For plants, vigilance comes at a cost

Plants struggle to grow and reproduce when also primed to ward off invaders, a new US study suggests.

In a classic case of an evolutionary trade-off, researchers from Michigan State University have shown that when a plant needs, or – as in this case – is forced to allocate large amounts of its energy to defence, other functions have to go without.

In a study published in the journal Proceedings of the National Academy of Sciences, a team led by Gregg Howe focused on the response of plants to herbivores, which depend on them for shelter and food. If a caterpillar starts munching on a leaf, a plant produces toxins that ward it off, and then stops when the threat passes.

However, by genetically breeding out 10 of the 13 repressor proteins, known as JAZ proteins, that would automatically shut down the defence mechanisms when they are not needed, the researchers produced a plant in continuous defence mode.

“It kept producing defence compounds, even in the absence of threats. As expected, it showed high resistance to insects,” says co-author Qiang Guo. “Unleashing this defence arsenal also provided protection against fungi that target plant tissues.”

On the down side, the growth rate of the modified plants was much slower that their normal counterparts – Guo says they could “see and measure the deficit” each day – and their reproductive success was compromised.

The plants produced a third fewer seeds, and these were smaller, of lower quality and contained fewer nutritional fats. They also had a different lipid make-up and germinated later than usual.

And this wasn’t due to lack of fuel. The researchers say the mutated plants received the same amount of energy compounds from photosynthesis as their natural counterparts. They just used it on defence.

“To illustrate that point, we fed the plant with sugar, a fuel source, and it partially recovered its growth,” Guo adds.