Abstract
Alternaria brassicae is an important necrotrophic pathogen that infects the Brassicaceae family. A. brassicae, like other necrotrophs, also secretes various proteinaceous effectors and metabolites that cause cell death to establish itself in the host. However, there has been no systematic study of A. brassicae effectors and their roles in pathogenesis. The availability of the genome sequence of A. brassicae in public domain has enabled the search for effectors and their functional characterization. Nep1-like proteins (NLPs) are a superfamily of proteins that induce necrosis and ethylene biosynthesis. They have been reported from a variety of microbes including bacteria, fungi, and oomycetes. In this study, we identified two NLPs from A. brassicae viz. AbrNLP1 and AbrNLP2 and functionally characterized them. Although both AbrNLPs were found to be secretory in nature, they localized differentially inside the plant. AbrNLP2 was found to induce necrosis in both host and non-host species, while AbrNLP1 could not induce necrosis in both species. Additionally, AbrNLP2 was shown to induce pathogen-associated molecular pattern (PAMP)-triggered immunity in both host and non-host species. Overall, our study indicates that AbrNLPs are functionally and spatially (subcellular location) distinct and may play different but important roles during the pathogenesis of A. brassicae.
Highlights
Plant pathogens secrete various proteins, secondary metabolites, and other small molecules to colonize the host by suppressing host defenses (Kamoun, 2006; Lindeberg et al, 2008)
Recent studies have revealed that necrotrophic pathogens secrete small proteinaceous effectors that can function as pathogenicity factors
We found two genes that contained the NPP1 domain viz. ABRSC02.1105 and ABRSC02.949 (GenBank accession numbers: MZ783062 and MZ783063)
Summary
Plant pathogens secrete various proteins, secondary metabolites, and other small molecules to colonize the host by suppressing host defenses (Kamoun, 2006; Lindeberg et al, 2008). The genus Alternaria contains many important plant pathogens causing diseases in agronomically important cereal, vegetable, oilseed, and fruit crops These species are known to be one of the major producers of host-specific toxins and metabolites that cause necrotic cell death and help in disease progression. The current study was undertaken to identify and functionally characterize the NLPs in A. brassicae, a notorious necrotrophic pathogen that mainly infects the Brassicaceae family and some plant species from other families (Ansari et al, 1990). We aimed to: (1) identify the NLPs in A. brassicae and their phylogenetic relationship with other known NLPs, (2) study the temporal expression pattern in the natural host—Brassica juncea, (3) determine their ability to cause necrosis in host as well as non-host species, (4) identify their subcellular localization, and (5) analyze if the NLPs in A. brassicae acted as PAMPs and induced PTI in the host
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