Plants don’t grow well when always on high alert

Plants make sacrifices to protect themselves from pests. Now a tiny weed that squeezes through sidewalk cracks is helping scientists understand those costs of always being on guard.

Nature’s greenery often fights insects and other plant-eaters by releasing bitter chemicals into their leaves. If a plant tastes bad, hungry bugs will avoid it. Arabidopsis (Ah-rab-ih-DOP-sis) is a commonly found member of the mustard family. Scientists often turn to it as the “lab rat” of the plant world. In the new study, researchers studied thale cress (Arabidopsis thaliana) to learn what this bitter defense costs the plant.

Putting extra energy into pumping protective chemicals through a plant’s veins leaves a plant less able to grow and reproduce, they now report.

All plants have a bundle of what are called JAZ genes in their DNA. Those genes provide the instructions to make JAZ proteins. These proteins help plants control how they use defensive chemicals. Over a 10-year span, the team bred together Arabidopsis plants that had been engineered to possess mutated JAZ genes. (Mutations are altered forms of some gene.) These plants ended up with 10 of its 13 JAZ genes disrupted, so they made less of those JAZ proteins.

Such plants were in defensive mode nearly all of the time. And they paid a price. They grew shorter than normal and were weaker. These plants also made fewer seeds that were viable (able to sprout). The plants also developed brown and withered leaves. This showed that the engineered plants were being starved of the carbon they need for growth. Carbon starvation happens when plants close up the tiny holes in their leaves known as stomata (Stoh-MAH-tah). Those holes let leaves breathe in the carbon dioxide they need to make food.

The scientists reported their findings online October 22 in the Proceedings of the National Academy of Sciences.

When plants keep their defenses turned on all the time, they use up energy. This leaves them with less energy to power growth and reproduction, the scientists explain.

“When plants use those resources for defense — in this case, a defense against insects — there is a major trade-off,” says Gregg Howe. An author of the new study, he’s a plant biologist at Michigan State University in East Lansing.

It’s not yet clear how the lessons learned from this plant might apply to other species, including the grains farmers grow for food, notes Georg Jander. He’s a chemical ecologist at Cornell University who was not involved in the new work.

Howe’s team hopes, however, that research like this will reveal new ways to protect crops from bugs without harming the plants or needing to douse fields in pesticides.