The enigma of environmental pH sensing in plants.

Environmental pH is a critical parameter for innumerable chemical reactions, myriad biological processes and all forms of life. The mechanisms that underlie the perception of external pH (pH _e ) have been elucidated in detail for bacteria, fungi and mammalian cells; however, little information is available on whether and, if so, how pH _e is perceived by plants. This is particularly surprising since hydrogen ion activity of the substrate is of paramount significance for plants, governing the availability of mineral nutrients, the structure of the soil microbiome and the composition of natural plant communities. Rapid changes in soil pH require constant readjustment of nutrient acquisition strategies, which is associated with dynamic alterations in gene expression. Referring to observations made in diverse experimental set-ups that unambiguously show that pH _e per se affects gene expression, we hypothesize that sensing of pH _e in plants is mandatory to prioritize responses to various simultaneously received environmental cues.