The 5' untranslated region of Bean pod mottle virus RNA2 tolerates unusually large deletions or insertions

A cowpea severe mosaic virus-based vector simplifies virus-induced gene silencing and foreign protein expression in soybean

BACKGROUND

Soybean gene functions cannot be easily interrogated through transgenic disruption (knock-out) of genes-of-interest, or transgenic overexpression of proteins-of-interest, because soybean transformation is time-consuming and technically challenging. An attractive alternative is to administer transient gene silencing or overexpression with a plant virus-based vector. However, existing virus-induced gene silencing (VIGS) and/or overexpression vectors suitable for soybean have various drawbacks that hinder their widespread adoption.

RESULTS

We describe the development of a new vector based on cowpea severe mosaic virus (CPSMV), a plus-strand RNA virus with its genome divided into two RNA segments, RNA1 and RNA2. This vector, designated FZ, incorporates a cloning site in the RNA2 cDNA, permitting insertion of nonviral sequences. When paired with an optimized RNA1 construct, FZ readily infects both Nicotiana benthamiana and soybean. As a result, FZ constructs destined for soybean can be first delivered to N. benthamiana in order to propagate the modified viruses to high titers. FZ-based silencing constructs induced robust silencing of phytoene desaturase genes in N. benthamiana, multiple soybean accessions, and cowpea. Meanwhile, FZ supported systemic expression of fluorescent proteins mNeonGreen and mCherry in N. benthamiana and soybean. Finally, FZ-mediated expression of the Arabidopsis transcription factor MYB75 caused N. benthamiana to bear brown leaves and purple, twisted flowers, indicating that MYB75 retained the function of activating anthocyanin synthesis pathways in a different plant.

CONCLUSIONS

The new CPSMV-derived FZ vector provides a convenient and versatile soybean functional genomics tool that is expected to accelerate the characterization of soybean genes controlling crucial productivity traits.

Virus-Induced Gene Silencing and Transient Gene Expression in Soybean (Glycine max) Using Bean Pod Mottle Virus Infectious Clones

Virus-induced gene silencing (VIGS) is a powerful and rapid approach for determining the functions of plant genes. The basis of VIGS is that a viral genome is engineered so that it can carry fragments of plant genes, typically in the 200 to 300 base pair size range. The recombinant viruses are used to infect experimental plants, and wherever the virus invades, the target gene or genes will be silenced. VIGS is thus transient, and in the span of a few weeks, it is possible to design VIGS constructs and then generate loss-of-function phenotypes through RNA silencing of the target genes. In soybean (Glycine max), the Bean pod mottle virus (BPMV) has been engineered to be valuable tool for silencing genes with diverse functions and also for over-expression of foreign genes. This protocol describes a method for designing BPMV constructs and using them to silence or transiently express genes in soybean. © 2016 by John Wiley & Sons, Inc.

The role of the 5' untranslated regions of Potyviridae in translation

The Potyviridae family relies on a cap-independent translation mechanism to facilitate protein expression. The genomic architecture of the viral RNAs of the Potyviridae family resembles those of the animal picornaviruses. The viral genomes lack a 5' cap structure. Instead, they have the viral protein VPg covalently linked to the 5' end of the RNA. The viral RNAs code for a single large polyprotein, which is then cleaved into several functional subunits. With their common genome organization with the Picornaviridae, it has been largely assumed that the members of the plant Potyviridae family share similar translation mechanism. We will describe the remarkably diverse translational enhancers identified within the family and their unique mechanisms of translation, from internal recruitment of the ribosomes to ribosomal scanning from the 5' end and the recruitment of the VPg in translation. The divergence among the potyviral translation enhancers is heightened with the recent discovery of Triticum mosaic virus, an atypical member of the Potyviridae family, for which its 5' leader by far exceeds the typical length of plant viral leaders and contains features typically found in animal viruses. Much remains to be learned on how these highly divergent elements enable potyviruses, which include some of the most damaging plant viruses, to take over the host translation apparatus. While no clear consensus sequence, structure or mechanism has been reported yet among the potyviral elements, more thorough studies are needed to fill in the gap of knowledge.