Melanins from two selected isolates of Pseudocercospora griseola grown in-vitro: Chemical features and redox activity

Pseudocercospora griseola is the causal agent of Angular Leaf Spot (ALS), a disease of common bean. Due to its coevolution with beans, two major groups have been defined, Andean (P. griseola f. griseola) and Mesoamerican (P. griseola f. mesoamericana). The aim of this study was to characterize the dark pigment, melanin, synthetized by a selected isolate of each genic group of P. griseola when grown on Potato-dextrose broth. P. griseola f. griseola isolate S3b and P. griseola f. mesoamericana isolate T4 produced 1.7 +/- 0.6 and 4.1 +/- 0.9 mg of melanin per g of dry biomass, respectively. Although both melanins possessed similar UV-visible absorption spectroscopic pattern, P. griseola f. mesoamericana isolate T4 melanin had a lower UV-visible absorption, higher reducing activity and metal chelating ability than melanin from P. griseola f. griseola isolate S3b. However, when the size of the sample was 10 mg S3b melanin had a higher content of free phenolic groups. Furthermore, cell wall polysaccharides modified in melanin the availability of active phenolic groups, which was dependent on the fungal isolate and the size of the sample. Therefore, the amount and chemical features of melanin as well as its deposition in mycelium walls within isolates is different, which might explain the different pigmentation and physiological behaviours of these representatives of the two groups of P. griseola.