Modulation of ABA responses by the protein kinase WNK8

Dry side of the core: a meta-analysis addressing the original nature of the ABA signalosome at the onset of seed imbibition

The timing of seedling emergence is a major agricultural and ecological fitness trait, and seed germination is controlled by a complex molecular network including phytohormone signalling. One such phytohormone, abscisic acid (ABA), controls a large array of stress and developmental processes, and researchers have long known it plays a crucial role in repressing germination. Although the main molecular components of the ABA signalling pathway have now been identified, the molecular mechanisms through which ABA elicits specific responses in distinct organs is still enigmatic. To address the fundamental characteristics of ABA signalling during germination, we performed a meta-analysis focusing on the Arabidopsis dry seed proteome as a reflexion basis. We combined cutting-edge proteome studies, comparative functional analyses, and protein interaction information with genetic and physiological data to redefine the singular composition and operation of the ABA core signalosome from the onset of seed imbibition. In addition, we performed a literature survey to integrate peripheral regulators present in seeds that directly regulate core component function. Although this may only be the tip of the iceberg, this extended model of ABA signalling in seeds already depicts a highly flexible system able to integrate a multitude of information to fine-tune the progression of germination.

Brassinosteroid-Insensitive 1-Associated Receptor Kinase 1 Modulates Abscisic Acid Signaling by Inducing PYR1 Monomerization and Association With ABI1 in Arabidopsis

Brassinosteroid-Insensitive 1-Associated Receptor Kinase 1 (BAK1) is a versatile kinase involved in many different plant developmental responses. Previously, we showed that BAK1 interacts with open stomata 1 (OST1), a cytoplasmic kinase, to promote abscisic acid (ABA)-induced stomatal closure. ABA is a plant hormone that primarily regulates stress responses and is recognized by the PYRABACTIN RESISTANCE1 (PYR1)/PYR1-LIKE (PYL)/REGULATORY COMPONENT OF ABA RECEPTORS (RCAR), which activates ABA signaling. Here, we demonstrated that BAK1 interacts with PYR1 and phosphorylates PYR1 in response to ABA in plants. We identified T137 and S142 of PYR1 as the phosphosites targeted by BAK1. Using phosphomimetic (PYR1DD) and phospho-dead (PYR1AA) PYR1 compared with wild-type PYR1, we showed that transgenic plants overexpressing a phosphomimetic PYR1 exhibited hypersensitivity to the inhibition of ABA-induced root growth and seed germination and increased ABA-induced stomatal closure and ABA-inducible gene expression. As underlying reasons for these phenomena, we further demonstrated that phosphorylated PYR1 existed in a monomeric form, in which ABA binding was increased, and the degree of complex formation with ABI1 was also increased. These results suggest that BAK1 positively modulates ABA signaling through interaction with PYR1, in addition to OST1.

Dynamic Responses of Antioxidant and Glyoxalase Systems to Seed Aging Based on Full-Length Transcriptome in Oat (Avena sativa L.)

Seed aging is a major challenge for food security, agronomic production, and germplasm conservation, and reactive oxygen species (ROS) and methylglyoxal (MG) are highly involved in the aging process. However, the regulatory mechanisms controlling the abundance of ROS and MG are not well characterized. To characterize dynamic response of antioxidant and glyoxalase systems during seed aging, oat (Avena sativa L.) aged seeds with a range of germination percentages were used to explore physiological parameters, biochemical parameters and relevant gene expression. A reference transcriptome based on PacBio sequencing generated 67,184 non-redundant full-length transcripts, with 59,050 annotated. Subsequently, eleven seed samples were used to investigate the dynamic response of respiration, ROS and MG accumulation, antioxidant enzymes and glyoxalase activity, and associated genes expression. The 48 indicators with high correlation coefficients were divided into six major response patterns, and were used for placing eleven seed samples into four groups, i.e., non-aged (Group N), higher vigor (Group H), medium vigor (Group M), and lower vigor (Group L). Finally, we proposed a putative model for aging response and self-detoxification mechanisms based on the four groups representing different aging levels. In addition, the outcomes of the study suggested the dysfunction of antioxidant and glyoxalase system, and the accumulation of ROS and MG definitely contribute to oat seed aging.

Molecular insights into the role of plant transporters in salt stress response

Salt stress disturbs the cellular osmotic and ionic balance, which then creates a negative impact on plant growth and development. The Na⁺ and Cl^- ions can enter into plant cells through various membrane transporters, including specific and non-specific Na⁺ , K⁺ , and Ca²⁺ transporters. Therefore, it is important to understand Na⁺ and K⁺ transport mechanisms in plants along with the isolation of genes, their characterization, the structural features, and their post-translation regulation under salt stress. This review summarizes the molecular insights of plant ion transporters, including non-selective cation transporters, cyclic nucleotide-gated cation transporters, glutamate-like receptors, membrane intrinsic proteins, cation proton antiporters, and sodium proton antiporter families. Further, we discussed the K⁺ transporter families such as high-affinity K⁺ transporters, HAK/KUP/KT transporters, shaker type K⁺ transporters, and K⁺ efflux antiporters. Besides the ion transport process, we have shed light on available literature on epigenetic regulation of transport processes under salt stress. Recent advancements of salt stress sensing mechanisms and various salt sensors within signaling transduction pathways are discussed. Further, we have compiled salt-stress signaling pathways, and their crosstalk with phytohormones.

With no lysine kinases: the key regulatory networks and phytohormone cross talk in plant growth, development and stress response

With No Lysine kinases (WNKs) are a distinct family of Serine/Threonine protein kinase with unique arrangement of catalytic residues in kinase domain. In WNK, an essential catalytic lysine requisite for attaching ATP and phosphorylation reaction is located in subdomain I, instead of subdomain II, which is essentially a typical feature of other Ser/Thr kinases. WNKs are identified in diverse organisms including multicellular and unicellular organisms. Mammalian WNKs are well characterized at structural and functional level, while plant WNKs are not explored much except few recent studies. Plant WNKs role in various physiological processes viz. ion maintenance, osmotic stress, pH homeostasis, circadian rhythms, regulation of flowering time, proliferation and organ development, and abiotic stresses are known, but the mechanisms involved are unclear. Plant WNKs are known to be involved in enhanced drought and salt stress response via ABA-signaling pathway, but the complete signaling cascade is yet to be elucidated. The current review will discuss the interplay between WNKs and growth regulators and their cross talks in plant growth and development. We have also highlighted the link between the stress phytohormones and WNK members in regulating abiotic stress responses in plants. The present review will provide an overall known mechanism on the involvement of WNKs in plant growth and development and abiotic stress response and highlight its role/applications in the development of stress-tolerant plants.

Upstream Open Reading Frame Mediated Translation of WNK8 Is Required for ABA Response in Arabidopsis

With no lysine (K) (WNK) kinases comprise a family of serine/threonine kinases belonging to an evolutionary branch of the eukaryotic kinome. These special kinases contain a unique active site and are found in a wide range of eukaryotes. The model plant Arabidopsis has been reported to have 11 WNK members, of which WNK8 functions as a negative regulator of abscisic acid (ABA) signaling. Here, we found that the expression of WNK8 is post-transcriptionally regulated through an upstream open reading frame (uORF) found in its 5′ untranslated region (5′-UTR). This uORF has been predicted to encode a conserved peptide named CPuORF58 in both monocotyledons and dicotyledons. The analysis of the published ribosome footprinting studies and the study of the frameshift CPuORF58 peptide with altered repression capability suggested that this uORF causes ribosome stalling. Plants transformed with the native WNK8 promoter driving WNK8 expression were comparable with wild-type plants, whereas the plants transformed with a similar construct with mutated CPuORF58 start codon were less sensitive to ABA. In addition, WNK8 and its downstream target RACK1 were found to synergistically coordinate ABA signaling rather than antagonistically modulating glucose response and flowering in plants. Collectively, these results suggest that the WNK8 expression must be tightly regulated to fulfill the demands of ABA response in plants.

Phosphorylation Site Motifs in Plant Protein Kinases and Their Substrates

Protein phosphorylation is an important cellular regulatory mechanism affecting the activity, localization, conformation, and interaction of proteins. Protein phosphorylation is catalyzed by kinases, and thus kinases are the enzymes regulating cellular signaling cascades. In the model plant Arabidopsis, 940 genes encode for kinases. The substrate proteins of kinases are phosphorylated at defined sites, which consist of common patterns around the phosphorylation site, known as phosphorylation motifs. The discovery of kinase specificity with a preference of phosphorylation of certain motifs and application of such motifs in deducing signaling cascades helped to reveal underlying regulation mechanisms, and facilitated the prediction of kinase-target pairs. In this mini-review, we took advantage of retrieved data as examples to present the functions of kinase families along with their commonly found phosphorylation motifs from their substrates.

Distinct Roles of N-Terminal Fatty Acid Acylation of the Salinity-Sensor Protein SOS3

The Salt-Overly-Sensitive (SOS) pathway controls the net uptake of sodium by roots and the xylematic transfer to shoots in vascular plants. SOS3/CBL4 is a core component of the SOS pathway that senses calcium signaling of salinity stress to activate and recruit the protein kinase SOS2/CIPK24 to the plasma membrane to trigger sodium efflux by the Na/H exchanger SOS1/NHX7. However, despite the well-established function of SOS3 at the plasma membrane, SOS3 displays a nucleo-cytoplasmic distribution whose physiological meaning is not understood. Here, we show that the N-terminal part of SOS3 encodes structural information for dual acylation with myristic and palmitic fatty acids, each of which commands a different location and function of SOS3. N-myristoylation at glycine-2 is essential for plasma membrane association and recruiting SOS2 to activate SOS1, whereas S-acylation at cysteine-3 redirects SOS3 toward the nucleus. Moreover, a poly-lysine track in positions 7-11 that is unique to SOS3 among other Arabidopsis CBLs appears to be essential for the correct positioning of the SOS2-SOS3 complex at the plasma membrane for the activation of SOS1. The nuclear-localized SOS3 protein had limited bearing on the salt tolerance of Arabidopsis. These results are evidence of a novel S-acylation dependent nuclear trafficking mechanism that contrasts with alternative subcellular targeting of other CBLs by S-acylation.

The flip side of phospho-signalling: Regulation of protein dephosphorylation and the protein phosphatase 2Cs

Protein phosphorylation is a key signalling mechanism and has myriad effects on protein function. Phosphorylation by protein kinases can be reversed by protein phosphatases, thus allowing dynamic control of protein phosphorylation. Although this may suggest a straightforward kinase-phosphatase relationship, plant genomes contain five times more kinases than phosphatases. Here, we examine phospho-signalling from a protein phosphatase centred perspective and ask how relatively few phosphatases regulate many phosphorylation sites. The most abundant class of plant phosphatases, the protein phosphatase 2Cs (PP2Cs), is surrounded by a web of regulation including inhibitor and activator proteins as well as posttranslational modifications that regulate phosphatase activity, control phosphatase stability, or determine the subcellular locations where the phosphatase is present and active. These mechanisms are best established for the Clade A PP2Cs, which are key components of stress and abscisic acid signalling. We also describe other PP2C clades and illustrate how these phosphatases are highly regulated and involved in a wide range of physiological functions. Together, these examples of multiple layers of phosphatase regulation help explain the unbalanced kinase-phosphatase ratio. Continued use of phosphoproteomics to examine phosphatase targets and phosphatase-kinase relationships will be important for deeper understanding of phosphoproteome regulation.