A University of Michigan study finds that plants growing in nutrient-rich soil may be more likely to defend themselves against insects.
The study, led by U-M scientist Mia Howard and published in the journal Oikos, showed that goldenrod that grew in fields fed annually with agricultural levels of nitrogen were more likely to "nod" as a defense mechanism against insects that lay eggs in the tip of their stem.
"One of the exciting things about this finding is that there are so many hypotheses and theories predicting that resources would affect the evolution of plant defenses, and here we have the first experimental evidence to show that it does," said Howard, an assistant professor in the U-M Department of Ecology and Evolutionary Biology.
The National Science Foundation, Michigan State University, the Battelle Foundation Fund at Denison University, and the United States Department of Agriculture supported this work.
Plants have many defense strategies against herbivores. One typical tactic is chemical defense. Many plants produce bitter compounds so their leaves are unpleasant to eat, or are otherwise off putting—or even toxic—to herbivores. These defenses can be complicated and expensive to assess, and their levels are often affected by the plant's current nutrient status, which makes it difficult to determine if a higher level of defense is because of evolution (genetics) or the level of nutrients the plant currently has access to, Howard said.
Goldenrod, a native North American plant that grows widely around the world, is susceptible to insects that lay their eggs in the top of the goldenrod's stem. The insects inject an egg into the stem, triggering the formation of a gall—a protective first home for the developing larva once the egg hatches, often in the form of a rosette of whirled leaves or a round ball formed in the stem.
To prevent these insects from laying their eggs in the tops of the plants, some goldenrod plants "nod" their stems during the period of time these flies are actively seeking host plants, curving their stems to hide their tips.
To study the nodding goldenrod, Howard followed plots of the plant growing at the W.K. Kellogg Biological Station at Michigan State University. There, researchers had been treating test plots with agricultural levels of nitrogen fertilizer over a period of three decades.
Howard surveyed six replicate fields, which had been naturally colonized by goldenrod. She compared plants in fertilized plots with each field with plants growing in adjacent, unfertilized plots with ambient levels of nutrients available. She found that there was a greater concentration of nodding goldenrod growing in the nutrient-rich plots.
Goldenrod that nods is a helpful test subject, Howard said. Nodding is a genetic characteristic: goldenrod without the genetic capability does not nod, and goldenrod that do nod do so regardless of how much nitrogen is in the soil, something the researchers confirmed in their study. By contrast, nodding is a strongly genetic trait, and the nodding goldenrods can be identified by just looking at them.
"If the plant is a nodder, it's going to nod. It's also really easy to phenotype because you can just look at it," Howard said. "This is really low tech. You can quickly do it in the field," Howard said.
However, because the goldenrod straighten back up after curling into a candy cane form, Howard said her team had to make multiple trips to the fields to survey the goldenrod populations. Researchers surmise that the goldenrod's nodding is timed to when apex gall-making herbivores are flying around, looking for a goldenrod in which they can lay their eggs. Once this time period has passed, the nodding goldenrods straighten back up.
"It's an unusual defense trait, but we found that these nodding types of goldenrods were much more common in these fertilized plots, which suggests that high nitrogen availability is selecting for increased nodding defenses," Howard said. "I think it's really exciting to be studying plant evolution in real time."
Howard says the study also points to the importance of long-term research projects.
"We were really fortunate that Kellogg biological station had started this experiment and continued to maintain it for three decades. Having these long term experiments is really valuable," Howard said. "You might not see a lot of differences in plant populations in the early years, but I think they are becoming a really good resource for studying evolution."
Study: " Three decades of experimental nitrogen fertilization increases the frequency of a defensive plant morph ." DOI: 10.1002/oik.11181
