“`html

A study from the University of Michigan reveals that plants thriving in nutrient-dense soil might be better equipped to protect themselves against insects.

Conducted by U-M researcher Mia Howard and featured in the journal Oikos, the research indicated that goldenrod plants nourished annually with nitrogen at agricultural levels exhibited a greater tendency to “nod” as a protective mechanism against insects that lay eggs in the tip of their stems.

“One of the thrilling aspects of this discovery is that there are numerous hypotheses and theories suggesting that resources would influence the evolution of plant defenses, and here we present the first empirical evidence to substantiate that claim,” remarked Howard, an assistant professor in the U-M Department of Ecology and Evolutionary Biology.

This research received funding from the National Science Foundation, Michigan State University, the Battelle Foundation Fund at Denison University, and the United States Department of Agriculture.

Plants have developed various defense mechanisms against herbivores. A common strategy is through chemical defenses, where many plants generate bitter substances to make their leaves less palatable, or even toxic—to deter herbivores. These defenses can be complex and costly to measure, with their effectiveness often influenced by the plant’s current nutritional condition, which complicates determining whether a heightened level of defense results from evolution (genetics) or the nutrients presently available to the plant, Howard explained.

Goldenrod, a plant native to North America that proliferates globally, is prone to insects that lay eggs within the top of the goldenrod’s stem. These insects insert an egg into the stem, prompting the formation of a gall—a protective environment for the developing larva once the egg hatches, often appearing as a rosette of whirled leaves or a spherical formation within the stem.

To thwart these insects from laying their eggs in the upper parts of the plants, some goldenrod specimens exhibit a “nodding” behavior during the time these flies search for hosts, bending their stems to conceal their tips.

To investigate the nodding goldenrod, Howard monitored plots of the plant at the W.K. Kellogg Biological Station at Michigan State University. There, researchers had administered nitrogen fertilizer at agricultural levels over a span of three decades.

Howard assessed six replicated fields, which had naturally been colonized by goldenrod. She evaluated plants in fertilized plots against those in adjacent, unfertilized plots with natural nutrient levels. A noticeable increase in nodding goldenrod was found in the nutrient-rich areas.

Goldenrod that exhibits nodding is a valuable subject for study, Howard noted. Nodding serves as a genetic trait: goldenrod lacking the genetic predisposition does not nod, while those capable of nodding do so independent of the nitrogen levels in the soil, a fact confirmed by the researchers. In contrast, nodding is a strictly genetic characteristic, easily identified by visual inspection.

“If a plant is a nodder, it will nod. It’s also very straightforward to phenotype since you can just observe it,” Howard stated. “This method is really simple. It can be quickly conducted in the field,” Howard commented.

However, since the goldenrod reverts to an upright position after bending into a crescent shape, Howard mentioned that her team needed to conduct multiple surveys of the fields to monitor goldenrod populations. Researchers hypothesize that the timing of goldenrod’s nodding corresponds with the period when dominant gall-making herbivores are flying, searching for suitable goldenrod for egg-laying. After this period elapses, the nodding goldenrods return to an upright position.

“It’s an unusual defensive trait, but we observed that these nodding varieties of goldenrods were significantly more prevalent in fertilized plots, suggesting that high nitrogen availability selects for increased nodding defenses,” Howard noted. “I find it incredibly fascinating to study plant evolution in real time.”

Howard emphasizes the significance of long-term research initiatives.

“We were fortunate that Kellogg Biological Station embarked on this experiment and has sustained it for thirty years. Long-term experiments like this are immensely valuable,” Howard stated. “While differences in plant populations may not be immediately apparent in the initial years, they evolve into valuable resources for studying evolution.”

“`