Mitogen-activated protein kinase (MAPK) cascades are key signaling modules downstream of receptors/sensors that perceive either endogenously produced stimuli such as peptide ligands and damage-associated molecular patterns (DAMPs) or exogenously originated stimuli such as pathogen/microbe-associated molecular patterns (P/MAMPs), pathogen-derived effectors, and environmental factors. In this review, we provide a historic view of plant MAPK research and summarize recent advances in the establishment of MAPK cascades as essential components in plant immunity, response to environmental stresses, and normal growth and development. Each tier of the MAPK cascades is encoded by a small gene family, and multiple members can function redundantly in a MAPK cascade. Yet, they carry out a diverse array of biological functions in plants. How the signaling specificity is achieved has become an interesting topic of MAPK research. Future investigation into the molecular mechanism(s) underlying the regulation of MAPK activation including the activation kinetics and magnitude in response to a stimulus, the spatiotemporal expression patterns of all the components in the signaling pathway, and functional characterization of novel MAPK substrates are central to our understanding of MAPK functions and signaling specificity in plants. This article is protected by copyright. All rights reserved.

Mitogen-activated protein kinase cascades in plant signaling.

Single-cell resolution of lineage trajectories in the Arabidopsis stomatal lineage and developing leaf.

Global Dynamic Molecular Profiling of Stomatal Lineage Cell Development by Single-Cell RNA Sequencing

A recent paper by Kidokoro et al. (2020) in The Plant Cell reported a transgene-dependent transcriptional silencing phenomenon in the dominant ice1-1 Arabidopsis mutant containing the CBF3-LUC reporter, and questioned whether ICE1 may regulate CBF genes and may be involved in plant cold response. Here, we evaluate available evidence supporting the involvement of ICE1 in plant cold response, and provide ChIP-seq data showing ICE1 binding to the promoters of CBF genes and other regulatory genes known to be critical for cold response as well as to the promoters of some COR genes.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/jipb.12918

The transcription factor ICE1 functions in cold stress response by binding to the promoters of CBF and COR genes

Dynamic cell states underlie flexible developmental programs, such as with the stomatal lineage of the Arabidopsis epidermis. Initial stages of the lineage feature asynchronous and indeterminate divisions modulated by environmental cues, enabling cell fate flexibility to generate the requisite density and pattern of stomata for a given environment. It remains unclear, however, how flexibility of cell fates is controlled. Here, we uncovered distinct models of cell state differentiation within Arabidopsis leaf tissue by leveraging single-cell transcriptomics and molecular genetics. Our findings resolved underlying heterogeneity within cell states of the flexible epidermal stomatal lineage, which appear to exist along a continuum, with progressive cell specification. Beyond the early stages of the lineage, we discovered that the core transcriptional regulator SPEECHLESS is required for cell fate commitment to yield stomatal guard cells. Overall, our work has refined the stomatal lineage paradigm and uncovered progressive cell state decisions along lineage trajectories in developing leaves.

/pdf/single-cell-resolution-of-lineage-trajectories-in-the-uo7xbr57n2.pdf

Single-Cell Resolution of Lineage Trajectories in the Arabidopsis Stomatal Lineage and Developing Leaf

Cell fate in eukaryotes is controlled by mitogen-activated protein kinases (MAPKs) that translate external cues into cellular responses. In plants, two MAPKs—MPK3 and MPK6—regulate diverse processes of development, environmental response and immunity. However, the mechanism that bridges these shared signalling components with a specific target remains unresolved. Focusing on the development of stomata—epidermal valves that are essential for gas exchange and transpiration—here, we report that the basic helix-loop-helix protein SCREAM functions as a scaffold that recruits MPK3/6 to downregulate SPEECHLESS, a transcription factor that initiates stomatal cell lineages. SCREAM directly binds to MPK3/6 through an evolutionarily conserved, yet unconventional, bipartite motif. Mutations in this motif abrogate association, phosphorylation and degradation of SCREAM, unmask hidden non-redundancies between MPK3 and MPK6, and result in uncontrolled stomatal differentiation. Structural analyses of MPK6 with a resolution of 2.75 A showed bipartite binding of SCREAM to MPK6 that is distinct from an upstream MAPKK. Our findings elucidate, at the atomic resolution, the mechanism that directly links extrinsic signals to transcriptional reprogramming during the establishment of stomatal cell fate, and highlight a unique substrate-binding mode adopted by plant MAPKs. Arabidopsis MPK3 and MPK6 are involved in many biological processes. How do they achieve their signalling specificity? Structural, biochemical and genetic approaches show that the bHLH protein SCREAM acts as a scaffold to recruit downstream substrates during stomatal development.

/pdf/bipartite-anchoring-of-scream-enforces-stomatal-initiation-km3xc8eydr.pdf

Bipartite anchoring of SCREAM enforces stomatal initiation by coupling MAP kinases to SPEECHLESS.

Cell-fate in eukaryotes is regulated by MAP Kinases (MAPKs) that translate external cues to cellular responses. In plants, two MAPKs, MPK3/6, regulate diverse processes of development, environmental response, and immunity. Yet, the mechanism bridging these shared signaling components with a specific target remains unresolved. Focusing on the development of stomata, epidermal valves for gas exchange and transpiration, we report here that the bHLH protein SCREAM functions as a scaffold by recruiting MPK3/6 to downregulate SPEECHLESS, a transcription factor initiating stomatal cell lineages. SCREAM directly binds with MPK3/6 through an evolutionarily-conserved yet unconventional bipartite motif. Mutations in this motif abrogate association, phosphorylation and degradation of SCREAM, unmask hidden non-redundancies between MPK3 and MPK6, and result in uncontrolled stomatal differentiation. Structural analyses of MPK6 at the 2.75A resolution unraveled bipartite binding of SCREAM with MPK6, that is distinct from an upstream MAPKK. Our findings elucidate, at the atomic resolution, the mechanism directly linking extrinsic signals to transcriptional reprogramming during the establishment of stomatal cell-fate, and highlight a unique substrate-binding mode adopted by plant MAPKs.

https://biorxiv.org/content/biorxiv/early/2019/03/24/587154.full-text.pdf

Bipartite anchoring of SCREAM enforces stomatal initiation by coupling MAP Kinases to SPEECHLESS

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Bipartite anchoring of SCREAM enforces stomatal initiation by coupling MAP kinases to SPEECHLESS.

Bipartite anchoring of SCREAM enforces stomatal initiation by coupling MAP Kinases to SPEECHLESS