Abstract
‘Candidatus Liberibacter asiaticus’ (CLas) is one of the causal agents of citrus Huanglongbing (HLB), a bacterial disease of citrus trees that greatly reduces fruit yield and quality. CLas strains produce an array of currently uncharacterized Sec-dependent secretory proteins. In this study, the conserved chromosomally encoded protein CLIBASIA_03875 was identified as a novel Sec-dependent secreted protein. We show that CLIBASIA_03875 contains a putative Sec- secretion signal peptide (SP), a 29 amino acid residue located at the N-terminus, with a mature protein (m3875) of 22 amino acids found to localize in multiple subcellular components of the leaf epidermal cells of Nicotiana benthamiana. When overexpressed via a Potato virus X (PVX)-based expression vector in N. benthamiana, m3875 suppressed programmed cell death (PCD) and the H2O2 accumulation triggered by the pro-apoptotic mouse protein BAX and the Phytophthora infestans elicitin INF1. Overexpression also resulted in a phenotype of dwarfing, leaf deformation and mosaics, suggesting that m3875 has roles in plant immune response, growth, and development. Substitution mutagenesis of the charged amino acid (D7, R9, R11, and K22) with alanine within m3875 did not recover the phenotypes for PCD and normal growth. In addition, the transiently overexpressed m3875 regulated the transcriptional levels of N. benthamiana orthologs of CNGCs (cyclic nucleotide-gated channels), BI-1 (Bax-inhibitor 1), and WRKY33 that are involved in plant defense mechanisms. To our knowledge, m3875 is the first PCD suppressor identified from CLas. Studying the function of this protein provides insight as to how CLas attenuates the host immune responses to proliferate and cause Huanglongbing disease in citrus plants.
Highlights
Upon invasion of the host, phytopathogens produce numerous compounds to facilitate their survival in the host
We identified a new Sec-dependent secretory protein CLIBASIA_03875 (GenBank No ACT57351.1), which is comprised of only 51 amino acids and includes a putative Sec-secretion signal peptide (SP) at the N-terminus of 29 amino acids
CLIBASIA_03875 was annotated in CLas psy62 [26] and Ishi-1 [29] and was a small putative chromosomal protein comprised of 51 amino acids
Summary
Upon invasion of the host, phytopathogens produce numerous compounds (e.g., metabolites) to facilitate their survival in the host. The resulting interaction between the PAMPs and PRRs promotes (1) transcriptional reprogramming, (2) protein phosphorylation, (3) the activation of ion channels, (4) the production of reactive oxygen intermediates, (5) cell wall reinforcement, and (6) the accumulation of antimicrobial compounds. These protective measures lead to PAMP-triggered immunity (PTI), the first inducible plant innate immunity [1,2]. ETI is a strong local defense response that often culminates in hypersensitive response (HR) This selective pressure may drive the evolution of mutants that either suppress ETI by acquiring additional effectors or avoid ETI entirely by shedding or diversifying the effectors that are recognized by the plant [1,3]
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