How plants’ breaking news help microbes and animals to stay alert
For millennia, humans have eased their aches with the help of plant molecules such as the famous salicylic acid, active ingredient of aspirin. These natural products are in fact a stress response of the plant when nutrient supplies in the soil become limited. Howitz and Sinclair (2008) propose that the beneficial effects of plant molecules are no coincidence but that animals (and humans) have learned to sense them and adjust their behavior accordingly; in other words – they are using the plant’s chemical chatter as an early warning system to be prepared for less food.
The authors coin this effect xenohormesis. Xenos is the Greek word for stranger; hormesis refers to the effect of a molecule that induces a mild stress and thereby activates cellular pathways that are beneficial. Xenohormesis describes how stress in one organism (the plant) induces the production of a molecule that activates beneficial pathways in another (the animal).
When a plant absorbs less nutrients through its roots, it produces higher levels of secondary metabolites – it is stressed. The stress molecules are secreted in small amounts to the nearest environment where bacteria and fungi listen to the news and respond: While some of them increase their efforts to help the plant take up nutrients (mostly symbionts) others raise their molecular shields to stay competitive. In both cases the plant molecule mediated a benefit. Similarly, these molecules can affect an organism when it feeds on the producing plant (or when it extracts and uses the active ingredient).
Howitz and Sinclair point out that one of the secrets of the success of molecules like salicylic acid is that instead of targeting just one protein it interacts with many key regulators in the cell thus, inducing a range of benefits. The current understanding is that these effects are due to a common origin, i.e. are caused because our common ancestors produced the same molecules and receptors for them. However, the authors argue that in some cases a plant product interacts with mammalian enzymes even when the animal lacks a structural relative. Therefore, they propose that mammalian enzymes and receptors have evolved binding sites for the plant’s stress molecules. By sensing the molecules they also sense the status of the environment, predict adverse conditions and adapt their feeding behavior to improve fitness and extend their life span.
To test their hypothesis, the authors suggest three main approaches: 1) compare how stress molecules produced in different far-related plants interact with proteins of the organisms that feed on them; 2) characterize the binding sites of the stress molecules to identify conserved domains and to distinguish between antagonistic and agonistic activity; 3) induce stress in model plants to examine the effect on life span of the feeding insects. Howitz and Sinclair conclude that with increasing understanding of the plant-animal interactions, natural products will celebrate a come-back in pharmaceutical research and help us to ease even more aches in the future.
Xenohormesis: Sensing the chemical cues of other species; Konrad T. Howitz and David A. Sinclair, Cell 133, May 2, 2008
The photo of the beautiful caterpillar was kindly provided by Kathrin Hack! Please check out more of her photography – www.naturfoto-hack.de