Abstract
Immunity of plants triggered by pathogen-associated molecular patterns (PAMPs) is based on the execution of an evolutionarily conserved defense response that includes the accumulation of pathogenesis-related (PR) proteins as well as multiple other defenses. The most abundant PR transcript of barley (Hordeum vulgare) leaf epidermis attacked by the powdery mildew fungus Blumeria graminis f. sp hordei encodes the germin-like protein GER4, which has superoxide dismutase activity and functions in PAMP-triggered immunity. Here, we show that barley GER4 is encoded by a dense cluster of tandemly duplicated genes (GER4a-h) that underwent several cycles of duplication. The genomic organization of the GER4 locus also provides evidence for repeated gene birth and death cycles. The GER4 promoters contain multiple WRKY factor binding sites (W-boxes) preferentially located in promoter fragments that were exchanged between subfamily members by gene conversion. Mutational analysis of TATA-box proximal W-boxes used GER4c promoter-beta-glucuronidase fusions to reveal their enhancing effects and functional redundancy on pathogen-induced promoter activity. The data suggest enhanced transcript dosage as an evolutionary driving force for the local expansion and functional redundancy of the GER4 locus. In addition, the GER4c promoter provides a tool to study signal transduction of PAMP-triggered immunity and to engineer strictly localized and pathogen-regulated disease resistance in transgenic cereal crops.
Highlights
Plants recognize attacking microorganisms by perceiving evolutionary conserved pathogen-associated molecular patterns (PAMPs), which leads to the execution of PAMP-triggered immunity (PTI; Jones and Dangl, 2006)
Barley and wheat GER4 genes are mainly expressed in leaf epidermis of B. graminis–attacked plants and in roots and coleoptiles of germinating barley seedlings (Wei et al, 1998; Zimmermann et al, 2006)
During the interaction of barley leaf epidermis with B. graminis, GER4 subfamily members represent the most abundant upregulated transcripts and are among the three to five most abundant transcripts of the entire analyzed transcriptome in this tissue, as determined from publicly available transcript profiling data (Table 1)
Summary
Plants recognize attacking microorganisms by perceiving evolutionary conserved pathogen-associated molecular patterns (PAMPs), which leads to the execution of PAMP-triggered immunity (PTI; Jones and Dangl, 2006) This response is evolutionarily conserved and includes the formation of local cell wall appositions and the accumulation of pathogenesis-related proteins as well as antimicrobial small molecules termed phytoalexins (Hammerschmidt, 1999; Dixon, 2001). Sp hordei (Bgh) has been studied in terms of defense-related genes and proteins (Caldo et al, 2004; Eckey et al, 2004; Dong et al, 2006), cell wall appositions (Wei et al, 1998; Huckelhoven et al, 1999; An et al, 2006), and key Germin-like proteins of barley have been described as pathogenesis-related proteins (Woo et al, 2000; Christensen et al, 2004; Zimmermann et al, 2006) They belong to the ubiquitously distributed cupin super family found in Archaeon, bacteria, and eukaryotes. The formation of reactive oxygen species may play a role in cell wall fortification against fungi at attempted penetration sites, as a signaling molecule
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