The active proton concentration – or pH – is a key parameter that defines every habitable environment. The preferred pH varies among species, making mechanisms that allow perceiving information on the prevailing proton activity mandatory to steer adequate responses that either direct migration along pH gradients or induce adaptive mechanisms to tune growth and development. Being sessile organisms lacking the recourse to migration, environmental pH (pHe) is of particular importance for plants, dictating the availability of essential mineral nutrients, the concentration of potentially toxic metal ions, and the composition of the microbiome. In addition, plants are challenged by dynamic changes of pHe, which requires constant recalibration of both cytosolic and apoplastic pH.

Limited information is available regarding as to how plants cope with the challenges associated with too acidic or too alkaline pHe. Alterations in ambient proton concentration requires adjustment of nutrient uptake patterns, reorientation of the growth-defense balance, and detoxification of excess metal in their environment. The proper functioning of ion channels, transporters, metabolic processes, intracellular trafficking, and endocytic turnover of cargo proteins is dependent on a precisely adjusted pH. Moreover, changes in apoplastic pH (and, possibly, also in cytosolic pH) induced by various biotic and abiotic stimuli are part of signaling cascades that negotiate the balance between growth and defense. This Research Topic aims at providing a comprehensive view on how plants perceive the pH of their environment and communicate this information to steer responses that adapt them to the prevailing conditions.

We endeavor to address this question by zooming in on an amalgamate of physiological processes affected by pHe, including but not limited to pH signaling, growth, pathogen defense, rhizotoxicity, and transport. Another focus is the interconnection of pH sensing with signal derived from other biotic and abiotic cues. Finally, we attempt to provide a recent update on tools and techniques to estimate pH within and outside cells. We invite contributions that consider the following topics:

 pH sensing and signaling

 pH-dependent transport processes

 Aluminum toxicity

 Bicarbonate tolerance

 Apoplastic pH control

 Cellular pH homeostasis

 Adaptation to alkaline soils

 Growth-defense balance

 Abiotic stress affecting cytosolic pH

 Organellar pH control

 Mineral nutrition at alkaline pH

 Systemic responses to pH

 Rhizotoxicity

 Composition and activity of the microbiome

 Rapid alkalinization factor (RALF) signaling

 Tools for pH determination

Descriptive studies that report responses to abiotic stress treatments without contributing to the mechanistic, or genetic, understanding of the responses observed will not be considered for peer review.

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Frontiers in Plant Science

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