Mangotoxin is an antimetabolite toxin that inhibits ornithine acetyl transferase, a key enzyme in the biosynthetic
pathway of ornithine and arginine and recently reported in strains of Pseudomonas syringae pv. syringae (Pss) isolated from
mango. Since symptoms on mango tissues are very difficult to reproduce, in this study the role of mangotoxin in Pss virulence
was addressed by analyzing the in planta growth and development of disease symptoms on tomato leaflets. Inoculation
experiments were carried out following several procedures using the wild-type strain Pss UMAF0158, two Tn5-mutant derivative
strains defective in mangotoxin production, and their complemented derivative strains in which mangotoxin production
is restored. The ability of the mangotoxin-defective mutants to grow in planta was similar, and their epiphytic survival on the
tomato leaf surface identical to the wild-type and complemented strains. However, both the disease index data of incidence
and the severity of necrotic symptoms indicated that mangotoxin-defective mutants were less virulent, indicating that mangotoxin
is a virulence factor. Furthermore, competition experiments showed that the survival values of the wild-type strain
were slightly but significantly higher than those of the mangotoxin-defective mutants, suggesting that mangotoxin production
would improve the epiphytic fitness of Pss.
