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Abstract
BackgroundPlant defensins represent a major innate immune protein superfamily that displays strong inhibitory effects on filamentous fungi. The total number of plant defensins in a conifer species is unknown since there are no sequenced conifer genomes published, however the genomes of several angiosperm species provide an insight on the diversity of plant defensins. Here we report the identification of five new defensin-encoding genes from the Picea glauca genome and the characterization of two of their gene products, named PgD5 and endopiceasin.ResultsScreening of a P. glauca EST database with sequences of known plant defensins identified four genes with homology to the known P. glauca defensin PgD1, which were designated PgD2-5. Whereas in the mature PgD2-4 only 7–9 amino acids differed from PgD1, PgD5 had only 64% sequence identity. PgD5 was amplified from P. glauca genomic DNA by PCR. It codes for a precursor of 77-amino acid that is fully conserved within the Picea genus and has similarity to plant defensins. Recombinant PgD5, produced in Escherichia coli, had a molecular mass of 5.721 kDa, as determined by mass spectrometry. The PgD5 peptide exhibited strong antifungal activity against several phytopathogens without any effect on the morphology of the treated fungal hyphae, but strongly inhibited hyphal elongation. A SYTOX uptake assay suggested that the inhibitory activity of PgD5 could be associated with altering the permeability of the fungal membranes. Another completely unrelated defensin gene was identified in the EST library and named endopiceasin. Its gene codes for a 6-cysteine peptide that shares high similarity with the fungal defensin plectasin.ConclusionsScreening of a P. glauca EST database resulted in the identification of five new defensin-encoding genes. PgD5 codes for a plant defensin that displays non-morphogenic antifungal activity against the phytopathogens tested, probably by altering membrane permeability. PgD5 has potential for application in the plant biotechnology sector. Endopiceasin appears to derive from an endo- or epiphytic fungal strain rather than from the plant itself.
Highlights
Plant defensins represent a major innate immune protein superfamily that displays strong inhibitory effects on filamentous fungi
Plant defensins are characterized as small globular, basic, cysteine-rich proteins (45–54 amino acids), containing a triple-stranded antiparallel β-sheet and one αhelix that are stabilized into a compact shape by four disulfide bridges [10,11,12,13,14]
Despite the tertiary structure being strongly conserved in plant defensins, the similarity on the primary sequence level is limited to eight cysteine residues, two glycines, one aromatic residue and a glutamic acid in the defined positions [17]
Summary
Plant defensins represent a major innate immune protein superfamily that displays strong inhibitory effects on filamentous fungi. Plant defensins are characterized as small globular, basic, cysteine-rich proteins (45–54 amino acids), containing a triple-stranded antiparallel β-sheet and one αhelix that are stabilized into a compact shape by four disulfide bridges [10,11,12,13,14]. These bridges form a cysteine-stabilized α-helix β-sheet motif (CSα/β) that is well conserved in peptides with antimicrobial activity. Variations in the amino acid sequences are reflected by small changes in the spatial display of the loops that contribute to the wide range of biological activities observed in these peptides, as a single amino acid substitution can change the spectrum of activity
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