Integrity of the Post-LRR Domain Is Required for TIR-NB-LRR Function.

Plants trigger appropriate defense responses, notably, through intracellular nucleotide-binding (NB) and leucine-rich repeat (LRR)-containing receptors (NLRs) that detect secreted pathogen effector proteins. In NLR resistance genes, the toll/interleukin-1 receptor (TIR)-NB-LRR proteins (TNLs) are an important subfamily, out of which approximately half the members carry a post-LRR (PL) domain of unknown role. We first investigated the requirement of the PL domain for TNL-mediated immune response by mutating the most conserved amino acids across PL domains of Arabidopsis thaliana TNLs. We identified several amino acids in the PL domain of RPS4, required for its ability to trigger a hypersensitive response to AvrRps4 in a Nicotiana tabacum transient assay. Mutating the corresponding amino acids within the PL domain of the tobacco TNL gene N also affected its function. Consequently, our results indicate that the integrity of the PL domain at conserved positions is crucial for at least two unrelated TNLs. We then tested the PL domain specificity for function by swapping PL domains between the paralogs RPS4 and RPS4B. Our results suggest that the PL domain is involved in their TNL pair specificity, 'off state' stability, and NLR complex activation. Considering genetically paired Arabidopsis TNLs, we finally compared the PL and TIR domains of their sensor and executor sequences, respectively. While TIR and PL domains from executors present complete motifs, sensors showed a lack of conservation with degenerated motifs. We here provide a contribution to the functional analysis of the PL domain in order to decipher its role for TNL function.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Host lignin composition affects haustorium induction in the parasitic plants Phtheirospermum japonicum and Striga hermonthica.

Parasitic plants in the family Orobanchaceae are destructive weeds of agriculture worldwide. The haustorium, an essential parasitic organ used by these plants to penetrate host tissues, is induced by host-derived phenolic compounds called haustorium-inducing factors (HIFs). The origin of HIFs remains unknown, although the structures of lignin monomers resemble that of HIFs. Lignin is a natural phenylpropanoid polymer, commonly found in secondary cell walls of vascular plants. We therefore investigated the possibility that HIFs are derived from host lignin. Various lignin-related phenolics, quinones and lignin polymers, together with nonhost and host plants that have different lignin compositions, were tested for their haustorium-inducing activity in two Orobanchaceae species, a facultative parasite, Phtheirospermum japonicum, and an obligate parasite, Striga hermonthica. Lignin-related compounds induced haustoria in P. japonicum and S. hermonthica with different specificities. High concentrations of lignin polymers induced haustorium formation. Treatment with laccase, a lignin degradation enzyme, promoted haustorium formation at low concentrations. The distinct lignin compositions of the host and nonhost plants affected haustorium induction, correlating with the response of the different parasitic plants to specific types of lignin-related compounds. Our study provides valuable insights into the important roles of lignin biosynthesis and degradation in the production of HIFs.

Haustorial Hairs Are Specialized Root Hairs That Support Parasitism in the Facultative Parasitic Plant Phtheirospermum japonicum.

A haustorium is the unique organ that invades host tissues and establishes vascular connections. Haustorium formation is a key event in parasitism, but its underlying molecular basis is largely unknown. Here, we use Phtheirospermum japonicum, a facultative root parasite in the Orobanchaceae, as a model parasitic plant. We performed a forward genetic screen to identify mutants with altered haustorial morphologies. The development of the haustorium in P. japonicum is induced by host-derived compounds such as 2,6-dimethoxy-p-benzoquinone. After receiving the signal, the parasite root starts to swell to develop a haustorium, and haustorial hairs proliferate to densely cover the haustorium surface. We isolated mutants that show defects in haustorial hair formation and named them haustorial hair defective (hhd) mutants. The hhd mutants are also defective in root hair formation, indicating that haustorial hair formation is controlled by the root hair development program. The internal structures of the haustoria in the hhd mutants are similar to those of the wild type, indicating that the haustorial hairs are not essential for host invasion. However, all the hhd mutants form fewer haustoria than the wild type upon infection of the host roots. The number of haustoria is restored when the host and parasite roots are forced to grow closely together, suggesting that the haustorial hairs play a role in stabilizing the host-parasite association. Thus, our study provides genetic evidence for the regulation and function of haustorial hairs in the parasitic plant.

A Plant Immune Receptor Detects Pathogen Effectors that Target WRKY Transcription Factors.

Defense against pathogens in multicellular eukaryotes depends on intracellular immune receptors, yet surveillance by these receptors is poorly understood. Several plant nucleotide-binding, leucine-rich repeat (NB-LRR) immune receptors carry fusions with other protein domains. The Arabidopsis RRS1-R NB-LRR protein carries a C-terminal WRKY DNA binding domain and forms a receptor complex with RPS4, another NB-LRR protein. This complex detects the bacterial effectors AvrRps4 or PopP2 and then activates defense. Both bacterial proteins interact with the RRS1 WRKY domain, and PopP2 acetylates lysines to block DNA binding. PopP2 and AvrRps4 interact with other WRKY domain-containing proteins, suggesting these effectors interfere with WRKY transcription factor-dependent defense, and RPS4/RRS1 has integrated a "decoy" domain that enables detection of effectors that target WRKY proteins. We propose that NB-LRR receptor pairs, one member of which carries an additional protein domain, enable perception of pathogen effectors whose function is to target that domain.

Two linked pairs of Arabidopsis TNL resistance genes independently confer recognition of bacterial effector AvrRps4.

Plant immunity requires recognition of pathogen effectors by intracellular NB-LRR immune receptors encoded by Resistance (R) genes. Most R proteins recognize a specific effector, but some function in pairs that recognize multiple effectors. Arabidopsis thaliana TIR-NB-LRR proteins RRS1-R and RPS4 together recognize two bacterial effectors, AvrRps4 from Pseudomonas syringae and PopP2 from Ralstonia solanacearum. However, AvrRps4, but not PopP2, is recognized in rrs1/rps4 mutants. We reveal an R gene pair that resembles and is linked to RRS1/RPS4, designated as RRS1B/RPS4B, which confers recognition of AvrRps4 but not PopP2. Like RRS1/RPS4, RRS1B/RPS4B proteins associate and activate defence genes upon AvrRps4 recognition. Inappropriate combinations (RRS1/RPS4B or RRS1B/RPS4) are non-functional and this specificity is not TIR domain dependent. Distinct putative orthologues of both pairs are maintained in the genomes of Arabidopsis thaliana relatives and are likely derived from a common ancestor pair. Our results provide novel insights into paired R gene function and evolution.

Structural basis for assembly and function of a heterodimeric plant immune receptor.

Cytoplasmic plant immune receptors recognize specific pathogen effector proteins and initiate effector-triggered immunity. In Arabidopsis, the immune receptors RPS4 and RRS1 are both required to activate defense to three different pathogens. We show that RPS4 and RRS1 physically associate. Crystal structures of the N-terminal Toll-interleukin-1 receptor/resistance (TIR) domains of RPS4 and RRS1, individually and as a heterodimeric complex (respectively at 2.05, 1.75, and 2.65 angstrom resolution), reveal a conserved TIR/TIR interaction interface. We show that TIR domain heterodimerization is required to form a functional RRS1/RPS4 effector recognition complex. The RPS4 TIR domain activates effector-independent defense, which is inhibited by the RRS1 TIR domain through the heterodimerization interface. Thus, RPS4 and RRS1 function as a receptor complex in which the two components play distinct roles in recognition and signaling.

HopAS1 recognition significantly contributes to Arabidopsis nonhost resistance to Pseudomonas syringae pathogens.

• Plant immunity is activated by sensing either conserved microbial signatures, called pathogen/microbe-associated molecular patterns (P/MAMPs), or specific effectors secreted by pathogens. However, it is not known why most microbes are nonpathogenic in most plant species. • Nonhost resistance (NHR) consists of multiple layers of innate immunity and protects plants from the vast majority of potentially pathogenic microbes. Effector-triggered immunity (ETI) has been implicated in race-specific disease resistance. However, the role of ETI in NHR is unclear. • Pseudomonas syringae pv. tomato (Pto) T1 is pathogenic in tomato (Solanum lycopersicum) yet nonpathogenic in Arabidopsis. Here, we show that, in addition to the type III secretion system (T3SS)-dependent effector (T3SE) avrRpt2, a second T3SE of Pto T1, hopAS1, triggers ETI in nonhost Arabidopsis. • hopAS1 is broadly present in P. syringae strains, contributes to virulence in tomato, and is quantitatively required for Arabidopsis NHR to Pto T1. Strikingly, all tested P. syringae strains that are pathogenic in Arabidopsis carry truncated hopAS1 variants of forms, demonstrating that HopAS1-triggered immunity plays an important role in Arabidopsis NHR to a broad-range of P. syringae strains.