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

Abstract

It has been reported previously that a 2b protein-defective mutant of the cucumber mosaic virus (CMV) Pepo strain (D2b) 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 D2b in the shoot meristem and leaf primordia (LP) of inoculated tobacco. Time-course histochemical observations revealed that D2b 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, D2b 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 D2b 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 D2b-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 D2b, whereas accumulation of D2b 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.

Relationship between viral distribution in the leaf primordia/young developing leaves and symptom severity in the fully expanded leaves of tobacco plants infected with Cucumber mosaic virus

“…We also showed that the CMV was distributed in most cells of the leaf primordia (2 to 3 mm) at 10 to 18 days postinoculation (dpi), but CMV was reduced in the leaf primordia at 21 dpi. This result suggested that uninfected leaf primordia at 21 dpi might develop as symptomless leaves and CMV-infected leaf primordia at 10 to 18 dpi might develop into mosaic (Sunpapao et al 2009) However, the relationship between CMV infection in the leaf primordia/young developing leaves stage and symptom severity in fully expanded leaves has not been elucidated. It is difficult to simultaneously analyze viral infections in the leaf primordia/young developing leaves and symptoms in fully expanded leaves using identical leaves.…”

“…Our previous studies showed that the 2b proteindefective mutant of CMV (Δ2b), which caused very mild symptoms on all upper leaves, only weakly invaded the leaf primordia of infected tobacco plants (Sunpapao et al 2009). This result suggested that the reduced infection of the leaf primordia by Δ2b resulted in the very mild symptoms and that the 2b, a RNA silencing suppressor (Goto et al 2007;Zhang et al 2006), involved in the CMV distribution in the young tiny leaves and in the cyclic symptom expression.…”

“…The Δ2b mutant caused mild symptom and only weakly invaded the leaf primordia of infected tobacco plants (Sunpapao et al 2009). We also showed that the CMV was distributed in most cells of the leaf primordia (2 to 3 mm) at 10 to 18 days postinoculation (dpi), but CMV was reduced in the leaf primordia at 21 dpi.…”

Studies on viral movement of Cucumber mosaic virus in infected plants

“…Tissue printing indicated that the areas infected by pepo increased faster than those infected by pepoD2b, whereas accumulation of pepoD2b in protoplasts was similar to that of pepo. These findings suggest that the 2b protein of the pepo CMV 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 (Sunpapao et al 2009). …”

The Road to RNA Silencing is Paved with Plant-Virus Interactions

“…Prior to this step, the virus is able to evade or suppress the RNA silencing mechanism. Apparently, the ability of RNA to enter the meristem is compromised by several VSR (Foster et al, 2002;Martínez-Hernández and Baulcombe, 2008;Sunpapao et al, 2009) and in some cases, RDR6 is involved in preventing RNAs from entering the meristem (Di Serio et al, 2010;Qu et al, 2005;Schwach et al, 2005). Additionally, in some cases, the level of RDR6 mRNA was down-regulated by the action of VSR (Zhang et al, 2008).…”