Plants use special hormones to defend against spider mites

The microscopic two-spotted spider mite, with its insatiable appetite for over 1,000 plant species, ranks among agriculture’s most formidable foes.

But recent research reveals that plant hormones aren’t defenseless victims. They’ve cunningly adapted a drought-survival mechanism to turn the tables on these pests, hindering their ability to feed and wreak havoc.

Stomata: Battleground for survival

Stomata act like the breathing pores of a plant. They are microscopic openings that allow essential gas exchange to occur. Carbon dioxide, needed for photosynthesis (the process by which plants turn sunlight into energy), enters through the stomata.

At the same time, oxygen, a byproduct of photosynthesis, exits through these pores. Stomata also play a critical role in regulating water loss from the plant.

Under normal conditions, stomata remain open to facilitate gas exchange. However, during a drought, when water conservation becomes a priority, plants have a clever trick up their sleeves.

A hormone called abscisic acid (ABA) is produced. This hormone acts as a signal to the stomata, instructing them to close. By shutting these tiny pores, the plant significantly reduces water loss through its leaves.

Interestingly, this same mechanism that helps plants survive drought also serves as a defense against harmful pests like spider mites. These tiny, sap-sucking creatures prefer to enter plant tissues through open stomata.

When stomata close in response to ABA, they essentially slam the door shut on these unwelcome visitors, making it much more difficult for spider mites to feed and damage the plant.

ABACUS2 biosensor: Plant responses and spider mite

This research was made possible through the combined efforts of scientists from two leading institutions: the University of Cambridge in the UK and the Centre for Plant Biotechnology and Genomics in Spain.

The team employed a sophisticated tool called the ABACUS2 biosensor to investigate the plant’s defense response. This advanced technology allowed them to measure precise changes in the levels of abscisic acid (ABA) within individual plant cells as they were being attacked by spider mites.

The ABACUS2 biosensor provided an incredibly detailed view of the plant’s internal defense mechanisms during the pest infestation.

Plant’s rapid reaction against spider mite

“Open stomata are natural openings where pests like mites insert their specialized feeding structures, called stylets, to pierce and then suck out the nutrient-rich contents from individual sub-epidermal cells”, elaborated Irene Rosa-Díaz, a researcher involved in the study.

The team discovered that plants waste no time in mounting their defense. Within hours of infestation, ABA levels spike, triggering stomatal closure. This swift action throws a wrench in the spider mites’ feeding plans.

Evidence of effectiveness

The researchers conducted several experiments to test their hypothesis, and the results were remarkably clear. Plants that were pre-treated with ABA – the hormone associated with drought response – had less damage from the spider mites. This suggests that preemptively triggering stomatal closure can make plants less vulnerable to attack.

Plants with naturally impaired production of ABA or with an abnormally high number of stomata proved to be much easier targets for the spider mites. This finding highlights the crucial role of both the ABA hormone and stomatal regulation in pest defense.

The search for the trigger

While the link between spider mite infestation and the plant’s defensive ABA response is established, scientists are still searching for the exact trigger. How does the plant initially sense that it’s under attack? This remains a key area of active research.

Researchers are exploring a range of intriguing possibilities to explain how a plant might detect the presence of spider mites.

Could the plant sense the tiny vibrations caused by the mites’ movements across its leaves as they feed? Do spider mites leave behind unique chemical traces in their saliva or elsewhere that act as a recognizable “scent” for the plant? Is it possible that the plant can directly detect the cellular damage caused by the mites’ feeding?

Answering this fundamental question will significantly advance our understanding of plant defense mechanisms and could open new avenues for developing targeted pest control strategies.

Natural pest control

If scientists pinpoint the initial mite-produced signal, a new era of pest control could unfold. Imagine treatments that pre-arm plants, mimicking the early stages of a drought response, and making them less attractive to invading pests. Breeding programs could also prioritize plants with favorable stomatal traits to boost resilience.

This research spotlights how even the most unassuming plants possess sophisticated survival strategies. By understanding the interplay between drought resistance and defense against biotic threats like spider mites, we stand to unlock a treasure trove of insights that could revolutionize agriculture.

The study is published in the journal Plant Physiology.

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