Nitrogen metabolism is one aspect of basic metabolism, which is still quite unknown in the field of plant-pathogen interactions. Evidence derived from previous studies conducted in our laboratory strongly suggests that during microbial pathogenesis an important nitrogen mobilization process takes place in diseased tissues. Here we describe the expression pattern of asparagine synthetase (AS; EC 6.3.5.4) in tomato leaves infected by the bacterial pathogen Pseudomonas syringae pv. tomato. Using an homologous AS cDNA probe isolated by RT-PCR from infected leaves, we have observed a high level induction of AS expression during the course of infection. Concomitantly, a single AS polypeptide also accumulated in response to bacterial infection. Furthermore, immunohistochemical analysis of AS in infected leaves revealed a strong immunostaining in phloem cells of the main vascular bundles and in secondary veins of the leaf blade. These data correlate with those previously reported for expression of a cytosolic isoform of glutamine synthetase (GS1) also induced during development of the infectious process. Taken together, our results suggest the existence of a GS1/AS pathway representing a metabolic route for transferring ammonium released from protein catabolism into asparagine, an amino acid that may have a major role in nitrogen mobilization from diseased tissues.

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Bacterial apical necrosis is a critical disease in the main production area of mango in Europe. It is caused by Pseudomonas syringae pv. syringae, and produces necrotic lesions on mango buds and leaves, causing severe yield losses due to a decrease of flowering and fruit set. A field study to evaluate control treatments against bacterial apical necrosis was carried out during three seasons on mango trees cv. Tommy Atkins in Huelva (Spain). Experimental treatments included Bordeaux mixture, fosetyl-Al, acibenzolar-S-methyl, gibberelic acid, silicon gel, a mixture between acibenzolar-S-methyl and Bordeaux mixture, and combined applications of fosetyl-Al with Bordeaux mixture or silicon gel. The treatments which caused a consistent reduction in bacterial apical necrosis symptoms at similar levels to the conventional treatment with Bordeaux mixture, were the plant resistance activator acibenzolar-S-methyl and the phosphonate derivative fosetyl-Al applied singly or in combination with other compounds, which could be alternative treatments. These treatments showed a significant decrease in the necrotic buds and/or leaves numbers; however, minor differences in P. syringae-like population levels were observed. The analysis of the inhibitory and bactericidal concentrations of cupric compounds against P. syringae strains isolated from mango tissues suggests that the commercial copper-based treatments with Bordeaux mixture used in the management of mango crops do not work in a bactericidal mode of action.

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