Abstract
Mitogen-activated protein kinase (MAPK) cascades are conserved eukaryote signaling modules where MAPKs, as the final kinases in the cascade, phosphorylate protein substrates to regulate cellular processes. While some progress in the identification of MAPK substrates has been made in plants, the knowledge on the spectrum of substrates and their mechanistic action is still fragmentary. In this focused review, we discuss the biological implications of the data in our original paper (Sustained mitogen-activated protein kinase activation reprograms defense metabolism and phosphoprotein profile in Arabidopsis thaliana; Frontiers in Plant Science 5: 554) in the context of related research. In our work, we mimicked in vivo activation of two stress-activated MAPKs, MPK3 and MPK6, through transgenic manipulation of Arabidopsis thaliana and used phosphoproteomics analysis to identify potential novel MAPK substrates. Here, we plotted the identified putative MAPK substrates (and downstream phosphoproteins) as a global protein clustering network. Based on a highly stringent selection confidence level, the core networks highlighted a MAPK-induced cellular reprogramming at multiple levels of gene and protein expression—including transcriptional, post-transcriptional, translational, post-translational (such as protein modification, folding, and degradation) steps, and also protein re-compartmentalization. Additionally, the increase in putative substrates/phosphoproteins of energy metabolism and various secondary metabolite biosynthesis pathways coincides with the observed accumulation of defense antimicrobial substances as detected by metabolome analysis. Furthermore, detection of protein networks in phospholipid or redox elements suggests activation of downstream signaling events. Taken in context with other studies, MAPKs are key regulators that reprogram cellular events to orchestrate defense signaling in eukaryotes.
Highlights
Since, plants are part of the ecological basis for oxygen production and food source of most lifeforms on earth, crop yield loss through stress conditions is an increasing threat to food security in view of the ever increasing human population and climate change
The sensing of potential pathogens or molecules released by microbes leads to complex signaling series of events, including ion fluxes, oxidative burst, activation of mitogenactivated protein kinase (MAPK) cascades, calcium decoding mechanisms (e.g., Calmodulin, calcium dependent protein kinases, Calcineurin B-like proteins, and their interacting kinases) (Romeis, 2001), hormonal control (Bari and Jones, 2009; Knogge et al, 2009) and defense-related gene expression (Boller and Felix, 2009; Figure 1A)
After treatment with conserved microbe-derived molecules, two main branches of MAPK cascades have been described in the model plant, Arabidopsis thaliana: one involving MKK4/5MPK3/6 (Asai et al, 2002) and the other, MEKK1-MKK1/2MPK4 (Ichimura et al, 2006)
Summary
Plants are part of the ecological basis for oxygen production and food source of most lifeforms on earth, crop yield loss through stress conditions is an increasing threat to food security in view of the ever increasing human population and climate change. A current challenge in MAPK research is to identify direct, in vivo MAPK substrates, their respective phosphorylation sites and elucidate how phosphorylation controls downstream signaling (Rasmussen et al, 2012) To this end, we performed phosphoproteomics studies on plants with simulated in vivo activation of MPK3 and MPK6 (Lassowskat et al, 2014), which was achieved by transgenic expression of a constitutively-active MKK5 (Lee et al, 2004). Such a strategy leads to only two activated MAPKs (Figure 1B) and enables us to focus on the direct substrates (and other downstream phosphoproteins) of only MPK3/MPK6. We will generalize these into four key concepts to summarize how MAPKs, in general, reprogram cellular biochemical activities to coordinately mount an appropriate (defense) response to stimuli
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