Why we should care about pathogenic viruses in our plants

Lee Poulsen wpoulsen@pacbell.net
Fri, 23 Jul 2010 06:36:34 PDT
Jacob Knecht wrote:
> Dear PBS members,
> Though I am not a professional pathologist, I am a passionate horticulturist
> and conservationist.  Ben Zonneveld brings up a good point.  Not all viruses
> reduce a plant's vigour nor are viruses responsible for many variegations
> (especially foliar) found in ornamental plants today.  Some viruses are now
> essential to modern biological research methods...

I enjoyed Jacob's essay and it caused me to think of a couple of points:

1. Not only are some microorganisms not bad, the "flora" in human 
gastrointestinal system for example, which we've known about for years, 
is considered to be essential. So much so, that scientists decided to 
sequence all of them and finally have done so, at least with a few 
sample humans, to try to put together the "other" human DNA sequence (or 
conglomeration of DNA) to find out what it is doing for us and how it 
differs from one part of the world to another or from one culture to 
another. I can't remember the figure, but something like 2 or 4 pounds 
(1 or 2 kilos) of the average adult's total weight isn't even really 
genetically ourself, but is all these other microorganisms. (I even have 
one friend, who in his treatment for a very rare form of cancer, had to 
have the entire flora killed. After they finished one stage of the 
treatment, they had to reintroduce as much of the flora as they could, 
and then wait for several weeks while he ate "normal" food so that the 
flora could grow and fill all of the former niches in his gut as well as 
allow for additional necessary flora from the food he ate to also enter 
his system and grow into the areas needed, before they could continue 
with the subsequent steps of his treatment.)

2. It is possible to eliminate virus from an infected plant. The 
University of California Citrus agency or division, in conjunction with 
the USDA I think, has a facility set up to do just this. (They also have 
an elaborate and extensive set-up for quarantining new citrus 
introductions from outside Calif. and maintaining virus-free budwood 
trees of most of the citrus varieties grown in the U.S.) Apparently, in 
a heated environment (greater than something like 105°F/40°C) the 
growing tips at the extremities of a plant can grow faster than the 
virus can travel to get to it. So they grow a plant in a special heated 
room and after it's grow for a while, they carefully slice off just the 
very newest cells at a growing tip. They then tissue culture these and 
grow the plant until buds form, which they then graft onto virus-free 
rootstock and eventually produce a virus-free version of the virused 
citrus variety. Pretty amazing.

3. Which leads me to a third comment/question: I am not a biologist, 
just someone very scientifically oriented and interested in many aspects 
of science. However, even though I've read and heard a lot about how the 
human body (and animals in general) fights viruses, antibodies, etc., I 
have utterly no knowledge about how or whether plants fight viruses. Do 
they produce antibodies? Do they have some other mechanism to do so? It 
would seem that they have to do something or else by now the entire 
plant kingdom would be either completely infiltrated with evolved 
viruses, or extinct. If there is such a mechanism, wouldn't it be 
possible to develop the equivalent or analog of vaccines for plants? 
They've made vaccines for pet animals and agricultural animals; have 
they tried creating the equivalent of vaccines for important commercial 
crops, and it not, is it because such a thing is not possible? (I 
realize that geophytes are far down the list in importance for making 
"vaccines" against viruses. But I'm curious about why I've never heard 
of medical methods to prevent virus infection in plants.)

--Lee Poulsen
Pasadena, California, USA - USDA Zone 10a

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