Calcium signaling during salt stress and in the regulation of ion homeostasis

Soil composition largely defines the living conditions of plants and represents one of their most relevant, dynamic, and complex environmental cues. The effective concentrations of many either tolerated or essential ions and compounds in the soil usually differ from the optimum that would be most suitable for plants. In this regard, salinity—caused by excess NaCl—represents a widespread adverse growth condition, but shortage of ions such as K⁺, NO₃⁻, and Fe²⁺ also restrains plant growth. During the past years, many components and mechanisms that function in the sensing and establishment of ion homeostasis have been identified and characterized. Here, we reflect on recent insights that extended our understanding of components and mechanisms which govern and fine-tune plant salt stress tolerance and ion homeostasis. We put special emphasis on mechanisms that allow for interconnection of the salt overly sensitive pathway with plant development and discuss newly emerging functions of Ca²⁺ signaling in salinity tolerance. Moreover, we review and discuss accumulating evidence for a central and unifying role for Ca²⁺ signaling and Ca²⁺-dependent protein phosphorylation in regulating sensing, uptake, transport, and storage processes of various ions. Finally, based on this cross-field inventory, we deduce emerging concepts and questions arising for future research.