Mitogen-activated protein kinase kinase kinase (MAPKKK) 4 from rapeseed (Brassica napus L.) is a novel member inducing ROS accumulation and cell death

The Abscisic Acid-Responsive Element Binding Factors (ABFs)-MAPKKK18 module regulates ABA- induced leaf senescence in Arabidopsis

The Mitogen-Activated Protein Kinase Kinase Kinase 18 (MAPKKK18) has been reported to play a role in abiotic stress priming in long-term abscisic acid (ABA) response including drought tolerance and leaf senescence. However, the upstream transcriptional regulators of MAPKKK18 remain to be determined. Here, we report ABA-Responsive Element (ABRE) Binding Factors (ABFs) as upstream transcription factors (TFs) of MAPKKK18 expression. Mutants of abf2, abf3, abf4 and abf2abf3abf4 dramatically reduced the transcription of MAPKKK18. Our electrophoresis mobility shift assay (EMSA) and dual-luciferase reporter assay demonstrated that ABF2, ABF3, and ABF4 bound to ABRE cis-elements within the promoter of MAPKKK18 to transactivate its expression. Furthermore, enrichments of the promoter region of MAPKKK18 by ABF2, ABF3, and ABF4 were confirmed by in vivo chromatin immunoprecipitation coupled with qPCR (ChIP-qPCR). Additionally, we found that mutants of mapkkk18 exhibited obvious delayed leaf senescence. Moreover, a genetic study showed that overexpression of ABF2, ABF3, and ABF4 in the background of mapkkk18 mostly phenocopied the stay-green phenotype of mapkkk18 and, expression levels of five target genes of ABFs, that is, NYE1, NYE2, NYC1, PAO, and SAG29 were attenuated as a result of MAPKKK18 mutation. These findings demonstrate that ABF2, ABF3, and ABF4 act as transcription regulators of MAPKKK18, and also suggest that, at least in part, ABA acts in priming leaf senescence via ABF-induced expression of MAPKKK18.

The Flavonoid Biosynthesis and Regulation in Brassica napus: A Review

Brassica napus is an important crop for edible oil, vegetables, biofuel, and animal food. It is also an ornamental crop for its various petal colors. Flavonoids are a group of secondary metabolites with antioxidant activities and medicinal values, and are important to plant pigmentation, disease resistance, and abiotic stress responses. The yellow seed coat, purple leaf and inflorescence, and colorful petals of B. napus have been bred for improved nutritional value, tourism and city ornamentation. The putative loci and genes regulating flavonoid biosynthesis in B. napus have been identified using germplasms with various seed, petal, leaf, and stem colors, or different flavonoid contents under stress conditions. This review introduces the advances of flavonoid profiling, biosynthesis, and regulation during development and stress responses of B. napus, and hopes to help with the breeding of B. napus with better quality, ornamental value, and stress resistances.

Mitogen-Activated Protein Kinase MAPKKK7 from Plasmodiophora brassicae Regulates Low-Light-Dependent Nicotiana benthamiana Immunity

MAPKKK is the largest family of mitogen-activated protein kinase (MAPK) cascades and is known to play important roles in plant pathogen interaction by regulating fungal cell proliferation, growth, and pathogenicity. Thus far, only a few have been characterized because of the functional redundancy of MAPKKKs. In this study, it is interesting that Plasmodiophora brassicae (Pb)MAPKKK7 was clustered into the A3 subgroup of plant MAPKKKs by a phylogenetic analysis and also with the BCK1 and STE groups of fungal MAPKKKs. PbMAPKKK7 function in reactive oxygen species accumulation and cell death in Nicotiana benthamiana was characterized. Agroinfiltration with the PbMAPKKK7 mutated protein kinase domain relieved these changes. Interestingly, the induction of cell death was dependent on light intensity. Transcriptional profiling analysis demonstrated that PbMAPKKK7 was highly expressed during cortex infection stages, indicating its important role in P. brassicae infection. These functional analyses of PbMAPKKK7 build knowledge of new roles of the MAPK cascade pathway in N. benthamiana and P. brassicae interactions.

Metabolomics Intervention Towards Better Understanding of Plant Traits

The majority of the most economically important plant and crop species are enriched with the availability of high-quality reference genome sequences forming the basis of gene discovery which control the important biochemical pathways. The transcriptomics and proteomics resources have also been made available for many of these plant species that intensify the understanding at expression levels. However, still we lack integrated studies spanning genomics–transcriptomics–proteomics, connected to metabolomics, the most complicated phase in phenotype expression. Nevertheless, for the past few decades, emphasis has been more on metabolome which plays a crucial role in defining the phenotype (trait) during crop improvement. The emergence of modern high throughput metabolome analyzing platforms have accelerated the discovery of a wide variety of biochemical types of metabolites and new pathways, also helped in improving the understanding of known existing pathways. Pinpointing the causal gene(s) and elucidation of metabolic pathways are very important for development of improved lines with high precision in crop breeding. Along with other-omics sciences, metabolomics studies have helped in characterization and annotation of a new gene(s) function. Hereby, we summarize several areas in the field of crop development where metabolomics studies have made its remarkable impact. We also assess the recent research on metabolomics, together with other omics, contributing toward genetic engineering to target traits and key pathway(s).

Transcriptome-wide identification of MAPKKK genes in bermudagrass (Cynodon dactylon L.) and their potential roles in low temperature stress responses

As upstream components of MAPK cascades, mitogen-activated protein kinase kinase kinases (MAPKKKs) act as adaptors linking upstream signaling steps to the core MAPK cascades. MAPK cascades are universal modules of signal transduction in eukaryotic organisms and play crucial roles in plant development processes and in responses to biotic and abiotic stress and signal transduction. Members of the MAPKKK gene family have been identified in several plants,however, MAPKKKs have not been systematically studied in bermudagrass (Cynodon dactylon L.). In this study, 55 potential CdMAPKKKs were produced from bermudagrass transcriptome data, of which 13 belonged to the MEKK, 38 to the Raf, and 4 to the ZIK subfamily. Multiple alignment and conserved motif analysis of CdMAPKKKs supported the evolutionary relationships inferred from phylogenetic analyses. Moreover, the distribution pattern in Poaceae species indicated that members of the MAPKKK family were conserved among almost all diploid species, and species-specific polyploidy or higher duplication ratios resulted in an expansion of the MAPKKK family. In addition, 714 co-functional links which were significantly enriched in signal transduction, responses to temperature stimuli, and other important biological processes of 55 CdMAPKKKs were identified using co-functional gene networks analysis; 30 and 19 co-functional genes involved in response to cold or heat stress, respectively, were also identified. Results of promoter analyses, and interaction network investigation of all CdMAPKKKs based on the rice homologs suggested that CdMAPKKKs are commonly associated with regulation of numerous biological processes. Furthermore, 12 and 13 CdMAPKKKs were significantly up- and downregulated, respectively, in response to low temperature stress; among them, six CdMAPKKKs were significantly induced by low temperature stress, at least at one point in time. This is the first study to conduct identification and functional analysis of the MAPKKK gene family in bermudagrass, and our results provide a foundation for further research on the functions of CdMAPKKKs in response to low temperature stress.

Engineering Multiple Abiotic Stress Tolerance in Canola, Brassica napus

Impacts of climate change like global warming, drought, flooding, and other extreme events are posing severe challenges to global crop production. Contribution of Brassica napus towards the oilseed industry makes it an essential component of international trade and agroeconomics. Consequences from increasing occurrences of multiple abiotic stresses on this crop are leading to agroeconomic losses making it vital to endow B. napus crop with an ability to survive and maintain yield when faced with simultaneous exposure to multiple abiotic stresses. For an improved understanding of the stress sensing machinery, there is a need for analyzing regulatory pathways of multiple stress-responsive genes and other regulatory elements such as non-coding RNAs. However, our understanding of these pathways and their interactions in B. napus is far from complete. This review outlines the current knowledge of stress-responsive genes and their role in imparting multiple stress tolerance in B. napus. Analysis of network cross-talk through omics data mining is now making it possible to unravel the underlying complexity required for stress sensing and signaling in plants. Novel biotechnological approaches such as transgene-free genome editing and utilization of nanoparticles as gene delivery tools are also discussed. These can contribute to providing solutions for developing climate change resilient B. napus varieties with reduced regulatory limitations. The potential ability of synthetic biology to engineer and modify networks through fine-tuning of stress regulatory elements for plant responses to stress adaption is also highlighted.

Sucrose Signaling Regulates Anthocyanin Biosynthesis Through a MAPK Cascade in Arabidopsis thaliana

Anthocyanin accumulation specifically depends on sucrose (Suc) signaling. However, the molecular basis of this process remains unknown. In this study, in vitro pull-down assays identified ETHYLENE-INSENSITIVE3 (EIN3), a component of both sugar signaling or/and metabolism. This protein interacted with YDA, and the physiological relevance of this interaction was confirmed by in planta co-immunoprecipitation, yeast two-hybrid (Y2H) assay, and bimolecular fluorescence complementation. Ethylene insensitive3-like 1 (eil1) ein3 double-mutant seedlings, but not ein3-1 seedlings, showed anthocyanin accumulation. Furthermore, ein3-1 suppressed anthocyanin accumulation in yda-1 plants. Thus, EMB71/YDA-EIN3-EIL1 may form a sugar-mediated gene cascade integral to the regulation of anthocyanin accumulation. Moreover, the EMB71/YDA-EIN3-EIL1 gene cascade module directly targeted the promoter of Transparent Testa 8 (TT8) by direct EIN3 binding. Collectively, our data inferred a molecular model where the signaling cascade of the YDA-EIN3-TT8 appeared to target TT8 via EIN3, thereby modulating Suc signaling-mediated anthocyanin accumulation.

An oilseed rape WRKY-type transcription factor regulates ROS accumulation and leaf senescence in Nicotiana benthamiana and Arabidopsis through modulating transcription of RbohD and RbohF

MAIN CONCLUSION

Overexpression of BnaWGR1 causes ROS accumulation and promotes leaf senescence. BnaWGR1 binds to promoters of RbohD and RbohF and regulates their expression. Manipulation of leaf senescence process affects agricultural traits of crop plants, including biomass, seed yield and stress resistance. Since delayed leaf senescence usually enhances tolerance to multiple stresses, we analyzed the function of specific MAPK-WRKY cascades in abiotic and biotic stress tolerance as well as leaf senescence in oilseed rape (Brassica napus L.), one of the important oil crops. In the present study, we showed that expression of one WRKY gene from oilseed rape, BnaWGR1, induced an accumulation of reactive oxygen species (ROS), cell death and precocious leaf senescence both in Nicotiana benthamiana and transgenic Arabidopsis (Arabidopsis thaliana). BnaWGR1 regulates the transcription of two genes encoding key enzymes implicated in production of ROS, that is, respiratory burst oxidase homolog (Rboh) D and RbohF. A dual-luciferase reporter assay confirmed the transcriptional regulation of RbohD and RbohF by BnaWGR1. In vitro electrophoresis mobility shift assay (EMSA) showed that BnaWGR1 could bind to W-box cis-elements within promoters of RbohD and RbohF. Moreover, RbohD and RbohF were significantly upregulated in transgenic Arabidopsis overexpressing BnaWGR1. In summary, these results suggest that BnaWGR1 could positively regulate leaf senescence through regulating the expression of RbohD and RbohF genes.

Leptosphaeria maculans Effector Protein AvrLm1 Modulates Plant Immunity by Enhancing MAP Kinase 9 Phosphorylation

Leptosphaeria maculans, the causal agent of blackleg disease in canola (Brassica napus), secretes an array of effectors into the host to overcome host defense. Here we present evidence that the L. maculans effector protein AvrLm1 functions as a virulence factor by interacting with the B. napus mitogen-activated protein (MAP) kinase 9 (BnMPK9), resulting in increased accumulation and enhanced phosphorylation of the host protein. Transient expression of BnMPK9 in Nicotiana benthamiana induces cell death, and this phenotype is enhanced in the presence of AvrLm1, suggesting that induction of cell death due to enhanced accumulation and phosphorylation of BnMPK9 by AvrLm1 supports the initiation of necrotrophic phase of L. maculans infection. Stable expression of BnMPK9 in B. napus perturbs hormone signaling, notably salicylic acid response genes, to facilitate L. maculans infection. Our findings provide evidence that a MAP kinase is directly targeted by a fungal effector to modulate plant immunity.

•

Leptosphaeria maculans effector AvrLm1 interacts with the Brassica napus MPK9 (BnMPK9)

•

AvrLm1 increases the accumulation and enhances the phosphorylation of BnMPK9

•

AvrLm1 enhances BnMPK9-dependent cell death in Nicotiana benthamiana

•

Stable expression of BnMPK9 in B. napus facilitates L. maculans infection

Comparative transcriptome analysis of two citrus germplasms with contrasting susceptibility to Phytophthora nicotianae provides new insights into tolerance mechanisms

Host perception of Phytophthora nicotianae switching to necrotrophy is fundamental for disease tolerance of citrus. It involves an HR-like response, strengthening of the cell wall structure and hormonal signaling. Stem rot caused by P. nicotianae is a worldwide disease of several important crops, including citrus. Given the growing awareness of chemical fungicides drawbacks, genetic improvement of citrus rootstocks remains the best alternative. However, the molecular basis underlying the successful response of resistant and/or tolerant genotypes remains poorly understood. Therefore, we performed a transcriptomic analysis to examine the differential defense response to P. nicotianae of two germplasms-tolerant sour orange (SO, Citrus aurantium) and susceptible Madam Vinous (MV, C. sinensis)-in both the biotrophic and necrotrophic phases of host-pathogen interaction. Our results revealed the necrotrophic phase as a decisive turning point, since it included stronger modulation of a number of genes implicated in pathogen perception, signal transduction, HR-like response, transcriptional reprogramming, hormone signaling, and cell wall modifications. In particular, the pathogen perception category reflected the ability of SO to perceive the pathogen even after its switch to necrotrophy, and thus to cope successfully with the infection, while MV failed. The concomitant changes in genes involved in the remaining functional categories seemed to prevent pathogen spread. This investigation provided further understanding of the successful defense mechanisms of C. aurantium against P. nicotianae, which might be exploited in post-genomic strategies to develop resistant Citrus genotypes.