BackgroundLeucine-rich repeat receptor-like kinases (LRR-RLKs) represent the largest subfamily of plant RLKs. The functions of most LRR-RLKs have remained undiscovered, and a few that have been experimentally characterized have been shown to have important roles in growth and development as well as in defense responses. Although RLK subfamilies have been previously studied in many plants, no comprehensive study has been performed on this gene family in Citrus species, which have high economic importance and are frequent targets for emerging pathogens. In this study, we performed in silico analysis to identify and classify LRR-RLK homologues in the predicted proteomes of Citrus clementina (clementine) and Citrus sinensis (sweet orange). In addition, we used large-scale phylogenetic approaches to elucidate the evolutionary relationships of the LRR-RLKs and further narrowed the analysis to the LRR-XII group, which contains several previously described cell surface immune receptors.ResultsWe built integrative protein signature databases for Citrus clementina and Citrus sinensis using all predicted protein sequences obtained from whole genomes. A total of 300 and 297 proteins were identified as LRR-RLKs in C. clementina and C. sinensis, respectively. Maximum-likelihood phylogenetic trees were estimated using Arabidopsis LRR-RLK as a template and they allowed us to classify Citrus LRR-RLKs into 16 groups. The LRR-XII group showed a remarkable expansion, containing approximately 150 paralogs encoded in each Citrus genome. Phylogenetic analysis also demonstrated the existence of two distinct LRR-XII clades, each one constituted mainly by RD and non-RD kinases. We identified 68 orthologous pairs from the C. clementina and C. sinensis LRR-XII genes. In addition, among the paralogs, we identified a subset of 78 and 62 clustered genes probably derived from tandem duplication events in the genomes of C. clementina and C. sinensis, respectively.ConclusionsThis work provided the first comprehensive evolutionary analysis of the LRR-RLKs in Citrus. A large expansion of LRR-XII in Citrus genomes suggests that it might play a key role in adaptive responses in host-pathogen co-evolution, related to the perennial life cycle and domestication of the citrus crop species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-2930-9) contains supplementary material, which is available to authorized users.

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.

A wide range of potentially plant pathogenic microorganisms are naturally present in the environment. Despite relying only on the innate immune system, plants are able to resist most of the pathogens. Plants employ a multi‐layered defence system in which the first layer triggers the basal resistance (pathogen‐associated molecular pattern‐triggered immunity [PTI]). The second layer occurs when a resistance protein (R protein) that mostly encodes nucleotide‐binding leucine‐rich repeat receptors (NLRs) recognises an effector molecule secreted by an adapted pathogen, leading to effector‐triggered immunity (ETI), which triggers the hypersensitive response (HR). More recently, ETI was shown to restore and potentiate PTI signalling components, leading to a robust immune response. Multiple mechanisms of regulation are employed to guarantee proper HR activation. NLR proteins can interact between them and form a heel‐like pentamer that anchors to the plasma membrane. Furthermore, NLRs and other proteins can cooperate with NLRs to propagate the immune signalling. Downstream to the recognition of the pathogen by the plant, a rapid cellular response is initiated involving the generation of signalling events that precedes the HR. Here, we summarise the mechanisms involved in HR and highlight new advances in the knowledge of the immune system signalling. We also approach the role of HR threshold during infection by biotrophic, necrotrophic and hemibiotrophic pathogens and the impact in plant fitness and the community of pathogens found in the environment.

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