Pseudocercospora griseola Causing Angular Leaf Spot on Phaseolus vulgaris Produces 1,8-Dihydroxynaphthalene-Melanin

Abstract

Pseudocercospora griseola is the causal agent of angular leaf spot of common bean (ALS). It has undergone parallel coevolution with its host and two major groups have been defined, "Andean" (P. griseola f. griseola) and "Mesoamerican" (P. griseola f. mesoamericana). The aim of this study was to analyze the nature and the level of the dark pigment synthesized by the representatives of each group. After 21 days of incubation on potato dextrose agar medium, P. griseola f. griseola isolate S3b developed colonies with diameters of 17.5 +/- 1.3 mm and concentric rings of pigmentation. Isolate T4 of P. griseola f. mesoamericana presented smaller colonies (9.9 +/- 0.3 mm) with a uniform dark-gray color. Both isolates, S3b and T4, produced the same pigment, a 1,8-dihydroxynaphthalene-melanin, although different in quantity and structural features as suggested by the IR spectrum. The P. griseola f. griseola isolate S3b had a higher growth rate and melanin content as well as smaller sensitivity to melanin synthesis inhibitors compared to the isolate T4 of P. griseola f. mesoamericana. These results suggest a possible link between melanin and growth in P. griseola.

Response of the fungus Pseudocercospora griseola f. mesoamericana to Tricyclazole

“…These polymeric substances might have a biological role such as virulence and/or survival against environmental stress factors (Bell and Wheeler 1986;Cooke and Whipps 1993;Chaskes et al 2008;Onofri et al 2008;Singaravelan et al 2008;Taborda et al 2008). Saparrat et al (2009) showed that Pseudocercopora griseola synthesizes DHN-melanins, a process that was inhibited by tricyclazole. Here, we confirmed that this inhibition leads to a reduction in fungal growth and pigmentation.…”

“…Concomitantly with this, the reddish pigment was released. Based on all these evidences, Saparrat et al (2009) hypothesized that the reddish pigments might be the soluble compounds released from fungal cultures grown in the presence of tricyclazole, which might be intermediate metabolites of melanin. Such a reduction in melanin synthesis, at least at certain concentrations of the inhibitor, affected mycelial growth; therefore, the melanin level in fungal wall may also be a determining factor in the organization of cell structure.…”

“…In this work, we found that such an intermediate metabolite had a MS/MS spectrum and a retro Diels-Alder like fragmentation pattern typical of flaviolin (McLafferty and Turecek 1993), which was the main metabolite associated with mycelium of P. griseola f. mesoamericana grown on medium with tricyclazole 100 ppm, probably as a result of the autoxidation of 1, 3, 6, 8-THN. Flaviolin might be a shunt product from the inhibition of reductases by tricyclazole (Zeng et al 2012), since isolate T4 synthesizes melanin through the DHNpathway (Saparrat et al 2009). …”

“…In contrast to other fungal species that need external precursors to synthesize this pigment (Franzen et al 2008), P. griseola synthesizes melanins constitutively, which might be related to morphogenesis, pathogenesis and/or survival strategies of the fungus (Elliott 1995;Langfelder et al 2003;Suryanarayanan et al 2004). Recently, Saparrat et al (2009) described that representatives of P. griseola from both gene pools synthesize 1,8-dihydroxynaphthalene (DHN) melanin. These same pigments have already been found in most Ascomycota, such as in Alternaria alternata (Carzaniga et al 2002), Bipolaris oryzae (Moriwaki et al 2004), Bipolaris sorokiniana (Kumar et al 2015), Cladosporium cladosporioides (Llorente et al 2012), Colletotrichum lagenarium, Magnaporthe grisea (Moriwaki et al 2004), Pseudocercospora fijiensis (Beltrán-García et al 2014) and Sporothrix schenckii (Romero-Martinez et al 2000).…”

“…1a) is a molecule that affects the DHN-melanin biosynthesis pathway through the inhibition of reductases that catalyze the transformation of 1, 3,6,3,6, to scytalone and of 1,3,8-trihydroxynaphthalene (1, 3, 8-THN) to vermelone (Bell and Wheeler 1986;Nobre and Ferreiro 2004). Saparrat et al (2009) and Llorente et al (2012) demonstrated that this fungicide reduced growth and melanization both on P. griseola and C. cladosporioides. Furthermore, this compound is used to control plant pathogens such as Magnaporthe grisea, Bipolaris oryzae, B. sorokiniana, Blumeria graminis, Colletotrichum lagenarium, C. lindemuthianum and Rhynchosporium secalis (Mares et al 2004;Kunova et al 2012;Kumar and Mukherjee 2013;Kumar et al 2015).…”