Monica Stein,a Jan Dittgen,b
Clara Sanchez-Rodrýguez,c Bi-Huei
Hou,a Antonio Molina,c Paul
Schulze-Lefert,b Volker Lipka,d
and Shauna Somervillea,1
Carnegie Institution, Department of Plant Biology,
Stanford, California, 94305.
Arabidopsis thaliana is
a host to the powdery mildew Erysiphe cichoracearum
and nonhost to Blumeria graminis f. sp hordei,
the powdery mildew pathogenic on barley (Hordeum
vulgare). Screening for Arabidopsis
mutants deficient in resistance to barley powdery
mildew identified PENETRATION3 (PEN3). pen3
plants permitted both increased invasion into epidermal
cells and initiation of hyphae by B. g. hordei,
suggesting that PEN3 contributes to defenses at the
cell wall and intracellularly. pen3 mutants
were compromised in resistance to the necrotroph Plectosphaerella
cucumerina and to two additional inappropriate
biotrophs, pea powdery mildew (Erysiphe pisi)
and potato late blight (Phytophthora infestans).
Unexpectedly, pen3 mutants were resistant
to E. cichoracearum. This resistance was
salicylic acid–dependent and correlated with
chlorotic patches. Consistent with this observation,
salicylic acid pathway genes were hyperinduced in
pen3 relative to the wild type. The phenotypes
conferred by pen3 result from the loss of
function of PLEIOTROPIC DRUG RESISTANCE8 (PDR8), a
highly expressed putative ATP binding cassette transporter.
PEN3/PDR8 tagged with green fluorescent protein localized
to the plasma membrane in uninfected cells. In infected
leaves, the protein concentrated at infection sites.
PEN3/PDR8 may be involved in exporting toxic materials
to attempted invasion sites, and intracellular accumulation
of these toxins in pen3 may secondarily activate
the salicylic acid pathway.
Erysiphe cichoracearum, Arabidopsis, Arabidopsis thaliana,
Blumeria graminis, Erysiphe pisi, Phytophthora