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Peronospora--not the oppressor

CRUTE at emrs.afrc.ac.uk CRUTE at emrs.afrc.ac.uk
Fri Jul 26 09:09:00 EST 1991

PERONOSPORA--not the oppressor I know!

As someone who has spent the past seventeen months investigating
the biology and genetics of the Arabidopsis/Peronospora pathosystem,
I am compelled to respond to an enthusiastic but wholey inaccurate
characterization by Jonathan Jones (24 July 1991) of the obligate
parasite Peronospora:  a "most merciless oppressor" of Arabidopsis.
After all, Peronospora would not live up to its name as an obligate
parasite if it were truly merciless, it would be extinct.
[Please note:  I have a high regard for oomycetes, and am therefore
sensitive to any undeserved, bad press they may receive in public].

Peronospora currently stands as the premier, coevolved parasite
known to infect Arabidopsis, as was so well illustrated by Koch
and Slusarenko, but there is absolutely nothing ruthless or crude
about this parasite.  In the wild, I have observed plants that grew
unhindered from the first true leaves into full-grown plants with
maturing pods, even though they were extensively infected and heavily
dressed the entire time with sporulation of the parasite.  If the plants
had not been sprayed with herbicide in the final days, they most
certainly would have released their bounty of seed. I have found
this also to be true under artificial conditions. From my observations,
systemic infection is common, but plants overwhelmed by the parasite
(unable to produce seed or killed outright) are rare either in the wild
or in the growth chamber.  This exceptional condition is influenced in
large part by an environment which favors the parasite (cool, damp and
humid conditions for a week or longer). In short, Arabidopsis has an
amazing ability to live with, and often to outgrow the parasite by
limiting the most prolific fungal growth to rosette leaves.

Working with the Arabidopsis/Peronospora pathosystem has been like
preparing a three-course meal:  the appetizer has wetted our appetites,
the maincourse promises to be long and satisfying, and we're all
looking forward to a colorful dessert beyond expectation.  From
microscopic examination and genetic evidence, we're just beginning to
unravel the fine-tuned host/parasite recognition involved in this
pathosystem.  The genetic diversity is tremendous.  For example, each
of the five Peronospora isolates we have tested thus far has proven
to be a different pathotype.  The core of research in this area is
led in the U.K. by Ian Crute (HRI-East Malling) with a project
funded by the AFRC-PMB Arabidopsis programme.  Jim Beynon at nearby
Wye College, Univ. of London joined our research effort in January,
and we have benefited from the start by encouragement and generous
support from Jeff Dangl (Germany).  Our work began independently
from Alan Slusarenko (Switzerland), but we are committed to a
coordinated effort with him towards the eventual isolation of
several genes for resistance to Peronospora--and more importantly,
what those genes are doing for the plant.  The effort grows steadily,
and Ian and I welcome the new addition of a postdoc to be hired
by the Sainsbury Laboratory.

You can learn more about fungal pathogens of Arabidopsis when Alan
speaks at the American Phytopath. Soc. meeting in St. Louis, Missouri
on Wednesday 20 August.  He will contribute to a symposium on the
pathology of Arabidopsis.  I'll be there as well to present a poster
on Peronospora and its next of kin Albugo.  If you are unable to
attend, but would appreciate learning more about these natural parasites
of Arabidopsis, then send me an EMail message any time but preferably
not while I'm visiting the states (7-28 Aug).

Eric B. Holub
Higher Scientific Officer
HRI-East Malling

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