the plant immune system
Some of the oldest and most impressive organisms on Earth are plants. Being able to thrive over hundreds to thousands of years in environments full of potentially harmful microbes requires a strong immune system. Plants have evolved macroscopic traits to combat environmental and biotic stresses, including thick outer layers of bark and waxy leaf cuticles, prickly spikes such as needles and thorns, and unsavoury chemical profiles that can deter herbivores. 

Should a microbe or pest breach one of those barriers, they will be met with microscopic defenses at the plant cell or tissue level. Plant cell walls are made up of sugars such as pectin that form a strong mesh that is difficult for pathogens to penetrate. Microbes that do live within plant tissues mostly colonize areas adjacent to plant cells called the apoplast.  If a plant cell recognizes a pathogen it often leads to localized cell death in order to save neighbouring tissues. A full immune response like this is costly and involves release of reactive molecules that can also damage the plant if not kept in check. Plant immune responses are therefore tightly controlled: immunity is turned on only when needed, and turned off as soon as the threat has cleared.
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Plant pests and diseases; CC0 1.0
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ZEISS Microscopy; CC BY-NC-ND 2.0
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Crescent Meadow, Sequoia National Forest; by Jerry Ting; CC BY-NC-SA 2.0

HOW DO CELLS RECEIVE AND PROCESS INFORMATION
​ABOUT THEIR EXTERNAL ENVIRONMENT?

Protein kinases: the phospho-modifiers

The plant anti-microbial immune system is gated by an armada of receptor proteins that bind foreign molecules and activate defence programs. The earliest experimentally-tractable responses following immune receptor activation include a burst of secondary messengers including calcium and reactive oxygen species that amplify the signal. A series of signaling cascades lead to changes in gene expression that result in further defenses being mounted - such as a thickening of the cell wall and the release of systemic signals that protect other areas of the plant. 
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Cellular signal transduction is largely mediated by protein kinases, enzymes that catalyze the attachment of phosphoryl groups to target proteins in a process called phosphorylation. Phosphorylation regulates protein function by influencing sub-cellular localization, binding partners, and, in the case of many enzymes, activity. Highly regulated phospho-relays pass messages received by proteins at the cell membrane to proteins in the nucleus to temporarily drive immune reprogramming and fight against disease. A lot of our work is aimed at understanding the molecular function of protein kinases in immune signaling pathways. 

​There are three main classes of intracellular signal transducing protein kinases:
  • Mitogen-activated protein kinases (MAPKs) - similar to ERK and Raf-like kinases in animals
  • Receptor-like cytoplasmic kinases (RLCKs) - similar to the RLK/Pelle family in animals 
  • Calcium-dependent protein kinases (CDPKs) - calcium-binding protein kinase family unique to plants and some protists
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We study proteins from all three superfamilies but recently our work has been largely centered around CDPKs and RLCKs. 

calcium-dependent protein kinases

CDPKs  are modular sensor-effector proteins that contain both a protein kinase domain (the ‘effector domain’ - coloured in grey to the right) and a bilobal calmodulin-like calcium-binding domain (the ‘sensor domain’ - coloured in pink), linked together by a hinge-like auto-inhibitory junction (coloured in black). The current model for activation posits that calcium binding to the CaM-like domain (CaMD) results in a conformational change that derepresses the kinase, allowing the CDPK to phosphorylate its substrates. 

​​We have learned a lot about the mechanism of CDPK activation through our work on CPK28. We recently discovered that phosphorylation of a single site in the kinase domain of CPK28 primes it for full calcium activation, which is important for its ability to rapidly respond to immune infection. This finding represents a step-change in our understanding of how phosphorylation can affect protein function, and we are excited to explore the role of phosphorylation on CDPK biochemistry more generally.   ​
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Created by Melissa Bredow in PyMol

read our work on this topic:
Bredow M, and Monaghan J. (2021) Differential regulation of the calcium-dependent protein kinase CPK28 by site-specific modification. Accepted in Plant Physiology. 

Bredow M, Bender KW, Johnson Dingee ​A, Holmes DR, Thomson A, Ciren D, Tanney C, Dunning KE, Trujillo M, Huber SC, Monaghan J. (2021) Phosphorylation-dependent subfunctionalization of the calcium-dependent protein kinase CPK28. PNAS. Preprint on BioRxiv. ​

Bredow M, Monaghan J. (2019) Regulation of plant immune siganling by calcium-dependent protein kinases. Molecular Plant-Microbe Interactions.

Bender K, Blackburn RK, Monaghan J, Derbyshire P, Menke FLH, Zipfel C, Goshe MB, Zielinski RE, Huber SC. (2017) Autophosphorylation-based calcium (Ca2+) sensitivity priming and Ca2+/Calmodulin inhibition of Arabidopsis thaliana Ca2+-dependent protein kinase 28 (CPK28). Journal of Biological Chemistry.

​​Monaghan J, Matschi S, Romeis T, Zipfel C. (2015) The calcium-dependent protein kinase CPK28 negatively regulates the BIK1-mediated PAMP-triggered calcium burst. Plant Signaling and Behavior, 10: 5, e1018497.

Monaghan J, Matschi S, Shorinola O, Rovenich H, Matei A, Segonzac C, Gro Malinovsky F, Rathjen J, MacLean D, Romeis T, Zipfel C. (2014) The calcium-dependent protein kinase CPK28 buffers plant immunity and regulates BIK1 turnover. Cell Host Microbe, 16 (5): 605-615.

e3 ligases: the UBIQuitin-modifiers

E3 ubiquitin ligases are enzymes that attach a small peptide called ubiquitin to target proteins in a process called ubiquitination. Proteins that are ubiquitinated can have different fates, depending on whether they are mono-, multi-mono, or poly-ubiquitinated. Poly-ubiquitinated proteins are shuttled to a cellular recycling machine called the 26S proteasome where they are degraded. There are hundreds of E3 ligases in plants that target proteins in multiple pathways and are essential for signal attenuation. In our lab, we study several E3 ligases that belong to the plant U-box (PUB) family. 

immune homeostasis

Pattern recognition receptor proteins (PRRs) line the plant cell membrane, scanning the external environment for anything that might cause damage. They are activated when they interact with 'danger signals' - classical examples are foreign molecules called microbe-associated molecular patterns (MAMPs) such as bacterial flagellin or fungal chitin. Many PRRs and/or their co-receptors are protein kinases that phosphorylate intracellular signaling kinases such as RLCKs and MAPKs, kick-starting cellular immunity. A well-studied convergent substrate of many receptors is the RLCK BIK1, which has been the focus of many labs across the world as it is major hub in immune pathways and is directly targeted by unrelated pathogens in order to suppress immune signaling and cause disease. 

While necessary for survival, sustained immune signaling is associated with considerable cellular damage, akin to chronic autoimmune diseases in humans, and is therefore exquisitely regulated both temporally and spatially. Our lab is interested in this aspect of immune signaling, which we call immune homeostasis.

Together with international collaborators, we found that the protein kinase CPK28 activates redundant ligases PUB25 and PUB26 that ubiquitinate BIK1 to mark it for proteasomal degradation. We think this regulation of BIK1 accumulation acts as a safeguard against inappropriately high immune signaling that may cause cellular damage.

In our ongoing work we are studying the role of additional PUBs in immune homeostasis and plant development, including a completely novel and unstudied subgroup that we anticipate will serve as an excellent model to understand the interplay between phosphorylation and ubiquitination more generally. ​
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Created by Melissa Bredow in BioRender

read our work on this topic:
Grubb LE, Derbyshire P, Dunning KE, Zipfel C, Menke FLH, Monaghan J. (2021) Large-scale identification of ubiquitination sites on membrane-associated proteins in Arabidopsis thaliana. Plant Physiology.  Preprint on BioRxiv. 

Monaghan J. (2018) Conserved degradation of orthologous RLCKs maintains immune homeostasis. Trends in Plant Science, 23: 555-558.

​​Wang J, Grubb LE, Wang J, Liang X, Li L, Gao C, Ma M, Feng F, Li M, Li L, Zhang X, Yu F, Xie Q, Chen S, Zipfel C, Monaghan J,  Zhou JM. (2018) A regulatory module controlling homeostasis of a plant immune kinase. Molecular Cell.

​Monaghan J, Matschi S, Romeis T, Zipfel C. (2015) The calcium-dependent protein kinase CPK28 negatively regulates the BIK1-mediated PAMP-triggered calcium burst. Plant Signaling and Behavior, 10: 5, e1018497.

Monaghan J, Matschi S, Shorinola O, Rovenich H, Matei A, Segonzac C, Gro Malinovsky F, Rathjen J, MacLean D, Romeis T, Zipfel C. (2014) The calcium-dependent protein kinase CPK28 buffers plant immunity and regulates BIK1 turnover. Cell Host Microbe, 16 (5): 605-615.

research facilities

Located in the BioSciences Complex at beautiful and historic Queen's University, our lab is well-equipped to enable molecular and cellular biology, plant genomics, large-scale phenotyping, and biochemistry. We have excellent plant growth facilities in our roof-top Phytotron, as well as access to the Confocal Microscopy Suite and the Genomics Core Facility in the Department of Biology. 

We are part of multiple communities on campus including the Plant Sciences Research Group (QueensUPlantSci), the Infection, Immunity & Inflammation Research Group (E1Q), and the Molecular, Cellular and Integrative Biology Research Group (MCIB), offering inter-disciplinary perspectives on our research and its impact beyond plant biology.

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For a full reading list please see our 'Publications' page

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Queen’s University is situated on the territory of the Haudenosaunee and Anishinaabek.

We are grateful to be able to live, learn, and play on these lands.