In lab tests, a plant called thale cress (shown) that was engineered to be nearly always on alert for leaf-eating predators tended to be smaller and produced fewer viable seeds than its normal kin.

A tiny weed that slithers up through sidewalk cracks is helping scientists understand the sacrifices that plants make to protect themselves from pests.

Most plants combat insects and other herbivores by sending out bitter chemicals through their leaves. Now by studying thale cress (Arabidopsis thaliana), a commonly found member of the mustard family, researchers found that energy spent pumping protective chemicals through a plant’s veins diminishes its ability to grow and successfully reproduce.

“When plants use those resources for defense — in this case, defense against insects — there is a major trade-off,” says Gregg Howe, a plant biologist at Michigan State University in East Lansing. He and his colleagues report their findings online October 22 in the Proceedings of the National Academy of Sciences.

All plants have a bundle of what are known as JAZ genes. Those genes provide the instructions to make JAZ proteins, which help plants control the use of the defensive chemicals. Over a decade, the team disrupted the activity of 10 of the 13 JAZ genes found in Arabidopsis plants to hinder production of those proteins.

As a result, the engineered plants were nearly permanently in defensive mode, which ultimately made them shorter, weaker and with fewer viable seeds than their normal counterparts, the scientists found. Brown and withered leaves also revealed that the engineered plants were starved of carbon, meaning they weren’t getting enough food. Maintaining such a defensive strategy consumes energy that the plant could otherwise use for growth or reproduction, the researchers speculate.

For now, it’s unclear how applicable these insights from Arabidopsis are to other plants, including major grain crops, says Georg Jander, a chemical ecologist at Cornell University. But through such research, the team hopes to illuminate new ways to protect crops from foraging insects without sacrificing crop yield or dousing fields in pesticides.