A novel microbial community restructuring strategy for enhanced hydrogen production using multiple pretreatments and CSTR operation.

To achieve rapid enrichment of the targeted hydrogen-producing bacterial population and reconstruction of the microbial community in the biological hydrogen-producing reactor, the activated sludge underwent multiple pretreatments using micro-aeration, alkaline treatment, and heat treatment. The activated sludge obtained from the multiple pretreatments was inoculated into the continuous stirred tank reactor (CSTR) for continuous operations. The community structure alteration and hydrogen-producing capability of the activated sludge were analyzed throughout the operation of the reactor. We found that the primary phyla in the activated sludge population shifted to Proteobacteria, Firmicutes, and Bacteroidetes, which collectively accounted for 96.69% after undergoing several pretreatments. This suggests that the multiple pretreatments facilitated in achieving the selective enrichment of the fermentation hydrogen-producing microorganisms in the activated sludge. The CSTR start-up and continuous operation of the biological hydrogen production reactor resulted in the reactor entering a highly efficient hydrogen production stage at influent COD concentrations of 4000 mg/L and 5000 mg/L, with the highest hydrogen production rate reaching 8.19 L/d and 9.33 L/d, respectively. The main genus present during the efficient hydrogen production stage in the reactor was Ethanoligenens, accounting for up to 33% of the total population. Ethanoligenens exhibited autoaggregation capabilities and a superior capacity for hydrogen production, leading to its prevalence in the reactor and contribution to efficient hydrogen production. During high-efficiency hydrogen production, flora associated with hydrogen production exhibited up to 46.95% total relative abundance. In addition, redundancy analysis (RDA) indicated that effluent pH and COD influenced the distribution of the primary hydrogen-producing bacteria, including Ethanoligenens, Raoultella, and Pectinatus, as well as other low abundant hydrogen-producing bacteria in the activated sludge. The data indicates that the multiple pretreatments and reactor's operation has successfully enriched the hydrogen-producing genera and changed the community structure of microbial hydrogen production.

Na2CO3-responsive mechanism insight from quantitative proteomics and SlRUB gene function in Salix linearistipularis seedlings.

Mongolian willow (Salix linearistipularis) is a naturally occurring woody dioecious plant in the saline soils of north-eastern China, which has a high tolerance to alkaline salts. Although transcriptomics studies have identified a large number of salinity-responsive genes, the mechanism of salt tolerance in Mongolian willow is not clear. Here, we found that in response to Na2CO3 stress, Mongolian willow regulates osmotic homeostasis by accumulating proline and soluble sugars and scavenges reactive oxygen species (ROS) by antioxidant enzymes and non-enzymatic antioxidants. Our quantitative proteomics study identified 154 salt-sensitive proteins mainly involved in maintaining the stability of the photosynthetic system and ROS homeostasis to cope with Na2CO3 stress. Among them, Na2CO3-induced rubredoxin (RUB) was predicted to be associated with 122 proteins for the modulation of these processes. The chloroplast-localized S. linearistipularis rubredoxin (SlRUB) was highly expressed in leaves and was significantly induced under Na2CO3 stress. Phenotypic analysis of overexpression, mutation and complementation materials of RUB in Arabidopsis suggests that SlRUB is critical for the regulation of photosynthesis, ROS scavenging and other metabolisms in the seedlings of Mongolian willow to cope with Na2CO3 stress. This provides more clues to better understand the alkali-responsive mechanism and RUB functions in the woody Mongolian willow.

H2O2 negatively regulates aluminum resistance via oxidation and degradation of the transcription factor STOP1.

Aluminum (Al) stress triggers the accumulation of hydrogen peroxide (H2O2) in roots. However, whether H2O2 plays a regulatory role in aluminum resistance remains unclear. In this study, we show that H2O2 plays a crucial role in regulation of Al resistance, which is modulated by the mitochondrion-localized pentatricopeptide repeat protein REGULATION OF ALMT1 EXPRESSION 6 (RAE6). Mutation in RAE6 impairs the activity of complex I of the mitochondrial electron transport chain, resulting in the accumulation of H2O2 and increased sensitivity to Al. Our results suggest that higher H2O2 concentrations promote the oxidation of SENSITIVE TO PROTON RHIZOTOXICITY 1 (STOP1), an essential transcription factor that promotes Al resistance, thereby promoting its degradation by enhancing the interaction between STOP1 and the F-box protein RAE1. Conversely, decreasing H2O2 levels or blocking the oxidation of STOP1 leads to greater STOP1 stability and increased Al resistance. Moreover, we show that the thioredoxin TRX1 interacts with STOP1 to catalyze its chemical reduction. Thus, our results highlight the importance of H2O2 in Al resistance and regulation of STOP1 stability in Arabidopsis (Arabidopsis thaliana).

Comparative Transcriptome Analysis between Resistant and Susceptible Pakchoi Cultivars in Response to Downy Mildew.

Downy mildew caused by the obligate parasite Hyaloperonospora brassicae is a devastating disease for Brassica species. Infection of Hyaloperonospora brassicae often leads to yellow spots on leaves, which significantly impacts quality and yield of pakchoi. In the present study, we conducted a comparative transcriptome between the resistant and susceptible pakchoi cultivars in response to Hyaloperonospora brassicae infection. A total of 1073 disease-resistance-related differentially expressed genes were identified using a Venn diagram. The Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that these genes were mainly involved in plant-pathogen interaction, plant hormone signal transduction, and other photosynthesis-related metabolic processes. Analysis of the phytohormone content revealed that salicylic acid increased significantly in the resistant material after inoculation with Hyaloperonospora brassicae, whereas the contents of jasmonic acid, abscisic acid, and 1-aminocyclopropane-1-carboxylic acid decreased. Exogenous salicylic acid treatment also significantly upregulated Hyaloperonospora brassicae-induced genes, which further confirmed a crucial role of salicylic acid during pakchoi defense against Hyaloperonospora brassicae. Based on these findings, we suggest that the salicylic-acid-mediated signal transduction contributes to the resistance of pakchoi to downy mildew, and PAL1, ICS1, NPR1, PR1, PR5, WRKY70, WRKY33, CML43, CNGC9, and CDPK15 were involved in this responsive process. Our findings evidently contribute to revealing the molecular mechanism of pakchoi defense against Hyaloperonospora brassicae.

Acute and chronic effects of sublethal neonicotinoid thiacloprid to Asian honey bee (Apis cerana cerana).

Pesticide pollution is one of the most important factors for global bee declines. Despite many studies have revealed that the most important Chinese indigenous species，Apis cerana, is presenting a high risk on exposure to neonicotinoids, the toxicology information on Apis cerana remain limited. This study was aimed to determine the acute and chronic toxic effects of thiacloprid (IUPAC name: {(2Z)-3-[(6-Chloro-3-pyridinyl)methyl]-1,3-thiazolidin-2-ylidene}cyanamide) on behavioral and physiological performance as well as genome-wide transcriptome in A. cerana. We found the 1/5 LC50 of thiacloprid significantly impaired learning and memory abilities after both acute and chronic exposure, nevertheless, has no effects on the sucrose responsiveness and phototaxis climbing ability of A. cerana. Moreover, activities of detoxification enzyme P450 monooxygenases and CarE were increased by short-term exposure to thiacloprid, while prolonged exposure caused suppression of CarE activity. Neither acute nor chronic exposure to thiacloprid altered honey bee AChE activities. To further study the potential defense molecular mechanisms in Asian honey bee under pesticide stress, we analyzed the transcriptomes of honeybees in response to thiacloprid stress. The transcriptomic profiles revealed consistent upregulation of immune- and stress-related genes by both acute or chronic treatments. Our results suggest that the chronic exposure to thiacloprid produced greater toxic effects than a single administration to A. cerana. Altogether, our study deepens the understanding of the toxicological characteristic of A. cerana against thiacloprid, and could be used to further investigate the complex molecular mechanisms in Asian honey bee under pesticide stress.

Cell type-specific proteomics uncovers a RAF15-SnRK2.6/OST1 kinase cascade in guard cells.

Multicellular organisms such as plants contain various cell types with specialized functions. Analyzing the characteristics of each cell type reveals specific cell functions and enhances our understanding of organization and function at the organismal level. Guard cells (GCs) are specialized epidermal cells that regulate the movement of the stomata and gaseous exchange, and provide a model genetic system for analyzing cell fate, signaling, and function. Several proteomics analyses of GC are available, but these are limited in depth. Here we used enzymatic isolation and flow cytometry to enrich GC and mesophyll cell protoplasts and perform in-depth proteomics in these two major cell types in Arabidopsis leaves. We identified approximately 3,000 proteins not previously found in the GC proteome and more than 600 proteins that may be specific to GC. The depth of our proteomics enabled us to uncover a guard cell-specific kinase cascade whereby Raf15 and Snf1-related kinase2.6 (SnRK2.6)/OST1(open stomata 1) mediate abscisic acid (ABA)-induced stomatal closure. RAF15 directly phosphorylated SnRK2.6/OST1 at the conserved Ser175 residue in its activation loop and was sufficient to reactivate the inactive form of SnRK2.6/OST1. ABA-triggered SnRK2.6/OST1 activation and stomatal closure was impaired in raf15 mutants. We also showed enrichment of enzymes and flavone metabolism in GC, and consistent, dramatic accumulation of flavone metabolites. Our study answers the long-standing question of how ABA activates SnRK2.6/OST1 in GCs and represents a resource potentially providing further insights into the molecular basis of GC and mesophyll cell development, metabolism, structure, and function.

Cysteine-rich receptor-like protein kinases: emerging regulators of plant stress responses.

Cysteine-rich receptor-like kinases (CRKs) belong to a large DUF26-containing receptor-like kinase (RLK) family. They play key roles in immunity, abiotic stress response, and growth and development. How CRKs regulate diverse processes is a long-standing question. Recent studies have advanced our understanding of the molecular mechanisms underlying CRK functions in Ca2+ influx, reactive oxygen species (ROS) production, mitogen-activated protein kinase (MAPK) cascade activation, callose deposition, stomatal immunity, and programmed cell death (PCD). We review the CRK structure-function relationship with a focus on the roles of CRKs in immunity, the abiotic stress response, and the growth-stress tolerance tradeoff. We provide a critical analysis and synthesis of how CRKs control sophisticated regulatory networks that determine diverse plant phenotypic outputs.

Genome-Wide Identification of the A20/AN1 Zinc Finger Protein Family Genes in Ipomoea batatas and Its Two Relatives and Function Analysis of IbSAP16 in Salinity Tolerance.

Stress-associated protein (SAP) genes-encoding A20/AN1 zinc-finger domain-containing proteins-play pivotal roles in regulating stress responses, growth, and development in plants. They are considered suitable candidates to improve abiotic stress tolerance in plants. However, the SAP gene family in sweetpotato (Ipomoea batatas) and its relatives is yet to be investigated. In this study, 20 SAPs in sweetpotato, and 23 and 26 SAPs in its wild diploid relatives Ipomoea triloba and Ipomoea trifida were identified. The chromosome locations, gene structures, protein physiological properties, conserved domains, and phylogenetic relationships of these SAPs were analyzed systematically. Binding motif analysis of IbSAPs indicated that hormone and stress responsive cis-acting elements were distributed in their promoters. RT-qPCR or RNA-seq data revealed that the expression patterns of IbSAP, ItbSAP, and ItfSAP genes varied in different organs and responded to salinity, drought, or ABA (abscisic acid) treatments differently. Moreover, we found that IbSAP16 driven by the 35 S promoter conferred salinity tolerance in transgenic Arabidopsis. These results provided a genome-wide characterization of SAP genes in sweetpotato and its two relatives and suggested that IbSAP16 is involved in salinity stress responses. Our research laid the groundwork for studying SAP-mediated stress response mechanisms in sweetpotato.

Highly sensitive characterization of non-human glycan structures of monoclonal antibody drugs utilizing tandem mass spectrometry.

Glycosylation is an important attribute of monoclonal antibodies (mAbs) for assessing manufacturing quality. Analysis of non-human glycans containing terminal galactose-α1,3-galactose and N-glycolylneuraminic acid is essential due to the potential immunogenicity and insufficient efficacy caused by mAb expression in non-human mammalian cells. Using parallel sequencing of isobaric glycopeptides and isomeric glycans that were separated by reversed-phase and porous graphitic carbon LC, we report a highly sensitive LC MS/MS method for the comprehensive characterization of low-abundance non-human glycans and their closely related structural isomers. We demonstrate that the straightforward use of high-abundance diagnostic ions and complementary fragments under the positive ionization low-energy collision-induced dissociation is a universal approach to rapidly discriminate branch-linkage structures of biantennary glycans. Our findings reveal the structural diversity of non-human glycans and sulfation of α-galactosylated glycans, providing both an analytical method and candidate structures that could potentially be used in the crucial quality control of therapeutic mAb products.

Genome-wide identification and expression analysis reveals spinach brassinosteroid-signaling kinase (BSK) gene family functions in temperature stress response.

BACKGROUND: Brassinosteroid (BR)- signaling kinase (BSK) is a critical family of receptor-like cytoplasmic kinase for BR signal transduction, which plays important roles in plant development, immunity, and abiotic stress responses. Spinach (Spinacia oleracea) is cold- tolerant but heat- sensitive green leafy vegetable. A study on BSK family members and BSKs- mediated metabolic processes in spinach has not been performed. RESULTS: We identified and cloned seven SoBSKs in spinach. Phylogenetic and collinearity analyses suggested that SoBSKs had close relationship with dicotyledonous sugar beet (Beta vulgaris) rather than monocotyledons. The analyses of gene structure and conserved protein domain/ motif indicated that most SoBSKs were relative conserved, while SoBSK6 could be a truncated member. The prediction of post-translation modification (PTM) sites in SoBSKs implied their possible roles in signal transduction, redox regulation, and protein turnover of SoBSKs, especially the N-terminal myristoylation site was critical for BSK localization to cell periphery. Cis-acting elements for their responses to light, drought, temperature (heat and cold), and hormone distributed widely in the promoters of SoBSKs, implying the pivotal roles of SoBSKs in response to diverse abiotic stresses and phytohormone stimuli. Most SoBSKs were highly expressed in leaves, except for SoBSK7 in roots. Many SoBSKs were differentially regulated in spinach heat- sensitive variety Sp73 and heat- tolerant variety Sp75 under the treatments of heat, cold, as well as exogenous brassinolide (BL) and abscisic acid (ABA). The bsk134678 mutant Arabidopsis seedlings exhibited more heat tolerance than wild- type and SoBSK1- overexpressed seedlings. CONCLUSIONS: A comprehensive genome- wide analysis of the BSK gene family in spinach presented a global identification and functional prediction of SoBSKs. Seven SoBSKs had relatively- conserved gene structure and protein function domains. Except for SoBSK6, all the other SoBSKs had similar motifs and conserved PTM sites. Most SoBSKs participated in the responses to heat, cold, BR, and ABA. These findings paved the way for further functional analysis on BSK- mediated regulatory mechanisms in spinach development and stress response.

Multilayered synergistic regulation of phytoalexin biosynthesis by ethylene, jasmonate, and MAPK signaling pathways in Arabidopsis.

Camalexin, an indolic antimicrobial metabolite, is the major phytoalexin in Arabidopsis thaliana, and plays a crucial role in pathogen resistance. Our previous studies revealed that the Arabidopsis mitogen-activated protein kinases MPK3 and MPK6 positively regulate pathogen-induced camalexin biosynthesis via phosphoactivating the transcription factor WRKY33. Here, we report that the ethylene and jasmonate (JA) pathways act synergistically with the MPK3/MPK6-WRKY33 module at multiple levels to induce camalexin biosynthesis in Arabidopsis upon pathogen infection. The ETHYLENE RESPONSE FACTOR1 (ERF1) transcription factor integrates the ethylene and JA pathways to induce camalexin biosynthesis via directly upregulating camalexin biosynthetic genes. ERF1 also interacts with and depends on WRKY33 to upregulate camalexin biosynthetic genes, indicating that ERF1 and WRKY33 form transcriptional complexes to cooperatively activate camalexin biosynthetic genes, thereby mediating the synergy of ethylene/JA and MPK3/MPK6 signaling pathways to induce camalexin biosynthesis. Moreover, as an integrator of the ethylene and JA pathways, ERF1 also acts as a substrate of MPK3/MPK6, which phosphorylate ERF1 to increase its transactivation activity and therefore further cooperate with the ethylene/JA pathways to induce camalexin biosynthesis. Taken together, our data reveal the multilayered synergistic regulation of camalexin biosynthesis by ethylene, JA, and MPK3/MPK6 signaling pathways via ERF1 and WRKY33 transcription factors in Arabidopsis.

The Construction and Exploration of a Comprehensive MicroRNA Centered Regulatory Network in Foxtail Millet (Setaria italica L.).

MicroRNA (miRNA) is an essential endogenous post-transcriptional regulatory factor, and foxtail millet (Setaria italica L.) is an ideal C4 model cereal that is a highly valuable crop in semiarid and arid areas. The Research on comprehensive and high confidence identification and annotation of foxtail millet miRNAs needs to be strengthened, and to our knowledge, there is no information on the regulatory network of foxtail millet miRNA. In this study, 136 high confidence miRNAs were identified through high-throughput sequencing of the small RNAs in seven tissues at the shooting and grain filling stages of foxtail millet. A total of 2,417 target genes were obtained by combining computational biology software and degradome sequencing methods. Furthermore, an analysis using transcriptome sequencing revealed the relationships between miRNAs and their target genes and simultaneously explored key regulatory modules in panicles during the grain filling stage. An miRNA regulatory network was constructed to explore the functions of miRNA in more detail. This network, centered on miRNAs and combining upstream transcriptional factors and downstream target genes, is primarily composed of feed forward loop motifs, which greatly enhances our knowledge of the potential functions of miRNAs and uncovers numerous previously unknown regulatory links. This study provides a solid foundation for research on the function and regulatory network of miRNAs in foxtail millet.

Pto Interaction Proteins: Critical Regulators in Plant Development and Stress Response.

Pto interaction (Pti) proteins are a group of proteins that can be phosphorylated by serine/threonine protein kinase Pto, which have diverse functions in plant development and stress response. In this study, we analyzed the phylogenetic relationship, gene structure, and conserved motifs of Pti1s and predicted the potential cis-elements in the promoters of Pti1 genes using bioinformatics methods. Importantly, we systematically summarized the diverse functions of Pti1s in tomato, rice, Arabidopsis, potato, apple, and cucumber. The potential cis-elements in promoters of Pti1s decide their functional diversity in response to various biotic and abiotic stresses. The protein kinase Pti1 was phosphorylated by Pto and then modulated the downstream signaling pathways for PTI and ETI in the disease insistence process. In addition, some transcription factors have been defined as Ptis (e.g., Pti4, Pti5, and Pti6) originally, which actually were ethylene-response factors (ERFs). Pti4, Pti5, and Pti6 were modulated by salicylic acid (SA), jasmonate (JA), and ethylene signaling pathways and regulated diverse defense-related gene expression to cope with Pst infection and insect wounding.

The ubiquitin-proteasome system regulates meiotic chromosome organization.

Meiotic crossover (CO) recombination is tightly regulated by chromosome architecture to ensure faithful chromosome segregation and to reshuffle alleles between parental chromosomes for genetic diversity of progeny. However, regulation of the meiotic chromosome loop/axis organization is poorly understood. Here, we identify a molecular pathway for axis length regulation. We show that the cohesin regulator Pds5 can interact with proteasomes. Meiosis-specific depletion of proteasomes and/or Pds5 results in a similarly shortened chromosome axis, suggesting proteasomes and Pds5 regulate axis length in the same pathway. Protein ubiquitination is accumulated in pds5 and proteasome mutants. Moreover, decreased chromosome axis length in these mutants can be largely rescued by decreasing ubiquitin availability and thus decreasing protein ubiquitination. Further investigation reveals that two ubiquitin E3 ligases, SCF (Skp­Cullin­F-box) and Ufd4, are involved in this Pds5­ubiquitin/proteasome pathway to cooperatively control chromosome axis length. These results support the hypothesis that ubiquitination of chromosome proteins results in a shortened chromosome axis, and cohesin­Pds5 recruits proteasomes onto chromosomes to regulate ubiquitination level and thus axis length. These findings reveal an unexpected role of the ubiquitin­proteasome system in meiosis and contribute to our knowledge of how Pds5 regulates meiotic chromosome organization. A conserved regulatory mechanism probably exists in higher eukaryotes.

Plant immunity suppression via PHR1-RALF-FERONIA shapes the root microbiome to alleviate phosphate starvation.

The microbiome plays an important role in shaping plant growth and immunity, but few plant genes and pathways impacting plant microbiome composition have been reported. In Arabidopsis thaliana, the phosphate starvation response (PSR) was recently found to modulate the root microbiome upon phosphate (Pi) starvation through the transcriptional regulator PHR1. Here, we report that A. thaliana PHR1 directly binds to the promoters of rapid alkalinization factor (RALF) genes, and activates their expression under phosphate-starvation conditions. RALFs in turn suppress complex formation of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) receptor through FERONIA, a previously-identified PTI modulator that increases resistance to certain detrimental microorganisms. Suppression of immunity via the PHR1-RALF-FERONIA axis allows colonization by specialized root microbiota that help to alleviate phosphate starvation by upregulating the expression of PSR genes. These findings provide a new paradigm for coordination of host-microbe homeostasis through modulating plant innate immunity after environmental perturbations.

Overexpression of PvWOX3a in switchgrass promotes stem development and increases plant height.

Switchgrass (Panicum virgatum L.) is an important perennial, noninvasive, tall ornamental grass that adds color and texture to gardens and landscapes. Moreover, switchgrass has been considered a forage and bioenergy crop because of its vigorous growth, low-input requirements, and broad geography. Here, we identified PvWOX3a from switchgrass, which encodes a WUSCHEL-related homeobox transcription factor. Transgenic overexpression of PvWOX3a in switchgrass increased stem length, internode diameter, and leaf blade length and width, all of which contributed to a 95% average increase in dry weight biomass compared with control plants. Yeast one-hybrid and transient dual-luciferase assays showed that PvWOX3a can repress the expression of gibberellin 2-oxidase and cytokinin oxidase/dehydrogenase through apparently direct interaction with their promoter sequences. These results suggested that overexpression of PvWOX3a could increase gibberellin and cytokinin levels in transgenic switchgrass plants, which promotes cell division, elongation, and vascular bundle development. We also overexpressed PvWOX3a in a transgenic miR156-overexpressing switchgrass line that characteristically exhibited more tillers, thinner internodes, and narrower leaf blades. Double transgenic switchgrass plants displayed significant increases in internode length and diameter, leaf blade width, and plant height but retained a tiller number comparable to that of plants expressing miR156 alone. Ultimately, the double transgenic switchgrass plants produced 174% more dry-weight biomass and 162% more solubilized sugars on average than control plants. These findings indicated that PvWOX3a is a viable potential genetic target for engineering improved shoot architecture and biomass yield of horticulture, fodder, and biofuel crops.

Phosphoproteomic Profiling Reveals Early Salt-Responsive Mechanisms in Two Foxtail Millet Cultivars.

Excess soluble salts in saline soils are harmful to most plants. Understanding the biochemical responses to salts in plants and studying the salt tolerance-associated genetic resources in nature will contribute to the improvement of salt tolerance in crops. As an emerging model crop, foxtail millet (Setaria italica L.) has been regarded as a novel species for stress resistance investigation. Here, the dynamic proteomic and phosphoproteomic profiling of two foxtail millet varieties of An04 and Yugu2 with contrasting salt tolerance characteristics were investigated under salt stress. In total, 10,366 sites representing to 2,862 proteins were detected and quantified. There were 759 and 990 sites corresponding to 484 and 633 proteins identified under salinity in An04 and Yugu2, respectively, and 1,264 and 1,131 phosphorylation sites corresponding to 789 and 731 proteins were identified between these two varieties before and after salt stress, respectively. The differentially-regulated phosphoproteins (DRPPs) were mainly involved in signal transduction, regulation of gene expression, translation, ion transport, and metabolism processes. Yugu2 possessed signal perception and transduction capabilities more rapidly and had a more intense response compared with An04 upon salinity. The sucrose metabolism pathway, in particularly, might play a vital role in salt response in foxtail millet, which not only provides UDP-glucose for the cellulose synthesis and energy production, but also promotes flavonoid related synthesis to enhance the salt tolerance ability. Over-expressing the phospho-mimic sucrose synthase (SuS) (SuS S10D ) in soybean roots enhanced salt tolerance compared with over-expressing SuS lines. The knowledge of this research will shed light on elucidating the mechanisms of salt response, and pave the way for crop varieties innovation and cultivation under salinity and stresses.

MPK3/6-induced degradation of ARR1/10/12 promotes salt tolerance in Arabidopsis.

Cytokinins are phytohormones that regulate plant development, growth, and responses to stress. In particular, cytokinin has been reported to negatively regulate plant adaptation to high salinity; however, the molecular mechanisms that counteract cytokinin signaling and enable salt tolerance are not fully understood. Here, we provide evidence that salt stress induces the degradation of the cytokinin signaling components Arabidopsis (Arabidopisis thaliana) response regulator 1 (ARR1), ARR10 and ARR12. Furthermore, the stress-activated mitogen-activated protein kinase 3 (MPK3) and MPK6 interact with and phosphorylate ARR1/10/12 to promote their degradation in response to salt stress. As expected, salt tolerance is decreased in the mpk3/6 double mutant, but enhanced upon ectopic MPK3/MPK6 activation in an MKK5DD line. Importantly, salt hypersensitivity phenotypes of the mpk3/6 line were significantly alleviated by mutation of ARR1/12. The above results indicate that MPK3/6 enhance salt tolerance in part via their negative regulation of ARR1/10/12 protein stability. Thus, our work reveals a new molecular mechanism underlying salt-induced stress adaptation and the inhibition of plant growth, via enhanced degradation of cytokinin signaling components.