Signal transduction in response to active oxygen species in Arabidopsis thaliana
Many environmental stresses result in increased generation of active oxygen species (AOS) in plant cells, leading to the induction of protective mechanisms. In this study, signalling components linking AOS perception to downstream responses were examined, with particular emphasis on H2O2 signalling. All AOS investigated had an early [Ca2+]cyt peak in common, but differed in other aspects of their Ca2+ signatures, indicating that the plant is able to discriminate between different types of AOS. An early event in AOS signal transduction may involve changes in the cellular redox balance as reduction of glutathione levels prior to stress application increased the height of the first [Ca2+]cyt peak. Inhibiting or enhancing the height of the H2O2-triggered Ca2+ signature lead to inhibition or enhancement of GST1 and APX1 induction, respectively, demonstrating that the Ca2+ signature is required for induction of genes encoding antioxidant enzymes. OX1, encoding a putative ser/thr kinase, was shown to be involved in signal transduction in response to H2O2-generating stresses. Transcript levels of OX1 were increased upon treatment with H2O2 and a range of abiotic and biotic stresses as well as ABA, all of which have been shown to result in H2O2 accumulation. Inhibition of stress-induced [Ca2+]cyt elevations inhibited OX1 induction, placing the OX1 kinase downstream of Ca2+ in the signalling chain. OX1 is required for full activation of AtMPKS and AtMPK6 in response to ozone fumigation, indicating that OX1 functions upstream of these MAP kinases. An ox1 null-mutant displayed enhanced susceptibility to infection with a virulent Peronospora parasitica isolate as well as reduced induction of several defence genes. In addition, the ox1 mutant exhibited shorter root hairs and an early flowering phenotype. AOS treatment induced several genes encoding AtERF transcription factors, but did not have an effect on other members of this family. Induction occurred in an ethylene-independent but Ca2+-dependent manner.