ORIGINAL ARTICLE
Beate Kiefer Æ Michael Riemann Æ Claudia Bu
¨
che
Hanns-Heinz Kassemeyer Æ Peter Nick
The host guides morphogenesis and stomatal targeting
in the grapevine pathogen
Plasmopara viticola
Received: 23 November 2001 / Accepted: 12 February 2002 / Published online: 10 April 2002
Ó Springer-Verlag 2002
Abstract The oomycete grape downy mildew (Plasmo-
para viticola Berk. & Curt. Ex de Bary) is a serious
pathogen of grapevine and spreads by extremely efficient
cycles of asexual propagation. The high efficiency must
involve efficient sensing of the host. We therefore ana-
lyzed the time course and morphology of the early de-
velopment of this pathogen in a host system, by infection
of leaf discs of grapevine (Vitis vinifera L. cv. Mu
¨
ller-
Thurgau), and in a host-free system. Host factors were
demonstrated to influence pathogen development in the
following ways: (i) the release of zoospores from mature
sporangia was accelerated, (ii) the morphogenesis of the
germ tube was coordinated, and (iii) the zoospores were
targeted to the stomata by factors that depended on
stomata closure. The findings show that the early devel-
opment of P. viticola is regulated, specifically and
coordinately, by factors originating from the host plant.
Keywords Host-pathogen interaction Æ Oomycete Æ
Plasmopara (downy mildew) Æ Polar cell growth Æ
Vitis (pathogen targeting)
Abbreviations ABA: abscisic acid Æ DAPI: 4¢,6-diamidi-
no-2-phenylindole Æ FDA: fluorescein diacetate
Introduction
The phytopathogenic oomycete downy mildew (Plas-
mopara viticola), originally inhabiting wild species of
Vitis in North America, was reported for the first time in
Europe in 1878 and since then has posed a major
problem in vineyards all over the world. At high hu-
midity, it can infect large areas within a short period of
time (Mu
¨
ller and Sleumer 1934) and causes huge losses
in yield by affecting leaves and young grapes. Sexual
propagation exists prior to overwintering of infectious
spores, which are of agronomic importance as they
provide the first source of inoculum in the year. How-
ever, it is the rapid sequence of asexual propagation
cycles (sometimes as short as 4 days) that is responsible
for the efficient spread of this oomycete during the
growing season (Vercesi et al. 1999).
Most of the development takes place within the host
leaf, impeding curative approaches to pathogen control.
For this reason and with respect to management of
fungicide resistance, strategies targeted to the early
stages of development appear to be more promising,
because then the pathogen is still found on the lower
surface of the leaves. Unfortunately, the cellular and
molecular events characterizing this early phase of de-
velopment are still far from being understood, which is
mostly due to the fact that, so far, P. viticola has turned
out to be recalcitrant to any attempt at host-free culti-
vation.
At high humidity and warm temperatures, the lemon-
shaped sporangia release several flagellate zoospores
that swarm within the thin water film covering the lower
leaf surface (Mu
¨
ller and Sleumer 1934). When the
zoospores encounter a stoma, they shed their flagella
and encyst. Subsequently, a germ tube emerges from
each spore and reaches into the substomatal cavity,
where it dilates into an infection vesicle. From this in-
fection vesicle, a primary hypha emerges (opposite to the
site of the spore), and develops a mycelium that spreads
within the leaf tissue, extending mainly into the inter-
cellular space of the spongy parenchyma and forming
haustoria that penetrate into the cells of the host.
Recently, a host-free system has been established that
allows the early development of P. viticola to be studied
from zoospore release until the formation of a germ tube
Planta (2002) 215: 387–393
DOI 10.1007/s00425-002-0760-2
M. Riemann Æ P. Nick (&)
Institut fu
¨
r Biologie II,
Albert-Ludwigs-Universita
¨
t Freiburg,
Scha
¨
nzlestrasse 1, 79104 Freiburg, Germany
E-mail: peter.nick@biologie.uni-freiburg.de
Fax: +49-761-2032612
B. Kiefer Æ C. Bu
¨
che Æ H.-H. Kassemeyer
Staatliches Weinbauinstitut,
Merzhauserstrasse 119,
79100 Freiburg, Germany