pH sensing: Why plants can’t have it all

It is the mechanisms by which plants sense the pH of their environment largely obscure. According to study revealed that plants employ an interlaced bimodal peptide receptor system to monitor the pH of the root apoplast, a readout that prioritizes either growth or defense.

As....Each andEvery hospitable habitat is defined by its pH, a property that can undergo dramatic changes in space and time. Information on ambient pH is critical for virtually all organisms which are under all circumstances, and often able to sense the external hydrogen ion concentration has become a decisive factor for survival. This is held in particular for plants, which lack motility and depend on rapid adaptive responses to change their environment.  Plants tightly govern the pH of the cytoplasm and also their organelles within a narrow range, constantly recalibrating the concentration ofthe protons to a signature value typically observed in the respective cell compartments.

This control does not end at the cell membrane but extends into the extracellular space of root cells, in which the hydrogen ion concentration remains stable even after exposure to media with differentpH1. Since the pH of the root environment dictates processes that are crucial for plant fitness and survival — such as cell wall properties and also the concentration of its essential or potentially toxic minerals —assuming that plants employ means to measure external pH is a tempting thought. Plants rapidly respond to changes in external pH with pronounced changes in transcriptional profiles, supporting this supposition2. Systems that can sense external pH and trigger responses that adapt the cells to the prevailing conditions have evolved in organisms across all kingdoms of life, including bacterial, yeast, and mammalian cells3. Conspicuously, no such system has been described in plants yet.

A recent study published in Cell appears to have solved the long-standing enigma of pH-sensing in plants4. Using Arabidopsis thaliana as a model, the authors discovered a bimodal pH-sensing system that uses information on the current pH of the apoplast to tip the balance between two essential tasks of root cells — growth and pathogen defense 

By connecting adjacent cells, the root apoplast acts as a hub for environmental information perceived from biotic and abiotic factors5. Changes in apoplastic were shown to be sufficient to alter root growth6,7, putting the mechanisms involved in decoding such information to steer downstream events in the spotlight. The apoplastic plant pH-sensing system described by Liu et al. consists of two components, each of which is comprised of a small peptide secreted from the cell interior into the apoplast and their respective receptors residing in the plasma membrane 

In line with classical acid growth theory, which states that low apoplastic pH supports growth by loosening the rigid cell wall surrounding plant cells8, the results from Liu et al. show that perception of acidity triggers a cascade of downstream events that, ultimately, supports growth and suppresses protection. At very low pH, a small peptide — referred to as root meristem growth factor (RGF) — is post-transcriptionally activated by sulfonation of a tyrosine residue, which establishes binding of the peptide to its receptor GPR and the co-receptor SERK, both residing in the root apical meristem9. Alkaline pH, this sulfotyrosine residue is

Above and beyond the mechanisms underlying perception of external proton levels, Liu et al. show that monitoring the pH in the plant apoplastoptimizes the trade-off between defense and growth, a compromise that prevents plants from allocating too many resources to the root apical meristem while fightingoff pathogens. Cessation of growth caused by elevated pH makes the cell wall more rigid and protective against harmful bacteria or other infectious agents. Decisions of how to allocate limited resources need to be carefully balanced to optimize the plant’s fitness........

Over the past decades......

• it remained unclear whether responses of plants to alterations in external pH are a ‘red herring’ (i.e., a clue leading away from more important questions), or indicative of the existence of a bona fide pH-sensing mechanism in plants.
• The latter scenario resembles ‘Chekhov's gun principle where a seemingly unimportant detail becomes significant later on, as, for example, a loaded gun mentioned at the beginning of a story that is fired at some point.
• The study by Li et al. decided their herring versus Chekhov's gun convention in Chekhov's favor but also raised the obvious question as to whether this is all there is.
• Alterations in apoplastic are triggered by and associated with disparate biological processes such as development, abiotic stress, and the availability of essential mineral nutrients, necessitating behavioral responses that are tailored to adapt the plant to thesespecific conditions. It is hard to imagine that such multifaceted tasks can