G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 173

Rac1

RAR1

PAMP

Activation

Transcriptional
regulation

RACK1

HSP90

SGT1

HSP70

NADPH oxidase

SA signaling

ROS

JA signaling

Rac GTPase-RboH
interaction

Activation of immune
responses

Fig. 3.4 Function of RAR1-SGT1-HSP90-HSP70 molecular chaperone complex in small
G-protein-triggered immunity in rice (Adapted from Thao et al. 2007 ; Nakashima et al. 2009 ;
Shirasu 2009 )

resistance to the rice blast pathogen M. oryzae and the bacterial blight pathogen
X. oryzae pv. oryzae (Thao et al. 2007 ). Constitutively active OsRac1 comple-
mented the loss of resistance, suggesting that OsRac1 and RAR1 are functionally
linked. Studies with OsRAR1-RNAi and treatment with geldanamycin, an HSP90-
specifi c inhibitor, showed that RAR1 and HSP90 are essential for the Rac1-
mediated enhancement of PAMP-triggered immune responses in rice cell cultures.
It was also shown that the function of HSP 90, but not RAR1, may be essential for
their association with the Rac1 complex. OsRac1 also regulates RAR1 expression
at both the mRNA and protein levels. It was found that Rac1, RAR1, HSP90, and
HSP70 form a complex in rice cells and these proteins play important roles in
plant innate immunity (Thao et al. 2007 ).
Silencing of the co-chaperone genes RAR1 and SGT1-2 involved in basal and R
gene-mediated defense resulted in susceptibility to Pseudomonas syringae in soybean