AGROBACTERIUM-MEDIATED INOCULATION OF PSTVD CDNAS ONTO TOMATO REVEALSTHE BIOLOGICAL EFFECT OF APPARENTLY LETHAL MUTATIONS

Potato spindle tuber viroid (PSTVd) mutants which contain alterations in the terminal loops of the rod-like native structure have previously been reported from our laboratory. PSTVd-P contains mutations at posi tions 2, 4, and 6 in the left terminal loop; PSTVd-R+, a sequence perm utation of PSTVd-R, contains the same mutations at positions 177 and 1 78 in the right terminal loop as PSTVd-R and contains in addition a 1- nucleotide G insertion at position 176. PSTVd-P, PSTVd-R, and PSTV-Rwere noninfectious when either cDNA or SP6-generated RNA transcripts w ere used as inoculum onto tomato cotyledons. In the current study, mut ant and wild-type PSTVd constructs were mobilized into Agrobacterium t umefaciens and used for stem inoculation of tomato plants. Agrobacteri um-mediated inoculation of the mutant and wild-type constructs has con firmed the inability of the PSTVd-P mutant to establish an infection. The PSTV-R+ mutant and/or sequence variants derived in vivo can establ ish an infection, although PSTVd-R+ progeny and replicative intermedia tes appear to be primarily restricted to the gall and root tissues of the plant and only occasionally are progeny detectable in the newly de veloping leaves. The reduced level of viroid accumulation from the PST Vd-R+ mutant appears to be consistent with the mutant viroid replicati ng/accumulating only in a limited number of cells or cell types. The m utations in the right terminal loop may alter interactions with specif ic host components and thereby disrupt the normal pattern of intercell ular transport of the viroid or limit its replication to a cell type b ut not abolish replication per se. (C) 1994 Academic Press,