Molecular and biological properties of an endornavirus infecting winged bean (Psophocarpus tetragonolobus)

Mycoviruses: Antagonistic Potential, Fungal Pathogenesis, and Their Interaction with Rhizoctonia solani

Mycoviruses, or fungal viruses, are prevalent in all significant fungal kingdoms and genera. These low-virulence viruses can be used as biocontrol agents to manage fungal diseases. These viruses are divided into 19 officially recognized families and 1 unclassified genus. Mycoviruses alter sexual reproduction, pigmentation, and development. Spores and fungal hypha spread mycoviruses. Isometric particles mostly encapsulate dsRNA mycoviruses. The widespread plant-pathogenic fungus Rhizoctonia solani, which has caused a rice sheath blight, has hosted many viruses with different morphologies. It causes significant crop diseases that adversely affect agriculture and the economy. Rice sheath blight threatens the 40% of the global population that relies on rice for food and nutrition. This article reviews mycovirology research on Rhizoctonia solani to demonstrate scientific advances. Mycoviruses control rice sheath blight. Hypovirulence-associated mycoviruses are needed to control R. solani since no cultivars are resistant. Mycoviruses are usually cryptic, but they can benefit the host fungus. Phytopathologists may use hypovirulent viruses as biological control agents. New tools are being developed based on host genome studies to overcome the intellectual challenge of comprehending the interactions between viruses and fungi and the practical challenge of influencing these interactions to develop biocontrol agents against significant plant pathogens.

Successful full-length genomic cloning and characterization of site-specific nick structures of Phytophthora endornaviruses 2 and 3 in yeast, Saccharomyces cerevisiae

Two endornaviruses, Phytophthora endornavirus 2 (PEV2) and Phytophthora endornavirus 3 (PEV3), have been discovered in pathogens targeting asparagus. In this study, we analyzed the nick structure in the RNA genomes of PEV2 and PEV3 in the host oomycetes. Northern blot hybridization using positive and negative strand-specific RNA probes targeting the 5' and 3' regions of PEV2 and PEV3 RNA genomes revealed approximately 1.0 kilobase (kb) RNA fragments located in the 5' regions of the two genomes. 3' RACE analysis determined that the size of the RNA fragments were 958 nucleotides (nt) for PEV2 and 968 nt for PEV3. We have successfully constructed full-length cDNA clones of the entire RNA genomes of PEV2 and PEV3 using a homologous recombination system in the yeast, Saccharomyces cerevisiae. These full-length cDNA sequences were ligated downstream of a constitutive expression promoter (TDH3) or a galactose-inducing promoter (GAL1) in the shuttle vector to enable the production of the full-length RNA transcripts of PEV2 and PEV3 in yeast cells. Interestingly, a 1.0 kb RNA fragment from the PEV3 positive-strand transcript was also detected with a 5'-region RNA probe, indicating that site-specific cleavage also occurred in yeast cells. Further, when PEV2 or PEV3 mRNA was overexpressed under the GAL1 promoter, yeast cell growth was suppressed. A fusion protein combining EGFP to the N-terminus of the full-length PEV2 ORF or C-terminus of the full-length PEV3 ORF was expressed, and allowed PEV2 and PEV3 ORFs to be successfully visualized in yeast cells. Expression of the fusion protein also revealed presence of heterogeneous bodies in the cells.

Complete nucleotide sequence of an alphaendornavirus isolated from common buckwheat (Fagopyrum esculentum)

A double-stranded RNA (dsRNA) of approximately 16 kbp was isolated from symptomless common buckwheat (Fagopyrum esculentum) plants. The size of the dsRNA suggested that it was the replicative form of an endornavirus. The dsRNA was sequenced, and it consisted of 15,677 nt, containing a single open reading frame that potentially encoded a polyprotein of 5190 aa. The polyprotein contained conserved domains for a viral methyltransferase, viral RNA helicase 1, MSCRAMM family adhesion SdrC, UDP-glycosyltransferase, and viral RNA-dependent RNA polymerase 2. A site-specific nick in the plus strand was detected near the 5' end of the dsRNA. BLASTp analysis showed that the polyprotein shared the highest identity with the polyprotein of winged bean endornavirus 1. Results of phylogenetic analysis supported placing this novel virus from common buckwheat, which was provisionally named "Fagopyrum esculentum endornavirus 1", in the genus Alphaendornavirus of the family Endornaviridae.

Identification of a novel endornavirus in Geranium carolinianum and occurrence within three agroecosystems

A putative endornavirus was detected in Carolina geranium (Geranium carolinianum) in Louisiana, USA. The virus was provisionally named Geranium carolinianum endornavirus 1 (GcEV1). The viral RNA was sequenced, and it consisted of 14,625 nt containing a single ORF coding a putative polyprotein of 4815 aa with conserved domains for a helicase 1, peptidase C97, glycosyl transferase GTB-type, and RNA-dependent RNA polymerase 2. The 5'end consisted of 130 nt while the 3'end consisted of 54 nt ending in nine cytosine residues. The closest relative to GcEV1 was Phaseolus vulgaris endornavirus 3. In phylogenetic analyses, GcEV1 clustered with members of the genus Alphaendornavirus. GcEV1 was detected in 57 of 60 G. carolinianum plants collected from three distinct agroecosystems. The virus was not detected in eight other species of the genus Geranium. There was no association of a particular phenotypic trait of the host with the presence or absence of the virus. GcEV1 was transmitted at a rate of 100% in seeds of a self-pollinated G. carolinianum plant.

Viruses Infecting the Plant Pathogenic Fungus Rhizoctonia solani

The cosmopolitan fungus Rhizoctonia solani has a wide host range and is the causal agent of numerous crop diseases, leading to significant economic losses. To date, no cultivars showing complete resistance to R. solani have been identified and it is imperative to develop a strategy to control the spread of the disease. Fungal viruses, or mycoviruses, are widespread in all major groups of fungi and next-generation sequencing (NGS) is currently the most efficient approach for their identification. An increasing number of novel mycoviruses are being reported, including double-stranded (ds) RNA, circular single-stranded (ss) DNA, negative sense (−)ssRNA, and positive sense (+)ssRNA viruses. The majority of mycovirus infections are cryptic with no obvious symptoms on the hosts; however, some mycoviruses may alter fungal host pathogenicity resulting in hypervirulence or hypovirulence and are therefore potential biological control agents that could be used to combat fungal diseases. R. solani harbors a range of dsRNA and ssRNA viruses, either belonging to established families, such as Endornaviridae, Tymoviridae, Partitiviridae, and Narnaviridae, or unclassified, and some of them have been associated with hypervirulence or hypovirulence. Here we discuss in depth the molecular features of known viruses infecting R. solani and their potential as biological control agents.

A Review on Current Status and Future Prospects of Winged Bean (Psophocarpus tetragonolobus) in Tropical Agriculture

Winged bean, Psophocarpus tetragonolobus (L.) DC., is analogous to soybean in yield and nutritional quality, proving a valuable alternative to soybean in tropical regions of the world. The presence of anti-nutritional factors and high costs associated with indeterminate plant habit have been major concerns in this crop. But occurrence of good genetic variability in germplasm collections offers precious resources for winged bean breeding. However, lack of germplasm characterization is hindering such efforts. From a genomic standpoint, winged bean has been little studied despite rapid advancement in legume genomics in the last decade. Exploiting modern genomics/breeding approaches for genetic resource characterization and the breeding of early maturing, high yielding, determinate varieties which are disease resistant and free of anti-nutritional factors along with developing consumer friendly value-added products of local significance are great challenges and opportunities in the future that would boost cultivation of winged bean in the tropics. We review past efforts and future prospects towards winged bean improvement.