Molecular Plant Pathology - Pathogen Profiles
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Downy mildew of Arabidopsis thaliana caused by Hyaloperonospora
parasitica (formerly Peronospora parasitica)
Alan J. Slusarenko* and Nikolaus L. Schlaich
Department of Plant Physiology (BioIII), RWTH Aachen, Worringerweg 1,
D-52056 Aachen, Germany
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| Summary: |
Downy mildew of Arabidopsis
is not a hugely destructive disease of an important crop plant, neither is
it of any economic importance. The most obvious symptom, the aerial
conidiophores, might, at a glance to the casual observer, be mistaken for
the trichomes normally present on the leaves. However, a huge research
effort is being devoted to this humble pathosystem which became
established as a laboratory model in the 1990s. Since then, enormous
progress has been made in cloning and characterizing major genes for
resistance (RPP genes) and in defining many of their downstream
signalling components, some of them RPP-gene specific. Resistance
is generally associated with an oxidative burst and a salicylic acid
dependent hypersensitive reaction type of programmed cell death.
Biological and chemical induction of systemic acquired resistance (SAR) in
Arabidopsis protecting against downy mildew were demonstrated early
on, and investigations of mutants have contributed fundamentally to our
understanding of host-pathogen interactions and the mechanisms of plant
defence. This review will attempt to collate the wealth of information
which has accrued with this pathosystem in the last decade and will
attempt to predict future research directions by drawing attention to some
still unanswered questions. |
| Taxonomy: |
Hyaloperonospora Constant. parasitica
(Pers.:Fr) Fr. (formerly Peronospora parasitica), Kingdom
Chromista, Phylum Oomycota, Order Peronosporales, Family Peronosporaceae,
Genus Hyaloperonospora, of which it is the type species. |
| Host range: |
Isolates infecting Arabidopsis
thaliana have so far proven to be non-pathogenic on other crucifers
tested but exist in a clear gene-for-gene relationship with different host
ecotypes. |
| Disease Symptoms: |
Infections are first apparent to
the naked eye as a carpet or 'down' of conidiophores covering the upper
and lower surfaces of leaves and petioles. This symptom is characteristic
of this group of diseases and lends it its name. |
| Useful Websites: |
http://ppathw3.cals.cornell.edu/PP644/references.htm
(links to references on Oomycetes) http://www.arabidopsis.org/
(TAIR, The Arabidopsis Information Resource). |
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Life cycle of Hyaloperonospora parasitica. (a) infections arise initially
from oospores germinating in the soil. (b) Plants are colonized by a
coenocytic, intercellularly growing mycelium which swells to fit the
intercellular spaces, giving it an irregular appearance. The hyphae put
out pear-shaped feeding organs called haustoria into host cells. After a
variable period of growth (1–2 weeks) conidiophores, bearing asexual,
spherical hyaline conidiospores (c) grow out of stomata. (d) On
germination, conidia initiate new rounds of infection. (e–g) Oospores
are formed concurrently with asexual spores. (e) The female sexual organs,
oogonia, contain an oosphere that is fertilized via a fertilization tube
growing through its outer wall from the male antheridium. (f ) The
fertilized oosphere develops into a mature oospore. (g) Oospores are very
profuse in infected leaves. The components of this diagram are not drawn
to scale but the fungal structures are illustrated photographically in
Koch and Slusarenko (1990). Reprinted from Mauch-Mani and Slusarenko
(1993) with permission from Elsevier Science Publishers.
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