Abstract
Harpin proteins are produced by plant-pathogenic Gram-negative bacteria and regulate bacterial pathogenicity by inducing plant growth and defence responses in non-hosts. HpaG-Xcm, a novel harpin protein, was identified from Xanthomonas citri pv. mangiferaeindicae, which causes bacterial black spot of mango. Here, we describe the predicted structure and functions of HpaG-Xcm and investigate the mechanism of heat resistance. The HpaG-Xcm amino acid sequence contains seven motifs and two α-helices, in the N- and C-terminals, respectively. The N-terminal α-helical region contains two heptads, which form the coiled-coil (CC) structure. The CC region, which is on the surface of HpaG-Xcm, forms oligomeric aggregates by forming hydrophobic interactions between hydrophobic amino acids. Like other harpins, HpaG-Xcm was heat stable, promoted root growth and induced a hypersensitive response (HR) and systemic acquired resistance in non-host plants. Subjecting HpaG-Xcm to high temperatures altered the gene expression induced by HpaG-Xcm in tobacco leaves, probably due to changes in the spatial structure of HpaG-Xcm. Phenotypic tests revealed that the high-temperature treatments reduced the HR and disease resistance induced by HpaG-Xcm but had little effect on growth promotion. These findings indicate that the stability of interactions between CC and plants may be associated with thermal stability of HpaG-Xcm.
Highlights
In recent years, bacterial black spot of mango, which can infect leaves, branches and fruits has become increasingly serious in the main mango-producing areas, severely impacting product quality and yield
Phylogenetic analysis revealed that HpaXcm and the nine harpin proteins from Xanthomonas contained seven conservative motifs and could be divided into two classes: HpaG-Xcm, HpaG-Xag, HpaG-Xam, Hpa1Xac, HpaXm, Hpa1Xoc, Hpa1Xoo-2 and Hpa1Xoo; and XopA-Xcv and HpaXcc (Fig. 2)
Seven motifs were found in all eight amino acid sequences of the first group, whereas only four and two motifs were found in XopA-Xcv and HpaXcc, respectively
Summary
Bacterial black spot of mango, which can infect leaves, branches and fruits has become increasingly serious in the main mango-producing areas, severely impacting product quality and yield. Many harpin proteins from Xanthomonas spp. have been reported from different hosts, including HpaG-Xag from X. axonopodis pv. The amino acid sequences of harpin proteins from different plant pathogenic bacteria are different, whereas homologues of Xanthomonas spp. harpins from different species are highly homologous[5,8,9]. The N-terminal α-helical regions of Hpa1Xoo and Hpa1Xoc contain one Cys each, whereas the Cys sites of HpaG-Xag, HpaXac and HpaXm are placed by a threonine (Thr) residue[6]. Both of the Xanthomonas spp. harpins contain two main α-helices, at the N- and C-terminals, respectively. Given that different aggregates seemed to be formed by the interactions between them, Tarafdar et al.[15] speculated that the natural polymerization state of harpins might be related to their thermal stability
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