Cytosolic calcium and pH signaling in plants under salinity stress

Abstract

Calcium is one of the essential nutrients for growth and development of plants. It is an important component of various structures in cell wall and membranes. Besides some fundamental roles under normal condition, calcium functions as a major secondary-messenger molecule in plants under different developmental cues and various stress conditions including salinity stress. Also changes in cytosolic pH, pH(cyt), either individually, or in coordination with changes in cytosolic Ca(2+) concentration, [Ca(2+)](cyt), evoke a wide range of cellular functions in plants including signal transduction in plant-defense responses against stresses. It is believed that salinity stress, like other stresses, is perceived at cell membrane, either extra cellular or intracellular, which then triggers an intracellular-signaling cascade including the generation of secondary messenger molecules like Ca(2+) and protons. The variety and complexity of Ca(2+) and pH signaling result from the nature of the stresses as well as the tolerance level of the plant species against that specific stress. The nature of changes in [Ca(2+)](cyt) concentration, in terms of amplitude, frequency and duration, is likely very important for decoding the specific downstream responses for salinity stress tolerance in planta. It has been observed that the signatures of [Ca(2+)](cyt) and pH differ in various studies reported so far depending on the techniques used to measure them, and also depending on the plant organs where they are measured, such as root, shoot tissues or cells. This review describes the recent advances about the changes in [Ca(2+)](cyt) and pH(cyt) at both cellular and whole-plant levels under salinity stress condition, and in various salinity-tolerant and -sensitive plant species.

Figure 1

A proposed model of salt-stress tolerance in plants at cellular level. The sensing of ionic stress induces a transient elevation of cytosolic Ca²⁺ and pH.,,, Cytosolic-Ca²⁺ elevation is attributed with the influx of Ca²⁺ from cell wall, as well as from the vacuole. ER and mitochondria may also contribute to cytosolic-Ca²⁺ elevation. The cytosolic-pH increase seems to be linked with vacuolar-pH decreases. Cytosolic Ca²⁺ elevation and pH increase, either independently or in coordination, induces the SOS pathway for ionic homeostasis through inhibition of Na⁺ entry, enhancing K⁺ uptake into the cell, and sequestration of cytosolic Na⁺ either into the apoplast or vacuole.,, Cell organelles, such as nucleus, mitochondria, ER etc., might also have their own Ca²⁺ elevation and subsequent signaling cascade for downstream responses., Osmotic stress, in contrast to ionic toxicity, may either decrease or increase cytosolic-Ca²⁺ level, which might be linked with the production of reactive oxygen species (ROS). ROS then activates the MAPK pathways for osmotic homeostasis and detoxification responses.