The GSK3/Shaggy-Like Kinase ASKα Contributes to Pattern-Triggered Immunity

The brassinosteroid-responsive protein OCTOPUS is a novel regulator of Arabidopsis thaliana immune signaling

Phloem is a critical tissue for transport of photosynthates and extracellular signals in vascular plants. However, it also represents an ideal environment for pathogens seeking access to valuable host nutrients. Although many vascular pathogens induce economically relevant crop damage, there is still little known about the mechanisms by which immune signaling operates through the phloem. An existing phosphoproteomic dataset was mined to identify proteins that were both phosphorylated in response to the defense-elicitor flagellin (flg22) and expressed in vascular cells. A single candidate, OCTOPUS (OPS), is polarly associated with the plasma membrane of sieve element cells and has been characterized as an inhibitor of brassinosteroid insensitive-2 in promotion of brassinosteroid-related phytohormone signaling. The observation that OPS is differentially phosphorylated in response to flg22 led us to the examine whether OPS may also regulate flg22-induced immune signaling. Two independent alleles of ops exhibited enhanced immunity outputs across multiple signaling branches of PAMP-triggered immunity (PTI), constitutively and in response to flg22 treatment. Together with our observation that interactions between OPS and brassinosteroid insensitive-2 were disrupted by induction of salicylic acid and depletion of brassinosteriod, these data support a model whereby OPS modulates brassinolide and immune signaling to control downstream responses. We present OPS as a novel addition to the list of proteins with documented roles in PAMP-PTI signaling. These results further indicate that immune signaling in the phloem may be a significant and unique component of the host detection and response to pathogens in vascular plants.

Regulatory network of GSK3-like kinases and their role in plant stress response

Glycogen synthase kinase 3 (GSK3) family members are evolutionally conserved Ser/Thr protein kinases in mammals and plants. In plants, the GSK3s function as signaling hubs to integrate the perception and transduction of diverse signals required for plant development. Despite their role in the regulation of plant growth and development, emerging research has shed light on their multilayer function in plant stress responses. Here we review recent advances in the regulatory network of GSK3s and the involvement of GSK3s in plant adaptation to various abiotic and biotic stresses. We also discuss the molecular mechanisms underlying how plants cope with environmental stresses through GSK3s-hormones crosstalk, a pivotal biochemical pathway in plant stress responses. We believe that our overview of the versatile physiological functions of GSK3s and underlined molecular mechanism of GSK3s in plant stress response will not only opens further research on this important topic but also provide opportunities for developing stress-resilient crops through the use of genetic engineering technology.

Glycogen synthase kinases in model and crop plants - From negative regulators of brassinosteroid signaling to multifaceted hubs of various signaling pathways and modulators of plant reproduction and yield

Glycogen synthase kinases, also known as SHAGGY-like Kinases (GSKs/SKs), are highly conserved serine/threonine protein kinases present both in animals and plants. Plant genomes contain multiple homologs of the GSK3 genes which participate in various biological processes. Plant GSKs/SKs, and their best known representative in Arabidopsis thaliana - Brassinosteroid Insentisive2 (BIN2/SK21) in particular, were first identified as components of the brassinosteroid (BR) signaling pathway. As phytohormones, BRs regulate a wide range of physiological processes in plants - from germination, cell division, elongation and differentiation to leaf senescence, and response to environmental stresses. The GSKs/SKs proteins belong to a group of several highly conserved components of the BR signaling which evolved early during evolution of this molecular relay. However, recent reports indicated that the GSKs/SKs proteins are also implicated in signaling pathways of other phytohormones and stress-response processes. As a consequence, the GSKs/SKs proteins became hubs of various signaling pathways and modulators of plant development and reproduction. Thus, it is very important to understand molecular mechanisms regulating activity of the GSKs/SKs proteins, but also to get insights into role of the GSKs/SKs proteins in modulation of stability and activity of various substrate proteins which participate in the numerous signaling pathways. Although elucidation of these aspects is still in progress, this review presents a comprehensive and detailed description of these processes and their implications for regulation of development, stress response, and reproduction of model and crop species. The GSKs/SKs proteins and their activity are modulated through phosphorylation and de-phosphorylation reactions which are regulated by various proteins. Importantly, both phosphorylations and de-phosphorylations may have positive and negative effects on the activity of the GSKs/SKs proteins. Additionally, the activity of the GSKs/SKs proteins is positively regulated by reactive oxygen species, whereas it is negatively regulated through ubiquitylation, deacetylation, and nitric oxide-mediated nitrosylation. On the other hand, the GSKs/SKs proteins interact with proteins representing various signaling pathways, and on the basis of the complicated network of interactions the GSKs/SKs proteins differentially regulate various physiological, developmental, stress response, and yield-related processes.

D6 protein kinase in root xylem benefiting resistance to Fusarium reveals infection and defense mechanisms in tung trees

Fusarium oxysporum, a global soil-borne pathogen, causes severe disease in various cultivated plants. The mechanism underlying infection and resistance remains largely elusive. Vernicia fordii, known as the tung tree, suffers from disease caused by F. oxysporum f. sp. fordiis (Fof-1), while its sister species V. montana displays high resistance to Fof-1. To investigate the process of infection and resistance ability, we demonstrated that Fof-1 can penetrate the epidermis of root hairs and then centripetally invade the cortex and phloem in both species. Furthermore, Fof-1 spread upwards through the root xylem in susceptible V. fordii trees, whereas it failed to infect the root xylem in resistant V. montana trees. We found that D6 PROTEIN KINASE LIKE 2 (VmD6PKL2) was specifically expressed in the lateral root xylem and was induced after Fof-1 infection in resistant trees. Transgenic analysis in Arabidopsis and tomato revealed that VmD6PKL2 significantly enhanced resistance in both species, whereas the d6pkl2 mutant displayed reduced resistance against Fof-1. Additionally, VmD6PKL2 was identified to interact directly with synaptotagmin (VmSYT3), which is specifically expressed in the root xylem and mediates the negative regulation responding to Fof-1. Our data suggested that VmD6PKL2 could act as a resistance gene against Fof-1 through suppression of VmSYT3-mediated negative regulation in the lateral root xylem of the resistant species. These findings provide novel insight into Fusarium wilt resistance in plants.

Versatile Physiological Functions of Plant GSK3-Like Kinases

The plant glycogen synthase kinase 3 (GSK3)-like kinases are highly conserved protein serine/threonine kinases that are grouped into four subfamilies. Similar to their mammalian homologs, these kinases are constitutively active under normal growth conditions but become inactivated in response to diverse developmental and environmental signals. Since their initial discoveries in the early 1990s, many biochemical and genetic studies were performed to investigate their physiological functions in various plant species. These studies have demonstrated that the plant GSK3-like kinases are multifunctional kinases involved not only in a wide variety of plant growth and developmental processes but also in diverse plant stress responses. Here we summarize our current understanding of the versatile physiological functions of the plant GSK3-like kinases along with their confirmed and potential substrates.

Early transcriptome changes induced by the Geminivirus C4 oncoprotein: setting the stage for oncogenesis

BACKGROUND

The Beet curly top virus C4 oncoprotein is a pathogenic determinant capable of inducing extensive developmental abnormalities. No studies to date have investigated how the transcriptional profiles differ between plants expressing or not expressing the C4 oncoprotein.

RESULTS

We investigated early transcriptional changes in Arabidopsis associated with expression of the Beet curly top virus C4 protein that represent initial events in pathogenesis via a comparative transcriptional analysis of mRNAs and small RNAs. We identified 48 and 94 differentially expressed genes at 6- and 12-h post-induction versus control plants. These early time points were selected to focus on direct regulatory effects of C4 expression. Since previous evidence suggested that the C4 protein regulated the brassinosteroid (BR)-signaling pathway, differentially expressed genes could be divided into two groups: those responsive to alterations in the BR-signaling pathway and those uniquely responsive to C4. Early transcriptional changes that disrupted hormone homeostasis, 18 and 19 differentially expressed genes at both 6- and 12-hpi, respectively, were responsive to C4-induced regulation of the BR-signaling pathway. Other C4-induced differentially expressed genes appeared independent of the BR-signaling pathway at 12-hpi, including changes that could alter cell development (4 genes), cell wall homeostasis (5 genes), redox homeostasis (11 genes) and lipid transport (4 genes). Minimal effects were observed on expression of small RNAs.

CONCLUSION

This work identifies initial events in genetic regulation induced by a geminivirus C4 oncoprotein. We provide evidence suggesting the C4 protein regulates multiple regulatory pathways and provides valuable insights into the role of the C4 protein in regulating initial events in pathogenesis.

It takes two to tango - molecular links between plant immunity and brassinosteroid signalling

In response to the invasion of microorganisms, plants actively balance their resources for growth and defence, thus ensuring their survival. The regulatory mechanisms underlying plant immunity and growth operate through complex networks, in which the brassinosteroid phytohormone is one of the central players. In the past decades, a growing number of studies have revealed a multi-layered crosstalk between brassinosteroid-mediated growth and plant immunity. In this Review, by means of the tango metaphor, we immerse ourselves into the intimate relationship between brassinosteroid and plant immune signalling pathways that is tailored by the lifestyle of the pathogen and modulated by other phytohormones. The plasma membrane is the unique stage where brassinosteroid and immune signals are dynamically integrated and where compartmentalization into nanodomains that host distinct protein consortia is crucial for the dance. Shared downstream signalling components and transcription factors relay the tango play to the nucleus to activate the plant defence response and other phytohormonal signalling pathways for the finale. Understanding how brassinosteroid and immune signalling pathways are integrated in plants will help develop strategies to minimize the growth-defence trade-off, a key challenge for crop improvement.

Cytosolic Glucose-6-Phosphate Dehydrogenase Is Involved in Seed Germination and Root Growth Under Salinity in Arabidopsis

Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) is the key regulatory enzyme in the oxidative pentose phosphate pathway (OPPP). The cytosolic isoforms including G6PD5 and G6PD6 account for the major part of the G6PD total activity in plant cells. Here, we characterized the Arabidopsis single null mutant g6pd5 and g6pd6 and double mutant g6pd5/6. Compared to wild type, the mutant seeds showed a reduced germination rate and root elongation under salt stress. The seeds and seedlings lacking G6PD5 and G6PD6 accumulate more reactive oxygen species (ROS) than the wild type under salt stress. Cytosolic G6PD (cy-G6PD) affected the expression of NADPH oxidases and the G6PD enzymatic activities in the mutant atrbohD/F, in which the NADPH oxidases genes are disrupted by T-DNA insertion and generation of ROS is inhibited, were lower than that in the wild type. The NADPH level in mutants was decreased under salt stress. In addition, we found that G6PD5 and G6PD6 affected the activities and transcript levels of various antioxidant enzymes in response to salt stress, especially the ascorbate peroxidase and glutathione reductase. Exogenous application of ascorbate acid and glutathione rescued the seed and root phenotype of g6pd5/6 under salt stress. Interestingly, the cytosolic G6PD negatively modulated the NaCl-blocked primary root growth under salt stress in the root meristem and elongation zone.

Involvement of G6PD5 in ABA response during seed germination and root growth in Arabidopsis

BACKGROUND

Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) functions in supply of NADPH, which is required for plant defense responses to stresses. However, whether G6PD functions in the abscisic acid (ABA) signaling pathway remains to be elucidated. In this study, we investigated the involvement of the cytosolic G6PD5 in the ABA signaling pathway in Arabidopsis.

RESULTS

We characterized the Arabidopsis single null mutant g6pd5. Phenotypic analysis showed that the mutant is more sensitive to ABA during seed germination and root growth, whereas G6PD5-overexpressing plants are less sensitive to ABA compared to wild type (WT). Furthermore, ABA induces excessive accumulation of reactive oxygen species (ROS) in mutant seeds and seedlings. G6PD5 participates in the reduction of H₂O₂ to H₂O in the ascorbate-glutathione cycle. In addition, we found that G6PD5 suppressed the expression of Abscisic Acid Insensitive 5 (ABI5), the major ABA signaling component in dormancy control. When G6PD5 was overexpressed, the ABA signaling pathway was inactivated. Consistently, G6PD5 negatively modulates ABA-blocked primary root growth in the meristem and elongation zones. Of note, the suppression of root elongation by ABA is triggered by the cell cycle B-type cyclin CYCB1.

CONCLUSIONS

This study showed that G6PD5 is involved in the ABA-mediated seed germination and root growth by suppressing ABI5.

Loss of cytosolic glucose-6-phosphate dehydrogenase increases the susceptibility of Arabidopsis thaliana to root-knot nematode infection

BACKGROUND AND AIMS

Root knot nematodes (RKNs, Meloidogyne spp.) are microscopic roundworms with a wide host range causing great economic losses worldwide. Understanding how metabolic pathways function within the plant upon RKN infection will provide insight into the molecular aspects of plant-RKN interactions. Glucose-6-phosphate dehydrogenase (G6PDH), the key regulatory enzyme of the oxidative pentose phosphate pathway (OPPP), is involved in plant responses to abiotic stresses and pathogenesis. In this study, the roles of Arabidopsis cytosolic G6PDH in plant-RKN interactions were investigated.

METHODS

Enzyme assays and western blotting were used to characterize changes in total G6PDH activity and protein abundance in wild-type Arabidopsis in response to RKN infection. The susceptibility of wild-type plants and the double mutant g6pd5/6 to RKNs was analysed and the expression of genes associated with the basal defence response was tested after RKN infection using quantitative reverse transcription PCR.

KEY RESULTS

RKN infection caused a marked increase in total G6PDH activity and protein abundance in wild-type Arabidopsis roots. However, the transcript levels of G6PDH genes except G6PD6 were not significantly induced following RKN infection, suggesting that the increase in G6PDH activity may occur at the post-transcriptional level. The double mutant g6pd5/6 with loss-of-function of the two cytosolic isoforms G6PD5 and G6PD6 displayed enhanced susceptibility to RKNs. Moreover, reactive oxygen species (ROS) production and gene expression involved in the defence response including jasmonic acid and salicylic acid pathways were suppressed in the g6pd5/6 mutant at the early stage of RKN infection when compared to the wild-type plants.

CONCLUSIONS

The results demonstrated that the G6PDH-mediated OPPP plays an important role in the plant-RKN interaction. In addition, a new aspect of G6PDH activity involving NADPH production by the OPPP in plant basal defence against RKNs is defined, which may be involved in ROS signalling.

Annotation and profiling of barley GLYCOGEN SYNTHASE3/Shaggy-like genes indicated shift in organ-preferential expression

GLYCOGEN SYNTHASE KINASE3/Shaggy-like kinases (GSKs) represent a highly conserved group of proteins found in all eukaryotes. In plants they are encoded by multigene families and integrate signaling of brassinosteroids, auxin and abscisic acid in wide range of physiological and developmental processes with a strong impact on plant responses to environmental and biotic factors. Based on comprehensively studied structures of 10 Arabidopsis thaliana GSK genes and encoded proteins we report identification and phylogenetic reconstruction of 7 transcriptionally active GSK genes in barley. We re-evaluated annotation of the GSK genes in the current barley genome (Hv_IBSC_PGSB_v2) and provided data that a single gene annotated in the previous barley genome ensemble should be retained in the current one. The novel structure of another GSK, predicted in Hv_IBSC_PGSB_v2 to encode both GSK and amine oxidase domains, was proposed and experimentally confirmed based on the syntenic region in Brachypodium distachyon. The genes were assigned to 4 groups based on their encoded amino acid sequences and protein kinase domains. The analysis confirmed high level of conservation of functional protein domains and motifs among plant GSKs and the identified barley orthologs. Each of the seven identified HvGSK genes was expressed indicating semi-constitutive regulation in all tested organs and developmental stages. Regulation patterns of GSKs from the indicated groups showed a shift in organ-preferential expression in A. thaliana and barley illustrating diversification of biological roles of individual HvGSKs in different plant species.

Site-specific phosphorylation of TRANSPARENT TESTA GLABRA1 mediates carbon partitioning in Arabidopsis seeds

Seed development is dependent on nutrients, such as a source of carbon, supplied by the parent plant. It remains largely unknown how these nutrients are distributed to zygotic and maternal tissues to coordinate storage of reserve compounds and development of protective tissues like seed coat. Here we show that phosphorylation of TRANSPARENT TESTA GLABRA1 (TTG1) is regulated by SHAGGY-like kinases 11/12 (SK11/12) and that this mediates carbon flow to fatty acid synthesis and seed coat traits in Arabidopsis seeds. SK11/12 phosphorylate TTG1 at serine 215, thus preventing TTG1 interaction with TRANSPARENT TESTA2. This compromises recruitment of TTG1 to the GLABRA2 locus and downregulates GLABRA2 expression, which enhances biosynthesis of fatty acids in the embryo, but reduces production of mucilage and flavonoid pigments in the seed coat. Therefore, site-specific phosphorylation of TTG1 by SK11/SK12 regulates carbon partitioning between zygotic and maternal sinks in seeds.

Post-translational regulation of plant immunity

Plants have evolved multi-layered molecular defense strategies to protect against pathogens. Plant immune signaling largely relies on post-translational modifications (PTMs) to induce rapid alterations of signaling pathways to achieve a response that is appropriate to the type of pathogen and infection pressure. In host cells, dynamic PTMs have emerged as powerful regulatory mechanisms that cells use to adjust their immune response. PTM is also a virulence strategy used by pathogens to subvert host immunity through the activities of effector proteins secreted into the host cell. Recent studies focusing on deciphering post-translational mechanisms underlying plant immunity have offered an in-depth view of how PTMs facilitate efficient immune responses and have provided a more dynamic and holistic view of plant immunity.

Analysis of homo- and hetero-dimerization among the three 6-phosphogluconate dehydrogenase isoforms of Arabidopsis

We recently described that all three 6-phosphogluconate dehydrogenase (6PGDH) isoforms of Arabidopsis (PGD) with similar length show dual localization: PGD1 and PGD3 in the cytosol and in plastids, and PGD2 in the cytosol and in peroxisomes. We set out to investigate heterodimer formation, however due to only weak homodimerization of all Arabidopsis PGD isoforms in yeast cells, we conducted further protein-protein interaction studies in planta to investigate homomer versus heteromer formation and their sub-cellular localization. Bimolecular fluorescence complementation (BiFC) analyses in co-transfected Arabidopsis protoplasts demonstrated that all PGD isoforms may form homo- and heterodimers. Notably, with free N-terminal ends, PGD1-PGD3 heterodimers were detected both in the cytosol and in the plastid stroma, but heterodimers with PGD2 only in the cytosol. This independently confirmed that PGD2 cannot enter plastids. On the other hand, with free C-terminal ends, PGD1-PGD2 and PGD3-PGD2 heterodimers were confined to the cytosol, indicating that only PGD2 homodimers are imported by peroxisomes. Together these findings suggest functional redundancy of PGD1 and PGD3 inside plastids, and relevance of PGD1-PGD2 or PGD3-PGD2 heterodimer formation in the cytosol: this could retain sufficient 6PGDH activity needed for NADPH provision, especially during stress defense and initiation of developmental responses.