VibA, a Homologue of a Transcription Factor for Fungal Heterokaryon Incompatibility, Is Involved in Antifungal Compound Production in the Plant-Symbiotic Fungus Epichloë festucae

Abstract

Symbiotic association of epichloae endophytes (Epichloë/Neotyphodium species) with cool-season grasses of the subfamily Pooideae confers bioprotective benefits to the host plants against abiotic and biotic stresses. While the production of fungal bioprotective metabolites is a well-studied mechanism of host protection from insect herbivory, little is known about the antibiosis mechanism against grass pathogens by the mutualistic endophyte. In this study, an Epichloë festucae mutant defective in antimicrobial substance production was isolated by a mutagenesis approach. In an isolated mutant that had lost antifungal activity, the exogenous DNA fragment was integrated into the promoter region of the vibA gene, encoding a homologue of the transcription factor VIB-1. VIB-1 in Neurospora crassa is a regulator of genes essential in vegetative incompatibility and promotion of cell death. Here we show that deletion of the vibA gene severely affected the antifungal activity of the mutant against the test pathogen Drechslera erythrospila. Further analyses showed that overexpressing vibA enhanced the antifungal activity of the wild-type isolate against test pathogens. Transformants overexpressing vibA showed an inhibitory activity on test pathogens that the wildtype isolate could not. Moreover, overexpressing vibA in a nonantifungal E. festucae wild-type Fl1 isolate enabled the transformant to inhibit the mycelial and spore germination of D. erythrospila. These results demonstrate that enhanced expression of vibA is sufficient for a nonantifungal isolate to obtain antifungal activity, implicating the critical role of VibA in antifungal compound production by epichloae endophytes. E pichloae endophytes (holomorphic Epichloë and anamorphic Neotyphodium) are clavicipitaceous fungi that maintain a systemic and constitutive symbiotic relationship with a broad spectrum of cool-season grasses of the subfamily Pooideae (1). In the mutualistic relationship between epichloae endophytes and coolseason grasses, the reported benefits include protection of the host plant from insect and vertebrate herbivores (2-5), resistance to diseases (6-9), and an increase in tolerance to abiotic stresses, such as drought (10, 11). The protective ability of some epichloae species makes them suitable agents of biological plant protection against economically important grass diseases and insect and small animal herbivores (12). This encouraged breeders to develop and eventually led to the release of "endophyte-enhanced" turfgrass and perennial ryegrass cultivars (13).The production and release of anti-insect metabolites are among the known mechanisms for how epichloae endophytes protect their hosts from insect herbivory. Epichloae endophytes in association with their grass hosts are noted to synthesize bioprotective alkaloids, such as peramine and lolines, and another class of compound, the janthitrems, which increase resistance of the plant hosts to insect feeding (2, 14-16). Moreover, the genetic basis of the biosynthesis of endophyte-derived anti-in...

Identification of ε-Poly-L-lysine as an Antimicrobial Product from an Epichloë Endophyte and Isolation of Fungal ε-PL Synthetase Gene

“…A culture filtrate was found to inhibit conidial germination of the plant pathogen D. erythrospila. It was previously shown that the activity was 10 times as high as that of the original E437 strain [14]. The active filtrates were subjected to three-steps purification: (1) cation exchange resin (Amberlite-IRC 76), (2) reversed-phase HPLC (ODS column), and (3) normal-phase HPLC (HILIC column), affording an active substance in the yield of 38.6 mg/L.…”

“…We previously identified E. festucae strain E437 as a producer of antifungal substance against several grass pathogens such as Drechslera erythrospila, Colletotrichum graminicola, and Bipolaris sorokiniana [13]. We have isolated a mutant that had lost antifungal activity by random mutagenesis, and a transcription factor VibA was identified as an essential factor for the production of the antifungal activity [14]. Fungal VibA homologs (also known as VIB-1) are NDT80/PhoG-type transcription factors that regulate a wide range of genes involved in the vegetative incompatibility, conidiation, formation of aerial hyphae, development of female reproductive structure, and production of extracellular protease [15][16][17].…”

“…Fungal VibA homologs (also known as VIB-1) are NDT80/PhoG-type transcription factors that regulate a wide range of genes involved in the vegetative incompatibility, conidiation, formation of aerial hyphae, development of female reproductive structure, and production of extracellular protease [15][16][17]. Overexpression of VibA gene promoted the production of an unknown antifungal substance against plant pathogens [14]; however, identification of the antifungal product was remained to be solved. The present study highlights the purification, chemical characterization, and biosynthesis of the unknown antifungal substance produced by the Epichloë endophyte.…”

The N‐acetylglucosamine catabolic gene cluster in Trichoderma reesei is controlled by the Ndt80‐like transcription factor RON1

“…The involvement in multiple, seemingly unrelated processes has been reported for VIB-1, including the regulation of protoperithecial development, vegetative incompatibility and programmed cell death, regulation of responses to nitrogen and carbon starvation, as well as plant cell wall degradation by carbon catabolite repression (Dementhon et al, 2006;Hutchison and Glass, 2010;Xiong et al, 2014). In the endophytic plant-symbiont Epichloë festucae (Sordariomycetes), VIB-1 is involved in the production of antifungal compounds (Niones and Takemoto, 2015).…”

Lolium perenne apoplast metabolomics for identification of novel metabolites produced by the symbiotic fungus Epichloë festucae

“…The spectrum of epichloëcyclins produced varies between different endophyte strains as a result of differences in encoded GigA sequences; for example, epichloëcyclins A–N have been characterized, but only A–E are present in plants infected with E. festucae strain Fl1 (Johnson et al , 2015). Epichloë species are also known to protect the host, to varying degrees, from a range of fungal pathogens (Gwinn & Gavin, 1992; Bonos et al , 2005; Clarke et al , 2006; Steinebrunner et al , 2008; Niones & Takemoto, 2014, 2015; Tian et al , 2017). In addition to protection from these biotic stresses, Epichloë endophytes also protect hosts from abiotic stresses such as drought (Arachevaleta et al , 1989; Hahn et al , 2007) although the physiological basis of this phenomenon is not known.…”