Of Silent Spring, salty soil, and responsive roots
“The most alarming of all man’s assaults upon the environment is the contamination of air, earth, rivers, and sea with dangerous and even lethal materials. This pollution is for the most part irrecoverable; the chain of evil it initiates not only in the world that must support life but in living tissues is for the most part irreversible.” from Silent Spring by Rachel Carson (1907-1964)
Rachel Carson fundamentally changed the way we view our relationship with nature. Carson was born 107 years ago this past week, on May 27th 1907. In the 1950s she became a full-time science writer, focusing on the wonders of the natural world. Her trilogy of books on the spectacles to be found in our planet’s seas won her both widespread respect and a U.S. National Book Award.
But Carson’s major, lasting impact came with the publication of her book, Silent Spring.
In Silent Spring, Carson detailed the ills wrought by synthetic chemicals on the natural environment. Specifically, she focused on the misuse of synthetic pesticides. She described these as “biocides” on account of their indiscriminate impacts on non-target organisms, including humans.
Carlson’s beautifully-worded text not only alerted the world to the existing problems with pervasive use of pesticides, but those that were likely to emerge in the future. She foretold the effects of bioaccumulation – as pesticides persisted in ecosystems and accumulated at each step up the food chain. She also warned of impending problems with both pest resistance and invasive species in weakened ecosystems.
Crucially, Carson’s Silent Spring drew a very firm connection between our health and wellbeing and environmental health and wellbeing. Humans were an integral part of nature, and our fates were inextricably tied to the fate of the natural world. Our actions, no matter how benign or beneficial we thought they might be, could have unintended consequences that could undermine our important interdependent relationship with nature.
More than 50 years after its publication, the overriding message contained within Silent Spring is every bit as relevant today as it was then. As the human population edges upwards from 7 billion people, our reliance on, and impact on, nature increases daily.
Our population has grown to such a size that our impact extends well beyond the introduction of new chemicals in the environment. Our actions now dramatically change the balance of even the most fundamental chemicals on the planet.
Increasingly salty, or saline, soils are becoming a reality the world over. Increased aridity in some regions has concentrated naturally occurring salt in the soil. In other regions, rising seawater has contaminated the groundwater with sea salt.
Humans have contributed to increased soil salinity both indirectly and directly. Land and water use shifts water tables, while human-influenced climatic changes impact precipitation and air temperature – all of which can contribute to elevated soil salinity.
While it seems counterintuitive, even irrigation of fields for crop production can contribute to soil salinity. All water applied to the soil contains some salt. Plants are unable to absorb this salt, and so it remains in the soil. Water that is not used by the plants is removed from the soil by evaporation. Between plant removal of water, coupled with evaporation and retention of salt in the soil, soil salinity can actually rise with irrigation.
Even more directly, salt is being flung onto the ground in ever increasing quantities for snow and ice management throughout frigid winters. Over the past fifty years, the application of salt on roads and walkways has increased dramatically. For example, in 1960, around 3 million metric tons of salt were spread on North American roadways; whereas, today, more than 20 million metric tons of salt are distributed on North American roads and walkways annually.
In the spring, rain runoff carries salt from the road and shoulders onto surrounding soils and into the water table.The cumulative effects of salt application to roadways and walkways are marked. Some freshwater river watersheds have seen salt levels rise between 100% and 250%. And these levels rise every year.
Increased soil salinity creates a significant problem. Salt is, generally speaking, toxic to plants. Many a school student has completed the classroom experiment showing that elevated salt concentrations are an effective means to kill plants.
As they are literally rooted in one place, plants cannot escape the toxic effects of salt. Elevated soil salinity therefore poses a risk to both ecosystem health and to crop production. In addition to finding ways to reduce soil salinity, we need to better understand how plants respond to salt to gain insights necessary for monitoring environmental impacts, sustaining ecosystem function, and preserving crop production.
Recent research by Won-Gyu Choi and his colleagues, in the laboratory of Simon Gilroy at the University of Wisconsin, has found a key element in plants’ initial responses to salt – calcium. When plants sense salt they create a “calcium wave” – an elevated concentration of calcium ions that passes from the point of salt perception, throughout the plant. The wave is created by the release of calcium that the plants store within their cells.
A novel system was used to observe changes in calcium within plant cells. Won-Gyu Choi and his colleagues engineered plants to make a protein that fluoresces when calcium is present. The amount that the protein fluoresces reflects the amount of calcium that is present in that location. Using this calcium-sensitive fluorescent protein, they were able to observe changes in calcium concentrations at the level of individual plant cells.
When Won-Gyu Choi and his colleagues exposed plant roots to a number of stresses, the plants generally responded by elevated calcium concentrations at the point of application of the stress. When roots were exposed to cold, touch, or oxidative stress, calcium levels increased at the point where the stress was applied. By contrast, when roots sensed salt, simple sodium chloride, calcium rose at the point of contact, but then this increase in calcium travelled throughout the rest of the plant, from the roots all the way to the shoots above ground.
The speed of the calcium wave was fast – travelling through two plant cells per second. This means that the salt stress signal was transmitted to other parts of the plant body very quickly.
After the passage of the calcium wave, plant tissues reconfigured their cellular functions – making new molecules that help the plant contend with salt stress. In particular, the plant made some new proteins that changed the way salt was moved through the plant body, so as to cause as little damage as possible. The plants also altered cell structures so as to create barriers to salt – again protecting cells from salt’s damaging effects. Won-Gyu Choi and colleagues were certain that it was the calcium wave that created this response by using chemicals that inhibited the passage of calcium.
Won-Gyu Choi and his colleagues speculated that calcium concentrations are elevated in the cell by being released from a special compartment within the cell. Calcium is stored within this compartment and then released to create the calcium wave. The release of calcium from the compartment requires the action of special proteins. These proteins form a pore that opens to release calcium from the compartment and thereby create the calcium wave.
Mutant plants where the pore proteins no longer functioned properly were used to determine the role of the pores in creating the calcium wave. In these mutants, the calcium wave didn’t travel properly in the plant anymore. In keeping with this, the mutant plants with the impaired pore proteins were unable to mount a defence against salt. Importantly, the mutant plants were also compromised in terms of their growth in the presence of salt relative to normal plants.
This research has uncovered an important mechanism that plants use to contend with salty soil.
The calcium wave that plants create in roots to let the rest of the plant know that salty times are ahead has striking similarities to how our nervous system functions. Calcium is also used to signal from one neuron to another when we experience stress – pain, for example. When you get salt in a wound, the signals your brain receives also have a calcium wave element to them. Plants are using an analogous system to transmit information about salt stress – but instead of being received at a centralised brain, they inform the entirety of the plant’s body.
Importantly, this research shows that evolution has equipped plants with means by which to contend with elevated salt. It shows that at least some plants can be expected to have an inbuilt defence against elevated soil salinity. In keeping with this, it is known that some plants are better able to cope with salty conditions better than others. Indeed, some plants seem to have what might be viewed as a preference for salty soils, in that they are able to thrive on such soils when competitors cannot. This said, such salt-tolerant plants, halophytes, represent just a small fraction of all plants – only 2%. Most plants cannot withstand the salt conditions that are emerging with human-created changes in soil salinity.
As with all natural systems, there is a limit to resiliency.
This is an important consideration given the ongoing increases in soil salinity around the world, and our dependency on the plants that must contend with the challenges to their growth and existence imposed by such soils. As Rachel Carson warned us over 50 years ago, even our best intents can have undesirable consequences. Frequently, these consequences underscore the beautiful, but finely balanced relationship that exists between the components of the natural world.
Our impact on soil salinity compels us to take a close look at how this in turn affects the growth and survival of plants. Perhaps this reflection provides us with a hopeful moment as we look to the future. It provides us with an opportunity to pause and be amazed by how such a seemingly simple an organism as a plant has such a sophisticated way to respond to a perturbation in its world. In turn, perhaps this will shape how we mitigate such perturbations, so as to preserve that which has amazed. As Carson herself said, in a message that is more hopeful than pessimistic:
“The more clearly we can focus our attention on the wonders and realities of the universe, the less taste we shall have for destruction.”
Let this be the greatest lesson we take from how plants grapple with a salty challenge.
Images: All photos by Malcolm M. Campbell
Choi WG, Toyota M, Kim SH, Hilleary R, & Gilroy S (2014) Salt stress-induced Ca2+ waves are associated with rapid, long-distance root-to-shoot signaling in plants. Proceedings of the National Academy of Sciences 111: 6497-6502
Findlay SE, & Kelly VR (2011) Emerging indirect and long‐term road salt effects on ecosystems. Annals of the New York Academy of Sciences1223:58-68
Ke C, Li Z., Liang Y, Tao W, & Du M (2013) Impacts of chloride de-icing salt on bulk soils, fungi, and bacterial populations surrounding the plant rhizosphere. Applied Soil Ecology 72:69-78