Calcium signaling in plant-microbe interaction
Calcium ion (Ca2+) is an important second messenger involved in many signaling pathways in plants. The concentration of free Ca2+ in the cytosol in a plant cell ([Ca2+]cyt) connects the extracellular stimuli, including the perception of microbe-associated molecular patterns (MAMPs) by pattern-recognition receptors (PRRs), to intracellular responses. Since Ca2+ is neither synthesized nor degraded by plants, [Ca2+]cyt is completely dependent on the entry of external source or the release of Ca2+ from its intracellular stores. However, because Ca2+ can react with phosphate, the energy source of life, its presence in the cytoplasm will prevent energy metabolism and other cellular activities from taking place. Thus, its concentration is tightly regulated by various proteins. In plant-microbe interaction, different microbes trigger different receptor proteins, causing distinctive Ca2+ elevation patterns, referred to as Ca2+ signature. Ca2+ signatures can vary in different aspects: amplitude, duration, frequency, spatial distribution, and times of cycle in [Ca2+]cyt changes. The Ca2+ signature produced by PRRs' perception of microbes can be decoded by downstream effectors, changing the expression of defense or symbiosis-related genes, resulting in various different responses by plants.
Perception of Microbes
The first step of a Ca2+ signaling event is the perception of microbes performed by PRRs, a type of receptor protein located on the plasma membrane of plant cells. PRRs are capable of recognizing MAMPs, molecules specific to certain classes of microbes that are present in the extracellular space.
All PRRs that can recognize MAMPs of bacteria studied so far are either receptor-like kinase or receptor-like protein. All of them are transmembrane receptors. One example of these PRRs is flagellin-sensitive 2 (FLS2), which recognizes the flagellin protein in bacteria flagellum. MAMPs of symbiotic bacteria can also be recognized by PRRs. A lysin-motif (LysM) receptor-like kinase, nodulation (nod) factor perception (NFP), can recognize lipochitooligosaccharide in the nod factors released by symbiotic bacteria such as rhizobia.
An example of PRRs capable of recognizing fungi is the LysM-receptor kinase 5 and chitin elicitor receptor kinase 1 receptor complex, which recognizes chitin in the cell wall of fungi. LysM receptor-like kinases also recognize lipochitooligosaccharide in Myc factors released by symbiotic fungi such as mycorrhizal fungi.
Formation of Ca2+ Signature
After a PRR precepts microbes, it triggers an influx of Ca2+ ion from outside the cell, resulting in a unique Ca2+ signature. The activated PRR has two mechanisms: it can either trigger the entry of Ca2+ by itself or by activating a downstream receptor. For the first type of mechanism, there are two possibilities. The first possibility is that it can form a complex with the Ca2+ ion channels on the membrane physically and regulate the activity of the channels. The second possibility is to produce a signaling molecule by itself to bind to Ca2+ permeable channels or Ca2+ pumps and activate them. The other mechanism is to activate a downstream receptor-like cytoplasmic kinase (RLCK). Many of the PRRs activate BIK1, a RLCK which activates a NADPH oxidase called RBOHD by phosphorylating it. Activated RBOHD produces reactive oxygen species (ROS) such as H2O2, and the ROS interacts with Ca2+ channels and causes an influx of extracellular Ca2+, although the precise mechanism is not known. The increase in [Ca2+]cyt can then trigger the release of Ca2+ from internal sources, further increasing [Ca2+]cyt.
When PRR detects the presence of nod factors or myc factors, Ca2+ signaling in both cytoplasm and nucleus can happen. The influx of Ca2+ into nucleus is mediated by both Ca2+ channeling proteins and ATP-powered Ca2+ pumps located on the nucleus membrane, and the influx of Ca2+ into cytoplasm is likely caused by the interaction of ROS with channel proteins.
Decoding of Ca2+ Signature
As [Ca2+]cyt changes, the Ca2+ signature is decoded by Ca2+ sensor proteins. There are various types of sensor proteins in plants that can bind to Ca2+ ions, and they respond to the rise in [Ca2+]cyt in two different ways. The first possible type includes Calmodulin (CaM) and CaM-like proteins (CMLs). Binding to Ca2+ changes their structures and enables them to bind to their targets, calcineurin b-like proteins (CBLs). CBLs then interact with CBL-interacting protein kinases (CIPKs), enzymes that are capable of phosphorylation. The second possible way a sensor can decode the Ca2+ signature is that the sensor has a kinase domain of its own and can phosphorylate other molecules directly. An example of this type of sensor is Ca2+-dependent protein kinases (CDPKs or CPKs).
This can cause changes at both transcriptional and post-translational levels. At the post-translational level, the phosphorylation events by CIPKs or CPKs activate RBOHD in the cell membrane and lead to a further increase in the level of ROS production. The ROS then further promotes the influx of Ca2+, forming a positive-feedback loop. CCaMK, a sensor protein involved in decoding the Ca2+ signal at post-translational level, is involved in plant’s response to symbiotic signals. It is found in a complex that activates a transcription factor called RAM1, which regulates arbuscular branching, a process in the symbiosis of plants with arbuscular mycorrhizae. At the transcriptional level, Ca2+-modified CaM can bind to transcription factors that promote the transcription of genes related to plant immunity, such as the gene for the synthesis of salicylic acid in plants, causing the whole plant to be ready for pathogen attacks. However, more research is needed to understand how the transcription factors lead to plants’ defense against pathogens.
Ca2+ signal as a negative regulator
Although Ca2+ has been evolved to be utilized as a ubiquitous secondary messenger in plants, it is, by its nature, still a toxin, and long-term exposure to high Ca2+ levels can eventually lead to harm, and even death, to plant cells. So, Ca2+ ions’ presence in the cytoplasm needs to be removed after the signaling event. In fact, Ca2+ signal can also act as a negative regulator that down regulates the influx of Ca2+. It can down regulate it from two processes: formation of Ca2+ signature and decoding of Ca2+ signature.
In the formation of the Ca2+ signature, some CPKs can bind to BIK1 and phosphorylate it, thus promoting its degradation. Because BIK1 is directly downstream of many of the PRRs, and because it promotes ROS production, the degradation of BIK1 basically stops ROS-dependent defense signaling in plants. What’s more, some Ca2+-modified CaM can bind to Ca2+ channel proteins, such as cyclic nucleotide-gated ion channels (CNGCs), leading to their inactivation, preventing the Ca2+ influx and preventing the Ca2+ signature from forming.
In the decoding of the Ca2+ signature, some transcription factors bound by Ca2+-modified CaM actually suppress the expression of defense-related genes of plants, such as the CaM-binding transcriptional factor 3 (CaMTA3), which suppresses the expression of both EDS1 and NDR1, proteins required for the activation of plant defenses, by binding to their promoters.
Ca2+ signaling plays an important role in the plant-microbe interaction. PRRs are capable of detecting different classes of microbes, leading to a series of phosphorylation events by themselves or downstream effectors, causing the influx of Ca2+, and generating the Ca2+ signature. The Ca2+ signature is then decoded at both post-translational and transcriptional levels. It also negatively regulates the influx of Ca2+. However, little is known about how different patterns of Ca2+ signature correlate with external stimuli and how they lead to effective immune responses to pathogen infection, and there are still plenty of areas for further research.
- Sanders Dale et al. “Calcium at the Crossroads of Signaling” 2002. The Plant Cell 14:401–S417.
- Vadassery, J. and Oelmüller, R. “Calcium signaling in pathogenic and beneficial plant microbe interactions” 2009. Plant Signaling & Behavior 4:1024-1027.
- Edel, Kai H. et al. “The Evolution of Calcium-Based Signalling in Plants” 2017. Current Biology 27(13):R667-R679.
- Carafoli, E. and Krebs, J. “Why calcium? How calcium became the best communicator” 2016. Journal of Biological Chemistry 291(40):20849–57.
- Yuan et al. “Calcium signatures and signaling events orchestrate plant-microbe interactions” 2017. Current Opinion in Plant Biology 38:173-183.
- Lu, You and Tsuda, Kenichi. “Intimate Association of PRR- and NLR-Mediated Signaling in Plant Immunity” 2020. Molecular Plant-Microbe Interactions 34(1):3-14.
- Segonzac, Cécile and Zipfel, Cyril. “Activation of plant pattern-recognition receptors by bacteria” 2011. Current Opinion in Microbiology 14(1): 54-61.
- Chinchilla, D. et al. “A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence” 2007. Nature 448: 497-500.
- Gough, Clare and Jacquet, Christophe. “Nod factor perception protein carries weight in biotic interactions” 2013. Trends in Plant Science 18(10): 566-574.
- Liu, T. et al. “Chitin-Induced Dimerization Activates a Plant Immune Receptor” 2012. Science 336(6085): 1160-1164.
- Gust, Andrea A. et al. “Plant LysM proteins: modules mediating symbiosis and immunity” 2012. Trends in Plant Science 17(8): 495-502.
- Keinath, Nana F. et al. “Live Cell Imaging with R-GECO1 Sheds Light on flg22- and Chitin-Induced Transient (Ca2+)cyt Patterns in Arabidopsis” 2015. Molecular Plant 8(8):1188-1200.
- Li, Lei et al. “The FLS2-Associated Kinase BIK1 Directly Phosphorylates the NADPH Oxidase RbohD to Control Plant Immunity” 2014. Cell Host & Microbe 15(3):329-338.
- Oldroyd, Giles E. D. “Speak, friend, and enter: signalling systems that promote beneficial symbiotic associations in plants” 2013. Nature Reviews Microbiology 11:252-263.
- Cardenas, Luis et al. “Fast, transient and speciﬁc intracellular ROS changes in living root hair cells responding to Nod factors (NFs)” 2008. The Plant Journal 56:802-813.
- Seybold, Heike et al. “Ca2+ signalling in plant immune response: from pattern recognition receptors to Ca2+ decoding mechanisms” 2014. New Phytologist 204:782-790.
- Oda, Takashi et al. “Structure of the N-terminal Regulatory Domain of a Plant NADPH Oxidase and Its Functional Implications” 2010. Journal of Biological Chemistry 285(2):1435-1445.
- Cheval, C. et al. “Calcium/calmodulin-mediated regulation of plant immunity” 2013. Biochimica et Biophysica Acta (BBA) - Molecular Cell Research 1833(7):1766-1771.
- Monaghan, J. et al. “The Calcium-Dependent Protein Kinase CPK28 Buffers Plant Immunity and Regulates BIK1 Turnover” 2014. Cell Host & Microbe 16(5):605-615.
- DeFalco, Thomas A. et al. “Opening the Gates: Insights into Cyclic Nucleotide-Gated Channel-Mediated Signaling” 2016. Trends in Plant Science 21(11):903-906.