Abstract
Xanthomonas citri subsp. citri causes citrus canker disease, which is characterized by the formation of water-soaked lesions, white or yellow spongy pustules and brown corky canker. In this work, we report the contribution of extracellular endoglucanase to canker development during infection. The ectopic expression of nine putative cellulases in Escherichia coli indicated that two endoglucanases, BglC3 and EngXCA, show carboxymethyl cellulase activity. Both bglC3 and engXCA genes were transcribed in X. citri subsp. citri, however, only BglC3 protein was detected outside the cell in western blot analysis. The deletion of bglC3 gene resulted in complete loss of extracellular carboxymethyl cellulase activity and delayed the onset of canker symptoms in both infiltration- and wound-inoculation assays. When growing in plant tissue, the cell density of bglC3 mutant was lower than that of the wild type. Our data demonstrated that BglC3 is an extracellular endoglucanase required for the full virulence of X. citri subsp. citri.
Highlights
Plant phytobacteria produce several extracellular enzymes that digest host plant primary cell walls, playing roles in bacterial pathogenicity
The deletion mutation of the bglC3 gene resulted in complete loss of extracellular carboxymethyl hydrolysis activity, reduction in cell growth in citrus plants and delayed canker symptom emergence
Among the nine cellulase candidates from Xcc 29–1, BglC3 and EngXCA showed carboxymethyl cellulose hydrolysis activity when ectopically expressed in E. coli BL21(DE3)
Summary
Plant phytobacteria produce several extracellular enzymes that digest host plant primary cell walls, playing roles in bacterial pathogenicity. Extracellular enzymes have been well characterized in Pectobacterium and Dickeya species because of their essential roles in soft rot symptom production [1]. The Pectobacterium and Dickeya species produce pectinase, cellulase and protease, leading to the maceration of plant tissues [2,3]. The production of these extracellular enzymes is controlled by a few transcriptional factors. In Xanthomonads, the endoglucanase engXCA and engXCB double mutant of X. campestris pv. Campestris show a five-fold reduction in cellulose-degradation suggesting additional candidates contribute to cellulase production [7]. A mutation in the engXCB homolog in X. oryzae pv. A mutation in the engXCB homolog in X. oryzae pv. oryzae results in an 87% reduction in leaf lesion lengths, representing a remarkable reduction in virulence [8]
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