Calcium in plants

Abstract

Calcium is an essential plant nutrient. It is required for various structural roles in the cell wall and membranes, it is a counter-cation for inorganic and organic anions in the vacuole, and the cytosolic Ca2+ concentration ([Ca2+]cyt) is an obligate intracellular messenger coordinating responses to numerous developmental cues and environmental challenges. This article provides an overview of the nutritional requirements of different plants for Ca, and how this impacts on natural flora and the Ca content of crops. It also reviews recent work on (a) the mechanisms of Ca2+ transport across cellular membranes, (b) understanding the origins and specificity of [Ca2+]cyt signals and (c) characterizing the cellular [Ca2+]cyt-sensors (such as calmodulin, calcineurin B-like proteins and calcium-dependent protein kinases) that allow plant cells to respond appropriately to [Ca2+]cyt signals.

Fig. 1. Calcium disorders in horticultural crops: (a) cracking in tomato fruit; (b) tipburn in lettuce; (c) calcium deficiency in celery; (d) blossom end rot in immature tomato fruit; (e) bitter pit in apples; (f) gold spot in tomato fruit with calcium oxalate crystals (inset). Photographs (A–E) are from the HRI collection and (F) is courtesy of Lim Ho (HRI‐Wellesbourne).

Fig. 2. The relationships between Ca concentration in the soil solution ([Ca²⁺]_ext) and (A) shoot dry weight and (B) shoot Ca content of two calcifuge species Juncus squarrosus (filled circles) and Nardus stricta (filled triangles), the mineral‐tolerant Siegelingia decumbens (open circles), and the calcicole species Origanum vulgare (filled squares). Plants were grown for approx. 4 weeks after germination in a quartz sand to which was added a complete mineral solution containing various concentrations of CaCl₂. Data are taken from Jefferies and Willis (1964), who observed (a) that Juncus squarrosus did not establish at [Ca²⁺]_ext ≥ 0·8 mm and (b) that Origanum vulgare showed symptoms of Ca deficiency and was unable to survive at [Ca²⁺]_ext less than about 0·8 mm.

Fig. 3. The relationships between root CEC, derived from a literature survey, and shoot calcium concentration, derived from both literature and experimental data, for angiosperm orders (Table 1). Circles represent shoot Ca content data from a literature survey. Filled circles and linear regression represent orders with n ≥ 3 species sampled. Triangles represent shoot Ca content estimated in a phylogenetically balanced experiment.

Fig. 4. Cartoon illustrating the subcellular location of Ca²⁺ transporters in Arabidopsis thaliana based on Sze et al. (2000), Sanders et al. (2002) and White et al. (2002). In the plasma membrane there are hyperpolarization‐activated Ca²⁺ channels (HACC, possibly encoded by annexin genes), depolarization‐activated Ca²⁺ channels (DACC, one of which may be encoded by TPC1), Ca²⁺‐permeable outward rectifying K⁺ channels (KORC, encoded by SKOR and GORK), voltage‐insensitive cation channels (VICC, probably encoded by the CNGC and GLR genes) and Ca²⁺‐ATPases (one of which is ACA8). Biochemical and electrophysiological evidence indicates that IP₃‐receptors, IP₆‐receptors, cADPR‐activated (ryanodine)‐receptors and two types of voltage‐gated Ca²⁺ channels (the depolarization‐activated SV channels and the hyperpolarization‐activated Ca²⁺ channels) are present in the tonoplast together with Ca²⁺‐ATPases (including ACA4) and H⁺/Ca²⁺‐antiporters encoded by the CAX genes. There is also biochemical and electrophysiological evidence for the presence of NAADP‐receptors, IP₃‐receptors, cADPR‐activated (ryanodine)‐receptors and depolarization‐activated Ca²⁺ channels in the endoplasmic reticulum (ER), together with Ca²⁺‐ATPases (including ECA1 and ACA2). The Ca²⁺‐ATPase ACA1 is located in the plastid inner membrane.