Virus-Mediated Overexpression of Two Allelic Protein Fragments Elicits Drastically Different Responses in Soybean

Soybean (Glycine max) is relatively recalcitrant to genetic manipulations; hence, it is often interrogated with transient means such as virus-induced gene silencing (VIGS). We earlier modified cowpea severe mosaic virus (CPSMV) to develop a soybean-friendly VIGS system referred to as QUIN-FZ. Here we report additional calibrations of this system. We enhanced the intra-bacterial stability of plasmid QUIN, which contained a CPSMV RNA1 cDNA embedded with four introns, by adding a fifth intron, resulting in PENTIN. We separately upgraded the plasmid FZ, which contained a modified CPSMV RNA2 cDNA with a cloning site in the middle of the viral polyprotein, by creating another cloning site within the 3' untranslated region, leading to ZY. We next used the new PENTIN-ZY system to investigate a putative soybean protein kinase designated QL18. Virus-mediated overexpression of two allelic, 147-amino-acid (aa) protein fragments, derived from two different QL18 orthologs, elicited drastically different responses in soybeans. While the fragment derived from soybean accession OX20-8 prevented the cognate virus from infecting top young leaves in at least 50% of inoculated seedlings, its allelic counterpart derived from soybean accession PI427105B elicited apical necrosis in 100% of soybean seedlings. By examining progeny viruses as well as viruses encoding chimeras of the two fragments, we identified more than a dozen mutations that abrogated these unique phenotypes. Our findings establish the PENTIN-ZY system as a versatile tool for overexpressing small proteins and protein fragments, accelerating their functional characterization.