Functional characterization of OsRacB GTPase--a potentially negative regulator of basal disease resistance in rice.
The rice genome contains at least seven expressed Rop small GTPase genes. Of these Rops, OsRac1 is the only characterized gene that has been implicated in disease resistance as a positive regulator. To our interest in finding a negative ROP regulator of disease resistance in rice, we applied a "phylogeny of function" approach to rice Rops, and identified OsRacB based on its close genetic orthologous relationship with the barley HvRacB gene, a known negative regulator of disease resistance. To determine the function of OsRacB, we isolated the OsRacB cDNA and conducted gene expression and transgenic studies. OsRacB, a single copy gene in the genome of rice, shared 98% identity with HvRacB at the amino acid level. Its mRNA was strongly expressed in leaf sheath (LS) and in panicles, but was very weakly expressed in young and mature leaves. The basal mRNA level of OsRacB in LS of two-week-old seedlings was strongly down-regulated upon wounding by cut and treatment with jasmonic acid. A dramatic down-regulation in the OsRacB transcripts was also found in plants inoculated with the blast pathogen, Magnaporthe grisea. Interestingly, transgenic rice plants over-expressing OsRacB showed increased symptom development in response to rice blast pathogens. Additionally, fluorescence microscopy of green fluorescent protein (GFP):OsRacB-transformed onion cells and Arabidopsis protoplasts revealed OsRacB association with plasma membrane (PM), suggesting that PM localization is required for proper function of OsRacB. Based on these results, we suggest that OsRacB functions as a potential regulator for a basal disease resistance pathway in rice.
Jung
Young-Ho
YH
Department of Molecular Biology, College of Natural Science, Sejong University, Seoul 143-747, Korea.
Agrawal
Ganesh Kumar
GK
Rakwal
Randeep
R
Kim
Jung-A
JA
Lee
Mi-Ok
MO
Choi
Pil Gyu
PG
Kim
Young Jin
YJ
Kim
Min-Jea
MJ
Shibato
Junko
J
Kim
Sun-Hyung
SH
Iwahashi
Hitoshi
H
Jwa
Nam-Soo
NS
D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D020082:Magnaporthe; D008969:Molecular Sequence Data; D020559:Monomeric GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D019697:Onions / Q000201:enzymology / Q000235:genetics; D012275:Oryza / Q000201:enzymology* / Q000276:immunology / Q000382:microbiology; D010802:Phylogeny; D010935:Plant Diseases; D018515:Plant Leaves / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified; D012333:RNA, Messenger / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
D010940:Plant Proteins; D012333:RNA, Messenger; D020559:Monomeric GTP-Binding Proteins