Characterization of Hpa1 from Xanthomonas axonopodis pv. citri in plant-pathogen interactions

SGRO, G:; FICARRA, F.A.; GOTTIG, N.; JORGELINA OTTADO

Congreso; XLVIII Reunión Anual de SAIB; 2012

Xanthomonas axonopodis pv. citri (Xac), the bacterium responsible for citrus canker, contains a gene in the hrp [for hypersensitive response (HR) and pathogenicity] cluster that encodes a harpin protein called Hpa1. Harpins are glycine-rich, cysteine-lacking, heat-stable proteins that trigger HR responses. Xac Hpa1 elicited HR in nonhost plants, whereas, in the host plant citrus, it elicited a weak defence response with no visible phenotype. Recombinant Hpa1 protein aggregated bacterial cells and co-infiltrations of Xac with Hpa1 in citrus leaves increased the number of cankers. To characterize the effect of Hpa1 during the disease, a Xachpa1 mutant was constructed, and infiltration of this mutant caused a smaller number of cankers. Also, the lack of hpa1 hindered bacterial aggregation both in culture medium and in planta. Moreover, analysis of citrus leaves infiltrated with Hpa1 revealed alterations in mesophyll morphology. We expressed the N-terminal and C-terminal regions and found that, although both regions elicited HR in nonhost plants, only the N-terminal region produced increased virulence and bacterial aggregation, supporting the role of this region of the protein as the main active domain. Our results suggest that, Hpa1 main roles in citrus canker are to alter leaf mesophyll structure and to aggregate bacterial cells, and thus increasing virulence and pathogen fitness.