Abstract
Cysteine-rich receptor-like kinases (CRKs) are transmembrane proteins that exhibit ectodomains containing the domain of unknown function 26 (DUF26). The CRKs form a large subfamily of receptor-like kinases in plants, and their possible functions remain to be elucidated. Several lines of evidence suggest that CRKs play important roles in plant defense responses to environmental stress, including plant immunity. We performed a genome-wide analysis of CRK encoding genes in soybean (Glycine max). We found 91 GmCRKs distributed in 16 chromosomes, and identified several tandem and segmental duplications, which influenced the expansion of this gene family. According to our phylogenetic analysis, GmCRKs are grouped in four clades. Furthermore, 12% of the members exhibited GmCRKs with a duplicated bi-modular organization of the ectodomains, containing four DUF26 domains. Expression analysis of GmCRKs was performed by exploring publicly available databases, and by RT-qPCR analysis of selected genes in soybean leaves responding to biotic stress signals. GmCRKs exhibited diverse expression patterns in leaves, stems, roots, and other tissues. Some of them were highly expressed in only one type of tissue, suggesting predominant roles in specific tissues. Furthermore, several GmCRKs were induced with PAMPs, DAMPs and the pathogens Phakopsora pachyrhizi and Phytophthora sojae. Expression profiles of several GmCRKs encoding highly similar proteins exhibited antagonist modes of regulation. The results suggest a fine-tuning control of GmCRKs transcriptional regulation in response to external stimuli, including PAMPs and DAMPs. This study offers a comprehensive view of the GmCRKs family in soybean, and provides a foundation for evolutionary and functional analysis of this family of plant proteins involved in the perception of pathogens and activation of plant immunity.
Highlights
Plants activate the immune system by perception of pathogen associated molecular patterns (PAMPs)/microbe-associated molecular patterns (MAMPs), such as bacterial flagellin or fungal chitin, or by perception of plant damage associated molecular patterns (DAMPs), such as oligogalacturonides (OGs)
Glyma20G138700, was excluded from the Soybean Cysteine-rich receptor-like kinases (CRKs)-family and transcriptional regulation by biotic stress signals analysis since it was classified as a pseudogene at NCBI, and no transmembrane domain could be identified by hydrophobicity plot analysis
Surface localized CRKs have been implicated in Arabidopsis plant immunity [21], and they have been proposed as possible candidates involved in reactive oxygen species (ROS) perception [15, 17]
Summary
Plants activate the immune system by perception of pathogen associated molecular patterns (PAMPs)/microbe-associated molecular patterns (MAMPs), such as bacterial flagellin or fungal chitin, or by perception of plant damage associated molecular patterns (DAMPs), such as oligogalacturonides (OGs). Plant resistance proteins recognize pathogen effectors and activate effector-triggered immunity (ETI) that usually is a stronger plant response leading to programmed cell death at the site of infection [4]. The leucine rich repeat (LRR)RLK FLS-2 from Arabidopsis binds bacterial flagellin (flg22) and triggers plant immunity leading to resistance to bacterial phytopathogens [8, 9]. FLS2 integrates hetero-oligomeric receptor signaling complexes with other RLKs such as BAK1 [10]. The Arabidopsis LysM-RLK CERK1 ectodomain binds fungal chitin, leading to CERK1 homo-dimerization that integrates a plasma membrane-signaling complex triggering plant immunity [11]. In rice chitin perception involves a hetero-oligomeric receptor complex containing both LysM-RLK OsCERK1 and RLP-CEBiP [1]. Arabidopsis Wak1-RLK perceives damage by detecting molecules such as OGs [12]
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