- OOMYCETE DISEASES OF CUCURBITS 287
Infection of plants by Peronosporales have been studied extensively,
especially the genusPhytophthora. WithPhytophthora, infection gen-
erally starts when motile zoospores released from sporangia reach a leaf
or root surface, encyst, and germinate (Kamoun et al. 1999a; Hardham
2001). Adhesion of the cyst to plant surface occurs rapidly following
zoospore encystment (Hardham and Mitchell 1998; Hardham 2001).
The cyst germinates, and the germ tube swells to form an appressorium
(plural: appressoria) that enables hyphal penetration in the plant tis-
sues. In root-infecting species, penetration can occur in-between cells
without forming an appressorium (Hardham 2001).
Penetration and colonization of host tissue involves the secretion of
a range of enzymes that break down physical cell wall barriers (McIn-
tyre and Hankin 1978). Several degradative enzymes such as cutinases,
proteases, endo- and exoglucanases, and chitinases have been identified
(Kamoun et al. 1999b; Qutob et al. 2000; McLeod et al. 2003). A fewPhy-
tophthoragenes encoding degradative enzymes have been character-
ized in detail, including genes encoding phospholipases (Nespoulous
et al. 1999), a훽-glucosidase/훽-xylosidase (Hardham 2001), exo-1,3-훽-
glucanases (McLeod et al. 2003), an endo-1,3-훽-glucanase (McLeod et al.
2003), and endopolygalacturonases (endo-PGs) (Gotesson et al. 2002;
Torto et al. 2002).
Phytophthoraeffectors that suppress host defense responses of plants
have been described in several pathosystems (Sanchez et al. 1992;
Yoshioka et al. 1995; Ham et al. 1997). Suppression of host defenses
can occur through the production of inhibitory proteins that target host
enzymes. These proteins, termed glucanase inhibitor proteins (GIPs),
show significant structural similarity to the trypsin class of serine
proteases but bear mutated catalytic residues and are proteolytically
nonfunctional as a consequence (Rose et al. 2002). GIPs are thought to
function as counter defensive molecules that inhibit the degradation of
훽-1,3- and훽-1,6-glucans in the pathogen cell wall and/or the release of
defense-eliciting oligosaccharides by host endo-훽-1,3-glucanases (Rose
et al. 2002). Another class of secreted inhibitory proteins, containing
one to three domains of Kazal-type serine protease inhibitors, was
identified by data mining ofPhytophthoraESTs (Torto et al. 2003).
One of these proteins, EPI1 fromP. infestans, was found to inhibit and
interact with tomato proteases, suggesting a novel type of defense–
counterdefense cross talk between plants andPhytophthora(Tian et al.
2005).
SeveralPhytophthoraeffector molecules are known to induce a vari-
ety of cellular defense responses in plants (Nurnberger et al. 1994;
Sacks et al. 1995; Villalba Mateos et al. 1997; Kamoun et al. 1998;