Molecular Plant Pathology – Pathogen Profiles

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

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) inArabidopsis 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: (TAIR, The Arabidopsis Information Resource).

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.