The 2b protein of cucumber mosaic virus is essential for viral infection of the shoot apical meristem and for efficient invasion of leaf primordia in infected tobacco plants
It has been reported previously that a 2b protein-defective mutant of the cucumber mosaic virus (CMV) Pepo strain (Δ2b) induces only mild symptoms in systemically infected tobacco plants. To clarify further the role of the 2b protein as an RNA silencing suppressor in mosaic symptom expression during CMV infection, this study monitored the sequential distribution of Δ2b in the shoot meristem and leaf primordia (LP) of inoculated tobacco. Time-course histochemical observations revealed that Δ2b was distributed in the shoot meristem at 7 days post-inoculation (p.i.), but could not invade shoot apical meristem (SAM) and quickly disappeared from the shoot meristem, whereas wild-type (Pepo) transiently appeared in SAM from 4 to 10 days p.i. In LP, Δ2b signals were detected only at 14 and 21 days p.i., whereas dense Pepo signals were observed in LP from 4 to 18 days p.i. Northern blot analysis showed that small interfering RNA (siRNA) derived from Δ2b RNA accumulated earlier in the shoot meristem and LP than that of Pepo. However, a similar amount of siRNA was detected in both Pepo- and Δ2b-infected plants at late time points. Tissue printing analysis of the inoculated leaves indicated that the areas infected by Pepo increased faster than those infected by Δ2b, whereas accumulation of Δ2b in protoplasts was similar to that of Pepo. These findings suggest that the 2b protein of the CMV Pepo strain determines virulence by facilitating the distribution of CMV in the shoot meristem and LP via prevention of RNA silencing and/or acceleration of cell-to-cell movement.
AnandalakshmiR.,
PrussG. J.,
GeX.,
MaratheR.,
MalloryA. C.,
SmithT. H.,
VanceV. B.1998; A viral suppressor of gene silencing in plants. Proc Natl Acad Sci U S A 95:13079–13084[CrossRef]
Diaz-PendonJ. A.,
LiF.,
LiW. X.,
DingS.-W.2007; Suppression of antiviral silencing by Cucumber mosaic virus 2b protein in Arabidopsis is associated with drastically reduced accumulation of three classes of viral small interfering RNAs. Plant Cell 19:2053–2063[CrossRef]
DingS.-W.,
RathjenJ. P.,
LiW.-X.,
SwansonR.,
HealyH.,
SymonsR. H.1995b; Efficient infection from cDNA clones of cucumber mosaic cucumovirus RNAs in a new plasmid vector. J Gen Virol 76:459–464[CrossRef]
FosterT. M.,
LoughT. J.,
EmersonS. J.,
LeeR. H.,
BowmanJ. L.,
ForsterR. L. S.,
LucasW. J.2002; A surveillance system regulates selective entry of RNA into the shoot apex. Plant Cell 14:1497–1508[CrossRef]
GotoK.,
KoboriT.,
KosakaY.,
NatsuakiT.,
MasutaC.2007; Characterization of silencing suppressor 2b of Cucumber mosaic virus based on examination of its small RNA-binding abilities. Plant Cell Physiol 48:1050–1060[CrossRef]
HosokawaD.,
WatanabeM.,
MoriK.1990; Distribution of tobacco mosaic virus in the apices of shoots and roots regenerated from virus-infected tobacco leaves in vitro culture. Ann Phytopathological Soc Jpn 56:276–280[CrossRef]
KasschauK. D.,
CarringtonJ. C.2001; Long-distance movement and replication maintenance functions correlate with silencing suppression activity of potyviral HC-Pro. Virology 285:71–81[CrossRef]
KoboriT.,
MiyagawaM.,
NishiokaK.,
OhkiS. T.,
OsakiT.2002; Amino acid 129 of Cucumber mosaic virus coat protein determines local symptom expression and systemic movement in Tetragonia expansa , Momordica charantia and Physalis floridana . J Gen Plant Pathol 68:81–88[CrossRef]
LinN. S.,
LangenbergW. G.1984; Distribution of barley stripe mosaic virus protein in infected wheat root and shoot tips. J Gen Virol 65:2217–2224[CrossRef]
Martín-HernándezA. M.,
BaulcombeD. C.2008; Tobacco rattle virus 16-kilodalton protein encodes a suppressor of RNA silencing that allows transient viral entry in meristems. J Virol 82:4064–4071[CrossRef]
MochizukiT.,
OhkiS. T.2004; Shoot meristem tissue of tobacco inoculated with Cucumber mosaic virus is infected with the virus and subsequently recovers from infection by RNA silencing. J Gen Plant Pathol 70:363–366[CrossRef]
MochizukiT.,
OhkiS. T.2005; Amino acid 129 in the coat protein of Cucumber mosaic virus primarily determines invasion of the shoot apical meristem of tobacco plants. J Gen Plant Pathol 71:326–332[CrossRef]
OhkiS. T.,
TakamiY.,
InouyeT.1990; Accumulation of double-stranded RNAs 3 and 4 in the symptomless leaves and green areas of mosaic-affected leaves of tobacco plants infected with cucumber mosaic virus. Ann Phytopathological Soc Jpn 56:213–218[CrossRef]
QuF.,
YeX.,
HouG.,
SatoS.,
ClementeT. E.,
MorrisT. J.2005; RDR6 has a broad-spectrum but temperature-dependent antiviral defense role in Nicotiana benthamiana . J Virol 79:15209–15217[CrossRef]
RobertsD. A.,
ChristieR. G.,
ArcherM. C.Jr1970; Infection of apical initials in tobacco shoot meristems by tobacco ring spot virus. Virology 42:217–220[CrossRef]
RyangB. S.,
KoboriT.,
MatsumotoT.,
KosakaY.,
OhkiS. T.2004; Cucumber mosaic virus 2b protein compensates for restricted systemic spread of Potato virus Y in doubly infected tobacco. J Gen Virol 85:3405–3414[CrossRef]
SaigaT.,
FujiwaraM.,
SaitohH.,
OhkiS. T.,
OsakiT.1998; Comparative analysis for replication and movement of cucumber mosaic virus in Cucumis figarei and C. melo . Ann Phytopathological Soc Jpn 64:255–263[CrossRef]
SaitohH.,
FujiwaraM.,
OhkiS. T.,
OsakiT.1999; The coat protein gene is essential for the systemic infection of cucumber mosaic virus in Cucumis figarei at high temperature. Ann Phytopathological Soc Jpn 65:248–253[CrossRef]
SchwachF.,
VaistijF. E.,
JonesL.,
BaulcombeD. C.2005; An RNA-dependent RNA polymerase prevents meristem invasion by Potato virus X and is required for the activity but not the production of a systemic silencing signal. Plant Physiol 138:1842–1852[CrossRef]
SoardsA. J.,
MurphyA. M.,
PalukaitisP.,
CarrJ. P.2002; Virulence and different local and systemic spread of Cucumber mosaic virus in tobacco are affected by the CMV 2b protein. Mol Plant Microbe Interact 15:647–653[CrossRef]
VoinnetO.,
PintoY. M.,
BaulcombeB. C.1999; Suppression of gene silencing: a general strategy used by diverse DNA and RNA viruses of plants. Proc Natl Acad Sci U S A 96:14147–14152[CrossRef]
YelinaN. E.,
SavenkovE. I.,
SolovyevA. G.,
MorozovS. Y.,
ValkonenJ. P.2002; Long-distance movement, virulence, and RNA silencing suppression controlled by a single protein in hordei- and potyviruses: complementary functions between virus families. J Virol 76:12981–12991[CrossRef]
The 2b protein of cucumber mosaic virus is essential for viral infection of the shoot apical meristem and for efficient invasion of leaf primordia in infected tobacco plants