Celebrating a Heroic Plant Part: the Casparian Strip
The turning of the New Year inspires me to take stock of what I’m grateful for. Giving thanks for what’s working and what inspires me often lends some guidance for the year ahead.
I’ll take this opportunity to sing praises of the beloved plants at the heart of A Nourishing Harvest. Specifically, I’ll explore the workings of a rather heroic plant part when it comes to dealing with environmental contamination: the Casparian strip.
Image: An orange California poppy flower (Eschscholzia californica) is decked out in a white superhero cape. Its green-gloved “hands” are in fists on its “hips.” It looks like a 5-year-old drew it. (Image created by Yours Truly.)
What’s the Casparian Strip?
Casparian strips are ring-like “cell wall impregnations” found in the roots of plants. These rings are found in the endodermis— a layer of cells that surrounds the vascular tissue at the center of the root. The strips contain lignin, which “makes for a very sturdy, chemically resistant structure, perfect for a protective barrier” (Geldner).
The Casparian strip acts like a checkpoint for minerals, water, and some contaminants. Since the strip is hydrophobic—impermeable to water—it’s well-suited to controlling the movement of water and inorganic salts. It prevents these substances from puttering freely in the space between cells, and forces them to move through the cell’s plasma membrane. This gives the plant more control over what passes from the roots to the stems and leaves, and how much (Geldner).
The nutritional profile of different soils varies widely, and the Casparian strip can help limit and modulate the flow of minerals from the soil into the plant accordingly (Palmer).
Toxic or unneeded minerals [including heavy metals] may be excluded (Simpson).
PFAS and Other Toxins: What the Casparian Strip Can Do
The Casparian strip came onto my radar while learning how PFAS chemicals interact with plants. As we know from previous articles in this PFAS series, longer-chain PFAS like PFOS and PFOA are much less likely to be transported from root to shoot than shorter-chain PFAS. The fact that they’re more likely to be bound or blocked by the Casparian strip is partly responsible for this (Liu).
In their 2020 research review, Costello and Lee note:
It is widely accepted that the Casparian strip…serves to constrain translocation of contaminants from roots to the central vasculature, thus the rest of the plant. The Casparian strip prevents passive transport of compounds across the endodermis, by sealing the nonliving spaces between cells and cell membranes…Trends discussed in PFAS plant uptake studies are consistent with this hypothesis that the Casparian strip constrains translocation of long-chain PFCAs, PFSAs, FOSAs, and Cl-PFAESs.
Plants with fewer or no Casparian strips, like carrots and radishes, are more likely to transport PFAS from their roots to their shoots and leaves. These veggies were found to have a significantly higher concentration of PFOS or PFOA, respectively, in their leaves than other veggie plants (Mei).
“The concentrations of PFOA in the aboveground tissues of radish plants grown in soil were 5–10-fold those of other plants (tomato, pea, and celery)" (Mei).
In two studies, the concentration of PFOS in carrot leaves was found to be several times higher than in lettuce leaves “because carrot also lacks the Casparian strip” (Mei).
Image: Wild carrot, also known as Queen Anne’s lace (Daucus carota) in full lacey bloom.
Heavy Metals
The Casparian strip can also prevent positively-charged metal ions from entering the root’s vascular tissue, thus impeding transport to above-ground plant parts. This includes heavy metals like cadmium (Chen) and lead (Seregin).
In one study of corn seedlings by Seregin et al., cadmium ions occurred at a significantly lower concentration at the center of the root than outside the Casparian strip barrier, indicating that the Casparian strip impeded the movement of cadmium ions. Corn seedlings can tolerate some level of cadmium, largely due to the presence of the Casparian strip (Chen).
A plant’s anatomy can actually adapt in the presence of heavy metals. This is true of the Casparian strip, as well as another protective plant part called the suberin lamella. The suberin lamella is another selective barrier that’s found in some plants. It’s a coating on endodermis cells made of a hydrophobic, protective substance called suberin (Costello and Lee).
In one study, corn roots exposed to cadmium developed suberin lamella (Costello & Lee), presumedly in response to this toxic stressor.
In a study of cotton plants, exposure to cadmium resulted in a significant increase in the expression of genes associated with the Casparian strip. This is a sign that Casparian strips in cotton roots grew “broader, wider and deeper in response to cadmium stress” (Chen).
Of course, plants do take up heavy metals, sometimes to a degree that’s dangerous to the plant and the plant eater. If you'd like to learn more about how this occurs and its relevance in agricultural plants, this article offers a nice overview.
When PFAS and cadmium are present together, interesting results ensue. As noted above, cadmium may increase the presence of suberin lamella, allowing the plant to reduce cadmium’s movement up the plant. This has the added benefit of decreasing the plant’s transport of PFAS, too (Costello and Lee). Since heavy metals have existed in soils for as long as plants, it makes sense that plants may be better adapted to responding to them. How handy that the same mechanisms may help a plant manage newfangled contaminants, too.
Layering multiple contaminants into the soil can have “negative” effects on the plant as well. PFAS-induced root damage may allow for increased uptake of cadmium. In one study, cadmium accumulation in rapeseed and wheat increased when PFAAs were present (Costello & Lee).
Herbicides: What the Casparian Strip Doesn’t Do So Well
As I noted above, widespread exposure to synthetic chemicals is a very recent phenomenon in biological history. Just like humans, plants haven’t had much time to adapt and evolve in this new milieu—and we release new chemicals into the environment each year. It’s not surprising that plant anatomy isn’t set up to blockade or effectively metabolize every toxin it encounters.
In the realms of agriculture, gardening, and foraging, a plant’s interaction with herbicides is of particular interest. Herbicides are widely applied to public and private land (lawns, farm fields, roadsides, parks, and more) to control weeds and invasive species. How do commonly-used herbicides interact with the Casparian strip?
In his article “Understanding Herbicides,” Joseph DiTomaso notes:
“The Casparian strip does not serve as an important barrier to absorption or translocation…The majority of soil-applied herbicides…ultimately accumulate in the shoot tissues, e.g., triazine and urea herbicides.”
Teryl Roper of the University of Wisconsin-Madison writes that some “herbicides may be taken up passively by roots, while others require the plant to expend energy to take them up. For the most part, herbicides enter plant roots passively,” apparently getting around the Casparian strip checkpoint.
Young seedlings are generally more susceptible to herbicide absorption than older plants, since they haven’t developed the Casparian strip or protective waxy cuticles yet (DiTomaso).
As with PFAS, it seems we’ve been moving away from pesticides with long carbon chains that accumulate in animal bodies and resist decomposition in the environment. DDT is one such chemical—a lipophilic (fat-loving) compound which was banned in 1972 after enthusiastic use in the 1950s and 1960s. As we move toward pesticides that are more easily broken down in the environment—which generally means they’re more water-soluble, just like short-chain PFAS—I suspect we’re also moving towards chemicals and metabolites that are more easily taken up by plants, and can more easily get by the Casparian strip. I’ll be interested in exploring this hypothesis more.
Image: A pile of freshly harvested turmeric root (Curcuma longa) is washed and ready to be used.
Consideration for Edible & Medicinal Roots
In their “Introduction to the Casparian Strip," Niko Geldner asks, “Why are Casparian strips located in an inner cell layer?…Why would you put an extracellular diffusion barrier so deep within the root as opposed to blocking diffusion at the epidermis? There is no good answer to this (but then, ‘why’ questions are always a bit unfair…).”
Niko muses that roots are a bit like an inside-out human digestive system. They have to balance the uptake of important nutrients with protecting the creature from unwanted contaminants. Niko notes:
Putting the diffusion barrier deep inside the root may allow the cortex to act like a ‘lobby’ where many things are admitted and can be perceived and selected for uptake. Only at the point where the vasculature begins (which is a direct highway to the precious leaves) would the Casparian strips of the endodermis then put up a strict diffusion barrier.
I have never been one to peel potatoes, carrots, and other root vegetables or herbs. I’ve been told that the outer layer of the root tends to possess a higher concentration of nutrients than the root’s flesh. Since I grow my garden without chemicals, and I buy organic produce, why peel my veggies?
I now understand that contaminants can accumulate in the outer layer of a root alongside beneficial nutrients. We’re basically eating the “lobby” that Geldner describes. Learning how commonplace environmental contaminants like PFAS are—even in organic gardens—is giving me pause when it comes to my no-peel ways. Are the nutrients in the peel of our root veggies worth ingesting whatever contaminants may also lie there?
Final Thoughts
Casparian strips are just one of numerous mechanisms and variables that affect a plant’s uptake of PFAS and other contaminants. If you’d like to dig deeper into this topic, clicking the journal articles under Sources below will give you a place to start.
Just like our heroic liver (and the other organs that contribute to toxin metabolism in the human body), the Casparian strip and suberin lamella are currently part of a strange chemistry experiment. As our exposure to modern contaminants stretches into a decades- and generations-long relationship, our plant and animal bodies will make their limits ever more clear. This New Year, I’ll toast to more research dollars dedicated to documenting the interactions between contaminants and creatures. May our government regulations fit the reality this data reveals.
Image: a snazzy elderberry cocktail (Sambucus nigra) in a glass, garnished with bright blue cornflower (Centaurea cyanus).
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Sources
Chen, Haodong; Yujung Li; et al. “Analysis of potential strategies for cadmium stress tolerance revealed by transcriptome analysis of upland cotton.” Scientific Reports. Vol 9, no. 86 (2019). Webpage.
Costello, M. Christina Schilling and Linda S. Lee. “Sources, Fate, and Plant Uptake in Agricultural Systems of Per- and Polyfluoroalkyl Substances.” Current Pollution Report (2020). Webpage.
DiTomaso, Joseph. “Understanding Herbicides: What They are and How They Work.” University of California Agriculture and Natural Resources. Viewed Dec 10, 2022. Webpage.
Geldner, Niko. “Casparian Strips.” Current Biology. Vol 23, no. 23. Viewed Oct 27, 2022 on Cell.com. Webpage.
Liu, Zhaoyang, Yonglong Lu et al. “Multiple crop bioaccumulation and human exposure of perfluoroalkyl substances around a mega fluorochemical industrial park, China: Implication for planting optimization and food safety.” Environment International. Vol 127, June 2019, pp. 671-684. Webpage.
Palmer, Linda. “Casparian Strip Formation in Rice." Plantae, American Society of Plant Biologists. Sept 25, 2019. Webpage.
Roper, Teryl. “Herbicide Physiology: Why Do I See What I See?” University of Wisconsin-Madison. Webpage.
Simpson, Michael G. “Evolution and Diversity of Vascular Plants." Plant Systematics (Third Edition), 2019. Webpage.
Song, Chengwei et al. “Development and chemical characterization of Casparian strips in the roots of Chinese fir (Cunninghamia lanceolata).” Trees. Vol 33, pp. 827–836 (2019). Webpage.
Seregin, I. V.; L. K. Shpigun et al. “Distribution and Toxic Effects of Cadmium and Lead on Maize Roots.” Russian Journal of Plant Physiology. Vol 51, pp. 525–533 (2004). Webpage.
