Not one, but two second messengers?

Confusingly, a second second messenger is not a third messenger.
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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.

Example of primary active transport, where energy from hydrolysis of ATP is directly coupled to the movement of a specific substance across a membrane independent of any other species.
Example of primary active transport, where energy from hydrolysis of ATP is directly coupled to the movement of a specific substance across a membrane independent of any other species. Image: LadyofHats Mariana Ruiz Villarreal / Wikipedia

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.

Aha, I get it now. Calcium and kalium (the Latin name for potassium and the reason why its chemical symbol is K and not P which is for phosphorus…). I guess it was plain to ‘c’ all along…

[Ed. – for more on the regulation of potassium transport and signaling in plants, see Yi Wang and Wei-Hua Wu’s article.]

References

WHITE, P. J. (2003). Calcium in Plants. Annals of Botany, 92(4), 487–511. https://doi.org/10.1093/aob/mcg164

Hepler, P. K. (2005). Calcium: A Central Regulator of Plant Growth and Development. THE PLANT CELL ONLINE, 17(8), 2142–2155. https://doi.org/10.1105/tpc.105.032508

Ranty, B., Aldon, D., Cotelle, V., Galaud, J.-P., Thuleau, P., & Mazars, C. (2016). Calcium Sensors as Key Hubs in Plant Responses to Biotic and Abiotic Stresses. Frontiers in Plant Science, 7. https://doi.org/10.3389/fpls.2016.00327

Edel, K. H., Marchadier, E., Brownlee, C., Kudla, J., & Hetherington, A. M. (2017). The Evolution of Calcium-Based Signalling in Plants. Current Biology, 27(13), R667–R679. https://doi.org/10.1016/j.cub.2017.05.020

Shabala, S. (2017). Signalling by potassium: another second messenger to add to the list? Journal of Experimental Botany, 68(15), 4003–4007. https://doi.org/10.1093/jxb/erx238

Pereira, A. (2016). Plant Abiotic Stress Challenges from the Changing Environment. Frontiers in Plant Science, 7. https://doi.org/10.3389/fpls.2016.01123

Ninfa, A. J. (2007). Regulation of carbon and nitrogen metabolism: Adding regulation of ion channels and another second messenger to the mix. Proceedings of the National Academy of Sciences, 104(11), 4243–4244. https://doi.org/10.1073/pnas.0700325104

Wang, Y., & Wu, W.-H. (2017). Regulation of potassium transport and signaling in plants. Current Opinion in Plant Biology, 39, 123–128. https://doi.org/10.1016/j.pbi.2017.06.006


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