Light and Plants: Light and the Anti-herbivore Defense in Plants
Plants possess a variety of defense mechanisms against attack by numerous herbivorous insects. They produce chemicals that are toxic to insects or inhibit digestion of insect food, or release volatile compounds into the air to attract natural enemies of the herbivorous insects. The insects that successfully disarm plant defense systems shall prevail in the natural environment (Figure 1).

Figure 1. A Colorado potato beetle feeding on potato leaves. Colorado potato beetles apply special bacteria on leaves while munching on leaf tissue, minimizing the level of plant defense. This feeding strategy made this insect species one of the most notorious insect pests of Solanaceous plants (e.g. potato, tomato, egg plant, etc.) in North America (Ref. Chung et al., PNAS 2013; Photo Credit: University of Maine).
It would be great for plants if plants can maintain the level of anti-herbivore defense highest all the time, but in that case, plants would consume too much resources, which would be used for proper growth and reproduction otherwise. Therefore, plants tend to focus their defense capacity on selective organs important in growth and reproduction.

Especially, plant defense is strong in the leaves located on the top part of plants because upper leaves take most of sunlight for photosynthesis and thus are highly valuable. Indeed, plants inhibit in shaded leaves the action of jasmonic acid, a plant hormone activating anti-herbivore defense, to distribute resources in proportion to the value of their organs (Figure 2).

Today’s take-home message: “Plants adopt the choice and concentration strategy when resources are limited, as we all humans do.”

Figure 2. The action of jasmonic acid (JA), the hormone involved in activation of anti-herbivore defense is inhibited in the leaves where shaded and thus its R:FR ratio is lowered (see Light Science Newsletter Vol. 4). In this way, plants can prevent too much resources from being consumed for defense in the leaves of low photosynthetic value (Ref. Ballaré, Annu. Rev. Plant Biol., 2014).
Jinwon Kim, Ph.D. 

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