The emerging role of apoplastic H⁺ in plant signaling

Protons (H⁺), commonly measured as pH, represent a fundamental physiological parameter in cellular systems. In plants, cytoplasmic H⁺ levels are tightly regulated to maintain homeostasis, whereas apoplastic H⁺ concentrations fluctuate dynamically in response to stress and developmental cues. This review discusses and scrutinizes the roles of apoplastic H⁺ fluctuations in transmitting stress and developmental cues, focusing on their interactions with established regulatory elements, such as phytohormones, H₂O₂, and peptide-receptor complexes, as well as other functional proteins, within stress and developmental signaling networks. Recent advances in understanding the mechanisms underlying apoplastic H⁺ as a pivotal regulator of plant adaptation and development open promising avenues for future research to enhance plant resilience and productivity under diverse environmental challenges.