Deletion of a MFS transporter-like gene in Cercospora nicotianae reduces cercosporin toxin accumulation and fungal virulence.
Many phytopathogenic Cercospora species produce a host-nonselective polyketide toxin, called cercosporin, whose toxicity exclusively relies on the generation of reactive oxygen species. Here, we describe a Cercospora nicotianae CTB4 gene that encodes a putative membrane transporter and provide genetic evidence to support its role in cercosporin accumulation. The predicted CTB4 polypeptide has 12 transmembrane segments with four conserved motifs and has considerable similarity to a wide range of transporters belonging to the major facilitator superfamily (MFS). Disruption of the CTB4 gene resulted in a mutant that displayed a drastic reduction of cercosporin production and accumulation of an unknown brown pigment. Cercosporin was detected largely from fungal hyphae of ctb4 disruptants, but not from the surrounding medium, suggesting that the mutants were defective in both cercosporin biosynthesis and secretion. Cercosporin purified from the ctb4 disruptants exhibited toxicity to tobacco suspension cells, insignificantly different from wild-type, whereas the disruptants formed fewer lesions on tobacco leaves. The ctb4 null mutants retained normal resistance to cercosporin and other singlet oxygen-generating photosensitizers, indistinguishable from the parental strain. Transformation of a functional CTB4 clone into a ctb4 null mutant fully revived cercosporin production. Thus, we propose that the CTB4 gene encodes a putative MFS transporter responsible for secretion and accumulation of cercosporin.
Choquer
Mathias
M
Citrus Research and Education Center, Institute of Food and Agricultural Sciences (IFAS), University of Florida, 700 Experiment Station Road, Lake Alfred, FL 33850, USA.
Lee
Miin-Huey
MH
Bau
Huey-Jiunn
HJ
Chung
Kuang-Ren
KR
D001203:Ascomycota / Q000235:genetics* / Q000378:metabolism / Q000472:pathogenicity*; D001483:Base Sequence; D004271:DNA, Fungal / Q000235:genetics; D005656:Fungal Proteins / Q000235:genetics* / Q000378:metabolism; D017353:Gene Deletion; D005800:Genes, Fungal; D026901:Membrane Transport Proteins / Q000235:genetics* / Q000378:metabolism; D009183:Mycotoxins / Q000378:metabolism; D010569:Perylene / Q000031:analogs & derivatives* / Q000378:metabolism; D010935:Plant Diseases / Q000382:microbiology; D014026:Nicotiana / Q000382:microbiology; D014774:Virulence / Q000235:genetics
D004271:DNA, Fungal; D005656:Fungal Proteins; D026901:Membrane Transport Proteins; D009183:Mycotoxins; C400126:cercosporin transporter protein, Cercospora; D010569:Perylene; C026363:cercosporin