Disruption of AtWNK8 enhances tolerance of Arabidopsis to salt and osmotic stresses via modulating proline content and activities of catalase and peroxidase

Screening and functional characterization of salt-tolerant NAC gene family members in Medicago sativa L

Introduction

Alfalfa is the most widely cultivated high-quality perennial leguminous forage crop in the world. In China, saline-alkali land represents an important yet underutilized land resource. Cultivating salt-tolerant alfalfa varieties is crucial for the effective development and utilization of saline-alkali soils and for promoting the sustainable growth of grassland-livestock farming in these regions. The NAC (NAM, ATAF, and CUC) family of transcription factors plays a key role in regulating gene expression in response to various abiotic stresses, such as drought, salinity and extreme temperatures, thereby enhancing plant stress tolerance.

Methods

This study evaluated the structure and evolutionary relationship of the members of the NAC-like transcription factor family in alfalfa using bioinformatics. We identified 114 members of the NAC gene family in the Zhongmu No.1 genome and classified them into 13 subclasses ranging from I to XIII. The bioinformatics analysis showed that subfamily V might be related to the response to salt stress. Gene expression analysis was conducted using RNA-seq and qRT-PCR, and MsNAC40 from subfamily V was chosen for further investigation into salt tolerance.

Results

MsNAC40 gene had an open reading frame of 990 bp and encoded a protein containing 329 amino acids, with a molecular weight of 3.70 KDa and a conserved NAM structural domain. The protein was hydrophilic with no transmembrane structure.After treating both the MsNAC40 overexpressing plants and the control group with 150 mmol/L NaCl for 15 days, physiological and biochemical measurements revealed that these plants had significantly greater height, net photosynthetic rate, stomatal conductance, and transpiration rate compared to the control group, while their conductivity was significantly lower. Additionally, the levels of abscisic acid in the roots and leaves, along with the activities of peroxidase, superoxide dismutase, and catalase in the leaves, were significantly higher in the overexpressing plants, whereas the malondialdehyde content was significantly lower. Moreover, the Na+ content in the overexpressing plants was significantly reduced, while the K+/Na+ ratio was significantly increased compared to the control group.

Discussion

These results indicated that the MsNAC40 gene improved the salt tolerance of Pioneer Alfalfa SY4D, but its potential mechanism of action still needs to be further explored.

A Series of Novel Alleles of Ehd2 Modulating Heading and Salt Tolerance in Rice

Rice (Oryza sativa L.) is a staple crop for nearly half of the global population and one of China's most extensively cultivated cereals. Heading date, a critical agronomic trait, determines the regional and seasonal adaptability of rice varieties. In this study, a series of mutants (elh5 to elh12) exhibiting extremely late heading under both long-day (LD) and short-day (SD) conditions were identified from an ethyl methanesulfonate (EMS) mutant library. Using MutMap and map-based cloning, the causative gene was identified as a novel allele of Ehd2/OsID1/RID1/Ghd10. Functional validation through CRISPR/Cas9 knockout and complementation assays confirmed its role in regulating heading. The elh6 mutation was found to cause intron retention due to alternative splicing. Ehd2 encodes a Cys-2/His-2-type zinc finger transcription factor with an IDD domain and transcriptional activity in yeast. Its expression peaks in developing leaves before heading and spikes during reproductive conversion. In elh6 mutants, delayed heading resulted from downregulating the Ehd1-Hd3a pathway genes. Salinity stress significantly hampers rice growth and productivity. Transcriptomic analysis of elh10 and ZH8015 seedlings exposed to salt stress for 24 h identified 5150 differentially expressed genes (DEGs) at the seedling stage, predominantly linked to stress response pathways. Ehd2 was revealed as a modulator of salt tolerance, likely through the regulation of ion transport, enzyme activity, and antioxidant systems. This study establishes Ehd2 as a pivotal factor in promoting heading while negatively regulating salt tolerance in rice.

OsWNK9 mitigates salt stress by promoting root growth and stomatal closure in rice

Salinity stress severely affects rice growth and reduces its productivity. With No Lysine Kinases (WNKs) are serine/threonine kinases emerging as potential candidate genes due to their involvement in various abiotic stress tolerance responses. However, studies providing mechanistic insights into the roles of WNKs in plants remain scarce. In the present study, OsWNK9-overexpressing rice lines showed strong tolerance to salinity stress. Overexpression of OsWNK9 also triggered the accumulation of abscisic acid (ABA) and restored indole-3-acetic acid (IAA) concentrations in roots, triggering stomatal closure in shoots and maintaining cell expansion of the root epidermal cells when challenged with salt treatment. The overexpression lines showed increased activity of antioxidant enzymes, which further mitigated ROS-mediated cellular damage under salinity stress. We also identified that OsWNK9 interacts with Receptor for Activated Kinase C1A (RACK1A), ABA-8'-hydroxylase, and (Vacuolar Type ATPase) V-Type ATPase. Taken together, our findings suggest that OsWNK9 expression is warranted under salinity stress and exerts its effects by interacting with its downstream targets and by increased accumulation of ABA and IAA, thereby regulating seed germination, stomatal activity, improved root growth, and ionic homeostasis, which all contribute to significantly higher yield produced per plant under long term salinity stress.

OsWNK9 regulates cadmium concentration in brown rice by restraining cadmium transport from straw to brown rice

Selecting and breeding rice cultivars that enable strong cadmium (Cd) accumulation in rice straw but low accumulation in brown rice is a promising way to achieve Cd phytoremediation as well as to ensure the food safety of rice. Herein, we isolated a gene OsWNK9 from the quantitative trait locus associated with reducing Cd translocation from rice straw to brown rice and decreasing the Cd concentration in brown rice (BRCdC). Continuous strong expression of OsWNK9 was observed in nodes and internode and was induced after Cd supply. OsWNK9 was localized in the rice cell nucleus and participated in the regulation of Cd transport in yeast. Two independent oswnk9 rice mutants were generated via CRISPR/Cas9 gene-editing and showed significantly higher BRCdC than that of the wild type (WT). The BRCdC of knockout oswnk9 mutants was 0.227 mg kg-1and 0.238 mg kg-1, increased by 14 % and 19 % compared with that of the WT due to the lower Cd allocation in the basal stem, internode, and node III, which was unrelated to Cd uptake. Interestingly, OsWNK9 could promote iron (Fe) accumulation in rice under Cd-contaminated conditions, suggesting that OsWNK9 is an ideal gene for Cd phytoremediation and Fe biofortification in rice to support safe food production.

Genome-Wide Identification and Expression Pattern Analysis of the WNK Gene Family in Apple under Abiotic Stress and Colletotrichum siamense Infection

With-no-lysine kinase (WNK) is a unique serine/threonine kinase family member. WNK differs from other protein kinases by not having a standard lysine in subdomain II of the universally preserved kinase catalytic region. Conversely, the amino acid lysine located in subdomain I plays a crucial role in its phosphorylation. The WNK family has been reported to regulate Arabidopsis flowering, circadian rhythm, and abiotic stress. Eighteen members of the WNK gene family were discovered in apples in this research, and they were primarily grouped into five categories on the phylogenetic tree. Conserved domains and motifs also confirmed their identity as members of the WNK family. Promoter cis-acting element analysis indicated their potential role in responses to both abiotic stress and phytohormones. Furthermore, qRT-PCR analysis showed that the expression of MdWNK family genes was stimulated to different extents by Colletotrichum siamense, NaCl, mannitol, ABA, JA, and SA, with Colletotrichum siamense being the most prominent stimulant. MdWNK family genes were expressed across all apple tissues, with young fruits showing the greatest expression and roots showing the least expression. The research offered detailed insights into the MdWNK gene family, serving as a crucial basis for investigating the biological roles of MdWNK genes.

Genome-wide identification and expression analysis of the WNK kinase gene family in soybean

WNK kinases are a unique class of serine/threonine protein kinases that lack a conserved catalytic lysine residue in the kinase domain, hence the name WNK (with no K, i.e., lysine). WNK kinases are involved in various physiological processes in plants, such as circadian rhythm, flowering time, and stress responses. In this study, we identified 26 WNK genes in soybean and analyzed their phylogenetic relationships, gene structures, chromosomal distribution, cis-regulatory elements, expression patterns, and conserved protein motifs. The soybean WNK genes were unevenly distributed on 15 chromosomes and underwent 21 segmental duplication events during evolution. We detected 14 types of cis-regulatory elements in the promoters of the WNK genes, indicating their potential involvement in different signaling pathways. The transcriptome database revealed tissue-specific and salt stress-responsive expression of WNK genes in soybean, the second of which was confirmed by salt treatments and qRT-PCR analysis. We found that most WNK genes were significantly up-regulated by salt stress within 3 h in both roots and leaves, except for WNK5, which showed a distinct expression pattern. Our findings provide valuable insights into the molecular characteristics and evolutionary history of the soybean WNK gene family and lay a foundation for further analysis of WNK gene functions in soybean.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01440-5.

Tobacco NtUBC1 and NtUBQ2 enhance salt tolerance by reducing sodium accumulation and oxidative stress through proteasome activation in Arabidopsis

The ubiquitin/proteasome system plays a crucial role in the regulation of plant responses to environmental stress. Here, we studied the involvement of the UBC1 and UBQ2 genes encoding a ubiquitin conjugating enzyme (E2) and ubiquitin extension protein, respectively, in the response to salt stress. Our results showed that the constitutive expression of tobacco NtUBC1 and NtUBQ2 in Arabidopsis thaliana improved salt tolerance, along with the lower Na+ level and higher K+/Na+ ratio compared to control plants. Moreover, the expression levels of sodium transporters, including AtHKT1 (High-Affinity K+ Transporter1) and AtSOS1 (Salt Overly Sensitive 1), were higher in NtUBC1- and NtUBQ2-Arabidopsis. However, the transcript level of AtNHX1 (Na+/H+ Exchanger 1) was similar between control and transgenic plants. After salt exposure, the activity of the 26S proteasome markedly increased in NtUBC1- and NtUBQ2-expressing plants; however, ubiquitinated protein levels decreased compared to control plants. Furthermore, higher activity of antioxidant enzymes and lower ROS production were observed in UBC1- and UBQ2-expressing plants. We further challenged atubc1, atubc2, and atubq2 single mutants and atubc1ubc2 double mutant lines with salt stress; interestingly, the salt sensitivity and sodium levels of the studied mutants were enhanced, while the potassium levels were reduced. However, the atubc1ubc2 double mutant illustrated a more severe phenotype than the single mutants, probably due to the redundant function of UBC1 and UBC2 in Arabidopsis. Taken together, NtUBC1 and NtUBQ2 enhance salt tolerance by enhancing 26S proteasome activity and reducing Na+ accumulation, ROS, and ubiquitinated/salt-denatured proteins.

Late Elongated Hypocotyl Positively Regulates Salt Stress Tolerance in Medicago truncatula

Abiotic stress, such as drought, osmotic, and salinity stresses, seriously affects plant growth and crop production. Studying stress-resistant genes that enhance plant stress tolerance is an efficient way to facilitate the breeding of crop species with high stress tolerance. In this study, we reported that the core circadian clock component, the LATE ELONGATED HYPOCOTYL (LHY) orthologue MtLHY, plays a positive role in salt stress response in Medicago truncatula. The expression of MtLHY was induced by salt stress, and loss-of-function mutants of MtLHY were shown to be hypersensitive to salt treatment. However, overexpression of MtLHY improved salt stress tolerance through a higher accumulation of flavonoids. Consistently, exogenous flavonol application improved the salt stress tolerance in M. truncatula. Additionally, MtLHY was identified as a transcriptional activator of the flavonol synthase gene, MtFLS. Our findings revealed that MtLHY confers plant salt stress tolerance, at least by modulating the flavonoid biosynthesis pathway, which provides insight into salt stress tolerance that links the circadian clock with flavonoid biosynthesis.

Achieving abiotic stress tolerance in plants through antioxidative defense mechanisms

Climate change has increased the overall impact of abiotic stress conditions such as drought, salinity, and extreme temperatures on plants. Abiotic stress adversely affects the growth, development, crop yield, and productivity of plants. When plants are subjected to various environmental stress conditions, the balance between the production of reactive oxygen species and its detoxification through antioxidant mechanisms is disturbed. The extent of disturbance depends on the severity, intensity, and duration of abiotic stress. The equilibrium between the production and elimination of reactive oxygen species is maintained due to both enzymatic and non-enzymatic antioxidative defense mechanisms. Non-enzymatic antioxidants include both lipid-soluble (α-tocopherol and β-carotene) and water-soluble (glutathione, ascorbate, etc.) antioxidants. Ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR) are major enzymatic antioxidants that are essential for ROS homeostasis. In this review, we intend to discuss various antioxidative defense approaches used to improve abiotic stress tolerance in plants and the mechanism of action of the genes or enzymes involved.

Cloning, homology modelling and expression analysis of Oryza sativa WNK gene family

With No Lysine kinases (WNKs) represents a gene family that encodes Ser/Thr kinases, with anomalous disposition of catalytic lysine residue in subdomain I. In plants, WNKs had been linked to circadian rhythm, photoperiodic response and abiotic stress tolerance with mechanism yet undeciphered. In the present study, full-length CDS sequences of rice WNKs (OsWNK1 to 8) were cloned from indica cultivar IR64. A total of six highly conserved kinase subdomains were identified. Comparative analysis of protein sequences from six different species of rice showed varying magnitudes of substitution (76.2 %), deletion (15.4 %), and addition (8.4 %) events. ConSurf analysis coupled with CASTp results identified functional residues that were clustered together in modelled 3-D structures. Among post-translational modifications (PTMs) studied, 87.7 % of phosphorylation sites were predicted. Mined protein-protein interactions (PPIs) depicted OsWNKs to interact notably with other OsWNK members and with key proteins like PRR95 involved in photoperiodic response and protein phosphatase like PP2C involved in ABA signalling. Gene duplication analysis revealed two paralogous duplicated gene pairs: WNK6-WNK9 and WNK7-WNK8. Oryza sativa showed maximum syntenic relationship with Sorghum bicolor among the compared species. OsWNKs showed differential transcript expression profiles on treatment with plant growth regulators indicating its versatile role in plant growth and development.

The Anthocyanin Accumulation Related ZmBZ1, Facilitates Seedling Salinity Stress Tolerance via ROS Scavenging

Anthocyanins are a class of antioxidants that scavenge free radicals in cells and play an important role in promoting human health and preventing many diseases. Here, we characterized a maize Bronze gene (BZ1) from the purple colored W22 introgression line, which encodes an anthocyanin 3-O-glucosyltransferase, a key enzyme in the anthocyanin synthesis pathway. Mutation of ZmBZ1 showed bronze-colored seeds and reduced anthocyanins in seeds aleurone layer, seedlings coleoptile, and stem of mature plants by comparison with purple colored W22 (WT). Furthermore, we proved that maize BZ1 is an aleurone layer-specific expressed protein and sub-located in cell nucleus. Real-time tracing of the anthocyanins in developing seeds demonstrated that the pigment was visible from 16 DAP (day after pollination) in field condition, and first deposited in the crown part then spread all over the seed. Additionally, it was transferred along with the embryo cell activity during seed germination, from aleurone layer to cotyledon and coleoptile, as confirmed by microscopy and real-time qRT-PCR. Finally, we demonstrated that the ZmBZ1 contributes to stress tolerance, especially salinity. Further study proved that ZmBZ1 participates in reactive oxygen scavenging (ROS) by accumulating anthocyanins, thereby enhancing the tolerance to abiotic stress.

Linker histone variant HIS1-3 and WRKY1 oppositely regulate salt stress tolerance in Arabidopsis

The salt overly sensitive (SOS) pathway plays an important role in plant salt stress; however, the transcriptional regulation of the genes in this pathway is unclear. In this study, we found that Linker histone variant HIS1-3 and WRKY1 oppositely regulate the salt stress response in Arabidopsis (Arabidopsis thaliana) through the transcriptional regulation of SOS genes. The expression of HIS1-3 was inhibited by salt stress, and the disruption of HIS1-3 resulted in enhanced salt tolerance. Conversely, the expression of WRKY1 was induced by salt stress, and the loss of WRKY1 function led to increased salt sensitivity. The expression of SOS1, SOS2, and SOS3 was repressed and induced by HIS1-3 and WRKY1, respectively, and HIS1-3 regulated the expression of SOS1 and SOS3 by occupying the WRKY1 binding sites on their promoters. Moreover, WRKY1 and HIS1-3 acted upstream of the SOS pathway. Together, our results indicate that HIS1-3 and WRKY1 oppositely modulate salt tolerance in Arabidopsis through transcriptional regulation of SOS genes.

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

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

Genome-wide identification and evolution of WNK kinases in Bambusoideae and transcriptional profiling during abiotic stress in Phyllostachys edulis

With-no-lysine (WNK) kinases play vital roles in abiotic stress response, circadian rhythms, and regulation of flowering time in rice, Arabidopsis, and Glycine max. However, there are no previous reports of WNKs in the Bambusoideae, although genome sequences are available for diploid, tetraploid, and hexaploid bamboo species. In the present study, we identified 41 WNK genes in five bamboo species and analysed gene evolution, phylogenetic relationship, physical and chemical properties, cis-elements, and conserved motifs. We predicted the structure of PeWNK proteins of moso bamboo and determined the exposed, buried, structural and functional amino acids. Real-time qPCR analysis revealed that PeWNK5, PeWNK7, PeWNK8, and PeWNK11 genes are involved in circadian rhythms. Analysis of gene expression of different organs at different developmental stages revealed that PeWNK genes are tissue-specific. Analysis of various abiotic stress transcriptome data (drought, salt, SA, and ABA) revealed significant gene expression levels in all PeWNKs except PeWNK11. In particular, PeWNK8 and PeWNK9 were significantly down- and up-regulated, respectively, after abiotic stress treatment. A co-expression network of PeWNK genes also showed that PeWNK2, PeWNK4, PeWNK7, and PeWNK8 were co-expressed with transcriptional regulators related to abiotic stress. In conclusion, our study identified the PeWNKs of moso bamboo involved in circadian rhythms and abiotic stress response. In addition, this study serves as a guide for future functional genomic studies of the WNK genes of the Bambusoideae.

AtUSP17 negatively regulates salt stress tolerance through modulation of multiple signaling pathways in Arabidopsis

AtUSP17 is a multiple stress-inducible gene that encodes a universal stress protein (USP) in Arabidopsis thaliana. In the present study, we functionally characterized AtUSP17 using its knock-down mutant, Atusp17, and AtUSP17-overexpression lines (WTOE). The overexpression of AtUSP17 in wild-type and Atusp17 mutant Arabidopsis plants resulted in higher sensitivity to salt stress during seed germination than WT and Atusp17 mutant lines. In addition, the WTOE and FC lines exhibited higher abscisic acid (ABA) sensitivity than Atusp17 mutant during germination. The exogenous application of ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) was able to rescue the salt hypersensitive phenotype of WTOE lines. In contrast, AgNO₃ , an ethylene action inhibitor, further blocked the effect of ACC during germination. The addition of ACC under salt stress resulted in reduced reactive oxygen species (ROS) accumulation, expression of ABA-responsive genes, improved proline synthesis, increased expression of positive regulators of ethylene signaling and antioxidant defense genes with enhanced antioxidant enzyme activities. The WTOE lines exhibited salt sensitivity even at the adult plant stage, while Atusp17 mutant exhibited higher salt tolerance with higher chlorophyll, relative water content and lower electrolyte leakage as compared with WT. The BAR interaction viewer database and available literature mining identified AtUSP17-interacting proteins, which include RGS1, RACK1C and PRN1 involved in G-protein signaling, which play a crucial role in salt stress responses. Based on the present study and available literature, we proposed a model in which AtUSP17 negatively mediates salt tolerance in Arabidopsis through modulation of ethylene, ABA, ROS, and G-protein signaling and responses.

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

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

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

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

Systems biology reveals key tissue-specific metabolic and transcriptional signatures involved in the response of Medicago truncatula plant genotypes to salt stress

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Τhe response of different Medicago truncatula genotypes to salt stress was examined.

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Systems biology revealed tissue-specific metabolic and transcriptional signatures.

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RFO metabolites linked with tolerance were identified.

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Several genes belonging to the TIR-NBS-LRR class were linked with hyper-sensitivity.

Salt stress is an important factor limiting plant productivity by affecting plant physiology and metabolism. To explore salt tolerance adaptive mechanisms in the model legume Medicago truncatula, we used three genotypes with differential salt-sensitivity: TN6.18 (highly sensitive), Jemalong A17 (moderately sensitive), and TN1.11 (tolerant). Cellular damage was monitored in roots and leaves 48 h after 200 mM NaCl treatment by measuring lipid peroxidation, nitric oxide, and hydrogen peroxide contents, further supported by leaf stomatal conductance and chlorophyll readings. The salt-tolerant genotype TN1.11 displayed the lowest level of oxidative damage, in contrast to the salt sensitive TN6.18, which showed the highest responses. Metabolite profiling was employed to explore the differential genotype-related responses to stress at the molecular level. The metabolic data in the salt tolerant TN1.11 roots revealed an accumulation of metabolites related to the raffinose pathway. To further investigate the sensitivity to salinity, global transcriptomic profiling using microarray analysis was carried out on the salt-stressed sensitive genotypes. In TN6.18, the transcriptomic analysis identified a lower expression of many genes related to stress signalling, not previously linked to salinity, and corresponding to the TIR-NBS-LRR gene class. Overall, this global approach contributes to gaining significant new insights into the complexity of stress adaptive mechanisms and to the identification of potential targets for crop improvement.

Biphasic activation of survival and death pathways in Arabidopsis thaliana cultured cells by sorbitol-induced hyperosmotic stress

Hyperosmotic stresses represent some of the most serious abiotic factors that adversely affect plants growth, development and fitness. Despite their central role, the early cellular events that lead to plant adaptive responses remain largely unknown. In this study, using Arabidopsis thaliana cultured cells we analyzed early cellular responses to sorbitol-induced hyperosmotic stress. We observed biphasic and dual responses of A. thaliana cultured cells to sorbitol-induced hyperosmotic stress. A first set of events, namely singlet oxygen (¹O₂) production and cell hyperpolarization due to a decrease in anion channel activity could participate to signaling and osmotic adjustment allowing cell adaptation and survival. A second set of events, namely superoxide anion (O₂^-) production by RBOHD-NADPH-oxidases and SLAC1 anion channel activation could participate in programmed cell death (PCD) of a part of the cell population. This set of events raises the question of how a survival pathway and a death pathway could be induced by the same hyperosmotic condition and what could be the meaning of the induction of two different behaviors in response to hyperosmotic stress.

Exogenous application of xanthine and uric acid and nucleobase-ascorbate transporter MdNAT7 expression regulate salinity tolerance in apple

BACKGROUND

Soil salinity is a critical threat to global agriculture. In plants, the accumulation of xanthine activates xanthine dehydrogenase (XDH), which catalyses the oxidation/conversion of xanthine to uric acid to remove excess reactive oxygen species (ROS). The nucleobase-ascorbate transporter (NAT) family is also known as the nucleobase-cation symporter (NCS) or AzgA-like family. NAT is known to transport xanthine and uric acid in plants. The expression of MdNAT is influenced by salinity stress in apple.

RESULTS

In this study, we discovered that exogenous application of xanthine and uric acid enhanced the resistance of apple plants to salinity stress. In addition, MdNAT7 overexpression transgenic apple plants showed enhanced xanthine and uric acid concentrations and improved tolerance to salinity stress compared with nontransgenic plants, while opposite phenotypes were observed for MdNAT7 RNAi plants. These differences were probably due to the enhancement or impairment of ROS scavenging and ion homeostasis abilities.

CONCLUSION

Our results demonstrate that xanthine and uric acid have potential uses in salt stress alleviation, and MdNAT7 can be utilized as a candidate gene to engineer resistance to salt stress in plants.

Phenotyping in Arabidopsis and Crops-Are We Addressing the Same Traits? A Case Study in Tomato

The convenient model Arabidopsis thaliana has allowed tremendous advances in plant genetics and physiology, in spite of only being a weed. It has also unveiled the main molecular networks governing, among others, abiotic stress responses. Through the use of the latest genomic tools, Arabidopsis research is nowadays being translated to agronomically interesting crop models such as tomato, but at a lagging pace. Knowledge transfer has been hindered by invariable differences in plant architecture and behaviour, as well as the divergent direct objectives of research in Arabidopsis versus crops compromise transferability. In this sense, phenotype translation is still a very complex matter. Here, we point out the challenges of "translational phenotyping" in the case study of drought stress phenotyping in Arabidopsis and tomato. After briefly defining and describing drought stress and survival strategies, we compare drought stress protocols and phenotyping techniques most commonly used in the two species, and discuss their potential to gain insights, which are truly transferable between species. This review is intended to be a starting point for discussion about translational phenotyping approaches among plant scientists, and provides a useful compendium of methods and techniques used in modern phenotyping for this specific plant pair as a case study.

Modulation of ABA responses by the protein kinase WNK8

Abscisic acid (ABA) regulates growth and developmental processes in response to limiting water conditions. ABA functions through a core signaling pathway consisting of PYR1/PYL/RCAR ABA receptors, type 2C protein phosphatases (PP2Cs), and SnRK2-type protein kinases. Other signaling modules might converge with ABA signals through the modulation of core ABA signaling components. We have investigated the role of the protein kinase WNK8 in ABA signaling. WNK8 interacted with PP2CA and PYR1, phosphorylated PYR1 in vitro, and was dephosphorylated by PP2CA. A hypermorphic wnk8-ct Arabidopsis mutant allele suppressed ABA and glucose hypersensitivities of pp2ca-1 mutants during young seedling development, and WNK8 expression in protoplasts suppressed ABA-induced reporter gene expression. We conclude that WNK8 functions as a negative modulator of ABA signaling.

Variations in Physiology and Multiple Bioactive Constituents under Salt Stress Provide Insight into the Quality Evaluation of Apocyni Veneti Folium

As one of the major abiotic stresses, salinity stress may affect the physiology and biochemical components of Apocynum venetum L. To systematically evaluate the quality of Apocyni Veneti Folium (AVF) from the perspective of physiological and the wide variety of bioactive components response to various concentrations of salt stress, this experiment was arranged on the basis of ultra-fast liquid chromatography tandem triple quadrupole mass spectrometry (UFLC-QTRAP-MS/MS) technology and multivariate statistical analysis. Physiological characteristics of photosynthetic pigments, osmotic homeostasis, lipid peroxidation product, and antioxidative enzymes were introduced to investigate the salt tolerance mechanism of AVF under salinity treatments of four concentrations (0, 100, 200, and 300 mM NaCl, respectively). Furthermore, a total of 43 bioactive constituents, including 14 amino acids, nine nucleosides, six organic acids, and 14 flavonoids were quantified in AVF under salt stress. In addition, multivariate statistical analysis, including hierarchical clustering analysis, principal component analysis (PCA), and gray relational analysis (GRA) was employed to systematically cluster, distinguish, and evaluate the samples, respectively. Compared with the control, the results demonstrated that 200 mM and 100 mM salt stress contributed to maintain high quality of photosynthesis, osmotic balance, antioxidant enzyme activity, and the accumulation of metabolites, except for total organic acids, and the quality of AVF obtained by these two groups was better than others; however, under severe stress, the accumulation of the oxidative damage and the reduction of metabolite caused by inefficiently scavenging reactive oxygen species (ROS) lead to lower quality. In summary, the proposed method may provide integrated information for the quality evaluation of AVF and other salt-tolerant Chinese medicines.

Nudge-nudge, WNK-WNK (kinases), say no more?

Contents Summary 35 I Overview of animal and plant WNK kinases 35 II. Structure: domains and topology 36 III. Phylogeny-evolutionary relationships 41 IV. Plant WNK kinase distribution and regulation of WNK expression and activity 41 V. Functions of WNK family members in physiology and development 41 VI. Say no more? Still many questions to be answered 45 Acknowledgements 46 References 46 SUMMARY: WITH NO LYSINE (WNK) kinases are serine/threonine kinases uniquely characterized by an anomalous placement of a catalytic lysine, hence their moniker. In animals, WNK protein kinases play critical roles in protein trafficking of components that mediate renal ion transport processes and regulate osmoregulation of cell volume. In plants, the WNK kinase gene family is larger and more diverse. Recent studies revealed WNK kinase roles in orchestrating the trafficking of an ion channel, a lipid kinase complex in animals, and a heterotrimeric G protein signaling component in plants that is necessary for signal transduction. For this reason, new attention is geared toward investigating the mechanisms adopted by WNK kinases to nudge intracellular proteins to their destinations. In this review, the functions of WNK kinases in protein trafficking are derived from what we have learned from the model organism Arabidopsis thaliana. To place this new idea in context, we provide the predicted WNK kinase structures, their predicted expression patterns, a speculated evolutionary pathway, and the regulatory roles of plant WNKs in transport processes and other physiologies. We brazenly predict that the WNK kinases in both plants and animals will soon be recognized as a nexus for trafficking-based signal transduction.

Expression of OsWNK9 in Arabidopsis conferred tolerance to salt and drought stress

With No Lysine (WNK) kinase belongs to ser/thr protein kinase group in which conserved catalytic lysine (K) residue of subdomain II is shifted to subdomain I. In this study, we cloned full-length coding region of WNK9 from Oryza sativa (OsWNK9) and performed in silico studies to confirm the presence of all kinase signature regulatory elements. The transcript analysis revealed that OsWNK9 was strongly down regulated under salinity, drought and ABA stress in shoots. Constitutive expression of OsWNK9 in Arabidopsis thaliana imparted increased tolerance to salt, drought, and ABA stress. Transgenic lines showed healthy phenotypes such as green leaves, achieved higher fresh weight and longer roots under salt, drought and ABA stress as compared to wild-type (WT). Transgenic plants showed better seed germination, higher chlorophyll retention and less water loss under salt and drought stress compared to WT. Promoter/gene expression studies revealed that OsWNK9 were expressed throughout plant tissues with higher expression in roots. Subcellular localization studies of OsWNK9 showed their presence in the nucleus. The transcript analysis of abiotic stress marker genes and ABA dependent genes showed they were highly expressed in transgenic lines compared to WT in response to salt and drought stress. The endogenous ABA level under salt and drought stress in transgenic lines was higher than WT. The results indicated that OsWNK9 may regulate salt and drought response in ABA dependent manner.

Environment-dependent regulation of spliceosome activity by the LSM2-8 complex in Arabidopsis

Spliceosome activity is tightly regulated to ensure adequate splicing in response to internal and external cues. It has been suggested that core components of the spliceosome, such as the snRNPs, would participate in the control of its activity. The experimental indications supporting this proposition, however, remain scarce, and the operating mechanisms poorly understood. Here, we present genetic and molecular evidence demonstrating that the LSM2–8 complex, the protein moiety of the U6 snRNP, regulates the spliceosome activity in Arabidopsis, and that this regulation is controlled by the environmental conditions. Our results show that the complex ensures the efficiency and accuracy of constitutive and alternative splicing of selected pre-mRNAs, depending on the conditions. Moreover, miss-splicing of most targeted pre-mRNAs leads to the generation of nonsense mediated decay signatures, indicating that the LSM2–8 complex also guarantees adequate levels of the corresponding functional transcripts. Interestingly, the selective role of the complex has relevant physiological implications since it is required for adequate plant adaptation to abiotic stresses. These findings unveil an unanticipated function for the LSM2–8 complex that represents a new layer of posttranscriptional regulation in response to external stimuli in eukaryotes.

Kale BoRACK1 is involved in the plant response to salt stress and Peronospora brassicae Gaumann

The receptor for activated C kinase 1 (RACK1) belongs to a protein subfamily containing a tryptophan-aspartic acid-domain (WD) repeat structure. Compelling evidence indicates that RACK1 can interact with many signal molecules and affect different signal transduction pathways. In this study, a kale (Brassica oleracea var. acephala f.tricolor) RACK1 gene (BoRACK1) was cloned by RT-PCR. The amino acid sequence of BoRACK1 had seven WD repeats in which there were typical GH (glycine-histidine) and WD dipeptides. Comparison with AtRACK1 from Arabidopsis revealed 87.1% identity at the amino acid level. Expression pattern analysis by RT-PCR showed that BoRACK1 was expressed in all analyzed tissues of kale and that its transcription in leaves was down-regulated by salt, abscisic acid, and H₂O₂ at a high concentration. Overexpression of BoRACK1 in kale led to a reduction in symptoms caused by Peronospora brassicae Gaumann on kale leaves. The expression levels of the pathogenesis-related protein genes, PR-1 and PRB-1, increased 2.5-4-fold in transgenic kale, and reactive oxygen species production was more active than in the wild-type. They also exhibited increased tolerance to salt stress in seed germination. H₂O₂ may also be involved in the regulation of BoRACK1 during seed germination under salt stress. Quantitative real-time PCR analyses showed that the transcript levels of BoRbohs genes were significantly higher in overexpression of BoRACK1 transgenic lines. Yeast two-hybrid assays showed that BoRACK1 could interact with WNK8, eIF6, RAR1, and SGT1. This study and previous work lead us to believe that BoRACK1 may form a complex with regulators of plant salt and disease resistance to coordinate kale reactions to pathogens.

Impact of salt stress, cell death, and autophagy on peroxisomes: quantitative and morphological analyses using small fluorescent probe N-BODIPY

Plant peroxisomes maintain a plethora of key life processes including fatty acid β-oxidation, photorespiration, synthesis of hormones, and homeostasis of reactive oxygen species (ROS). Abundance of peroxisomes in cells is dynamic; however mechanisms controlling peroxisome proliferation remain poorly understood because measuring peroxisome abundance is technically challenging. Counting peroxisomes in individual cells of complex organs by electron or fluorescence microscopy is expensive and time consuming. Here we present a simple technique for quantifying peroxisome abundance using the small probe Nitro-BODIPY, which in vivo fluoresces selectively inside peroxisomes. The physiological relevance of our technique was demonstrated using salinity as a known inducer of peroxisome proliferation. While significant peroxisome proliferation was observed in wild-type Arabidopsis leaves following 5-hour exposure to NaCl, no proliferation was detected in the salt-susceptible mutants fry1-6, sos1-14, and sos1-15. We also found that N-BODIPY detects aggregation of peroxisomes during final stages of programmed cell death and can be used as a marker of this stage. Furthermore, accumulation of peroxisomes in an autophagy-deficient Arabidopsis mutant atg5 correlated with N-BODIPY labeling. In conclusion, the technique reported here enables quantification of peroxisomes in plant material at various physiological settings. Its potential applications encompass identification of genes controlling peroxisome homeostasis and capturing stress-tolerant genotypes.

Predicted Functional Implications of Phosphorylation of Regulator of G Protein Signaling Protein in Plants

Heterotrimeric G proteins function in development, biotic, and abiotic stress responses, hormone signaling as well as sugar sensing. We previously proposed that discrimination of these various external signals in the G protein pathway is accomplished in plants by membrane-localized receptor-like kinases (RLKs) rather than G-protein-coupled receptors. Arabidopsis thaliana Regulator of G Signaling protein 1 (AtRGS1) modulates G protein activation and is phosphorylated by several RLKs and by WITH-NO-LYSINE kinases (WNKs). Here, a combination of in vitro kinase assays, mass spectrometry, and computational bioinformatics identified and functionally prioritized phosphorylation sites in AtRGS1. Phosphosites for two more RLKs (BRL3 and PEPR1) were identified and added to the AtRGS1 phosphorylation profile. Bioinformatics analyses revealed that RLKs and WNK kinases phosphorylate plant RGS proteins within regions that are conserved across eukaryotes and at a high frequency. Four phospho-sites among 14 identified are proximal to equivalent mammalian phosphosites that impact RGS function, including: pS437 and pT267 in GmRGS2, and pS339 and pS436 in AtRGS1. Based on these analyses, we propose that pS437 and pS436 regulate GmRGS2 and AtRGS1 protein interactions and/or localization, whereas pT267 is important for modulation of GmRGS2 GAP activity and localization. Moreover, pS339 most likely affects AtRGS1 activation.

Comparative shotgun proteomic analysis of wild and domesticated Opuntia spp. species shows a metabolic adaptation through domestication

The Opuntia genus is widely distributed in America, but the highest richness of wild species are found in Mexico, as well as the most domesticated Opuntia ficus-indica, which is the most domesticated species and an important crop in agricultural economies of arid and semiarid areas worldwide. During domestication process, the Opuntia morphological characteristics were favored, such as less and smaller spines in cladodes and less seeds in fruits, but changes at molecular level are almost unknown. To obtain more insights about the Opuntia molecular changes through domestication, a shotgun proteomic analysis and database-dependent searches by homology was carried out. >1000 protein species were identified and by using a label-free quantitation method, the Opuntia proteomes were compared in order to identify differentially accumulated proteins among wild and domesticated species. Most of the changes were observed in glucose, secondary, and 1C metabolism, which correlate with the observed protein, fiber and phenolic compounds accumulation in Opuntia cladodes. Regulatory proteins, ribosomal proteins, and proteins related with response to stress were also observed in differential accumulation. These results provide new valuable data that will help to the understanding of the molecular changes of Opuntia species through domestication.

BIOLOGICAL SIGNIFICANCE

Opuntia species are well adapted to dry and warm conditions in arid and semiarid regions worldwide, and they are highly productive plants showing considerable promises as an alternative food source. However, there is a gap regarding Opuntia molecular mechanisms that enable them to grow in extreme environmental conditions and how the domestication processes has changed them. In the present study, a shotgun analysis was carried out to characterize the proteomes of five Opuntia species selected by its domestication degree. Our results will help to a better understanding of proteomic features underlying the selection and specialization under evolution and domestication of Opuntia and will provide a platform for basic biology research and gene discovery.

Ascorbate Peroxidase and Catalase Activities and Their Genetic Regulation in Plants Subjected to Drought and Salinity Stresses

Hydrogen peroxide (H2O2), an important relatively stable non-radical reactive oxygen species (ROS) is produced by normal aerobic metabolism in plants. At low concentrations, H2O2 acts as a signal molecule involved in the regulation of specific biological/physiological processes (photosynthetic functions, cell cycle, growth and development, plant responses to biotic and abiotic stresses). Oxidative stress and eventual cell death in plants can be caused by excess H2O2 accumulation. Since stress factors provoke enhanced production of H2O2 in plants, severe damage to biomolecules can be possible due to elevated and non-metabolized cellular H2O2. Plants are endowed with H2O2-metabolizing enzymes such as catalases (CAT), ascorbate peroxidases (APX), some peroxiredoxins, glutathione/thioredoxin peroxidases, and glutathione sulfo-transferases. However, the most notably distinguished enzymes are CAT and APX since the former mainly occurs in peroxisomes and does not require a reductant for catalyzing a dismutation reaction. In particular, APX has a higher affinity for H2O2 and reduces it to H2O in chloroplasts, cytosol, mitochondria and peroxisomes, as well as in the apoplastic space, utilizing ascorbate as specific electron donor. Based on recent reports, this review highlights the role of H2O2 in plants experiencing water deficit and salinity and synthesizes major outcomes of studies on CAT and APX activity and genetic regulation in drought- and salt-stressed plants.

The plant kinome

Plant kinases are one of the largest protein families in Arabidopsis. There are almost 600 membrane-located receptor kinases and almost 400 soluble kinases with distinct functions in signal transduction. In this minireview we discuss phylogeny and functional context of prominent members from major protein kinase subfamilies in plants.

Growth attenuation under saline stress is mediated by the heterotrimeric G protein complex

BACKGROUND

Plant growth is plastic, able to rapidly adjust to fluctuation in environmental conditions such as drought and salinity. Due to long-term irrigation use in agricultural systems, soil salinity is increasing; consequently crop yield is adversely affected. It is known that salt tolerance is a quantitative trait supported by genes affecting ion homeostasis, ion transport, ion compartmentalization and ion selectivity. Less is known about pathways connecting NaCl and cell proliferation and cell death. Plant growth and cell proliferation is, in part, controlled by the concerted activity of the heterotrimeric G-protein complex with glucose. Prompted by the abundance of stress-related, functional annotations of genes encoding proteins that interact with core components of the Arabidopsis heterotrimeric G protein complex (AtRGS1, AtGPA1, AGB1, and AGG), we tested the hypothesis that G proteins modulate plant growth under salt stress.

RESULTS

Na+ activates G signaling as quantitated by internalization of Arabidopsis Regulator of G Signaling protein 1 (AtRGS1). Despite being components of a singular signaling complex loss of the Gβ subunit (agb1-2 mutant) conferred accelerated senescence and aborted development in the presence of Na+, whereas loss of AtRGS1 (rgs1-2 mutant) conferred Na+ tolerance evident as less attenuated shoot growth and senescence. Site-directed changes in the Gα and Gβγ protein-protein interface were made to disrupt the interaction between the Gα and Gβγ subunits in order to elevate free activated Gα subunit and free Gβγ dimer at the plasma membrane. These mutations conferred sodium tolerance. Glucose in the growth media improved the survival under salt stress in Col but not in agb1-2 or rgs1-2 mutants.

CONCLUSIONS

These results demonstrate a direct role for G-protein signaling in the plant growth response to salt stress. The contrasting phenotypes of agb1-2 and rgs1-2 mutants suggest that G-proteins balance growth and death under salt stress. The phenotypes of the loss-of-function mutations prompted the model that during salt stress, G activation promotes growth and attenuates senescence probably by releasing ER stress.

AtWNK9 is regulated by ABA and dehydration and is involved in drought tolerance in Arabidopsis

WNK (with no lysine [K]) kinases play important regulatory roles in flowering, as well as salt and osmotic stress tolerance in plants. Here, we report that AtWNK9, a member of the Arabidopsis WNK gene family, was induced by exogenous abscisic acid (ABA) treatment and dehydration stress. Overexpression of AtWNK9 from the cauliflower mosaic virus 35S promoter in Arabidopsis resulted in increased sensitivity to ABA, strong inhibition of primary root elongation, increased proline accumulation, reduced stomatal aperture, and a reduced rate of water loss. In addition, plant survival under drought stress was improved compared to wild type. In contrast, a mutant with a T-DNA insertion in AtWNK9 showed reduced ABA sensitivity and an increased rate of water loss; further, it showed increased susceptibility to drought stress. The transcription of a number of ABA signaling components, including ABI1, ERA1, ABI3, and ABF3, was up-regulated in AtWNK9 transgenic plants and down-regulated in the wnk9 mutant in response to ABA. Some ABA-responsive and biosynthetic genes, as well as other drought-related genes, were altered at various levels in AtWNK9 transgenic plants and wnk9 mutants under dehydration stress. Overall, these findings suggest that AtWNK9 plays a positive role in ABA signaling and improves drought tolerance in transgenic Arabidopsis.

Deciphering early events involved in hyperosmotic stress-induced programmed cell death in tobacco BY-2 cells

Hyperosmotic stresses represent one of the major constraints that adversely affect plants growth, development, and productivity. In this study, the focus was on early responses to hyperosmotic stress- (NaCl and sorbitol) induced reactive oxygen species (ROS) generation, cytosolic Ca(2+) concentration ([Ca(2+)]cyt) increase, ion fluxes, and mitochondrial potential variations, and on their links in pathways leading to programmed cell death (PCD). By using BY-2 tobacco cells, it was shown that both NaCl- and sorbitol-induced PCD seemed to be dependent on superoxide anion (O2·(-)) generation by NADPH-oxidase. In the case of NaCl, an early influx of sodium through non-selective cation channels participates in the development of PCD through mitochondrial dysfunction and NADPH-oxidase-dependent O2·(-) generation. This supports the hypothesis of different pathways in NaCl- and sorbitol-induced cell death. Surprisingly, other shared early responses, such as [Ca(2+)]cyt increase and singlet oxygen production, do not seem to be involved in PCD.