136, 2438–2442. While CaM, CMLs, CBLs, and CDPKs contain EF-hand Ca2+-binding motifs, CIPKs are activated by the interaction with CBLs (Kudla et al., 2018). The transport proteins responsible for the uptake of K+, AKT1 (Arabidopsis K+ transporter 1), and HAK5 (high-affinity K+ transporter 5) in Arabidopsis are both regulated via the same Ca2+ decoding complex consisting of CBL1/9 and CIPK23 (Xu et al., 2006; Ragel et al., 2015; Behera et al., 2017). (1999). Acid soils with calcium contents of 500 pounds per acre or less are deficient for legumes, especially peanuts, alfalfa, clovers, and soybeans. USA 97, 9323–9328. doi: 10.1105/tpc.4.9.1123, Riveras, E., Alvarez, J. M., Vidal, E. A., Oses, C., Vega, A., and Gutiérrez, R. A. Calcium can also be added to soil fertility programs and applied in irrigation water to ensure adequate levels are spoon fed to the plant. Genet. doi: 10.1111/tpj.13057. It is needed in large amounts by all plants for the formation of cell walls and cell membranes, and it plays a vital role in soil structure, according to an article on HelenaChemical.com. (San Diego: Academic Press). 11, 539–548. An endogenous peptide signal in Arabidopsis activates components of the innate immune response. (2011). In addition, voltage-independent channels (VICs) have been described. doi: 10.1016/S1360-1385(01)02124-0, Miedema, H., Demidchik, V., Véry, A.-A., Bothwell, J. H. F., Brownlee, C., and Davies, J. M. (2008). Plant Physiol. Calcium signals: the lead currency of plant information processing. Calcium is an essential element in plants. (2015) could finally show a nitrate-induced, NRT1.1- and phospholipase C-dependent Ca2+ signal. To protect themselves against this biotic stress, they have evolved a two-layered immune signaling network providing defense. doi: 10.1105/tpc.17.00136, Wang, Y., Kang, Y., Ma, C., Miao, R., Wu, C., Long, Y., et al. The author confirms being the sole contributor of this work and has approved it for publication. In principle, both the apoplastic and the symplastic pathway could be available for this movement, but, as discussed earlier, Ca2+ concentrations in the cytosol need to be in the submicromolar range to ensure the ability of the cell to generate a Ca2+ signal. A common model suggests that Ca2+ moves apoplastically from the epidermis through the cortex until it reaches the casparian strip of the endodermis (White, 2001). The influx of Ca2+ constitutes one of the earliest responses in the induced cascades. (2003), they found that the dwarf appearance of the mutant was dependent on high calcium supply, as was the suppression of HR. Calcium-dependent protein kinases play an essential role in a plant defence response. Soils with favorable pH levels are normally not deficient in calcium. (2002). Imaging of the Yellow Cameleon 3.6 indicator reveals that elevations in cytosolic Ca2+ follow oscillating increases in growth in root hairs of Arabidopsis. No use, distribution or reproduction is permitted which does not comply with these terms. Without proper levels of calcium, shelf life of fruits such as cantaloupes and tomatoes can be reduced significantly. Sci. New Phytol. Two spatially and temporally distinct Ca2+ signals convey Arabidopsis thaliana responses to K+ deficiency. Plant Cell 22, 508–522. doi: 10.1093/aob/mcg164, Wilkins, K. A., Matthus, E., Swarbreck, S. M., and Davies, J. M. (2016). Also, despite few examples, surprisingly little is known about how the involvement of calcium in one plant aspect is affecting another and whether the same or different genes are involved in two processes, as, for example, those taking up Ca2+ as a nutrient in roots and those responsible for influx during signal generation. Calcium requirement for ethylene-dependent responses. Calcium is only xylem mobile, meaning it can only move up the plant, and once in place, it cannot be remobilized and moved to new developing tissues. The Arabidopsis dnd1 “defense, no death” gene encodes a mutated cyclic nucleotide-gated ion channel. This review will describe the current knowledge of the mechanisms involved in uptake and transport of calcium as well as cellular homeostasis and signal generation, describing known genes involved and discussing possible implications the plant’s nutritional status with regard to calcium might have on immunity. Biophys. Trends Plant Sci. Different sets of proteins may determine the signature in response to different cues, although some components might be involved in several responses. Two voltage-dependent calcium channels co-exist in the apical plasma membrane of Arabidopsis thaliana root hairs. The casparian strip forms a barrier around endodermal cells, which consists mainly of suberin and lignin and prevents movement of water and solutes in order to inhibit the uptake of unwanted or toxic substances (Schreiber et al., 1999). Transient, sustained, or oscillatory rises in the cytosolic Ca2+ concentration thereby serve as a signal, which is decoded into downstream responses (McAinsh and Pittman, 2009; Dodd et al., 2010; Thor and Peiter, 2014). The RPM1 plant disease resistance gene facilitates a rapid and sustained increase in cytosolic calcium that is necessary for the oxidative burst and hypersensitive cell death. Acad. Mineral nutrition of higher plants. Calcium’s Role In Plant Nutrition Research shows soluble calcium fertilizer plays vital role in production of high-quality crops. The presence of these export proteins differs in different organelles. Plant Physiol. Plant Physiol. Conserved microbial patterns (pathogen-associated molecular patterns or PAMPs) or host-derived damage-associated molecular patterns (DAMPs) are recognized by surface-localized pattern-recognition receptors (PRRs), initiating a signaling cascade leading to induction of downstream responses such as defense gene expression, stomatal closure, and callose deposition at the infection site. Plants are exposed to a variety of pathogens, ranging from bacteria to fungi and oomycetes, and are also attacked by insects. In addition, different kinds of biotic stress also induce signaling cascades, in which calcium serves as second messenger. Curr. Silencing of NtCDPK2 resulted in impairment of gene-for-gene-dependent HR (Romeis et al., 2001). Advances and current challenges in calcium signaling. Plant Cell 29, 1460–1479. While earlier, these channels were described based on their electrophysiological characteristics (Demidchik et al., 2002; White et al., 2002; Miedema et al., 2008), advances in the sequencing of genomes have led to the identification of different channel families and the functional characterization of specific members thereof. Schreiber, L., Hartmann, K., Skrabs, M., and Zeier, J. *Correspondence: Kathrin Thor, email@example.com, Front. (2015). 1564, 299–309. Tomato plants cultivated in nutrient solution with high calcium content show increased resistance against infection by Ralstonia solanacearum (Yamazaki et al., 2000) and increasing calcium supply to soybean plants grown in the field reduced Phytophthora stem rot (Sugimoto et al., 2010). Hepler, P. K. (2005). Bacterial disease resistance in Arabidopsis through flagellin perception. It serves as a constituent of cell walls and membranes and thus contributes to the structure of cells and the upholding of physical barriers against pathogens. Plant Cell 17, 2142–2155. doi: 10.1073/pnas.95.13.7819, Zhang, S., Pan, Y., Tian, W., Dong, M., Zhu, H., Luan, S., et al.