Abstract
Plants employ the innate immune system to discriminate between self and invaders through two types of immune receptors, one on the plasma membrane and the other in the intracellular space. The immune receptors on the plasma membrane are pattern recognition receptors (PRRs) that can perceive pathogen-associated molecular patterns (PAMPs) or host-derived damage-associated molecular patterns (DAMPs) leading to pattern-triggered immunity (PTI). Particular pathogens are capable of overcoming PTI by secreting specific effectors into plant cells to perturb different components of PTI signalling through various mechanisms. Most of the immune receptors from the intracellular space are the nucleotide-binding leucine-rich repeat receptors (NLRs), which specifically recognize pathogen-secreted effectors to mediate effector-triggered immunity (ETI). In this review, we will summarize recent progress in structural studies of PRRs, NLRs, and effectors, and discuss how these studies shed light on ligand recognition and activation mechanisms of the two types of immune receptors and the diversified mechanisms used by effectors to manipulate plant immune signalling.
Highlights
As a PAMP molecule, bacterial flagellin can be recognized by plant and animal immune receptors (Gomez-Gomez & Boller, 2000; Hayashi et al, 2001; Zipfel et al, 2006)
Many PAMPs have been identified and the best characterized examples are flg22, EF-Tu, chitin, peptidoglycan, and lipopolysaccharides, which are recognized by FLS2 (GomezGomez & Boller, 2000; Zipfel et al, 2004), EFR (Zipfel et al, 2006), chitin elicitor receptor kinase 1 (CERK1) and LYK3/5 (Miya et al, 2007; Wan et al, 2008), LYM1/LYM3 (Willmann et al, 2011), and LORE (Ranf et al, 2015; Kutschera et al, 2019), respectively (Fig. 2)
The D1 domain of flagellin is recognized by TLR5 in animals (Fig. 3a) (Donnelly & Steiner, 2002; Yoon et al, 2012), whereas a conserved 22-amino acid fragment flg22 released by multi-step hydrolysis (Buscaill et al, 2019) is recognized by the LRR-RLK FLS2 in Arabidopsis (Zipfel et al, 2004; Sun et al, 2013)
Summary
Many PAMPs have been identified and the best characterized examples are flg, EF-Tu, chitin, peptidoglycan, and lipopolysaccharides, which are recognized by FLS2 (GomezGomez & Boller, 2000; Zipfel et al, 2004), EFR (Zipfel et al, 2006), CERK1 and LYK3/5 (Miya et al, 2007; Wan et al, 2008), LYM1/LYM3 (Willmann et al, 2011), and LORE (Ranf et al, 2015; Kutschera et al, 2019), respectively (Fig. 2). The chitin oligosaccharides from the degraded fungal cell wall can be recognized as a PAMP by the RLK chitin elicitor receptor kinase 1 (CERK1) and lysin-motif receptor-like kinase 5 (LYK5) in Arabidopsis (Miya et al, 2007), and by OsCERK1 and RLP OsCEBiP in rice (Kaku et al, 2006). All these receptors have the LysM extracellular domain, which is a conserved carbohydrate-binding module found in all kingdoms of life (Buist et al, 2008). AtCERK1 engages with the LysM-type RLPs, LYM1 and LYM3, which enables perception of peptidoglycans (PGNs), triggering immunity to bacterial pathogens (Willmann et al, 2011)
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