A holistic view on plant effector-triggered immunity presented as an iceberg model
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Cellular and Molecular Life Sciences
INVITED REVIEW
A holistic view on plant effector‑triggered immunity presented as an iceberg model Hans Thordal‑Christensen1 Received: 18 December 2019 / Revised: 10 March 2020 / Accepted: 30 March 2020 © The Author(s) 2020
Abstract The immune system of plants is highly complex. It involves pattern-triggered immunity (PTI), which is signaled and manifested through branched multi-step pathways. To counteract this, pathogen effectors target and inhibit individual PTI steps. This in turn can cause specific plant cytosolic nucleotide-binding leucine-rich repeat (NLR) receptors to activate effectortriggered immunity (ETI). Plants and pathogens have many genes encoding NLRs and effectors, respectively. Yet, only a few segregate genetically as resistance (R) genes and avirulence (Avr) effector genes in wild-type populations. In an attempt to explain this contradiction, a model is proposed where far most of the NLRs, the effectors and the effector targets keep one another in a silent state. In this so-called “iceberg model”, a few NLR-effector combinations are genetically visible above the surface, while the vast majority is hidden below. Besides, addressing the existence of many NLRs and effectors, the model also helps to explain why individual downregulation of many effectors causes reduced virulence and why many lesion-mimic mutants are found. Finally, the iceberg model accommodates genuine plant susceptibility factors as potential effector targets. Keywords Plant immunity · Pathogen effectors · Nucleotide-binding leucine-rich repeat receptors · Lesion mimic mutants · Susceptibility Abbreviations AAA-ATPase ATPases Associated with diverse cellular Activities-ATPase ABC ATP-binding cassette ACC 1-AminoCyclopropane-1-Carboxylic acid ACD11 Accelerated Cell Death11 ADR1 Activated Disease Resistance ALD1 AGD2-Like Defense response protein1 AMSH3 Associated Molecule with the SH3 domain of STAM 3 Apaf1 Apoptotic protease activating factor1 Avr Avirulence BAK1 Brassinosteroid insensitive1-Associated receptor Kinase1 CBP60g Calmodulin-Binding Protein 60G CC Coiled Coil CED4 CEll Death abnormality4 CERK1 Chitin Elicitor Receptor Kinase1 * Hans Thordal‑Christensen [email protected] 1
Department of Plant and Environmental Sciences, Copenhagen Plant Science Centre, University of Copenhagen, 1871 Frederiksberg C, Denmark
CNL CC-NLR CPR1 Constitutive expresser of PathogenesisRelated genes1 CRCK3 Calmodulin-binding Receptor-like cytoplasmic Kinase3 CTR1 Constitutive Triple Response1 EDS1/5 Enhanced Disease Susceptibility1/5 EFR EF-Tu Receptor EF-Tu Elongation factor-Tu EHM ExtraHaustorial Membrane EIN2 Ethylene INsensitive2 ER Endoplasmic Reticulum ESCRT Endosomal Sorting Complexes Required for Transport Et Ethylene ETI Effector-Triggered Immunity ETR1 EThylene Response1 ETS Effector-Triggered Susceptibility EXO70B1 EXOcyst complex component70B1 FLS2 Flagellin Sensing2 FMO1 Flavin-dependent MonoOxygenase1 HopAI1/Z2 Hrp-dependent Outer ProteinAI1/Z2 HR Hypers
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