In animals,
aconitase is a bifunctional
protein. When an
iron-
sulfur cluster is present in its catalytic center,
aconitase displays enzymatic activity; when this cluster is lost, it switches to an
RNA-binding protein that regulates the translatability or stability of certain transcripts. To investigate the role of
aconitase in plants, we assessed its ability to bind
mRNA. Recombinant
aconitase failed to bind an
iron responsive
element (IRE) from the human
ferritin gene. However, it bound the
5' UTR of the Arabidopsis chloroplastic CuZn
superoxide dismutase 2 (CSD2)
mRNA, and this binding was specific. Arabidopsis
aconitase knockout (KO) plants were found to have significantly less
chlorosis after treatment with the
superoxide-generating compound,
paraquat. This phenotype correlated with delayed induction of the
antioxidant gene GST1, suggesting that these KO lines are more tolerant to oxidative stress. Increased levels of CSD2 mRNAs were observed in the KO lines, although the level of CSD2
protein was not affected. Virus-induced gene silencing (VIGS) of
aconitase in Nicotiana benthamiana caused a 90% reduction in
aconitase activity,
stunting, spontaneous necrotic lesions, and increased resistance to
paraquat. The silenced plants also had less cell death after transient co-expression of the AvrPto and Pto
proteins or the
pro-apoptotic protein Bax. Following inoculation with Pseudomonas syringae pv. tabaci carrying avrPto,
aconitase-silenced N. benthamiana plants expressing the Pto transgene displayed a delayed hypersensitive response (HR) and supported higher levels of bacterial growth. Disease-associated cell death in N. benthamiana inoculated with P. s. pv. tabaci was also reduced. Taken together, these results suggest that
aconitase plays a role in mediating oxidative stress and regulating cell death.