RNA and capsid accumulation in cowpea protoplasts that are resistant to cowpea mosaic virus strain SB

Participation of the Cowpea mosaic virus protease in eliciting extreme resistance

Extreme resistance of Arlington line cowpea (Vigna unguiculata) to Cowpea mosaic virus (CPMV) is under control of a dominant locus designated Cpa. We transiently expressed, using Tomato bushy stunt virus (TBSV) vectors and Agrobacterium tumefaciens, in nearly isogenic Cpa/Cpa and cpa/cpa cowpea lines, sequences from RNA1, the larger of two CPMV genomic RNAs. Activation of a Cpa-specific response mapped to the CPMV 24K protease (24KPro). Mutational analysis of the 24KPro gene implicated protease activity, rather than 24KPro structure, in Cpa-mediated recognition of CPMV invasion. A 24KPro with alanine replacing the active site cysteine [24KPro(C-A)], but not wildtype 24KPro, accumulated after agroinfiltration of the corresponding binary vector constructions into Cpa/Cpa cowpea. In cpa/cpa cowpea, both protease versions accumulated, with 24KPro(C-A) in greater abundance. Thus, enzymically active 24KPro was recognized by both cowpea genotypes, but in Cpa/Cpa cowpea the suppression of 24KPro accumulation was very strong, consistent with extreme resistance to CPMV.

Not as they seem

I recount the early influences that directed me toward a career in research and then describe some efforts investigating Cowpea mosaic virus and the satellite RNA of Tobacco ringspot virus. These descriptions have a common theme of surprise, how things often can be not as they are expected to be. Finally, I examine the widely held belief that a plant transgene derived from a distant taxonomic source presents a greater risk than a transgene derived from a closely related plant and contend that this also is a situation in which things may not be as they initially seem.

Inhibition of cleavage of a plant viral polyprotein by an inhibitor activity present in wheat germ and cowpea embryos

In rabbit reticulocyte lysate, the bottom component RNA of cowpea mosaic virus directs the synthesis of a 200,000-molecular-weight precursor protein (200K protein) that is cleaved during synthesis by a reticulocyte enzyme to form a 32K protein and a 170K protein. Cleavage of the 200K protein was found to be effectively inhibited by inhibitor activity in wheat germ and cowpea embryo extracts. The inhibitor was nondialyzable, precipitatable by ammonium sulfate, and partially stable at high temperatures. The activity appeared to be specific in that it caused no inhibition of the secondary cleavage reactions (cleavage of the 170K protein) at concentrations that were sufficient to cause complete inhibition of the primary cleavage reaction (cleavage of the 200K protein).

Nucleotide sequence of satellite tobacco ringspot virus RNA and its relationship to multimeric forms

Tobacco ringspot virus, a member of the nepovirus group, supports the increase and encapsidation of coinoculated satellite tobacco ringspot virus RNA (STobRV RNA). The nucleotide sequence of the unit length STobRV RNA, found here to be 359 nucleotide residues for the budblight strain, occurs also in multimeric, repetitive sequence forms. These are able to undergo an autolytic processing reaction to generate biologically active, unit length STobRV RNA (G. A. Prody, J. T. Bakos, J. M. Buzayan, I. R. Schneider, and G. Bruening,1984, In "Abstracts of the 3rd Cold Spring Harbor RNA Processing Meeting, May 16-20,1 984," p. 8). We determined the nucleotide sequence of the monomeric STobRV RNA by combining results from partial enzymatic digestions of the RNA, partial chemical cleavage of cDNA transcribed from the RNA, and analyses of cDNA clones. Other analyses gave the terminal residues of monomeric STobRV RNA: a cytosine-2':3'-cyclic phosphodiester and a 5' terminal adenosine. The terminal residues of monomeric RNA and their adjacent nucleotide sequences are consistent with the sequence in the junction region of dimeric RNA, derived from transcripts and cDNA clones, and with the formation of two monomeric STobRV RNAs upon autolysis of dimer, without the gain or loss of a nucleotide residue.

Evidence for participation of RNA 1-encoded elicitor in Cowpea mosaic virus-mediated concurrent protection

The cowpea (Vigna unguiculata) line Arlington, inoculated with Cowpea mosaic virus (CPMV), showed no symptoms, and no infectivity or accumulation of capsid antigen was detected at several days after inoculation. Coinoculation, but not sequential inoculation, of CPMV with similar concentrations of another Comovirus; Cowpea severe mosaic virus (CPSMV), resulted in reduced numbers of CPSMV-induced lesions. This apparent, CPMV-mediated reduction in number of CPSMV-induced infection centers was termed concurrent protection. We report results obtained by inoculating two nearly isogenic cowpea lines derived from a CPMV-susceptible cowpea crossed to Arlington, one line CPMV-susceptible and the other resistant. The CPMV virions B and M, encapsidating genomic RNAs 1 and 2, respectively, were extensively purified by gradient centrifugation. In the CPMV-resistant cowpea, either CPMV or CPMV B affected concurrent protection against CPSMV and against two distinct non-Comoviruses: Cherry leafroll virus and Southern bean mosaic virus. Adding CPMV M to the inoculum did not enhance CPMV-B-mediated protection. CPMV B was ineffective in protecting CPMV-susceptible cowpea. We postulate that CPMV-mediated concurrent protection is elicited in CPMV-resistant cowpea by a CPMV RNA-1-encoded factor and acts to reduce accumulation or spread of CPMV and certain coinoculated challenging viruses in or from the inoculated cell. Coinoculated CPMV did not protect CPMV-resistant cowpea against Tomato bushy stunt virus or Cucumber mosaic virus.

Resistance of Capsicum annuum 'Avelar' to pepper mottle potyvirus and alleviation of this resistance by co-infection with cucumber mosaic cucumovirus are associated with virus movement

Capsicum annuum cv. Avelar plants resist systemic infection by the Florida isolate of pepper mottle potyvirus (PepMoV-FL). Immuno-tissue blot analysis for detection of PepMoV-FL infection in selected stem segments revealed that virus moved down the stem in external phloem, and, over time, accumulated to detectable levels throughout stem sections (appearing to accumulate in external and internal phloem) taken from below the inoculated leaf. At 21 days post-inoculation, PepMoV-FL was detected in stem segments one or two internodes above the inoculated leaf; however, no virus was observed in internal phloem in stem segments beyond these internodes. In contrast to these observations, PepMoV-FL was detected in the internal phloem of all internodes of the stem located above the inoculated leaf, with subsequent movement into non-inoculated leaves, in Avelar plants co-infected with PepMoV-FL and cucumber mosaic cucumovirus (CMV-KM). No apparent enhancement of PepMoV-FL accumulation occurred in protoplasts inoculated with PepMoV-FL alone versus a mixed inoculum of PepMoV-FL and CMV-KM. These findings confirm earlier observations that potyvirus movement up the stem of Capsicum species occurs via internal phloem. It is also shown that PepMoV-FL does not accumulate to detectable levels in internal phloem in the stem of Avelar plants, thereby limiting its movement to within the inoculated leaf and lower portions of the stem; however, co-infection of Avelar plants with CMV-KM alleviates this restricted movement, allowing PepMoV-FL to invade young tissues systemically.

Formation of circular satellite tobacco ringspot virus RNA in protoplasts transiently expressing the linear RNA

The most abundant form of the satellite RNA of tobacco ringspot virus (sTRSV RNA) is a linear, unit length molecule of 359 nucleotide residues, designated L-(+)M. A postulated replication scheme for the satellite RNA has as its first, and apparently virus-independent, step the ligation of L-(+)M into the corresponding circular form C-(+)M. We transiently expressed L-(+)M wild type and L-(+)M mutants in tobacco protoplasts using an African cassava mosaic geminivirus vector. Measured extents of C-(+)M accumulation were correlated with computer-predicted folding to suggest wild-type secondary structure elements that might be deleted without reducing ligation. A 127-nucleotide residue mutant L-(+)M was created by replacing, with 7 and 3 residues, respectively, nucleotide residues 53-211 and 268-350, each of which was predicted to form a set of three adjacent imperfect stem-loops in wild-type L-(+)M. The mutant L-(+)M was found to be extensively ligated to C-(+)M in protoplasts and to retain a calculated helix of the wild-type molecule that incorporates the 3' terminal sequence. A trinucleotide in the 3' region was mutated so as to disrupt and restore, respectively, the calculated helix, reducing and restoring, respectively, C-(+)M formation. These results suggest that the 3' stem contributes to the suitability of the small L-(+)M molecules as a substrate for a protoplast RNA ligase and that computed folding of sTRSV RNA may be predictive of sTRSV RNA structure in vivo.

Nucleotide sequence and genetic map of cowpea severe mosaic virus RNA 2 and comparisons with RNA 2 of other comoviruses

We report the nucleotide sequence of cowpea severe mosaic comovirus (CPSMV) genomic RNA 2. The molecule is composed of 3732 nucleotide (nt) residues, exclusive of the polyadenylate at the 3' end. Only one of the six reading frame registers has a long open reading frame, from nt 255 to nt 3260 in the polarity of encapsidated RNA and corresponding to a polyprotein of 1002 amino acid residues (aa). As has been reported for other comoviruses, a second in-frame AUG, at nt position 531, apparently also initiates translation, at least in vitro. Multiple alignments of the deduced CPSMV polyprotein aa sequence with those of bean pod mottle comovirus (BPMV), cowpea mosaic comovirus (CPMV), and red clover mottle comovirus (RCMV) were consistent with a similar size for each of the three genes: the putative movement protein, beginning at the second in-frame AUG, the large coat protein (L), and the small coat protein. Identical nucleotide sequences in the terminal noncoding regions of RNA 2 of the four viruses are limited to 9 nt at the 5' end and the 3' polyadenylate. However, extensive similarities in sequence and potential structure were found. For all three genes and the 5' untranslated region, CPSMV and BPMV are more similar to each other than either is to CPMV or RCMV, the last two being similar to each other. Observed similarities predict that both cleavage sites in the CPSMV RNA 2 polyprotein are at glutamine-serine dipeptides. A sequence of 16 aa at the amino terminus of L, determined by automated Edman degradation, matched a region of the deduced aa sequence in the polyprotein and is consistent with cleavage at the predicted glutamine-serine dipeptide.

Evidence for spontaneous circle formation in the replication of the satellite RNA of tobacco ringspot virus

Replication of the satellite RNA of tobacco ringspot virus (sTobRV RNA) has been postulated to require rolling circle transcription. The expected product of rolling circle transcription, multimeric sTobRV RNA, is known to undergo self-cleavage in vitro to release unit-length sTobRV RNA. A spontaneous, efficient, not enzymically-catalyzed in vitro circularization reaction is characteristic of unit-length sTobRV RNA of the less abundant, (-) polarity. We mutated sTobRV RNA at two sites that are distant from each other in the polyribonucleotide chain. A third form of the sTobRV RNA was mutated at both sites. Multimeric forms of the one-site mutants of sTobRV(+)RNA and sTobRV(-)RNA showed, respectively, undiminished and slightly diminished self-cleavage, whereas the spontaneous circularization of each one-site-mutated, unit-length sTobRV(-)RNA was greatly reduced, compared to the reactions of wild-type sTobRV RNA and the two-site mutant. The two-site mutant and the wild-type sTobRV RNAs replicated with similar efficiency. They reduced the titer of, and severity of, symptoms induced by coinoculated tobacco ringspot virus (TobRV). When coinoculated with TobRV, neither one-site mutant increased or provided protection against TobRV. Rather, each induced a substantial accumulation of what is apparently an endogenous form of sTobRV RNA. Our results are consistent with the formation of circular sTobRV(-)RNA as an essential step in sTobRV RNA replication.

Identification of a non-junction phosphodiester that influences an autolytic processing reaction of RNA

Oligoribonucleotides with specific sequences derived from the satellite RNA of tobacco ringspot virus undergo autolytic cleavage at the CpA phosphodiester that is the junction between unit sequences of multimeric satellite RNA. Buzayan et al. (Nucleic Acids Res., 16, 4009-4023 (1988)) showed that an oligoribonucleotide with 97 satellite RNA-derived nucleotide residues self-cleaved with greatly reduced efficiency when it was synthesized in vitro from adenosine-5'-O-(1-thiotriphosphate) (abbreviated rATP alpha S) and three rNTPs. No other substitution of one rNTP by the corresponding rNTP alpha S had this effect, suggesting that a phosphorothioate CpA junction inhibits self-cleavage. Here, we replaced the usual CpA junction of a small self-cleaving oligoribonucleotide with a CpU junction. Self-cleavage of this molecule was reduced not only by rUTP alpha S-substitution, as expected, but also by partial and complete rATP alpha S-substitution. By analysis of the locations of rAMPS residues in cleavage products derived from partially rATP alpha S-substituted oligoribonucleotides, we identified A26 as the residue contributing the non-junction phosphorothioate diester that most strongly inhibited self-cleavage. Manganese ions strongly stimulated the self-cleavage of the rATP alpha S-substituted, CpU-junction oligoribonucleotide but was less effective when the junction was CpA.

Two autolytic processing reactions of a satellite RNA proceed with inversion of configuration

Both polarities of the satellite RNA of tobacco ringspot virus occur in infected cells in multimeric forms which are capable of autolytic processing, using different sequences and structures [Feldstein, P.A., et al., Proc. Nat. Acad. Sci. USA (1990) 87 (in press)]. These transesterification reactions generate a 2',3'-cyclophosphate and a 5'-hydroxyl as the two new end groups. Cleavage is at a CpA for the (+) polarity RNA and at an ApG for the (-) polarity RNA. We enzymically synthesized oligoribonucleotides with processing capability and with specific 35S-labeled phosphorothioate diesters in the Rp configuration. After processing had occurred, the terminal nucleoside-2',3'-cyclophosphorothioate diester residues were recovered from the appropriate product by digestion with nuclease and phosphatase. Comparisons with specially prepared endo- and exoisomer reference compounds by thin layer chromatography and autoradiography revealed that the [35S]cytidine- and [35S]adenosine-2',3'-cyclophosphorothioate both were endo-isomers. The results are consistent with transesterification occurring by an inline SN2(P) attack of the 2'-hydroxyl group in the autolytic processing reactions of both polarities of the satellite RNA.

Resistance to TMV in transgenic plants results from interference with an early event in infection

Constitutive expression of the tobacco mosaic virus (TMV) coat protein (CP) gene in transgenic tobacco plants results in inhibition of disease symptom development following inoculation with TMV. Evidence is presented here that this protection is also observed in leaf mesophyll protoplasts isolated from these plants. Protoplasts were resistant to infection by TMV at concentrations of 10 microgram/ml to 1 mg/ml when introduced by either electroporation or polyethylene glycol-mediated inoculation. There was little protection against infection by TMV RNA and the protection was lost as the concentration of TMV RNA in the inoculum increased. When virus was incubated briefly at pH 8.0 prior to inoculation, protection broke down in a manner similar to that observed following RNA inoculation. Analogous results were obtained in experiments with whole plants. Because virus treated in this manner has presumably lost little or no CP, these results suggest that expression of the TMV CP gene in transgenic plant cells prevents TMV from uncoating. A model is presented for the mechanism of this blockage which relates these results to early events in TMV infection.