensity of droughts have had dramatic effects on grassland ecosystems; hypoxia, water stress, and mechanical impedance to root growth all change when the water content of the soil is altered. Multi-factor experiments have contributed to quantifying these potential changes and have provided important information on how water affects ecosystem processes under global change. Grasslands, while they hold an irreplaceable role in their contribution to climate change mitigation and adaptation, land and ecosystem health, resilience, biological diversity regimes, global productivity, and water cycles; they are also some of the most sensitive to drought.  Declines in grassland productivity had been driven by increased dryness over the last four decades.(4) For grasslands, their ecosystem has the ability to buffer impacts from drought and large amounts of soil C (carbon) in its surface. Short periods of drought adversely effect root biomass, litter decomposition rates, and short-term CO2 fluxes, but increases soil nutrient retention, soil fertility, and longer term C fixation rates. Droughts may have different impacts on aboveground and belowground productivity or communities. Grassland ecosystems are able to withstand moderate drought and still manage to maintain ecosystem functions. However, severe, extended droughts may induce catastrophic effects that could resemble the 1930s Dust Bowl in North America. Droughts can significantly alter the elasticity to new disturbances, such as insects, disease, or the next drought. (2)Recognition and prediction of the drought stress is significant to help managers prevent the emergence of undesirable states and promote the management of grassland C cycling. (5)

Grasslands are important because of the amount C cycling which occurs within them. Further studies will provide more evidence for the proper management and protection of these valued lands. Because of the intricacies of drought and soil effects on grasslands, continued research and a framework to track the specific changes due to drought stress will be necessary to provide the information needed to maintain these lands in a world that is increasing in temperature due to carbon dioxide pollution.

References

1. White, Donald A., ed. Drought: A Global Assessment. New York: Routledge Publishers, 2000.
   
   Reichstein, M.; Bahn, M.; Ciais, P.; Frank, D.; Mahecha, M.D.; Seneviratne, S.I.; Zscheischler, J.; Beer, C.; Buchmann, N.; Frank, D.C. Climate extremes and the carbon cycle. Nature 2013, 500, 287–295
2. De Boeck, H.J.; Dreesen, F.E.; Janssens, I.A.; Nijs, I. Whole-system responses of experimental plant communities to climate extremes imposed in different seasons. New Phytol. 2011, 189, 806–817.
3. Gibson, D.J. Grasses and Grassland Ecology; Oxford University Press: Oxford, UK, 2009; pp. 1–300.
4. Shi, X.; Zhao, D.; Wu, S.; Shi, W.; Dai, E.; Wang, W. Climate change risks for net primary production of ecosystems in China. Hum. Ecol. Risk Assess. 2016, 22, 1091–1105.
5. Lei, T.; Wu, J.; Li, X.; Geng, G.; Shao, C.; Zhou, H.; Wang, Q.; Liu, L. A new framework for evaluating the impacts of drought on net primary productivity of grassland. Sci. Total Environ. 2015, 536, 161–172.

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.