Nutrients are essential for plant growth and can also affect disease severity. Calcium has a role in plant disease resistance. It acts as a signal and transmits information in all eukaryotes (Allan et al., 2022). Calcium has dual function, it is a structural component for plant cell wall and membrane but also serves as a second messenger in many developmental and physiological processes (Thor 2019). Calcium must be available in sufficient amount. Roots take up calcium from the soil solution and transports it to the shoot via xylem. It may move across the root through the cytoplasm of the cells connected by plasmodesmata (the symplast) or through the spaces between the cells (the apoplast) (White and Broadley 2003). How the symplast and the apoplast pathway contribute to the delivery of calcium ion (Ca2+) to the xylem is not known (White 2001). The symplastic Ca2+ fluxes (for root nutrition and cell signaling) and apoplastic Ca2+ fluxes (for transfer to the shoot) would enable the root to fulfill the demand of the shoot for calcium without compromising the intracellular [Ca2+]cyt (cytosolic calcium) signals (White 2001). Calcium enters plant cells through Ca2+-permeable ion channels in their plasma membranes (White 2001; White and Broadley 2003).
Calcium influx in plant cells is required either for nutritional or signaling purposes. Nutritional mode involves passive Ca2+ supply via ion channels for structural and metabolic needs and relies on constitutive Ca2+ influx channels. Whereas the signaling mode depends on a rapid and transient ion-channel mediated increase in cytosolic Ca2+ referred to as a Ca2+ signal (Demidchik et al., 2018). This signal is essential for decoding internal and external stimuli, transducing them into physiological and gene expression responses (Demidchik et al., 2018).
Calcium acts as a second messenger in plant cells. Cytosolic free calcium concentration changes rapidly in response to various endogenous and environmental cues. Elevation in calcium concentration in plant cells upon pathogen infection is an early signaling event for plant defense responses (Zhang et al., 2014). The exogenous calcium treatment enhance resistance against Phoma arachidicola causing peanut web blotchby improving stability of cell membrane as well trigger production and accumulation of the reactive oxygen species inducing plant defense responses (Yan et al., 2024).
Microbes release various factors which are necessary for recognition by plants. In plant-fungal pathogen interactions, chitins, glucans, lipids, fatty acids (glycol-) proteins or peptides activate defense gene expression in the plant cell (Vadassery and Oelmuller 2009). During the plant-fungal pathogen interactions receptor-mediated cytoplasmic calcium elevation induces defense gene via the activation of ion fluxes at the plasma membrane, an oxidative burst and mitogen-activated protein kinases (MAPKs) activation (Vadassery and Oelmuller 2009). Phytopathogenic bacteria inject an array of effector proteins into host cells to alter host physiology and assist the infection process. Few of these effectors may trigger disease resistance because of recognition in plant cell by cytoplasmic immune receptor (Cui et al., 2009). The pathogenic attack induces calcium ion accumulation in the plant cell, resulting in calcium signature that mediate biotic signaling.
Calcium signaling and its associated machinery consists of calcium channel, calcium pumps and calcium binding proteins which have been identified. The signal encoding calcium channels and calcium pumps generate a variety of calcium transient in response to external stimuli, thus shaping the calcium signature (Allan et al., 2022). Large repertoire of calcium binding proteins exist that can decode calcium signature into specific responses. In general, Ca2+ dependent signaling pathways rely on transient elevations in cytosolic Ca2+ and certain stimuli cause a specific spatial and temporal increase of Ca2+ which is known as “calcium signature” (Qudeimat and Frank 2009). Multiple calcium-binding proteins are involved in plant immunity regulation (Yang et al., 2017). The change in Ca2+ concentration is sensed by several Ca2+ sensors or Ca2+ binding proteins. The role of nuclear Ca2+ signals in plant immune responses is linked with various calcium sensors, such as calmodulins (CaMs), CaM-like proteins and calcium-dependent protein kinases (CDPKs or CPKs). Utilizing these Ca2+ sensitive proteins located within the nucleus, nuclear Ca2+ signals can influence the transcriptional reprograming triggered by pathogen infection (Wang et al., 2024).
In plants [Ca2+]cyt signaling can take place through complex patterns such as waves and oscillations (Lecourieux et al., 2006). Guard cells perceive different chemical produced metabolically in response to abiotic and biotic stresses, integrate the signal into reactive oxygen species and calcium signature, converting these signatures into stomatal movements by regulating turgor pressure (Murata et al., 2015). Stimulus-specific calcium oscillations are necessary for stomatal closure (Allen et al., 2000). Chloroplast has a role in plant immunity as a site to produce salicylic acid and jasmonic acid mediating activation of plant immunity (Nomura et al., 2012). The study made by Nomura et al.(2012) show pathogen-associated molecular pattern (PAMP) signals are quickly relayed to chloroplast and evoke specific Ca2+ signature in the stroma. Chloroplast-localized protein calcium-sensing receptor (CAS) is involved in stromal Ca2+ transient and is responsible for PAMP-induced basal resistance and resistance (R)-gene mediated hypersensitive cell death (Nomura et al., 2012). In addition, CDPK or CPK regulate plant immune responses, both to PAMP and effectors (Yang et al., 2017). The multifaceted functions of CDPK in the complex immune and stress signaling include oxidative burst, stomatal movements, hormonal signaling and gene regulation (Boudsocq and Sheen 2012).
Different pathogens can induce calcium accumulation in the cytosol called calcium signatures [Ca2+]cyt. These calcium signature further control the diverse defense responsive proteins in the intercellular environment. The calcium signature then activates CDPK, CaMs and calcineurin B-like protein to impart defense signaling within the cells (Bhar 2023). CPKs are important Ca2+ sensors for decoding immune Ca2+ signals and CPKs can be found in plasma membrane, cytosol, nucleus, vacuolar membrane, peroxisomes, lipid bodies, endoplasmic reticulum and trans-Golgi network, which allows them to sense Ca2+ signals in subcellular regions and have specific interactions with their targets (Wang et al., 2024).
References:
Allan, C., Morris, R. J. and Meisrimler, C-N. 2022 Encoding Transmission, Decoding and Specificity of Calcium Signals in Plants. J. Exp. Bot. 73(11): 3372 – 3385
doi: 10.1093/jxb/erac105
Allen, G. J., Chu, S. P., Schumacher, K., Shimazaki, C. T., Vafeados, D., Kemper, A., Hawke, S. D., Tallman, G., Tsien, R. Y., Harper, J. F., Chory, J. and Schroeder, J. I. 2000 Alteration of Stimulus-Specific Guard Cell Calcium Oscillation and Stomatal Closing in Arabidopsis det3 Mutant. Science 289(5488): 2338 2342
doi: 10.1126/science.289.5488.2338
Bhar, A., Chakraborty, A. and Roy, A. 2023 The Captivating Role of Calcium in Plant-Microbe Interaction. Front. Plant Sci. 14: 1138252
doi: 10.3389/fpls.2023.1138252
Boudsocq, M. and Sheen, J. 2012 CDPKs in Immune and Stress Signaling. Trends Plant Sci. 18(1): 30 – 40
doi: 10.1016/j.tplants.2012.08.008
Cui, H., Xiang, T. and Zhou, J-M. 2009 Plant Immunity: A Lesson from Pathogenic Bacterial effector Proteins. Cell Microbiol. 11(10): 1453 – 1461
doi: 10.1111/j.1462-5822.2009.01359.x
Demidchik, V., Shabala, S., Isayenkov, S., Cuin, T. A. and Pottosin, I. 2018 Calcium Transport Across Plant Membranes: Mechanisms and Functions. New Phytologist 220(1): 49 – 69
doi.org/10.1111/nph.15266
Lecourieux, D., Ranjeva, R. and Pugin, A. 2006 Calcium in Plant Defence-Signaling Pathways. New Phytologist 171(2): 249 – 269
doi.org/10.1111/j.1469-8137.2006.01777.x
Murata, Y., Mori, I. C. and Munemasa, S. 2015 Diverse Stomatal Signaling and the Signal Integration Mechanism. Annu. Rev. Plant Biol. 66: 369 – 392
doi: 10.1146/annurev-arplant-043014-114707
Nomura, H., Komori, T., Uemura, S., Kanda, Y., Shimotani, K., Nakai, K., Furuichi, T., Takebayashi, K., Sugimoto, T., Sano, S., Suwastika, I. N., Fukusaki, E., Yoshioka, H., Nakahira, Y. and Shiina, T. 2012 Chloroplast-Mediated Activation of Plant Immune Signaling in Arabidopsis. Nature Communications 3: 926
Qudeimat, E. and Frank, W. 2009 Ca2+ Signatures. Plant Signal Behav. 4(4): 350 – 352
doi: 10.4161/psb.4.4.8218
Thor, K. 2019 Calcium-Nutrient and Messenger. Front. Plant Sci. 10: 440
doi: 10.3389/fpls.2019.00440
Vadassery, J. and Oelmuller, R. 2009 Calcium Signaling in Pathogenic and Beneficial Plant Microbe Interactions. Plant Signal behave. 4(11): 1024 – 1027
doi: 10.4161/psb.4.11.9800
Wang, Q., Cang, X., Yan, H., Zhang, Z., Li, W., He, J., Zhang, M., Lou, L., Wang, R. and Chang, M. 2024 Activating Plant Immunity: The Hidden Dance of Intracellular Ca2+ Stores. New Phytologist 242(6): 2430 – 2439
doi.org/10.1111/nph.19717
White, P. J. 2001 The Pathways of Calcium Movement to the Xylem. J Exp. Bot. 52(358): 891 – 899
doi: 10.1093/exbot/52.358.891
White, P. J. and Broadley, M. R. 2003 Calcium in Plants. Ann. Bot. 92(4): 487 – 511
doi: 10.1093/aob/mcg164
Yan, L., Liu, S., Li, R., Li, Z., Piao, J. and Zhou, R. 2024 Calcium Enhances the Resistance against Phoma arachidicola by Improving Cell Membrane Stability and Regulating Reactive oxygen Species Metabolism in Peanut. BMC Plant Biol. 24(1): 501
doi: 10.1186/s12870-024-05222-1
Yang, D-L., Shi, Z., Bao, Y., Yan, J., Yang, Z., Yu, H., Li, Y., Gou, M., Wang, S., Zou, B., Xu, D., Ma, Z., Kim, J. and Hua, J. 2017 Calcium Pumps and Interacting BON1 Protein Modulate Calcium Signature, Stomatal Closure and Plant Immunity. Plant Physiol. 175(1): 424 – 437
doi: 10.1104/pp.17.00495
Zhang, L., Du, L. and Poovaiah, B. W. 2014 Calcium Signaling and Biotic Defense Responses in Plants. Plant Signal Behav. 9(11): e973818
doi: 10.4161/15592324.2014.973818
BIOLOGICAL PROCESSES- A NATURE'S GIFT 