The calcium conundrum. Both versatile nutrient and specific signal.

Versatility and specificity are usually mutually exclusive terms. However, as we discuss calcium’s role in plant nutrition, we are obliged to contrast the plethora of general housekeeping functions of this element against the ability of calcium (Ca) to impart signaling specificity during biological responses (Marschner, 1995). Plants have evolved to rely on the unique properties of Ca for a range of structural, enzymatic, and signaling functions. In this brief update, I will attempt to touch upon the various roles of Ca in plant nutrition, growth, and development. Ca is a relatively large divalent cation, and in contrast to other macronutrients, a high proportion of total Ca is found in the cell walls (apoplast). Some of this Ca is associated to the cell wall, while another portion is exchangeable at the plasma membrane. Additionally, Ca can be found at high concentrations within the vacuole of plant cells. In some tissues of particular plants, Ca can be more than 10% of the dry weight and cause no deleterious effects to plant growth (Marschner, 1995). Juxtaposed against these concentrations of total calcium, the cytosolic-free Ca activity within these cells remains around the 0.1 to 0.2 mM level. In this update, the first portion will detail the general role of Ca in plant growth and development. In themiddlesection,wewill focusonhowalmost imperceptible fluctuations in Ca within the cytosol may be translated into plant growth and adaptation. This then leads to the final part of this broad review, to a discussion regarding how biologists are attempting to manipulate Ca levels to improve plant productivity and human nutrition.