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Copper Reveals Its Role in Exploding Plants — and in the Miracle of Man

Photo: Flowers of Cardamine hirsuta, by Aelwyn, CC BY-SA 3.0 , via Wikimedia Commons.

Plants and animals have clever ways of getting things done. Studying them gives scientists a never-ending treasure hunt.

Copper Fireworks

The exploding pods of the popping cress send the plant’s seeds flying in all directions, as far as a meter from the parent. Also called hairy bittercress, Cardamine hirsuta is a small herb with edible leaves growing in a basal rosette. It looks otherwise unremarkable with its tiny 2-mm flowers on short stalks — that is, until one walks through a patch of ripe ones and hears the sound of dozens of popping seed pods and feels them hitting the legs. 

Scientists investigating the exploding seed pods of C. hirsuta have found that the element copper is involved. The news from the Max Planck Institute, “Copper makes seed pods explode,” tells how three distinct genes are required to lay down lignin in a polar arrangement inside the slender seed pods. The pods have a “unique mechanical design” comprising three stiff rods of lignin connected by hinges. Lignin is the main ingredient of lignocellulose that makes stems and wood stiff. As the pods dry out, tension is created due to the asymmetry. The slightest touch or breeze is enough to pull the trigger on these “extraordinary exploding structures.” Within three milliseconds, the rods rapidly curl, flinging the seeds outward. Primary investigator Angela Hay explains,

“The mechanical design that allows these pods to explode depends on lignin being laid down in a precise patternin this single layer of cells. We know little about what controls this pattern of lignin deposition, and so we set out to identify the genes that control this process. We found three genes that are required to lignify the cell wall in exploding seed pods. These genes code for enzymes, called laccases, that polymerize lignin. When C. hirsuta plants lack all three laccase genes, they also lack lignin in this specific cell type.” [Emphasis added.]

The SPL7 gene encodes a protein that regulates copper levels in plants. The scientists found that copper-deficient soils produced seed pods that could not disperse seeds nearly as far. 

Interestingly, laccases are copper binding proteins that depend on copper for their function. “The link between these two findings is copper“, says Hay. “Plants need SPL7 to cope when there’s too little copper in the soil, and laccases need to bind copper for their enzymatic activity. Since lignin is critical for the mechanics of exploding seed pods, and copper-requiring laccases regulate this lignification, this makes seed dispersal dependent on the control of copper levels by SPL7.”

The details are published by Hay and colleagues in PNAS, “Explosive seed dispersal depends on SPL7 to ensure sufficient copper for localized lignin deposition via laccases.” 

Biological Copper

Like zinc, copper is “an essential metal in biology” that must be accessible to plants and animals at the earth’s surface. In humans, copper (element 29), though only amounting to 50 to 120 milligrams in the body, performs numerous critical functions. It is found working in our organs and metabolic processes, including the brain, heart, liver, lipid membranes the immune system, melanin in the skin, cross-linking of collagen, and much more. Because of its redox potential, copper works alongside iron in cytochrome C oxidase, which sits right at the key step of the electron transport chain as the final electron acceptor from oxygen, contributing to the proton motive force that runs ATP synthase. (See Michael Denton, The Miracle of the Cell, Chapter 6, “No Biology without Metals”). As with zinc also, the body has homeostatic mechanisms to ensure the proper levels of copper, since both deficiency and excess can cause problems. The PNAS paper says,

The cycling of Cu between an oxidized (Cu2+) and a reduced state (Cu1+) is used in many different redox reactions and electron transport. Thus, Cu is an essential micronutrient in nearly all eukaryotic organisms. However, when Cu ions are present in excess, redox cycling can catalyze the production of highly toxic ROS and cause cellular damage. Therefore, Cu homeostasis is tightly controlled in plants

As with other metals in the body, copper must be transported with care. Specialized transporters bring it across the membrane where chaperones route it to sites of copper-requiring enzymes. 

Dietary sources of copper include beef liver and shellfish, seeds and nuts, wheat-bran cereals, whole-grain products, and chocolate, showing once again that plants are the predominant importers of copper from the soil.

Darwin or Design?

Despite the reference to “mechanical design” above, the press release and the paper Darwinize the process, saying, “Plants have evolved numerous strategies to spread their seeds widely…. a few rare plants — such as the popping cress Cardamine hirsuta — have evolved exploding seed pods that propel their seeds in all directions.” They could have just said “plants exhibit” these phenomena and left out the magical thinking. After all, they admit that “large families of genes are involved in lignin polymerization in plant cell walls,” and “the process of lignification itself… remains little understood.” 

Out with a Bang

There’s another species of small herbaceous plant that launches its seeds with explosive force: the storksbill or filaree (Ergonium cicutarium). Though classified in a different order and family, this tiny plant also builds slender pods in which tension builds. When ready to launch, the sides of the pods split apart explosively, sending the seeds outward away from the parent. Attached to each seed is a slender filament called an awn, which is designed to respond to changes in humidity. After launch, the awn quickly coils into a spiral shape that rolls along the ground as humidity changes between day and night. In time-lapse videos, the seed appears to be searching for gaps in the soil. When it finds one, it actually drills itself into the ground! Illustra Media created a short video about this phenomenon. I served that project as a specimen hunter, collecting some of the seeds that were filmed for the production.

Scientific papers about the storksbill do not say whether copper laccases are active in the construction of the storksbill seed pods, or whether lignin molecules are deposited in a polar arrangement. One article mentions that storksbill is usually found in soils high in copper and zinc, so it’s likely that similar processes are involved at a biochemical level. A biotechnology paper states that laccases, which are copper oxidases, are implicated in lignin biosynthesis. It appears safe to assume that these natural spring-loaded rockets and drills would never be able to fascinate us unless a shiny metal, copper, made its way from geology into biology.

Copper Availability

As Michael Denton mentions in The Miracle of Man, the biologically essential metals like copper, zinc, and iron would not be available for life without plate tectonics, erosion of rocks by rivers and glaciers, and the delivery of metal-bearing minerals to soils. Even living on the right sized planet is a requirement. Denton speaks of the “prior fitness” of the environment for complex life, which involves the atmosphere, the crust, the star we orbit, the size and shape of the human body that permits fire-making and metallurgy, the properties of atoms, the unique characteristics of water, and more. Any one of these requirements, if not met, would yield a world that is sterile or at least devoid of complex beings able to use technology. 

Now we have also seen that enzymes that know how to handle copper, like laccases and cytochrome c oxidase, carefully transport the metal to sites where it is needed, at the right times and in the right amounts — sometimes with explosive results. Those enzymes would never be constructed without the information-rich genetic code, and molecular machines able to transcribe the code, translate the transcripts, and chaperone the resulting polypeptides to fold into the proper shape. Those pre-designed shapes are able to accept and gently caress the metal ions kept in store and delivered by other molecular machines.

This “mountain of scientific evidence,” writes Denton, has come together in our time to provide “compelling evidence that we do indeed, after all, occupy a central place in the great cosmic drama of being.”