Hb. Scholthof et al., THE CAPSID PROTEIN GENE OF TOMATO BUSHY STUNT VIRUS IS DISPENSABLE FOR SYSTEMIC MOVEMENT AND CAN BE REPLACED FOR LOCALIZED EXPRESSION OF FOREIGN GENES, Molecular plant-microbe interactions, 6(3), 1993, pp. 309-322
The requirement of the capsid protein for viability of tomato bushy st
unt virus (TBSV), a small, spherical plant virus, was analyzed by inac
tivating the corresponding gene in full-length cDNA clones followed by
bioassays of in vitro-generated transcripts. The results demonstrated
that the coat protein of TBSV is not involved in replication or gene
expression in protoplasts. In addition, cell-to-cell spread of viral R
NA as well as long-distance movement in plants occurred in the absence
of coat protein expression. Symptoms on systemic tobacco hosts in the
absence of an intact coat protein gene were quite typical of a TBSV i
nfection, except that most infected plants survived, whereas infection
with the wild-type virus resulted in a lethal necrosis. Viral RNA in
which the coat protein gene was replaced with a reporter gene expresse
d this foreign gene at high levels in protoplasts as well as in the in
oculated leaves of plants. Expression of the reporter gene was greatly
reduced in upper leaves of systemically infected hosts because the no
nviral sequences were rapidly deleted from the genome during the proce
ss of systemic infection. The capsid protein of TBSV was not responsib
le for the elicitation of necrotic lesions on Chenopodium amaranticolo
r. Analyses of beta-glucuronidase expression by TBSV on inoculated lea
ves of this local lesion host suggested that replication and cell-to-c
ell spread were not affected prior to 1 day postinoculation. The hyper
sensitive response is probably initiated at some time after establishm
ent of a localized infection and limited cell-to-cell movement.