Molecular Characterization of an Oomycete-Responsive PR-5 Protein Gene from <Emphasis Type="Italic">Zingiber zerumbet</Emphasis>

The tropical spice crop ginger (Zingiber officinale Roscoe) is highly susceptible to soft rot disease caused by the necrotrophic oomycete Pythium aphanidermatum (Edson) Fitzp. However, Zingiber zerumbet (L.) Smith, a wild relative of ginger, is resistant to P. aphanidermatum and has been proposed as a potential donor for soft rot resistance to Z. officinale. We identified a member of the pathogenesis-related protein group 5 (PR-5) gene family in Z. zerumbet that is expressed constitutively but upregulated in response to infection by P. aphanidermatum. Expression of this gene was upregulated as early as 1.5 h post inoculation (hpi) with the pathogen, peaked at 6 hpi, declined by 9 hpi, and again peaked at 15 hpi before declining at 48 hpi. A cDNA of this PR-5 gene, designated as ZzPR5, encodes a 226-amino-acid predicted protein with a calculated pI of 5.05. The N terminus of this protein contains a 22-amino-acid signal peptide, suggesting that the protein may show apoplastic accumulation like other acidic PR-5 proteins. Phylogenetic analysis revealed high similarity between ZzPR5 and PR-5 proteins reported from other plant species, especially from other Zingiberales. Molecular modeling of ZzPR5 protein revealed an acidic surface cleft, a feature characteristic of glycoside hydrolases and antifungal PR-5 proteins. In molecular docking studies, a linear polymeric molecule of (1,3)-β-d-glucan, a major constituent of the oomycete cell wall, fitted favorably into the surface cleft of ZzPR5 and interacted with acidic amino acids known to be involved in glucan hydrolysis, suggesting a potential antioomycete activity for ZzPR5 protein. Elucidation of the molecular mechanism of ZzPR5 may provide important insight toward engineering soft rot resistance into the obligatory asexual ginger.