The heterotrimeric G protein α subunit acts upstream of the small GTPase Rac in disease resistance of rice

^*Laboratory of Plant Molecular Genetics, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma 630-0101, Japan; ^†Department of Biosciences, Fukui Prefectural University, 4-1-1 Kenjyojima, Fukui 910-1195, Japan; and ^‡Faculty of Agriculture, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan

Edited by Frederick M. Ausubel, Harvard Medical School, Boston, MA, and approved July 17, 2002 (received for review April 24, 2002)

Abstract

We used rice dwarf1 (d1) mutants lacking a single-copy Gα gene and addressed Gα's role in disease resistance. d1 mutants exhibited a highly reduced hypersensitive response to infection by an avirulent race of rice blast. Activation of PR gene expression in the leaves of the mutants infected with rice blast was delayed for 24 h relative to the wild type. H₂O₂ production and PR gene expression induced by sphingolipid elicitors (SE) were strongly suppressed in d1 cell cultures. Expression of the constitutively active OsRac1, a small GTPase Rac of rice, in d1 mutants restored SE-dependent defense signaling and resistance to rice blast. Gα mRNA was induced by an avirulent race of rice blast and SE application on the leaf. These results indicated the role of Gα in R gene-mediated disease resistance of rice. We have proposed a model for the defense signaling of rice in which the heterotrimeric G protein functions upstream of the small GTPase OsRac1 in the early steps of signaling.

Footnotes

↵ § To whom reprint requests should be addressed. E-mail: simamoto{at}bs.aist-nara.ac.jp.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations:

SE,

sphingolipid elicitors;

HR,

hypersensitive response;

GUS,

β-glucuronidase