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 PacCKLAP2 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.