Functional analysis of the melanin biosynthesis genes ALM1 and BRM2-1 in the tomato pathotype of Alternaria alternata

Abstract

The tomato pathotype of Alternaria alternata (A. arborescens) produces the dark brown to black pigment melanin, which accumulates in the cell walls of hyphae and conidia. Melanin has been implicated as a pathogenicity factor in some phytopathogenic fungi. Here, two genes of the tomato pathotype for melanin biosynthesis, ALM1 and BRM2-1, which encode a polyketide synthetase and a 1,3,8-trihydroxynaphthalene (THN) reductase, respectively, have been cloned and disrupted in the pathogen. The gene-disrupted mutants, alm1 and brm2-1, had albino and brown phenotypes, respectively. The wild-type and the mutants caused the same necrotic lesions on the leaves after inoculation with spores. These results suggest that melanin is unlikely to play a direct role in pathogenicity in the tomato pathotype A. alternata. Scanning electron microscopy revealed that the conidia of both mutants have much smoother surfaces in comparison to the wild-type. The conidia of those mutants were more sensitive to UV light than those of the wild-type, demonstrating that melanin confers UV tolerance.

Activation of Melanin Synthesis in Alternaria infectoria by Antifungal Drugs

“…Consequently, we conclude that A. infectoria synthesizes DHN-melanin, which is similar to the melanins described to be synthesized by other Alternaria spp. (15)(16)(17)(18)(19)(20)(21). This claim was corroborated by the identification in A. infectoria of the genes encoding enzymes involved in DHN-melanin synthesis (2), trihydroxynaphthalene reductase, polyketide synthase, and scytalone dehydratase (see the supplemental material).…”

“…In Alternaria spp., only the DHN-melanin pathway for melanin synthesis has been described (15)(16)(17)(18)(19)(20)(21), and the gene cluster required for melanin synthesis was cloned from Alternaria alternata (15). In Alternaria spp., melanin is present in the conidia and is confined to the outer region of the primary cell walls and the septa, which delimit individual spore cells in the multicellular conidium.…”

“…Since Alternaria spp. naturally produce unmelanized appressoria, it is not surprising that A. alternata and A. brassicicola mutants that are deficient in melanin production retain their pathogenicity toward their hosts, which are usually plants (19,20,29).…”

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“…The fungal melanin may found outside the cell wall in fibres or granules compose of the secretion of phenols outside to oxidize externally by released enzymes from fungus later (Bell and Wheeler, 1986;Nosanchuk et al, 2015). A. alternata is known to produce melanin and compiles it in cellular walls of the fungal mycelium, as for its location within the cell wall of conidia; it is confirmed by the use of electron microscope studies and other studies (Camp-bell, 1969;Carzaniga et al, 2002;Kheder et al, 2012). In other fungi, melanin exists only in fungal mycelium and the spores are free of it, such as the Magnaporthe grisea (Kawamura et al, 1997).…”

Globally distributed root endophyte Phialocephala subalpina links pathogenic and saprophytic lifestyles

“…Gene cluster involved in melanin synthesis were predicted using protein (i) sequences of PKS genes experimentally shown to be involved in melanin synthesis such as G. lozoyensis PKS1 (AAN59953.1; [85]), Alternaria alternate alm (BAK64048.1; [84, 118]) and Alb1/PksP of Aspergillus fumigatus (XP_756095.1; [86]), (ii) additional genes involved in the melanin synthesis pathway such as scytalone dehydratases (BC1G_144888) and hydroxynaphthalene reductases (BC1G_04230) [55] and (iii) by comparing orthologous gene clusters including candidate PKS derived from QuartetS analysis for the 14 ascomycetous species with the presence of melanization in the respective species. In addition, the NRPS genes putatively involved in siderophore synthesis were annotated.…”