The Quest for MAP Kinase Substrates: Gaining Momentum

Ethylene-MPK8-ERF.C1-PR module confers resistance against Botrytis cinerea in tomato fruit without compromising ripening

The plant hormone ethylene plays a critical role in fruit defense against Botrytis cinerea attack, but the underlying mechanisms remain poorly understood. Here, we showed that ethylene response factor SlERF.C1 acts as a key regulator to trigger the ethylene-mediated defense against B. cinerea in tomato fruits without compromising ripening. Knockout of SlERF.C1 increased fruit susceptibility to B. cinerea with no effect on ripening process, while overexpression enhanced resistance. RNA-Seq, transactivation assays, EMSA and ChIP-qPCR results indicated that SlERF.C1 activated the transcription of PR genes by binding to their promoters. Moreover, SlERF.C1 interacted with the mitogen-activated protein kinase SlMPK8 which allowed SlMPK8 to phosphorylate SlERF.C1 at the Ser174 residue and increases its transcriptional activity. Knocking out of SlMPK8 increased fruit susceptibility to B. cinerea, whereas overexpression enhanced resistance without affecting ripening. Furthermore, genetic crosses between SlMPK8-KO and SlERF.C1-OE lines reduced the resistance to B. cinerea attack in SlERF.C1-OE fruits. In addition, B. cinerea infection induced ethylene production which in turn triggered SlMPK8 transcription and enhanced the phosphorylation of SlERF.C1. Overall, our findings reveal the regulatory mechanism of the 'Ethylene-MPK8-ERF.C1-PR' module in resistance against B. cinerea and provide new insight into the manipulation of gray mold disease in fruits.

Comprehensive analysis of MAPK gene family in upland cotton (Gossypium hirsutum) and functional characterization of GhMPK31 in regulating defense response to insect infestation

KEY MESSAGE

The transcriptomic, phenotypic and metabolomic analysis of transgenic plants overexpressing GhMPK31 in upland cotton revealed the regulation of H2O2 burst and the synthesis of defensive metabolites by GhMPK31. Mitogen-activated protein kinases (MAPKs) are a crucial class of protein kinases, which play an essential role in various biological processes in plants. Upland cotton (G. hirsutum) is the most widely cultivated cotton species with high economic value. To gain a better understanding of the role of the MAPK gene family, we conducted a comprehensive analysis of the MAPK gene family in cotton. In this study, a total of 55 GhMPK genes were identified from the whole genome of G. hirsutum. Through an investigation of the expression patterns under diverse stress conditions, we discovered that the majority of GhMPK family members demonstrated robust responses to abiotic stress, pathogen stress and pest stress. Furthermore, the overexpression of GhMPK31 in cotton leaves led to a hypersensitive response (HR)-like cell death phenotype and impaired the defense capability of cotton against herbivorous insects. Transcriptome and metabolomics data analysis showed that overexpression of GhMPK31 enhanced the expression of H2O2-related genes and reduced the accumulation of defensive related metabolites. The direct evidence of GhMPK31 interacting with GhRBOHB (H2O2-generating protein) were found by Y2H, BiFC, and LCI. Therefore, we propose that the increase of H2O2 content caused by overexpression of GhMPK31 resulted in HR-like cell death in cotton leaves while reducing the accumulation of defensive metabolites, ultimately leading to a decrease in the defense ability of cotton against herbivorous insects. This study provides valuable insights into the function of MAPK genes in plant resistance to herbivorous insects.

A B-box transcription factor OsBBX17 regulates saline-alkaline tolerance through the MAPK cascade pathway in rice

Rice OsBBX17 encodes a B-box zinc finger transcription factor in which the N-terminal B-box structural domain interacts with OsMPK1. In addition, it directly binds to the G-box of OsHAK2 and OsHAK7 promoters and represses their transcription. Under saline-alkaline conditions, the expression of OsBBX17 was inhibited. Meanwhile, activation of the OsMPK1-mediated mitogen-activated protein kinase cascade pathway caused OsMPK1 to interact with OsBBX17 and phosphorylate OsBBX17 at the Thr-95 site. It reduced OsBBX17 DNA-binding activity and enhanced saline-alkaline tolerance by deregulating transcriptional repression of OsHAK2 and OsHAK7. Genetic assays showed that the osbbx17-KO had an excellent saline-alkaline tolerance, whereas the opposite was in OsBBX17-OE. In addition, overexpression of OsMPK1 significantly improved saline-alkaline tolerance, but knockout of OsMPK1 caused an increased sensitivity. Further overexpression of OsBBX17 in the osmpk1-KO caused extreme saline-alkaline sensitivity, even a quick death. OsBBX17 was validated in saline-alkaline tolerance from two independent aspects, transcriptional level and post-translational protein modification, unveiling a mechanistic framework by which OsMPK1-mediated phosphorylation of OsBBX17 regulates the transcription of OsHAK2 and OsHAK7 to enhance the Na+ /K+ homeostasis, which partially explains light on the molecular mechanisms of rice responds to saline-alkaline stress via B-box transcription factors for the genetic engineering of saline-alkaline tolerant crops.

Integrated Transcriptome and Metabolome Analyses Reveal Details of the Molecular Regulation of Resistance to Stem Nematode in Sweet Potato

Stem nematode disease can seriously reduce the yield of sweet potato (Ipomoea batatas (L.) Lam). To explore resistance mechanism to stem nematode in sweet potato, transcriptomes and metabolomes were sequenced and compared between two sweet potato cultivars, the resistant Zhenghong 22 and susceptible Longshu 9, at different times after stem nematode infection. In the transcriptional regulatory pathway, mitogen-activated protein kinase signaling was initiated in Zhenghong 22 at the early stage of infection to activate genes related to ethylene production. Stem nematode infection in Zhenghong 22 also triggered fatty acid metabolism and the activity of respiratory burst oxidase in the metabolic pathway, which further stimulated the glycolytic and shikimic pathways to provide raw materials for secondary metabolite biosynthesis. An integrated analysis of the secondary metabolic regulation pathway in the resistant cultivar Zhenghong 22 revealed the accumulation of tryptophan, phenylalanine, and tyrosine, leading to increased biosynthesis of phenylpropanoids and salicylic acid and enhanced activity of the alkaloid pathway. Stem nematode infection also activated the biosynthesis of terpenoids, abscisic acid, zeatin, indole, and brassinosteroid, resulting in improved resistance to stem nematode. Finally, analyses of the resistance regulation pathway and a weighted gene co-expression network analysis highlighted the importance of the genes itf14g17940 and itf12g18840, encoding a leucine-rich receptor-like protein and 1-aminocyclopropane-1-carboxylate synthase, respectively. These are candidate target genes for increasing the strength of the defense response. These results provide new ideas and a theoretical basis for understanding the mechanism of resistance to stem nematode in sweet potato.

The MEKK1-MKK1/2-MPK4 cascade phosphorylates and stabilizes STOP1 to confer aluminum resistance in Arabidopsis

Aluminum (Al) toxicity can seriously restrict crop production on acidic soils, which comprise 40% of the world's potentially arable land. The zinc finger transcription factor STOP1 has a conserved and essential function in mediating plant Al resistance. Al stress induces STOP1 accumulation via post-transcriptional regulatory mechanisms. However, the upstream signaling pathway involved in Al-triggered STOP1 accumulation remains unclear. Here, we report that the MEKK1-MKK1/2-MPK4 cascade positively regulates STOP1 phosphorylation and stability. Mutations of MEKK1, MKK1/2, or MPK4 lead to decreased STOP1 stability and Al resistance. Al stress induces the kinase activity of MPK4, which interacts with and phosphorylates STOP1. The phosphorylation of STOP1 reduces its interaction with the F-box protein RAE1 that mediates STOP1 degradation, thereby leading to enhanced STOP1 stability and Al resistance. Taken together, our results suggest that the MEKK1-MKK1/2-MPK4 cascade is important for Al signaling and confers Al resistance through phosphorylation-mediated enhancement of STOP1 accumulation in Arabidopsis.

Analysis of metabolic and transcription levels provides insights into the interactions of plant hormones and crosstalk with MAPKs in the early signaling response of cherry tomato fruit induced by the yeast cell wall

Yeast cell walls (YCW) are promising bio-based elicitors for controlling post-harvest fruit decay. In this study, 1% YCW induction increased the resistance of cherry tomato fruits, reducing disease incidence by 66%. This study aimed to explore the interaction of hormones and crosstalk with MAPKs (mitogen-activated protein kinases) in the early response of resistance regulation in cherry tomato fruits treated with YCW and U0126. We analyzed the temporal changes in hormone content, the expression of critical genes involved in phytohormone biosynthesis, and signal transduction in cherry tomato fruits response to the induction. Results revealed that jasmonic acid (JA) and brassinosteroids (BR) significantly regulated early resistance response in fruit induced by 1% YCW. The salicylic acid (SA) pathway is inhibited by the activation of the JA pathway. JA and SA signaling pathway crosstalk with the MAPK3 pathway. BR plays an essential role in the regulation of fruit resistance. The BR pathway may function independently when JA/SA and MAPK3 pathways are inhibited.

Mitogen-activated protein kinase TaMPK3 suppresses ABA response by destabilising TaPYL4 receptor in wheat

Abscisic acid (ABA) receptors are considered as the targeted manipulation of ABA sensitivity and water productivity in plants. Regulation of their stability or activity will directly affect ABA signalling. Mitogen-activated protein kinase (MAPK) cascades link multiple environmental and plant developmental cues. However, the molecular mechanism of ABA signalling and MAPK cascade interaction remains largely elusive. TaMPK3 overexpression decreases drought tolerance and wheat sensitivity to ABA, significantly weakening ABA's inhibitory effects on growth. Under drought stress, overexpression lines show lower survival rates, shoot fresh weight and proline content, but higher malondialdehyde levels at seedling stage, as well as decreased grain width and 1000 grain weight in both glasshouse and field conditions at the adult stage. TaMPK3-RNAi increases drought tolerance. TaMPK3 interaction with TaPYL4 leads to decreased TaPYL4 levels by promoting its ubiquitin-mediated degradation, whereas ABA treatment diminishes TaMPK3-TaPYL interactions. In addition, the expression of ABA signalling proteins is impaired in TaMPK3-overexpressing wheat plants under ABA treatment. The MPK3-PYL interaction module was found to be conserved across monocots and dicots. Our results suggest that the MPK3-PYL module could serve as a negative regulatory mechanism for balancing appropriate drought stress response with normal plant growth signalling in wheat.

WRKY transcription factors: a promising way to deal with arsenic stress in rice

Arsenic (As) is a global carcinogenic contaminant, and is one of the significant environmental constraints that limits the development and yield of crop plants. It is always tagged along with rice as rice takes up As and tends to accumulate it in grains. This amassment makes a way for As to get into the food chain that leads to unforeseen human health risks. Being viewed as parallel with toxicity, As in rice is an important global risk that calls for an urgent solution. WRKY Transcription Factors (TFs) seems to be promising in this area. The classical and substantial progress in the molecular mechanism of WRKY TFs, strengthened the understanding of innovative solutions for dealing with As in rice. Here, we review the potential of WRKY TFs under As stressed rice as a genetic solution and also provide insights into As and rice. Further, we develop an understanding of WRKY TF gene family and its regulation in rice. To date, studies on the role of WRKY TFs under As stressed rice are lacking. This area needs to be explored more so that this gene family can be utilized as an effective genetic tool that can break the As cycle to develop low or As free rice cultivar.

MAPKs trigger antiviral immunity by directly phosphorylating a rhabdovirus nucleoprotein in plants and insect vectors

Signaling by the evolutionarily conserved mitogen-activated protein kinase or extracellular signal-regulated kinase (MAPK/ERK) plays critical roles in converting extracellular stimuli into immune responses. However, whether MAPK/ERK signaling induces virus immunity by directly phosphorylating viral effectors remains largely unknown. Barley yellow striate mosaic virus (BYSMV) is an economically important plant cytorhabdovirus that is transmitted by the small brown planthopper (SBPH, Laodelphax striatellus) in a propagative manner. Here, we found that the barley (Hordeum vulgare) MAPK MPK3 (HvMPK3) and the planthopper ERK (LsERK) proteins interact with the BYSMV nucleoprotein (N) and directly phosphorylate N protein primarily on serine 290. The overexpression of HvMPK3 inhibited BYSMV infection, whereas barley plants treated with the MAPK pathway inhibitor U0126 displayed greater susceptibility to BYSMV. Moreover, knockdown of LsERK promoted virus infection in SBPHs. A phosphomimetic mutant of the N Ser290 (S290D) completely abolished virus infection because of impaired self-interaction of BYSMV N and formation of unstable N-RNA complexes. Altogether, our results demonstrate that the conserved MAPK and ERK directly phosphorylate the viral nucleoprotein to trigger immunity against cross-kingdom infection of BYSMV in host plants and its insect vectors.

OsMAPK6 phosphorylates a zinc finger protein OsLIC to promote downstream OsWRKY30 for rice resistance to bacterial blight and leaf streak

Rice OsLIC encoding a CCCH zinc finger transcription factor plays an important role in immunity. However, the immune signaling pathways that OsLIC-involved and the underlying mechanisms that OsLIC-conferred resistance against pathogens are largely unclear. Here, we show that OsLIC, as a substrate for OsMAPK6, negatively regulates resistance to Xanthomonas oryzae pv. oryzae (Xoo) and X. oryzae pv. oryzicola (Xoc) by directly suppressing OsWRKY30 transcription. Biochemical assays showed that OsLIC bound to OsWRKY30 promoter and suppressed its transcription. Genetic assays confirmed that the osilc knockout mutants and OsWRKY30-overexpressing plants exhibited enhanced resistance to Xoo and Xoc, knocking out OsWRKY30 in the oslic mutants attenuated the resistance against bacterial pathogens. OsMAPK6 physically interacted with and phosphorylated OsLIC leading to decreased OsLIC DNA-binding activity, therefore, overexpression of OsLIC partially suppressed OsMAPK6-mediated rice resistance. In addition, both OsMAPK6-phosphorylated activation of OsLIC and phosphorylation-mimic OsLIC^5D had reduced DNA-binding activity towards OsWRKY30 promoter, thereby promoting OsWRKY30 transcription. Collectively, these results reveal that OsMAPK6-mediated phosphorylation of OsLIC positively regulates rice resistance to Xoo and Xoc by modulating OsWRKY30 transcription, suggesting that OsMAPK6-OsLIC-OsWRKY30 module is an immune signaling pathway in response to the bacterial pathogens.

MPK3 and MPK6 control salicylic acid signaling by up-regulating NLR receptors during pattern- and effector-triggered immunity

Arabidopsis thaliana mitogen-activated protein kinases 3 and 6 (MPK3/6) are activated transiently during pathogen-associated molecular pattern-triggered immunity (PTI) and durably during effector-triggered immunity (ETI). The functional differences between these two kinds of activation kinetics and how they coordinate the two layers of plant immunity remain poorly understood. Here, by suppressor analyses, we demonstrate that ETI-mediating nucleotide-binding domain leucine-rich repeat receptors (NLRs) and the NLR signaling components NDR1 and EDS1 can promote the salicylic acid sector of defense downstream of MPK3 activity. Moreover, we provide evidence that both sustained and transient MPK3/6 activities positively control the expression of several NLR genes, including AT3G04220 and AT4G11170. We further show that NDR1 and EDS1 contribute to the up-regulation of these two NLRs in both an ETI and a PTI context. Remarkably, whereas in ETI MPK3/6 activities are dependent on NDR1 and EDS1, they are not in PTI, suggesting crucial differences in the two signaling pathways. Finally, we demonstrate that expression of the NLR AT3G04220 is sufficient to induce expression of defense genes from the salicylic acid branch. Overall, this study expands our knowledge of MPK3/6 functions during immunity and provides new insights into the intricate interplay of PTI and ETI.

Low temperature inhibits anthocyanin accumulation in strawberry fruit by activating FvMAPK3-induced phosphorylation of FvMYB10 and degradation of Chalcone Synthase 1

Low temperature causes poor coloration of strawberry (Fragaria sp.) fruits, thus greatly reducing their commercial value. Strawberry fruits accumulate anthocyanins during ripening, but how low temperature modulates anthocyanin accumulation in plants remains largely unknown. We identified MITOGEN-ACTIVATED PROTEIN KINASE3 (FvMAPK3) as an important negative regulator of anthocyanin accumulation that mediates the poor coloration of strawberry fruits in response to low temperature. FvMAPK3 activity was itself induced by low temperature, leading to the repression of anthocyanin accumulation via two mechanisms. Activated FvMAPK3 acted as the downstream target of MAPK KINASE4 (FvMKK4) and SUCROSE NONFERMENTING1-RELATED KINASE2.6 (FvSnRK2.6) to phosphorylate the transcription factor FvMYB10 and reduce its transcriptional activity. In parallel, FvMAPK3 phosphorylated CHALCONE SYNTHASE1 (FvCHS1) to enhance its proteasome-mediated degradation. These results not only provide an important reference to elucidate the molecular mechanisms underlying low-temperature-mediated repression of anthocyanin accumulation in plants, but also offer valuable candidate genes for generating strawberry varieties with high tolerance to low temperature and good fruit quality.

Phosphorylation of transcription factor bZIP21 by MAP kinase MPK6-3 enhances banana fruit ripening

Ripening of fleshy fruits involves both diverse post-translational modifications (PTMs) and dynamic transcriptional reprogramming, but the interconnection between PTMs, such as protein phosphorylation and transcriptional regulation, in fruit ripening remains to be deciphered. Here, we conducted a phosphoproteomic analysis during banana (Musa acuminata) ripening and identified 63 unique phosphopeptides corresponding to 49 proteins. Among them, a Musa acuminata basic leucine zipper transcription factor21 (MabZIP21) displayed elevated phosphorylation level in the ripening stage. MabZIP21 transcript and phosphorylation abundance increased during banana ripening. Genome-wide MabZIP21 DNA binding assays revealed MabZIP21-regulated functional genes contributing to banana ripening, and electrophoretic mobility shift assay, chromatin immunoprecipitation coupled with quantitative polymerase chain reaction, and dual-luciferase reporter analyses demonstrated that MabZIP21 stimulates the transcription of a subset of ripening-related genes via directly binding to their promoters. Moreover, MabZIP21 can be phosphorylated by MaMPK6-3, which plays a role in banana ripening, and T318 and S436 are important phosphorylation sites. Protein phosphorylation enhanced MabZIP21-mediated transcriptional activation ability, and transient overexpression of the phosphomimetic form of MabZIP21 accelerated banana fruit ripening. Additionally, MabZIP21 enlarges its role in transcriptional regulation by activating the transcription of both MaMPK6-3 and itself. Taken together, this study reveals an important machinery of protein phosphorylation in banana fruit ripening in which MabZIP21 is a component of the complex phosphorylation pathway linking the upstream signal mediated by MaMPK6-3 with transcriptional controlling of a subset of ripening-associated genes.

The mitogen-activated protein kinase kinase TaMKK5 mediates immunity via the TaMKK5-TaMPK3-TaERF3 module

Sharp eyespot disease, caused by the soil-borne fungus Rhizoctonia cerealis, seriously threatens production of wheat (Triticum aestivum). Despite considerable advances in understanding the mechanisms of mitogen-activated protein kinase (MAPK) cascades in innate immunity in model plant species, the roles of MAPK cascades in wheat are unknown. In this study, we identified a wheat MAPK kinase TaMKK5, located on chromosome 6B, and deciphered its functional role in the innate immune responses to R. cerealis attack. The TaMKK5-6B transcript level was elevated after R. cerealis infection and was higher in resistant wheat genotypes compared to susceptible genotypes. Overexpressing TaMKK5-6B increased resistance to sharp eyespot and upregulated the expression of multiple defense-related genes in wheat, including the MAPK gene TaMPK3, the ethylene response factor gene TaERF3, the calcium-dependent protein kinase gene TaCPK7-D, the glutathione s-transferase-1 gene TaGST1, Defensin, and Chitinase 2, while TaMKK5 knock-down compromised the resistance and repressed the expression of these defense-related genes. Bimolecular fluorescence complementation, yeast two-hybrid, pull-down, and phosphorylation assays showed that TaMKK5 physically interacted with TaMPK3, and phosphorylated and activated TaMPK3, and that TaMPK3 interacted with and phosphorylated TaERF3. The TaMKK5-TaMPK3 cascade modulates the expression of TaGST1, Defensin, and Chitinase 2 through TaERF3. Collectively, TaMKK5 mediates resistance to sharp eyespot through the TaMKK5-TaMPK3-TaERF3 module and by upregulating the expression of defense-related genes in wheat. This study provides insights into the role of the wheat MAPK cascades in innate immunity. TaMKK5-6B is a promising gene for breeding wheat cultivars that are resistant to sharp eyespot.

ZmMPK5 phosphorylates ZmNAC49 to enhance oxidative stress tolerance in maize

Mitogen-activated protein kinase (MPK) is a critical regulator of the antioxidant defence system in response to various stimuli. However, how MPK directly and exactly regulates antioxidant enzyme activities is still unclear. Here, we demonstrated that a NAC transcription factor ZmNAC49 mediated the regulation of antioxidant enzyme activities by ZmMPK5. ZmNAC49 expression is induced by oxidative stress. ZmNAC49 enhances oxidative stress tolerance in maize, and it also reduces superoxide anion generation and increases superoxide dismutase (SOD) activity. A detailed study showed that ZmMPK5 directly interacts with and phosphorylates ZmNAC49 in vitro and in vivo. ZmMPK5 directly phosphorylates Thr-26 in NAC subdomain A of ZmNAC49. Mutation at Thr-26 of ZmNAC49 does not affect the interaction with ZmMPK5 and its subcellular localisation. Further analysis found that ZmNAC49 activates the ZmSOD3 expression by directly binding to its promoter. ZmMPK5-mediated ZmNAC49 phosphorylation improves its ability to bind to the ZmSOD3 promoter. Thr-26 of ZmNAC49 is essential for its transcriptional activity. In addition, ZmSOD3 enhances oxidative stress tolerance in maize. Our results show that phosphorylation of Thr-26 in ZmNAC49 by ZmMPK5 increased its DNA-binding activity to the ZmSOD3 promoter, enhanced SOD activity and thereby improved oxidative stress tolerance in maize.

Rice calcium/calmodulin-dependent protein kinase directly phosphorylates a mitogen-activated protein kinase kinase to regulate abscisic acid responses

Calcium (Ca2+)/calmodulin (CaM)-dependent protein kinase (CCaMK) is an important positive regulator of abscisic acid (ABA) and abiotic stress signaling in plants and is believed to act upstream of mitogen-activated protein kinase (MAPK) in ABA signaling. However, it is unclear how CCaMK activates MAPK in ABA signaling. Here, we show that OsDMI3, a rice (Oryza sativa) CCaMK, directly interacts with and phosphorylates OsMKK1, a MAPK kinase (MKK) in rice, in vitro and in vivo. OsDMI3 was found to directly phosphorylate Thr-25 in the N-terminus of OsMKK1, and this Thr-25 phosphorylation is OsDMI3-specific in ABA signaling. The activation of OsMKK1 and its downstream kinase OsMPK1 is dependent on Thr-25 phosphorylation of OsMKK1 in ABA signaling. Moreover, ABA treatment induces phosphorylation in the activation loop of OsMKK1, and the two phosphorylations, in the N-terminus and in the activation loop, are independent. Further analyses revealed that OsDMI3-mediated phosphorylation of OsMKK1 positively regulates ABA responses in seed germination, root growth, and tolerance to both water stress and oxidative stress. Our results indicate that OsMKK1 is a direct target of OsDMI3, and OsDMI3-mediated phosphorylation of OsMKK1 plays an important role in activating the MAPK cascade and ABA signaling.

Update on the Roles of Rice MAPK Cascades

The mitogen-activated protein kinase (MAPK) cascades have been validated playing critical roles in diverse aspects of plant biology, from growth and developmental regulation, biotic and abiotic stress responses, to phytohormone signal transduction or responses. A classical MAPK cascade consists of a MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK), and a MAPK. From the 75 MAPKKKs, eight MAPKKs, and 15 MAPKs of rice, a number of them have been functionally deciphered. Here, we update recent advances in knowledge of the roles of rice MAPK cascades, including their components and complicated action modes, their diversified functions controlling rice growth and developmental responses, coordinating resistance to biotic and abiotic stress, and conducting phytohormone signal transduction. Moreover, we summarize several complete MAPK cascades that harbor OsMAPKKK-OsMAPKK-OsMAPK, their interaction with different upstream components and their phosphorylation of diverse downstream substrates to fulfill their multiple roles. Furthermore, we state a comparison of networks of rice MAPK cascades from signal transduction crosstalk to the precise selection of downstream substrates. Additionally, we discuss putative concerns for elucidating the underlying molecular mechanisms and molecular functions of rice MAPK cascades in the future.

Regulation of glucosinolate biosynthesis

Glucosinolates are secondary defense metabolites produced by plants of the order Brassicales, which includes the model species Arabidopsis and many crop species. In the past 13 years, the regulation of glucosinolate synthesis in plants has been intensively studied, with recent research revealing complex molecular mechanisms that connect glucosinolate production with responses to other central pathways. In this review, we discuss how the regulation of glucosinolate biosynthesis is ecologically relevant for plants, how it is controlled by transcription factors, and how this transcriptional machinery interacts with hormonal, environmental, and epigenetic mechanisms. We present the central players in glucosinolate regulation, MYB and basic helix-loop-helix transcription factors, as well as the plant hormone jasmonate, which together with other hormones and environmental signals allow the coordinated and rapid regulation of glucosinolate genes. Furthermore, we highlight the regulatory connections between glucosinolates, auxin, and sulfur metabolism and discuss emerging insights and open questions on the regulation of glucosinolate biosynthesis.

Induction of γ-aminobutyric acid plays a positive role to Arabidopsis resistance against Pseudomonas syringae

Gamma-aminobutyric acid (GABA) is an important metabolite which functions in plant growth, development, and stress responses. However, its role in plant defense and how it is regulated are largely unknown. Here, we report a detailed analysis of GABA induction during the resistance response to Pseudomonas syringae in Arabidopsis thaliana. While searching for the mechanism underlying the pathogen-responsive mitogen-activated protein kinase (MPK)3/MPK6 signaling cascade in plant immunity, we found that activation of MPK3/MPK6 greatly induced GABA biosynthesis, which is dependent on the glutamate decarboxylase genes GAD1 and GAD4. Inoculation with Pseudomonas syringae pv tomato DC3000 (Pst) and Pst-avrRpt2 expressing the avrRpt2 effector gene induced GAD1 and GAD4 gene expression and increased the levels of GABA. Genetic evidence revealed that GAD1, GAD2, and GAD4 play important roles in both GABA biosynthesis and plant resistance in response to Pst-avrRpt2 infection. The gad1/2/4 triple and gad1/2/4/5 quadruple mutants, in which the GABA levels were extremely low, were more susceptible to both Pst and Pst-avrRpt2. Functional loss of MPK3/MPK6, or their upstream MKK4/MKK5, or their downstream substrate WRKY33 suppressed the induction of GAD1 and GAD4 expression after Pst-avrRpt2 treatment. Our findings shed light on both the regulation and role of GABA in the plant immunity to a bacterial pathogen.

Sustained Incompatibility between MAPK Signaling and Pathogen Effectors

In plants, Mitogen-Activated Protein Kinases (MAPKs) are important signaling components involved in developemental processes as well as in responses to biotic and abiotic stresses. In this review, we focus on the roles of MAPKs in Effector-Triggered Immunity (ETI), a specific layer of plant defense responses dependent on the recognition of pathogen effector proteins. Having inspected the literature, we synthesize the current state of knowledge concerning this topic. First, we describe how pathogen effectors can manipulate MAPK signaling to promote virulence, and how in parallel plants have developed mechanisms to protect themselves against these interferences. Then, we discuss the striking finding that the recognition of pathogen effectors can provoke a sustained activation of the MAPKs MPK3/6, extensively analyzing its implications in terms of regulation and functions. In line with this, we also address the question of how a durable activation of MAPKs might affect the scope of their substrates, and thereby mediate the emergence of possibly new ETI-specific responses. By highlighting the sometimes conflicting or missing data, our intention is to spur further research in order to both consolidate and expand our understanding of MAPK signaling in immunity.

The Winner Takes It All: Auxin-The Main Player during Plant Embryogenesis

In plants, the first asymmetrical division of a zygote leads to the formation of two cells with different developmental fates. The establishment of various patterns relies on spatial and temporal gene expression, however the precise mechanism responsible for embryonic patterning still needs elucidation. Auxin seems to be the main player which regulates embryo development and controls expression of various genes in a dose-dependent manner. Thus, local auxin maxima and minima which are provided by polar auxin transport underlie cell fate specification. Diverse auxin concentrations in various regions of an embryo would easily explain distinct cell identities, however the question about the mechanism of cellular patterning in cells exposed to similar auxin concentrations still remains open. Thus, specification of cell fate might result not only from the cell position within an embryo but also from events occurring before and during mitosis. This review presents the impact of auxin on the orientation of the cell division plane and discusses the mechanism of auxin-dependent cytoskeleton alignment. Furthermore, close attention is paid to auxin-induced calcium fluxes, which regulate the activity of MAPKs during postembryonic development and which possibly might also underlie cellular patterning during embryogenesis.

Survey of Sensitivity to Fatty Acid-Amino Acid Conjugates in the Solanaceae

Plants perceive insect herbivores via a sophisticated surveillance system that detects a range of alarm signals, including herbivore-associated molecular patterns (HAMPs). Fatty acid-amino acid conjugates (FACs) are HAMPs present in oral secretions (OS) of lepidopteran larvae that induce defense responses in many plant species. In contrast to eggplant (Solanum melongena), tomato (S. lycopersicum) does not respond to FACs present in OS from Manduca sexta (Lepidoptera). Since both plants are found in the same genus, we tested whether loss of sensitivity to FACs in tomato may be a domestication effect. Using highly sensitive MAP kinase (MAPK) phosphorylation assays, we demonstrate that four wild tomato species and the closely related potato (S. tuberosum) do not respond to the FACs N-linolenoyl-L-glutamine and N-linolenoyl-L-glutamic acid, excluding a domestication effect. Among other genera within the Solanaceae, we found that bell pepper (Capsicum annuum) is responsive to FACs, while there is a differential responsiveness to FACs among tobacco (Nicotiana) species, ranging from strong responsiveness in N. benthamiana to no responsiveness in N. knightiana. The Petunia lineage is one of the oldest lineages within the Solanaceae and P. hybrida was responsive to FACs. Collectively, we demonstrate that plant responsiveness to FACs does not follow simple phylogenetic relationships in the family Solanaceae. Instead, sensitivity to FACs is a dynamic ancestral trait present in monocots and eudicots that was repeatedly lost during the evolution of Solanaceae species. Although tomato is insensitive to FACs, we found that other unidentified factors in M. sexta OS induce defenses in tomato.

Banana MaBZR1/2 associate with MaMPK14 to modulate cell wall modifying genes during fruit ripening

Banana MaBZR1/2 interact with MaMPK14 to enhance the transcriptional inhibition of cell wall modifying genes including MaEXP2, MaPL2 and MaXET5. Fruit ripening and softening, the major attributes to perishability in fleshy fruits, are modulated by various plant hormones and gene expression. Banana MaBZR1/2, the central transcription factors of brassinosteroid (BR) signaling, mediate fruit ripening through regulation of ethylene biosynthesis, but their possible roles in fruit softening as well as the underlying mechanisms remain to be determined. In this work, we found that MaBZR1/2 directly bound to and repressed the promoters of several cell wall modifying genes such as MaEXP2, MaPL2 and MaXET5, whose transcripts were elevated concomitant with fruit ripening. Moreover, yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays indicated that MaBZR1/2 physically interacted with a mitogen-activated protein kinase MaMPK14, and this interaction strengthened the MaBZR1/2-mediated transcriptional inhibitory abilities. Collectively, our study provides insight into the mechanism of MaBZR1/2 contributing to fruit ripening and softening, which may have potential for banana molecular improvement.

Phosphoproteomic Profiling Reveals the Importance of CK2, MAPKs and CDPKs in Response to Phosphate Starvation in Rice

Phosphorus is one of the most important macronutrients required for plant growth and development. The importance of phosphorylation modification in regulating phosphate (Pi) homeostasis in plants is emerging. We performed phosphoproteomic profiling to characterize proteins whose degree of phosphorylation is altered in response to Pi starvation in rice root. A subset of 554 proteins, including 546 down-phosphorylated and eight up-phosphorylated proteins, exhibited differential phosphorylation in response to Pi starvation. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis with the differentially phosphorylated proteins indicated that RNA processing, transport, splicing and translation and carbon metabolism played critical roles in response to Pi starvation in rice. Levels of phosphorylation of four mitogen-activated protein kinases (MAPKs), including OsMAPK6, five calcium-dependent protein kinases (CDPKs) and OsCK2β3 decreased in response to Pi starvation. The decreased phosphorylation level of OsMAPK6 was confirmed by Western blotting. Mutation of OsMAPK6 led to Pi accumulation under Pi-sufficient conditions. Motif analysis indicated that the putative MAPK, casein kinase 2 (CK2) and CDPK substrates represented about 54.4%, 21.5% and 4.7%, respectively, of the proteins exhibiting differential phosphorylation. Based on the motif analysis, 191, 151 and 46 candidate substrates for MAPK, CK2 and CDPK were identified. These results indicate that modification of phosphorylation profiles provides complementary information on Pi-starvation-induced processes, with CK2, MAPK and CDPK protein kinase families playing key roles in these processes in rice.

The MPK8-TCP14 pathway promotes seed germination in Arabidopsis

The accurate control of dormancy release and germination is critical for successful plantlet establishment. Investigations in cereals hypothesized a crucial role for specific MAP kinase (MPK) pathways in promoting dormancy release, although the identity of the MPK involved and the downstream events remain unclear. In this work, we characterized mutants for Arabidopsis thaliana MAP kinase 8 (MPK8). Mpk8 seeds presented a deeper dormancy than wild-type (WT) at harvest that was less efficiently alleviated by after-ripening and gibberellic acid treatment. We identified Teosinte Branched1/Cycloidea/Proliferating cell factor 14 (TCP14), a transcription factor regulating germination, as a partner of MPK8. Mpk8 tcp14 double-mutant seeds presented a deeper dormancy at harvest than WT and mpk8, but similar to that of tcp14 seeds. MPK8 interacted with TCP14 in the nucleus in vivo and phosphorylated TCP14 in vitro. Furthermore, MPK8 enhanced TCP14 transcriptional activity when co-expressed in tobacco leaves. Nevertheless, the stimulation of TCP14 transcriptional activity by MPK8 could occur independently of TCP14 phosphorylation. The comparison of WT, mpk8 and tcp14 transcriptomes evidenced that whereas no effect was observed in dry seeds, mpk8 and tcp14 mutants presented dramatic transcriptomic alterations after imbibition with a sustained expression of genes related to seed maturation. Moreover, both mutants exhibited repression of genes involved in cell wall remodeling and cell cycle G1/S transition. As a whole, this study unraveled a role for MPK8 in promoting seed germination, and suggested that its interaction with TCP14 was critical for regulating key processes required for germination completion.

The mitogen-activated protein kinase 4-phosphorylated heat shock factor A4A regulates responses to combined salt and heat stresses

Heat shock factors regulate responses to high temperature, salinity, water deprivation, or heavy metals. Their function in combinations of stresses is, however, not known. Arabidopsis HEAT SHOCK FACTOR A4A (HSFA4A) was previously reported to regulate responses to salt and oxidative stresses. Here we show, that the HSFA4A gene is induced by salt, elevated temperature, and a combination of these conditions. Fast translocation of HSFA4A tagged with yellow fluorescent protein from cytosol to nuclei takes place in salt-treated cells. HSFA4A can be phosphorylated not only by mitogen-activated protein (MAP) kinases MPK3 and MPK6 but also by MPK4, and Ser309 is the dominant MAP kinase phosphorylation site. In vivo data suggest that HSFA4A can be the substrate of other kinases as well. Changing Ser309 to Asp or Ala alters intramolecular multimerization. Chromatin immunoprecipitation assays confirmed binding of HSFA4A to promoters of target genes encoding the small heat shock protein HSP17.6A and transcription factors WRKY30 and ZAT12. HSFA4A overexpression enhanced tolerance to individually and simultaneously applied heat and salt stresses through reduction of oxidative damage. Our results suggest that this heat shock factor is a component of a complex stress regulatory pathway, connecting upstream signals mediated by MAP kinases MPK3/6 and MPK4 with transcription regulation of a set of stress-induced target genes.

Early Evolution of the Mitogen-Activated Protein Kinase Family in the Plant Kingdom

Mitogen-activated protein kinase (MAPK) pathways are central cellular signalling mechanisms in all eukaryotes. They are key regulators of the cell cycle and stress responses, yet evolution of MAPK families took markedly different paths in the animal and plant kingdoms. Instead of the characteristic divergence of MAPK types in animals, in plants an expanded network of ERK-like MAPKs has emerged. To gain insight into the early evolution of the plant MAPK family we identified and analysed MAPKs in 13 representative species across green algae, a large and diverse early-diverging lineage within the plant kingdom. Our results reveal that the plant MAPK gene family emerged from three types of progenitor kinases, which are ubiquitously present in algae, implying their formation in an early ancestor. Low number of MAPKs is characteristic across algae, the few losses or duplications are associated with genome complexity rather than habitat ecology, despite the importance of MAPKs in environmental signalling in flowering plants. ERK-type MAPKs are associated with cell cycle regulation in opisthokont models, yet in plants their stress-signalling function is more prevalent. Unicellular microalgae offer an excellent experimental system to study the cell cycle, and MAPK gene expression profiles show CDKB-like peaks around S/M phase in synchronised Chlamydomonas reinhardtii cultures, suggesting their participation in cell cycle regulation, in line with the notion that the ancestral eukaryotic MAPK was a cell cycle regulator ERK-like kinase. Our work also highlights the scarcity of signalling knowledge in microalgae, in spite of their enormous ecological impact and emerging economic importance.