Nitric oxide (NO), a diffusible gas molecule, is a major signal molecule in both plants
and animals and regulates a plethora of biological processes. S-nitrosylation, a post-translation
modification, is conducted by NO, which covalently attaches protein cysteine thiols and forms
an S-nitroso thiol. S-nitrosylation plays an important role in plant development and plant
immune systems. In Arabidopsis thaliana, S-nitrosoglutathione (GSNO) is the major NO donor
for S-nitrosylation, and GSNO reductase (GSNOR) indirectly controls the S-nitrosylation level
by turning over the GSNO. An A. thaliana T-DNA insertion mutant gsnor1-3 shows the loss
of GSNOR activity and increases the S-nitrosylation level, resulting in loss of apical dominance,
reduction of SA accumulation, increased hypersensitive response (HR) cell death and reduced
disease resistance against virulence, avirulence and non-host pathogens. Interestingly, loss of
GSNOR in Drosophila melanogaster, an animal model system, reduces the resistance against
gram-positive and fungal pathogens.
Catalase is an antioxidant enzyme and regulates the redox environment through
scavenging the hydrogen peroxide (H2O2) to oxygen and water. Previous work in our lab had
discovered two gsnor1-3 suppressor mutants, gsnor1-3 spl7 and gsnor1-3 spl8, which restore
the loss of apical dominance and partially restore disease resistance. These two suppressor
mutants were then identified as the point mutation in CAT3. CAT3, one of the three CAT genes
in Arabidopsis, expresses catalase specifically in vascular tissues. To further extend the
suppression of cat3 in gsnor1-3, the mutations in CAT3 and its paralogs CAT2 and CAT1, as
well as other redox-related genes in gsnor1-3 background, were generated. In the
developmental phenotype, only the gsnor1-3 cat3 showed significant changes compared with
gsnor1-3. The disease susceptibility and HR cell death in gsnor1-3 cat3 were less than gsnor1-
3 and similar to wild-type. Moreover, the redox-related genes and CAT3 paralog mutations in
gsnor1-3 background showed no significant changes in disease resistance against virulence
pathogen compared with gsnor1-3 plant. Meanwhile, an SA-dependent (salicylic acid)
defence-related gene (PR1, pathogenesis-related gene 1) showed the early expression in
gsnor1-3 cat3 plant compared with gsnor1-3 plant. Results of developmental and disease-related
phenotypes suggest the suppression effects which turn-over the malfunction in gsnor1-
3 are highly specific to CAT3.
The previous report demonstrates that the hydroxyl radical, a reactive oxygen species
by-product from H2O2, decomposes GSNO to oxidised glutathione in vitro. The interaction of
GSNO and hydroxyl radical may be the possible mechanism of how cat3 suppresses gsnor1-
3. Therefore, we speculated less amount of GSNO in gsnor1-3 cat3 plant than in gsnor1-3 plant
and lower level of hydroxyl radicals in gsnor1-3 cat3 plant than in cat3 plant. To evaluate our
hypothesis, the content hydroxyl and GSNO were analysed in wild-type, gsnor1-3, cat3 and
gsnor1-3 cat3 plants. The total S-nitrosylated protein, which indicates the GSNO content in
vivo, was less in gsnor1-3 cat3 than in gsnor1-3. Furthermore, the level of hydroxyl radical in
gsnor1-3 cat3 was lower than cat3. Accordingly, the reduction of hydroxyl radical in gsnor1-
3 cat3 may occur due to the reaction with GSNO and vice versa.
Similar to what has been found in Arabidopsis, D. melanogaster also reported partial
restoration of the immunodeficiency phenotypes of gsnor knock-out flies with an additional
mutation in CAT gene. Interestingly, the content of hydroxyl radical in gsnor-/- cat-/- line was
less than cat+/-. Collectively, our results suggest an interaction of hydroxyl radical and GSNO
may happen both in Arabidopsis and Drosophila. Further research is needed to clarify the
interaction between hydroxyl radical and GSNO in Arabidopsis as well as in Drosophila