Rice Genetics IV - IRRI books - International Rice Research Institute

Role of the small GTPase Rac in disease resistance of riceOsRac gene family in riceRac belongs to the Rho subfamily of small GTPase and constitutes a gene family inplants (Valster et al 2000). Although some of these genes were shown to play a role inthe control of pollen tube growth (Lin and Yang 1997), the functions of most of thesegenes are not known. Because Rac plays an important role in the regulation of theNADPH oxidase in mammalian phagocytic cells, we sought rice expressed sequencetags (ESTs) having homology with human Rac and found three such sequences calledOsRac (Kawasaki et al 1999). They had deduced amino acid sequences approximately60% identical with those of human Rac proteins. More recently, we isolated fourmore genes in rice that are homologous with the human Rac genes. Rice has fourgroups of Rac genes, OsRac1–4, and OsRac2 has four closely related members. Thus,in total, rice has at least seven Rac-related genes and most are expressed in bothleaves and roots.ROS production by the constitutive active OsRac1The small GTPase such as Rac takes two forms within a cell. The GTP-bound form isan active form and it transmits signals to downstrean effectors and the GDP-boundinactive form is not active. These two forms cycle in the cell depending on the presenceor absence of various signals (Fig. 1).We made a constitutive active form of OsRac1 by substituting glycine at position19 with valine and fused it with the CaMV 35S promoter (Fig. 2). The construct wasintroduced into seed-derived calli of the wild type (cv. Kinmaze) and a lesion-mimicmutant of rice, sl (Sekiguchi lesion) (Kiyosawa 1970), by Agrobacterium-mediatedtransformation. In the transformed cell lines of the wild type and the sl mutant, H 2 O 2production was demonstrated by staining with diaminobenzidine (DAB). The ob-SignalsInactiveGDPGTPGEFGDPActiveGTPRacGDP-bound formPiGAPRacGTP-bound formSignalsFig. 1. Two forms of the small GTPase Rac within a cell. Theinactive GDP (guanosine diphosphate)-bound form is convertedto the active GTP (guanosine triphosphate)-bound formby the help of GEF (guanine nucleotide exchange factor). Theactive form is changed to the inactive form by the help ofGAP (GTPase-activating protein).324 Shimamoto et al