Abstract
Mycoviruses are wide spread throughout almost all groups of fungi but only a small number of mycoviruses can attenuate the growth and virulence of their fungal hosts. Alternaria alternata is an ascomycete fungus that causes leaf spot diseases on various crop plants. In this study, we identified a novel ssRNA mycovirus infecting an A. alternata f. sp. mali strain isolated from an apple orchard in China. Sequence analyses revealed that this virus is related to hypoviruses, in particular to Wuhan insect virus 14, an unclassified hypovirus identified from insect meta-transcriptomics, as well as other hypoviruses belonging to the genus Hypovirus, and therefore this virus is designed as Alternaria alternata hypovirus 1 (AaHV1). The genome of AaHV1 contains a single large open-reading frame encoding a putative polyprotein (∼479 kDa) with a cysteine proteinase-like and replication-associated domains. Curing AaHV1 from the fungal host strain indicated that the virus is responsible for the slow growth and reduced virulence of the host. AaHV1 defective RNA (D-RNA) with internal deletions emerging during fungal subcultures but the presence of D-RNA does not affect AaHV1 accumulation and pathogenicities. Moreover, AaHV1 could replicate and confer hypovirulence in Botryosphaeria dothidea, a fungal pathogen of apple white rot disease. This finding could facilitate better understanding of A. alternata pathogenicity and is relevant for development of biocontrol methods of fungal diseases.
Highlights
Mycoviruses are widely distributed across all major groups of phytopathogenic fungi (Ghabrial and Suzuki, 2009; Pearson et al, 2009)
We identified and characterized a novel hypovirulence-inducing hypovirus from A. alternata designated as Alternaria alternata hypovirus 1 (AaHV1)
For viral double-stranded RNA (dsRNA) quantification, total RNA was prepared from A. alternata mycelia cultured on potato dextrose agar (PDA) covered with cellophane as described by Andika et al (2017b)
Summary
Mycoviruses (fungal viruses) are widely distributed across all major groups of phytopathogenic fungi (Ghabrial and Suzuki, 2009; Pearson et al, 2009). Many unclassified hypoviruses (or hypo-like viruses) have recently been discovered from other filamentous fungi, e.g., Sclerotinia sclerotiorum (Xie et al, 2011; Hu et al, 2014; Khalifa and Pearson, 2014; Marzano et al, 2015) Valsa ceratosperma (Yaegashi et al, 2012), Fusarium graminearum (Wang et al, 2013; Li et al, 2015), Phomopsis longicolla (Koloniuk et al, 2014), Macrophomina phaseolina (Marzano and Domier, 2016), Botrytis cinerea (Hao et al, 2018), and Rosellinia necatrix (Arjona-Lopez et al, 2018). Alternaria alternata chrysovirus 1 has been described to alter host fungus growth, while it enhances the pathogenicity of fungal host on plants, most likely through the induction of host-specific toxin production (Okada et al, 2018). We demonstrated that AaHV1 confers hypovirulence in other plant phytogenic fungi
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