By David N. Leff
Remember the time, a few years ago, when people talked to their houseplants and played music for their cut flowers?
Be that as it may, it's a fact that some plants seem to have immune defenses resembling those of the animal kingdom. A case in point: Members of the Brassica botanical family, notably rapeseed, can turn on genes that silence or suppress their natural enemy, cauliflower mosaic virus (a.k.a. CaMV).
"CaMV is very host-specific," molecular plant virologist Nadia Al-Kaff, told BioWorld Today. "It only infects some plants, not all." Al-Kaff is a post-doctoral fellow at the John Innes Center, in Norwich, U.K.
"Some other viruses," she continued, "have a very wide host range, but CaMV infects only two members of the Crucifera family — Brassica and Tobacco."
Al-Kaff is first author of a paper in today's Science, dated March 27, 1998, titled "Transcriptional and posttranscriptional plant gene silencing in response to a pathogen."
"Plants show a variety of responses to pathogens," she observed. "Susceptible plants develop symptoms of disease."
When CaMV attacks oilseed rape (Brassica napus or B. oleracea), the plant's green leaf veins fade to a washed-out yellow or white, for lack of chlorophyll. Between these anemic veins deep-green islands of leaf tissue remain.
But the rape plant fights back. "We found in previous research that a wild-type B. oleracea," Al-Kaff recounted, "has a defense mechanism against systemic viral infection. It is active against CaMV to suppress it, so the virus is not replicating in that plant.
"Following that work," she went on, "we studied the B. napus oilseed rape plant, which," she explained, "is a hybrid between B. oleracea and B. rapa." The former species recovers from the symptoms of infection; the latter does not.
"CaMV contains a 35S promoter," Al-Kaff recounted, "which scientists use to drive transgenes, or any gene they want to put in the plants."
She continued: "We constructed three transgenes, T1, T2 and T3, with different combinations of 35S viral promoters and terminators. T1 had both, T2 the promoter only, T3 neither."
Al-Kaff and her co-authors then asked the question: "If we took transgenic plants which contain either the promoter or terminator of the virus or both, what will happen to that transgene at transfection?"
Does Virus Trigger Its Own Transfection?
The answer they obtained: "This defense mechanism is also present in the B. napus hybrid. Therefore the plant suppresses the virus as well as the transgene, because of nucleic-acid homology between the two sequences."
In plants containing the T1 construct, flanked by homologous viral DNA sequences, the transgene was indeed silenced in response to CaMV infection. No suppression occurred in noninfected plants.
Antiviral resistance by the plant, her Science paper pointed out, "is elicited by interaction between incompatible [i.e., resistant] plants and pathogens containing the appropriate genes."
To confirm that suppression of viral replication was independent of the transgene, the co-authors examined CaMV replication in infected leaves of transgenic and nontransgenic plants. They determined viral replication and gene expression were independent of the presence of transgenes, whereas changes in transgene activity depended on the CaMV being present.
Suppression of viral replication by the plant's defenses takes place in both transcriptional and posttranscriptional silencing phases.
"In the transcriptional expression," Al-Kaff recalled, "we detected no RNA transcripts in the nucleus, nor of course in the cells' cytoplasm. But in posttranscriptional silencing, we found RNA in the nucleus."
She added, "Until now, silencing is not a clear thing. We don't understand the mechanism itself yet. We cannot for example say this mechanism will happen in any other plants."
Nor does she envision any practical application in rapeseed farming.
What's The Genetic Underpinning?
"Brassica in general is a very important crop," Al-Kaff observed. "The main thing for us is to identify the genetic basis of viral silencing that is happening in these plants. Because we have found hosts that do this kind of thing, we are searching now for one that doesn't.
"Second," she continued, "We want to know exactly what will happen to a transgene containing the 35S viral promoter, especially oilseed rape, if for example it's infected."
She and her team are now "doing a lot of experiments to infect various oilseed rape, or Brassica neighbors, with CaMV. And we are asking: Are there different isolates or variants of the virus which will not cause this phenomenon?"
Commenting on the British investigators' report, molecular plant virologist Ramon Jordan told BioWorld Today: "A take-home message is that they are showing that, with the appropriate sequences engineered in the plant, one could turn the respective gene, or the target gene, off and on. It leads to the potential for controlling any number of genes in plant expression."
Jordan is a research leader at the U.S. National Arboretum laboratory in Beltsville, Md.
He concluded: "They're not talking so much about engineering the plant to have a specific gene product, but rather the appropriate sequences to have the gene turned on, turned off — and when."
Plant virologist Steve Howell, vice president of research at the Boyce-Thompson Institute for Plant Research, in Ithaca, New York, commented: "Their paper raises the interesting issue of why does a plant virus become self-limiting." *