The Gram-negative bacterium Xanthomonas axonopodis pv. citri, the causal agent of citrus canker, is a major threat to the citrus industry worldwide. Although this is a leaf spot pathogen, it bears genes highly related to degradation of plant cell walls, which are typically found in plant pathogens that cause symptoms of tissue maceration. Little is known on Xac capacity to cause disease and hydrolyze cellulose. We investigated the contribution of various open reading frames on degradation of a cellulose compound by means of a global mutational assay to selectively screen for a defect in carboxymethyl cellulase (CMCase) secretion in X. axonopodis pv. citri. Screening on CMC agar revealed one mutant clone defective in extracellular glycanase activity, out of nearly 3,000 clones. The insertion was located in the xpsD gene, a component of the type II secretion system (T2SS) showing an influence in the ability of Xac to colonize tissues and hydrolyze cellulose. In summary, these data show for the first time, that X. axonopodis pv. citri is capable of hydrolyzing cellulose in a T2SS-dependent process. Furthermore, it was demonstrated that the ability to degrade cellulose contributes to the infection process as a whole.
This study describes the use of electroporation for transforming Xanthomonas axonopodis pv. citri (Xac), the causal agent of citrus (Citrus spp.) canker. It also evaluates the methodology used for this species under different electrical parameters. The bacterium used in the study (Xac 306) was the same strain used for recent complete sequencing of the organism. The use of a plasmid (pUFR047, gentamycin r) is reported here to be able to replicate in cells of Xac. Following the preparation and resuspension of competent cells of Xac at a density of ~4 x 10(10) cfu/ml, in 10% glycerol, and the addition of the replicative plasmid, an electrical pulse was applied to each treatment. Selection of transformants showed a high efficiency of transformation (1.1 x 10(6) transformants/mug DNA), which indicates an effective, and inverse, combination between electrical resistance (50 W) and capacitance (50 µF) for this species, with an electrical field strength of 12.5 kV.cm-1 and 2.7-ms pulse duration. Besides the description of a method for electroporation of Xac 306, this study provides additional information for the use of the technique on studies for production of mutants of this species.
RESUMONa busca por novos indutores de resistência contra a vassoura-de-bruxa no cacaueiro avaliou-se o efeito de vários nutrientes, acibenzolar-S-metil (ASM) e a combinação desses nutrientes com ASM. Os produtos e as misturas foram pulverizados 30 dias antes da inoculação nas mudas do clone SIC-23. Foram utilizados os produtos comerciais Supa-potássio (silicato de potássio), Hortifós PK (fosfito de potássio) e Broadacre Mn (sulfato de manganês), testados nas dosagens 2,5 mL; 5,0 mL; 10 mL/L de água, isoladamente ou combinados com ASM (0,2g/L). O experimento foi conduzido no delineamento de blocos ao acaso, no esquema fatorial, com quatro repetições de doze plantas/parcela. A incidência da doença foi avaliada 60 dias após a inoculação. Somente o ASM promoveu controle significativo da vassoura-de-bruxa. Os nutrientes aplicados isoladamente não apresentaram efeito na severidade da doença. Por outro lado, o efeito protetor do ASM desapareceu quando este foi misturado ao Supa-potássio , na dose de 5 ou 10 mL/L. Termos para indexação:Resistência induzida, ASM, fosfitos, silicatos, manganês, Crinipellis perniciosa. ABSTRACTAiming at improving the level of induction of resistance in cocoa, various nutrients, acibenzolar-S-methyl (ASM) and their combination were tested on cocoa seedlings, clone SIC-23, 30 days before inoculation. The commercial products Supa-potássio (potassium silicate), Hortifós PK (potassium phosphite) and Broadacre Mn (manganese sulfate) were sprayed at doses of 2.5, 5.0 mL and 10.0 mL per liter of water, combined or not, with ASM (0.2 g/L). The experiment was set in a randomized block design, in a factorial scheme, with four replicates of twelve plants each. Disease incidence was assessed 60 days after inoculation. Only ASM promoted significant control of the disease. Nutrients alone had no effect on disease incidence. On the other hand, the protective effect of ASM disappeared when this product was mixed to Supa-potássio at 5 or 10 mL/L.
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