Abstract
Roots are subjected to a range of abiotic stresses as they forage for water and nutrients. Cytosolic free calcium is a common second messenger in the signaling of abiotic stress. In addition, roots take up calcium both as a nutrient and to stimulate exocytosis in growth. For calcium to fulfill its multiple roles must require strict spatio-temporal regulation of its uptake and efflux across the plasma membrane, its buffering in the cytosol and its sequestration or release from internal stores. This prompts the question of how specificity of signaling output can be achieved against the background of calcium’s other uses. Threats to agriculture such as salinity, water availability and hypoxia are signaled through calcium. Nutrient deficiency is also emerging as a stress that is signaled through cytosolic free calcium, with progress in potassium, nitrate and boron deficiency signaling now being made. Heavy metals have the capacity to trigger or modulate root calcium signaling depending on their dose and their capacity to catalyze production of hydroxyl radicals. Mechanical stress and cold stress can both trigger an increase in root cytosolic free calcium, with the possibility of membrane deformation playing a part in initiating the calcium signal. This review addresses progress in identifying the calcium transporting proteins (particularly channels such as annexins and cyclic nucleotide-gated channels) that effect stress-induced calcium increases in roots and explores links to reactive oxygen species, lipid signaling, and the unfolded protein response.
Highlights
Plant roots are exposed to a variety of abiotic stresses as they navigate the soil, foraging for nutrients and water
Components common to different abiotic stresses are emerging such as Arabidopsis CIPK23 in K+ and nitrate deprivation
Receptor like kinases such as FERONIA or Proline-rich Extensinlike Receptor Kinase4 (PERK4) have emerged as new components in [Ca2+]cyt signaling and perhaps other related proteins will be found to have a role in abiotic stress signaling
Summary
Plant roots are exposed to a variety of abiotic stresses as they navigate the soil, foraging for nutrients and water. Specificity of [Ca2+]cyt signaling is determined by the amplitude and duration (and possible oscillation) of the [Ca2+]cyt increase, often referred to as the “signature”’ (McAinsh and Pittman, 2009), that is elicited by the stimulus This signature would be driven by the opening of plasma membrane (PM) and endomembrane Ca2+-permeable channels and terminated by the activity of Ca2+ efflux transporters in those membranes, plus Ca2+-binding proteins, to restore the resting [Ca2+]cyt of 100–200 nM. Calmodulins (CaMs) and Calmodulinlike proteins (CMLs) are encoded by multi-gene families in plants They lack kinase domains, suggesting these proteins must target others with enzymatic activity. Root cells will express specific complements of these genes and their transcription can change under abiotic stress (Dinneny et al, 2008; Roy et al, 2008), with the implication that stress resets the [Ca2+]cyt signaling system. The candidate channels for elevating [Ca2+]cyt in roots will be introduced and the downstream consequences of the signal will be reviewed
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