Reference: Van Wyk, J. I., B. A. Krimmel, L. Crova, and I. S. Pearse. 2019. Plants trap pollen to feed predatory arthropods as an indirect resistance against herbivory. Ecology 100(11):e02867. DOI: 10.1002/ecy.2867
The last decade has brought fascinating dispatches from the botanical world—plants communicate, we have learned. Plants cooperate. Plants sense immediate threats and react to defend themselves. And plants even leverage ancient political wisdom in their machinations against enemies.
In a botanical take on the adage “the enemy of my enemy is my friend,” plants protect themselves against plant-eating insects by enticing the enemies of their pests (predatory insects or spiders, in this case) with offerings of shelter and food.
Like any political intrigue, this can lead to messy relationships. In some cases plants will entrap insects by producing a sticky sap, and then serve those wayward visitors up to predators as a free meal. With this incentive, the predators might stick around and eat other unwelcome, damaging insects that the plant is trying to oust.
Sometimes these negotiations require plants to make harsh compromises in service to a bigger goal. Cotton plants, for one, will tolerate low-impact insects like aphids eating some leaves if those pests attract predators (ants, in this case) that assail more harmful enemies like caterpillars. The most aggressive herbivores can be damaging enough to plants that such complex give-and-take interactions become worthwhile.
A recent study has uncovered another thread in this web of collusion, finding that plants defend themselves by capturing resources from other plants, then offering the hijacked goods to predatory arthropods that help rid them of pests.
Protein-packed pollen grains are the hot commodity in this transaction. Researchers, led by Jennifer Van Wyk at UC Davis, observed that fine hairs coating the leaves of turkey mullein (a plant so luxuriously fuzzy it has been called “cowboy toilet paper”) could trap pollen from the wind or the legs of insects. Pollen is nutritious enough to be sought after as a supplemental food source by predatory insects and spiders, hinting that it could make an effective bribe for a mullein plant seeking defenders.
To see if trapping pollen boosts plan defenses, researchers dusted pollen on the leaves of select turkey mullein plants and found that, compared to control plants, the pollen recipients were colonized by more predators and as a consequence suffered less damage from herbivorous pests.
The hairs on plant leaves can serve other purposes too, such as defense. While we don’t yet understand every function of this plant multitool, it’s clear there’s more to mullein leaves than their soft and fuzzy side.