Calcium is a macronutrient and has many important roles in the biology of plants – e.g. physiological, biochemical, and structural. Additional to those, and for many years now, one has been used to the notion that calcium is a so-called second messenger for plants. As a second messenger calcium is the tangible means by which external ‘factors’ are ultimately converted into intracellular actions with physiological and biochemical consequences for the well-being of the plant.
Given the importance of that mediating role, one might have imagined that a back-up plan would have evolved in the fullness of evolutionary time. Well, guess what! Yep, it now seems that calcium is not the only second messenger in plants. In an open access viewpoint article Sergey Shabala expresses the view that potassium should also be viewed as a second inorganic second messenger.
What prompts this suggestion is the recognition that the efflux of potassium in response to salt-stress could act as a metabolic signal that slows growth and elicits defence responses in response to this abiotic stress. Although a second messenger role for potassium may not be as widespread as that for calcium – yet! – it’s certainly a point of view, and shows that there’s still more to learn about the interaction between inorganic elements and organic life forms.
By way of achieving some uniformity among all living things, a role of potassium as a second messenger in plants extends the notion beyond prokaryotes where this element is seen as a second messenger in bacteria controlling gene expression and enzyme activity. Akin to the way that calcium is a second messenger in plants, animals and micro-organisms.
[Ed. – for more on the regulation of potassium transport and signaling in plants, see Yi Wang and Wei-Hua Wu’s article.]