Abstract
Cross-protection is a promising measure to control plant viral diseases. Reverse genetics had been recently adopted to generate attenuated mutants that have potential in cross-protection. But studies on the variability of the progeny viruses of the attenuated mutants are scarce. Sugarcane mosaic virus (SCMV; genus Potyvirus, family Potyviridae) is the prevalent virus inducing maize dwarf mosaic disease in China. Here, we showed that the substitution of arginine with isoleucine in the FRNK motif at position 184 of helper component-proteinase (HC-Pro) abolished its RNA silencing suppression (RSS) activity, drastically reduced the virulence and accumulation level of SCMV, and impaired the synergism between SCMV and maize chlorotic mottle virus. The attenuated mutant could protect maize plants from a severe infection of SCMV. However, a spontaneous mutation of glycine at position 440 to arginine in HC-Pro rescued the virulence and synergism with maize chlorotic mottle virus of SCMV and the RSS activity of HC-Pro. Similar results were obtained with tobacco vein banding mosaic virus and watermelon mosaic virus. These results provide novel evidence for the complementary mutation of potyviruses in maintaining the HC-Pro RSS activity and potyviral virulence and remind us of evaluating the potential risk of attenuated mutants thoroughly before applying for the control of plant viral diseases via cross-protection.
Highlights
Cross-protection is the phenomenon that plants infected with one mild strain of a virus will escape or prevent the infection by closely related severe viruses (Kunkel, 1955)
The corresponding amino acid of R184 was mutated to I or K and that of K186 was mutated to alanine (A) or R in helper component-proteinase (HC-Pro) derived from the progeny of Sugarcane mosaic virus (SCMV) mutants (Supplemental Figure 1)
These results indicated that R184 and K186 of the FRNK motif were involved in the virulence and accumulation levels of SCMV in maize plants
Summary
Cross-protection is the phenomenon that plants infected with one mild strain of a virus will escape or prevent the infection by closely related severe viruses (Kunkel, 1955). It has been used for the control of more than 30 viruses including citrus tristeza virus (CTV), papaya ringspot virus (PRSV), potato virus X (PVX), potato virus Y (PVY), sugarcane mosaic virus (SCMV), and tobacco mosaic. The availability and safety of mild strains (or attenuated mutants) are two limiting factors affecting the application of cross-protection in practice. The potential mild strains or attenuated mutants should have no synergism with other viruses infecting the target host plants. Studies on the safety of progeny viruses of the attenuated mutants are scarce
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