Abstract
Abiotic and biotic factors induce oxidative stress involving the production and scavenging of reactive oxygen species (ROS). This review is a survey of well-known and possible roles of serine-threonine protein phosphatases in plant oxidative stress signaling, with special emphasis on PP2A. ROS mediated signaling involves three interrelated pathways: (i) perception of extracellular ROS triggers signal transduction pathways, leading to DNA damage and/or the production of antioxidants; (ii) external signals induce intracellular ROS generation that triggers the relevant signaling pathways and (iii) external signals mediate protein phosphorylation dependent signaling pathway(s), leading to the expression of ROS producing enzymes like NADPH oxidases. All pathways involve inactivation of serine-threonine protein phosphatases. The metal dependent phosphatase PP2C has a negative regulatory function during ABA mediated ROS signaling. PP2A is the most abundant protein phosphatase in eukaryotic cells. Inhibitors of PP2A exert a ROS inducing activity as well and we suggest that there is a direct relationship between these two effects of drugs. We present current findings and hypotheses regarding PP2A-ROS signaling connections related to all three ROS signaling pathways and anticipate future research directions for this field. These mechanisms have implications in the understanding of stress tolerance of vascular plants, having applications regarding crop improvement.
Highlights
Reactive oxygen species (ROS) are produced in all eukaryotic cells as a consequence of both internal metabolic events and external stressors
Chandra and Low [71] have shown that inhibition of protein phosphatase activities by calyculin A (CA) and okadaic acid (OA) induced oxidative burst mimicking the effects of pathogens in soybean
One of the most interesting studies related to the effects of CA in plants showed that this inhibitor overstimulates the phosphorylation of OsPHB protein in cdr1 lesion mimic rice mutants, and this is leading to cell death [74]
Summary
Reactive oxygen species (ROS) are produced in all eukaryotic cells as a consequence of both internal metabolic events and external stressors. ROS producing and scavenging directly at the level of cell wall is not well known, but in general terms of apoplast, ROS produced mainly by the activity of NADPH oxidases will lead to a systemic oxidative burst by their cell-to-cell spread as seen during pathogen attack. External ROS, mainly superoxide and H2O2 produced by RBHOs trigger signal transduction pathways that alter gene expression leading to oxidative stress responses (see references [2,13,14] for an examples). ABA is a well-known mediator of intracellular H2O2 generation in guard cells [13] and Pathway 3 has a crucial role in this (Subchapter 3.3.) All these pathways involve protein phosphorylation dependent steps, mainly the mitogen activated protein kinase (MAPK) cascades. Types of Serine-Threonine Protein Phosphatases and ROS-Related Effects of PP2A Inhibitors in Higher Plants
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