Summary. Transgenic tobacco
(Nicotiana tabacum) plants of (i) cv. Samsun NN
containing the cauliflower mosaic virus 35S constitutive promoter linked to a
defective replicase (DR) gene derived from cucumber mosaic virus (CMV)
subgroup I isolate Fny, and (ii) cv. Xanthi containing the CaMV 35S promoter
linked to the coat protein (CP) gene of CMV subgroup I isolate C were tested
for resistance to various Australian isolates of CMV. The tobacco plants were
challenged with 3 CMV subgroup 1 isolates (BNRR, BMR and B6) using sap
inoculation. When used to challenge non-transgenic tobacco plants with 5
subgroup II CMV isolates from lupins (LY, LCH, LAcc, LGu and LD), this
inoculation method did not result in systemic infection so graft inoculation
was used instead to challenge transgenic plants with these 5 isolates. When
plants of the line with the DR gene were challenged with the 3 subgroup I
isolates, extreme resistance was revealed as none showed symptoms and CMV was
not detectable by ELISA. When the same 3 isolates were inoculated to the 3
lines with the CP gene, resistance was characterised by fewer plants becoming
virus infected, delayed systemic movement and, in the plants that were
infected, partial remission of symptoms plus somewhat decreased virus
concentration. Challenge of transgenic plants with DR or CP with the 5
subgroup II isolates resulted in fewer plants becoming infected. Actual
numbers of plants infected varied with line and subgroup II isolate and the DR
gene was as effective as the CP gene at decreasing infection. With subgroup II
isolate LY, infection was associated with remission of symptoms and with the
other 4 isolates with delayed systemic movement. Thus the DR gene approach was
more effective than the CP approach in obtaining extreme resistance against
Australian subgroup I isolates of CMV. These results suggest that introducing
a similar DR gene construct made from a subgroup II isolate from lupins into
commercial lupin cultivars may be a suitable strategy for obtaining extreme
resistance to subgroup II isolates from lupins.
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
