Plant Peroxisomal Protein Kinases Implicated in Stress-Related Responses
Our understanding of the peroxisome proteome has expanded, leading to newly assigned functions and shedding new light on the involvement of peroxisomes in stress-related responses. Peroxisomes have also been found to enlarge and proliferate in response to intra- and extracellular cues. Despite our improved knowledge of peroxisome biogenesis and functions, the control of these events by post-translational modifications (PTM) is poorly studied. PTMs, specifically protein phosphorylation, are expected to fine-tune the signaling cascades that regulate peroxisome metabolism and interactions with other organelles. In this chapter, we discuss the reported types of stress responses and roles of peroxisome-related protein kinases that impact plant peroxisomes, and provide new perspectives about peroxisome functions, many of which are regulated posttranslationally.
Overview of peroxisome-mediated stress responses. Peroxisomes respond to several stresses by forming peroxules, which then potentially interact with mitochondria, chloroplast and the ER, and by proliferation to increase their number. The peroxisomal membrane-associated penetration 2 (PEN2) protein is suggested to generate toxic molecules that can be secreted in the apoplast at the infection site, to stop fungal infection. The probable roles for CDPK1 (calcium-dependent kinase 1) and CTR1 (constitutive triple response 1) that are involved in calcium- and ethylene-dependent responses are also indicated. Peroxisome proliferation requires PEX11 for extension and the PMD1, DRP3, and FIS1 for constriction and fission. EIN2: Ethylene Insensitive 2, PEX11: Peroxin 11, PMD1: Peroxisomal and mitochondrial division factor 1, DRP3: Dynamin related protein 3, FIS1: Fission 1.
Plant peroxisomes play crucial roles in metabolism and ROS detoxification. Peroxisomes are highly dynamic and interact with other organelles (mitochondria and ER) under stress conditions. Peroxisomes respond rapidly to stress through enlargement, peroxule formation, proliferation, and decreased dynamics. Accumulating evidence indicates essential roles for peroxisomal proteins in innate immunity and response to abiotic stresses. Therefore, we propose that there is a multilayered signal transduction network that finetunes peroxisome-related processes. The vast peroxisomal phosphoproteome suggests critical roles of protein phosphorylation in the regulation of peroxisome responses. Arabidopsis peroxisomes have been shown to harbor several protein phosphatases and kinases, include receptor kinases that are upregulated upon stress. Peroxisomal targeting of CDPK1 and CTR1 implicate roles for peroxisomes in calcium- and ethylene-mediated responses. Further investigation of those protein kinases may reveal the linkage of peroxisome regulatory processes and stress responses.