Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 415

Pseudomonas syringae pv. tomato and to the oomycete pathogen Hyaloperonospora
arabidopsidis infections. In pepper plants, silencing of CaRING1 conferred enhanced
susceptibility to avirulent Xanthomonas campestris pv. vesicatoria infection
accompanied by reduced expression of SA-dependent PR-1 gene expression (Lee
et al. 2011 ). These results suggest that the E3 ubiquitin ligase positively regulates
SA accumulation and SA-mediated defense responses.
By contrast, ubiquitin-proteasome system has been shown to negatively regulate
SA levels in tobacco. Transgenic tobacco plants expressing an inhibitor of ubiquitin-
dependent protein degradation have been developed (Conrath et al. 1998 ). These
plants constitutively accumulated enhanced levels of salicylic acid and/or its
glucoside. These transgenic plants showed enhanced resistance to Tobacco mosaic
virus (Conrath et al. 1998 ). The results suggest that ubiquitin -proteasome pathway
negatively regulates SA accumulation in the defense signaling in tobacco.

10.4.2 Role of an E3 Ubiquitin Ligase, OsRHC1,

in SA-Dependent NPR1 Signaling

A novel RING zinc fi nger protein (OsRHC1) was detected in rice and it was identifi ed
as an E3 ubiquitin ligase. Its function was dependent on the ubiquitin-mediated
protein degradation via the 26S proteasome (Cheung et al. 2007 ). OsRHC1 cDNA
from rice in transgenic Arabidopsis thaliana enhanced the defense response toward
Pseudomonas syringae pv. tomato , suggesting its role in defense signaling in rice
(Cheung et al. 2007 ). The defense response effects were neutralized in an npr1 muta-
tion background, suggesting that the function of OSRHC1 is dependent on presence
of the key defense regulator NPR1 (Cheung et al. 2007 ). NPR1 is master regulator of
SA signaling and an important regulator of responses downstream of SA (Mou et al.
2003 ; Zhang et al. 2003 ; Chern et al. 2008 ). The activity of NPR1 has been shown as
a prerequisite for the functioning of OsRHC1. Therefore, it is suggested that OsRHC1
acts either upstream from the NPR1 in the signal transduction pathway or it acts on
a negative regulator of the NPR1 pathway (Cheung et al. 2007 ).

10.4.3 Role of SON1 (F-Box Protein in E3 Ubiquitin-Ligase

Complex) in SA-Mediated Immune Responses

The gene SON1 (for Suppressor of NIM1-1) was cloned from Arabidopsis thaliana .
It was found to encode a protein containing an F-box motif, an element found in the
E3 ubiquitin-ligase complex. The gene negatively regulates defense signaling system
(Kim and Delaney 2002 ). The son1 mutant exhibited resistance response. The son1
plants that contain a functional NPR1 gene revealed a constitutive increase in PR
genes expression. In contrast, son1 npr1 double mutant plants do not show induction
of PR gene expression (Kim and Delaney 2002 ). These results suggest that SON1
represses NPR1-dependent PR gene expression. SON1, the ubiquitin-ligase, may
target for degradation of different regulators of PR gene expression. Candidates for