PAMPs, PRRs, effectors and R-genes associated with citrus-pathogen interactions

Abstract

Background: Recent application of molecular-based technologies has considerably advanced our understanding of complex processes in plant-pathogen interactions and their key components such as PAMPs, PRRs, effectors and R-genes. To develop novel control strategies for disease prevention in citrus, it is essential to expand and consolidate our knowledge of the molecular interaction of citrus plants with their pathogens.

Scope: This review provides an overview of our understanding of citrus plant immunity, focusing on the molecular mechanisms involved in the interactions with viruses, bacteria, fungi, oomycetes and vectors related to the following diseases: tristeza, psorosis, citrus variegated chlorosis, citrus canker, huanglongbing, brown spot, post-bloom, anthracnose, gummosis and citrus root rot.

Keywords: Citrus immunity; and citrus root rot; anthracnose; brown spot; citrus canker; citrus psorosis; citrus variegated chlorosis (CVC); gummosis; huanglongbing (HLB); post-bloom; tristeza of citrus.

Fig. 1.

Domain structures of Citrus PRRs and R-genes.

Fig. 2.

Parallel between the classical pattern-triggered immunity (PTI)/effector-triggered immunity (ETI) response and the framework of RNA silencing activation and suppression. (A) Upon virus attack, RNA-dependent RNA polymerases (RDR) produce dsRNA, a virus-associated molecular pattern (VAMP). Similar to PTI, the RNA silencing machinery coordinated by Dicer and RNA induced silencing complex (RISC) recognize and process viral PAMPs, forming the first layer of defence. (B) Viruses have acquired viral supressors of RNA silencing (VSRs) as effectors that suppress host defence, resulting in effector-triggered susceptibility (ETS). (C) In turn, plants developed resistance (R) proteins that recognize viral effectors and activate ETI.

Fig. 3.

Schematic representation of X. fastidiosa interaction with resistant and susceptible genotypes.

Fig. 4.

PAMPs and effectors in X. citri. The known PAMPs for X. citri flagellin (flg22), adhesion and lipopolysaccharides (LPS) are present and enter the host cell. The T3SS-delivered effectors, such as PthA4 and its homologues, are injected into the host cell and travel to the nucleus, where they can act as transcriptional regulators.

Fig. 5.

General overview of T3SS-delivered effectors found in citrus-canker causing Xanthomonas species. The centre of the Venn diagram represents the core effectors found among these genomes (see box on the right), with these Xanthomonas core effectors shown in red.

Fig. 6.

An interaction model between citrus and ‘Candidatus Liberibacter asiaticus’. Reported components of the liberibacter possibly associated with its virulence mechanisms regarding suppression of host immunity and manipulation of its physiology. Flagellin components induce blockage of phloem plasmodesmata and impair sap flow between cells. Starch grains accumulate in chloroplasts in response to several changes in enzymatic activities. Salicylic acid might be broken down into catechol by hydroxylases and reactive oxygen species are suppressed by the activity of peroxidases of the bacteria.

Fig. 7.

Schematic representation of target sites of toxins produced by A. alternata. The putative target site of ACT-toxins in tangerine is the plasma membrane. After the infection of citrus leaves, induction of fast lipid peroxidation and accumulation of hydrogen peroxide (H₂O₂) occurs, resulting in cell death. The target of ACR-toxin is the mitochondrion, which after contact with the host plant causes a rapid increase in electrolyte leakage and consequent cell death. Ch, chloroplast; ER, endoplasmic reticulum; Gl, Golgi apparatus; Mt, mitochondrion; Nu, nucleus; Pm, plasma membrane; Va, vacuole.

Fig. 8.

Oomycete pathogens of citrus release apoplastic effectors such as NEP-like, ParA1, NPP1 and CBEL, which can elicit plant responses and necrosis and/or cytoplasmic effectors, such as PSE1 (RxLR effector) and PcCRN4 (Crinkler effector), which use the plant machinery (a translocator) to invade the cytoplasm and interfere with auxin production or suppress plant immunity, respectively.

Fig. 9.

Conserved RxRL domain in P. parasitica effectors.

Fig. 10.

Conserved LxLFLAK domain in P. parasitica CRN effectors.

Fig. 11.

(A) Nymphs and (B) adults of Diaphorina citri, the vector of Candidatus Liberibacter spp., the causal agent of HLB.

Fig. 12.

Macugonalia leucomelas, one of the sharpshooter vectors of CVC disease.

Fig. 13.

Schematic representation of a citrus cell and its general molecular interactions with the main fungi, viruses, vectors, oomycetes, bacteria and unculturable bacteria. Fungi: The infection process of C. acutatum requires proteins Klap 1 for appressorium development and infection on leaves of key lime. Proteins PacC^KLAP2 are required for pH adjustment in the transition from biotrophic hyphae to necrotrophic. Spore (SP), appressorium (AP), biotrophic hyphae (BH), necrotrophy hyphae (NH). The conidia produced by both pathotypes of A. alternata germinate quickly and begin to produce toxins. The putative target site of ACT-toxins in tangerine is the plasma membrane and of ACR-toxin is the mitochondrion, both leading to cell death. Virus:Citrus tristeza virus (CTV) deploys three effectors – p20, p23 and coat protein (CP) – with activities of viral suppressors of RNA silencing (VSR) to overcome host resistance. Citrus psorosis virus (CPsV) 24K protein physically interacts with pre-miRNA in the nucleus and induces the expression of genes involved in disease symptom development, suggesting a putative VSR function. Vectors: During feeding, vectors deliver putative effectors through saliva in stylets into the plant intercellular space which can modulate host plant recognition, volatile emission and defence. Oomycetes: Oomycete pathogens of citrus can secrete two types of proteins: apoplastic and cytoplasmic effectors. The apoplastic effectors, such as NEP-like, ParA1, NPP1 and CBEL, are frequently related to plant responses and necrosis elicitation. The cytoplasmic effectors, such as PSE1 (RxLR effector) and PcCRN4 (Crinkler effector), may interfere with the physiology of plants (auxin production) or suppress plant immunity, respectively. Bacteria:X. citri is able to inject into the plant host cell effectors such as PthA, which heads towards the cell nucleus where it controls gene regulation. Xylella fastidiosa may have effector molecules that are secreted into the plant, but none have been characterized to date. Unculturable bacteria: ‘Candidatus Liberibacter asiaticus’ might quench H₂O₂ accumulation and signalling events by secretion of peroxidase enzyme during early stages of infection. A functional salicylate hydroxylase (SahA) predicted in the ‘CaLas’ genome converts salicylic acid (SA) into catechol and might suppress SA-mediated defence.