The Botanical Counterattack, And an Arms Race Unseen

Roses use their thorny exteriors to fend off herbivores.
Roses use their thorny exteriors to fend off herbivores.

By Cody Sullivan
BU News Service

Underneath a swaying canopy of gold, burnt orange, and crimson runs the sun dappled Meadow Road.  The leaves of exotic Chinese maple trees litter the lawns surrounding Arnold Arboretum’s thoroughfare.  The distant groundskeepers, with their roaring lawn mowers and wire rakes, have yet to manicure the maples’ lawns.  Leaving the cover of trees behind, and emerging into the crisp air of a bright autumn afternoon, an elegant display of roses and arching trellises awaits me

Strolling onto the lawns of the Bradley Rosaceous Collection, I am surrounded by botanical bastions.  Separated from the grassy walkways, by barricades of flat stones, are roses in all forms: leafy plants, bristling bushes and woody trees. But the deep green leaves, and flowers ranging from pale pink to blood red, cannot conceal the rose’s thorns.  With their defensive lances at the ready, these rose bushes are prepared for the herbivorous onslaught plants face every day, all around the world.

The rose standing guard is armed with a few large and sturdy thorns, while across the grass pathway is a bush densely covered in small, almost hair-like thorns.  Both variations on the thorny theme will prick, pierce, and even impale herbivores.  A thorn stabbing into an animal’s delicate gums, or shredding the tender tongue, is enough to discourage would-be herbivores.

Roses represent a group of plants that use mechanical defenses against herbivores –– and they’re not the only ones: berry bushes are another well-known example of a species that uses its structure to repel attacks, and even a tree trunk’s bark blocks invasions. But physical defenses can’t keep all predators away. As I examine this rose’s defenses, I notice that some of its arrowhead shaped leaves have holes, despite their piercing protection.

Insects, which nimbly avoid rose thorns, commonly munch rose leaves, and seem to have the upper hand in the plant-herbivore war.  But not so fast: plants have evolved, and consistently update, a cache of chemical toxins and deterrents used to fend off tiny herbivores capable of avoiding thorns.

Deep inside each individual leaf, beneath its cuticle, and behind the barriers of the cell wall is a plant cell’s chemical war factory.  The cytoplasm of plant cells converts essential, everyday molecules into toxins, irritants, and bitter tasting defensive compounds.

All cells, whether in a plant or an animal, produce basic molecular building blocks, chiefly amino acids, nucleic acids, or fatty acids.  These “primary metabolites” enter the military production line and the addition of a hydroxide molecule here, or a carbon group there, modify these compounds into secondary metabolites which function as biologic defensive weapons.  A plant’s artillery comes in three forms: phenolics, terpenoids, or nitrogen containing compounds.  All three types work as either unpleasant tasting deterrents, or toxins that cause a range of symptoms from mild burning, to severe pain or nausea, and sometimes even death – the cyanide in hemlock is a secondary compound that can kill when administered in high enough doses.

When implementing their chemical defenses, plants use two different strategies.  The first strategy is to wait for attack, then react.  Chemicals used in this approach are called phytoalexins, and are only produced after an herbivore, fungus, or even a pathogen has latched onto a leaf and begun its assault.  The second strategy is to always be prepared for attack.  A plant that subscribes to this tactic constantly produces chemical weapons and stores them inside its cells, commonly in vacuoles, and then releases these phytoanticapans once the fight has commenced.

Each time a battle occurs between an herbivore and a plant, an exchange of information occurs.  Both sides come away from an encounter experienced with their enemy’s weaponry, spurring on a never ending arms race.  Plants will develop a new chemical only to have herbivores evolve a resistance to it, rendering that defense ineffective, and prompting the plant to create another new compound.  This arms race has resulted in around 100,000 different metabolite defenses evolving in plants worldwide.

Gazing at the chewed-up rose, I am reminded by the insect bites that even with a plant’s formidable defenses, mechanical and chemical, they still sometimes lose the battle.  Plants have a particularly difficult time surviving attacks when their aggressors are invasive pests or herbivores that a plant has not evolved a specific toxin for.  With nowhere to run, a plant’s only option is to stand and fight, however futile the attempt.

This unseen arms race and silent warfare is waged all across the pristine and orderly Bradley Rosaceous Collection.  Yet the war is not contained to just roses, or to Jamaica Plain’s Arnold Arboretum: globally, the battle rages.

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