An apple sucrose transporter MdSUT2.2 is a phosphorylation target for protein kinase MdCIPK22 in response to drought

Haplotype-resolved genome assembly and genome-wide association study identifies the candidate gene closely related to sugar content and tuber yield in Solanum tuberosum

As an important noncereal food crop grown worldwide, the genetic improvement of potato in tuber yield and quality is largely constrained due to the lacking of a high-quality reference genome and understanding of the regulatory mechanism underlying the formation of superior alleles. Here, a chromosome-scale haplotype-resolved genome assembled from an anther-cultured progeny of 'Ningshu 15', a tetraploid variety featured by its high starch content and drought resistance was presented. The assembled genome size was 1.653 Gb, with a contig N50 of approximately 1.4 Mb and a scaffold N50 of 61 Mb. The long terminal repeat assembly index score of the two identified haplotypes of 'Ningshu 15' was 11.62 and 11.94, respectively. Comparative genomic analysis revealed that positive selection occurred in gene families related to starch, sucrose, fructose and mannose metabolism, and carotenoid biosynthesis. Further genome-wide association study in 141 accessions identified a total number of 53 quantitative trait loci related to fructose, glucose, and sucrose content. Among them, a tonoplast sugar transporter encoding gene, StTST2, closely associated with glucose content was identified. Constitutive expression of StTST2 in potato and Arabidopsis increased the photosynthetic rate, chlorophyll and sugar content, biomass tuber and seed production in transgenic plants. In addition, co-immunoprecipitation assays demonstrated that StTST2 directly interacted with SUT2. Our study provides a high-quality genome assembly and new genetic locus of potato for molecular breeding.

Membrane transporters in Plants: Key players in abiotic and biotic stress tolerance and nutritional transport

Various abiotic and biotic stressors, including water extremes, temperature fluctuations, salinity, and heavy metals, pathogens and diseases significantly reduce global crop yields. Rapid plant responses are essential for adapting and minimizing metabolic losses. In this context, plant transporters (PTs) are essential for modulating stress responses by enabling the passage of diverse molecules and ions through the plasma membrane. Plant transporters play a pivotal role in regulating water and facilitating nutrient uptake, maintaining cellular equilibrium including osmotic regulation, detoxification, biofortification and orchestrating source-to-sink dynamics across different environmental stages in plants. In this review, we delved into recent discoveries concerning diverse transporter families such as ABC, MATE, NRAMP, SWEET, Symporters, STP, KUP, COPT/Ctr, NPF, NRT, PHT, YSL, ZIP and STP. Understanding the functions of these transporters is paramount for elucidating stress tolerance mechanisms and enhancing crop resilience through breeding and gene editing. These specialized plant membrane transporters play a crucial role in securing sustainable economic yields and maintaining high-quality produce, particularly in challenging growth conditions. We explored their contributions to plant robust growth via their crucial role in NPK and secondary metabolite transport. Through an integrated analysis of transporter dynamics during stress, we unveiled the nexus between nutrient management and stress resilience. We also clustered promising techniques that has been achieved to identify PTs such as function-driven screens, phenotype-driven screens and in silico-based approaches and provide a comprehensive overview of these transporters, offering valuable insights for the research community. This review also discusses future prospects for the use of bioinformatic computational tools in constructing signaling networks to improve our understanding of the behavior of transporters under abiotic and biotic stress. In this review, we highlight examples with case studies that illustrated how new technology and computational tools has been utilized in advanced identification and characterization of PTs functions. By strategically manipulating these transporters, we can pave the way for the development of "Plants for the Future."

How vacuolar sugar transporters evolve and control cellular sugar homeostasis, organ development and crop yield

Sugar exchange among different subcellular compartments is central for achieving cellular sugar homeostasis and directly affects the yield and quality of many horticultural and field crops. While a portion of photosynthesis-originated sugars is metabolized through glycolysis upon entering the cytosol, the remainder is reversibly channelled to the vacuole, mediated by different families of vacuolar sugar transporter (VST) located on the vacuolar membrane, the tonoplast. Historically, sugar transporters operating on plasma membranes have been studied more than those on tonoplasts. Recently, however, several breakthroughs have shed light on (1) the distinct roles of VSTs in plant development and stress responses and (2) how seemingly unrelated classes of VSTs act together to modulate sugar influx into and efflux from the vacuoles. Here we evaluate these advances, analyse the evolution of VSTs and identify knowledge gaps and future directions for better understanding and manipulation of cytosolic-vacuolar sugar exchange to optimize plant performance.

Major facilitator family transporters specifically enhance caffeyl alcohol uptake during C-lignin biosynthesis

The mode of transport of lignin monomers to the sites of polymerization in the apoplast remains controversial. C-Lignin is a recently discovered form of lignin found in some seed coats that is composed exclusively of units derived from caffeyl alcohol. RNA-seq and proteome analyses identified a number of transporters co-expressed with C-lignin deposition in the seed coat of Cleome hassleriana. Cloning and influx/efflux analysis assays in yeast identified two low-affinity transporters, ChPLT3 and ChSUC1, that were active with caffeyl alcohol but not with the classical monolignols p-coumaryl, coniferyl, and sinapyl alcohols, consistent with molecular modeling and docking studies. Expression of ChPLT3 in Arabidopsis seedlings enhanced root growth in the presence of caffeyl alcohol, and expression of ChPLT3 and ChSUC1 correlated with lignin C-unit content in hairy roots of Medicago truncatula. We present a model, consistent with phylogenetic and evolutionary considerations, whereby passive caffeyl alcohol transport may be supplemented by hitchhiking on secondary active transporters to ensure the synthesis of C-lignin, and inhibition of synthesis of G-lignin, in the apoplast.

Protein post-translational modifications (PTMS) unlocking resilience to abiotic stress in horticultural crops: A review

Horticultural crops are extensively cultivated throughout the world as crucial economical crops, encompassing fruits, vegetables, ornamentals, medicinal and beverage plants, for purposes such as food supply, special nutrition provision, medical application or aesthetic enjoyment. However, abiotic stress triggered by extreme climate change, such as excessive salt and prolonged drought, directly leads to the decline of nutritional quality of horticultural crops, contributing to the shortage of high-quality fruits. Post-translational modifications of proteins, such as phosphorylation and ubiquitination, can alter protein characteristics by adding specific groups to amino acids, which in turn impacts protein stability to regulate plant growth and development as well as environmental stress. Consequently, the revelation of the molecular mechanism of horticultural crops response to abiotic stress at post-translational modification level (PTMs) has always attracted a lot of scholars, as it is crucial for the development and breeding of climate-resilient apple varieties. At PTMs level, this review focuses on summarizing research advancements in horticultural crops responses to environmental stress, including drought, salt, cold, high temperature and iron (Fe) deficiency, with emphasis on sucrose non-fermentative 1 (SNF1) associated protein kinases (SnRKs) and mitogen-activated protein kinase (MAPK) cascade mediated phosphorylation, E3 ubiquitin ligases and BTB/TAZ subfamily BT2 mediated ubiquitination, SIZ1 SUMO E3 ligase mediated sumoylation, Nitric oxide (NO) mediated S-nitrosylation, and other forms of PTMs including protein glycosylation and lysine acetylation. In conclusion, this review adopts protein modification as an entry point to illuminate the mechanism of key genes regulating abiotic stress at PTMs level, providing a foundation for the cultivation of horticultural crops with superior resistance.

Rootstocks and drought stress impact the composition and functionality of grapevine rhizosphere bacterial microbiota

The microbiota, a component of the plant holobiont, plays an active role in the response to biotic and abiotic stresses. Nowadays, with recurrent drought and global warming, a growing challenge in viticulture is being addressed by different practices, including the use of adapted rootstocks. However, the relationships between these practices, abiotic stress and the composition and functions of the rhizosphere microbiota remain to be deciphered. This study aimed to unravel the impact of five rootstocks, water management and the combination of both on the rhizosphere bacterial microbiota in grapevines using shotgun metagenomics approach. The results showed that drought impacted the diversity, composition and functionality of the rhizosphere bacterial community. The genera Mycolicibacterium, Mycobacterium and Rhodococcus, and the bacterial functions, including DNA damage repair, fatty acid synthesis, sugar and amino acid transport, oxidative stress reduction, toxin synthesis and detoxification of exogenous compounds were significantly enriched under drought conditions. Rootstocks also significantly affected the rhizosphere bacterial richness but its influence on diversity and functionality compared to water management was weaker. Some taxa and function could be linked to water managements applied. The interaction between rootstocks and water management further influenced the rhizosphere composition, especially under drought conditions, where distinct clustering was observed for specific rootstocks. The results highlight the importance of conducting multifactorial studies to better understand their impact on shaping functional rhizosphere bacterial communities. This study paves the way for future research on beneficial bacterial inoculation and genetic engineering of rootstock to cope with drought stress.

The sugar transporter proteins in plants: An elaborate and widespread regulation network-A review

In higher plants, sugars are the primary products of photosynthesis, where CO2 is converted into organic carbon within the mesophyll cells of leaves. These sugars serve as a critical source of carbon skeletons for the biosynthesis of essential cellular compounds, energy production, and as osmotic and signaling molecules. Plant sugar transporter proteins play a key role in facilitating the long-distance translocation of sugars from source to sink organs, thereby controlling their distribution and accumulation across the plant. Over the past decade, substantial progress has been achieved in identifying the functions of individual genes linked to sugar transporters; however, the diverse regulatory mechanisms influencing these transporters remain insufficiently explored. This review consolidates current and previous research on the functions of sugar transporter proteins, focusing on their involvement in phloem transport pathways their impact on crop yield, cross-talk with other signals, and plant-microbe interactions. Furthermore, we propose future directions for studying the mechanisms of sugar transporter proteins and their potential applications in agriculture, with the goal of improving sugar utilization efficiency in crops and ultimately increasing crop yield.

Posttranslational regulation of plant membrane transporters

The movement of substances across biological membranes is often constrained by physical or energetic barriers, requiring the action of transporter proteins embedded within the lipidic bilayer. These transporters also provide finely tuned regulation of substrate fluxes, essential for maintaining cellular function under both normal and stress conditions. Consequently, transporters are subject to multiple levels of tight regulation, including posttranslational modifications (PTMs). Here, we review the current knowledge on PTMs affecting plant membrane transporters and their impact on protein function. The attachment of chemical groups to protein residues enables rapid modulation of transporter functions, influencing a wide range of protein characteristics. Phosphorylation stands out as the most common PTM, affecting transporter attributes such as activation status, localization and substrate specificity. In turn, ubiquitination acts as a signal for downregulation, either by targeting the transporters for proteasomal degradation or by triggering their endocytosis and subsequent vacuolar sorting. The roles of other, less common PTMs remain unclear, as limited examples exist and recent advances have been sparse. The complex dynamics of substrate transport, which require precise flux magnitudes and directions, appear to demand multi-layered control of the associated transporters. In consequence, further research is needed to investigate individual PTMs affecting transporters, as well as the interplay of multiple PTMs on a single transporter, to better understand how gradual modulation of protein function is achieved.

CBL1/CIPK23 phosphorylates tonoplast sugar transporter TST2 to enhance sugar accumulation in sweet orange (Citrus sinensis)

Fruit taste quality is greatly influenced by the content of soluble sugars, which are predominantly stored in the vacuolar lumen. However, the accumulation and regulation mechanisms of sugars in most fruits remain unclear. Recently, we established the citrus fruit vacuole proteome and discovered the major transporters localized in the vacuole membrane. Here, we demonstrated that the expression of tonoplast sugar transporter 2 (CsTST2) is closely associated with sugar accumulation during sweet orange (Citrus sinensis) ripening. It was further demonstrated that CsTST2 had the function of transporting hexose and sucrose into the vacuole. Overexpression of CsTST2 resulted in an elevation of sugar content in citrus juice sac, calli, and tomato fruit, whereas the downregulation of its expression led to the reduction in sugar levels. CsTST2 was identified as interacting with CsCIPK23, which binds to the upstream calcium signal sensor protein CsCBL1. The phosphorylation of the three serine residues (Ser277, Ser337, and Ser354) in the loop region of CsTST2 by CsCIPK23 is crucial for maintaining the sugar transport activity of CsTST2. Additionally, the expression of CsCIPK23 is positively correlated with sugar content. Genetic evidence further confirmed that calcium and CsCIPK23-mediated increase in sugar accumulation depends on CsTST2 and its phosphorylation level. These findings not only unveil the functional mechanism of CsTST2 in sugar accumulation, but also explore a vital calcium signal regulation module of CsCBL1/CIPK23 for citrus sweetness quality.

Apple vacuolar sugar transporters regulated by MdDREB2A enhance drought resistance by promoting accumulation of soluble sugars and activating ABA signaling

Soluble sugars are not only an important contributor to fruit quality, but also serve as the osmotic regulators in response to abiotic stresses. Early drought stress promotes sugar accumulation, while specific sugar transporters govern the cellular distribution of the sugars. Here, we show that apple plantlets accumulate soluble sugars in leaf tissues under drought stress. Transcriptional profiling of stressed and control plantlets revealed differential expression of several plasma membrane-or vacuolar membrane-localized sugar transporter genes. Among these, four previously identified vacuolar sugar transporter (VST) genes (MdERDL6-1, MdERDL6-2, MdTST1, and MdTST2) showed higher expression under drought, suggesting their roles in response to drought stress. Promoter cis-elements analyses, yeast one-hybrid, and dual-luciferase tests confirmed that the drought-induced transcription factor MdDREB2A could promote the expression of MdERDL6-1/-2 and MdTST1/2 by binding to their promoter regions. Moreover, overexpressing of each of these four MdVSTs alone in transgenic apple or Arabidopsis plants accumulated more soluble sugars and abscisic acid (ABA), and enhanced drought resistance. Furthermore, apple plants overexpressing MdERDL6-1 also showed reduced water potential, facilitated stomatal closure, and reactive oxygen species scavenging under drought conditions compared to control plants. Overall, our results suggest a potential strategy to enhance drought resistance and sugar accumulation in fruits through manipulating the genes involved in vacuolar sugar transport.

The high-affinity pineapple sucrose transporter AcSUT1B, regulated by AcCBF1, exhibited enhanced cold tolerance in transgenic Arabidopsis

Sucrose transporter (SUT) plays essential roles in plant growth and development, as well as responses to diverse abiotic stresses. However, limited information about the function of SUT was available in pineapple, an important tropical fruit crop with crassulacean acid metabolism. Here, four AcSUT genes were identified in pineapple genome, and divided into three clades according to the phylogenetic analysis. The expression profiles of AcSUTs were systemically examined, and they were all localized to plasma membrane. Transport activity assay by two-electrode voltage clamp of Xenopus oocytes showed that AcSUT1A and AcSUT1B were capable of transporting a range of glucosides, and they were exhibited high affinity for sucrose with Km value of 0.09 mM and 0.41 mM at pH 5.0, respectively. Overexpression of the cold-induced AcSUT1B conferred enhanced cold tolerance in transgenic Arabidopsis. DNA-protein interaction analysis further demonstrated that AcCBF1 directly binds the CRT/DRE element of the AcSUT1B promoter and activated its expression. Heterologous expression of AcCBF1 in Arabidopsis also increased cold tolerance. In this study, we investigated the transport activities of AcSUTs in pineapple and identified the AcCBF1-AcSUT1B module involved in cold stress, which provided new insights into the molecular mechanism of the cold response in pineapple.

Characterization of CBL-CIPK signaling networks and their response to abiotic stress in sugarcane

Calcineurin B-like proteins (CBLs) perceive calcium signals triggered by abiotic stress and interact with CBL-interacting protein kinases (CIPKs) to form a complex signal network. This study identified 21 SsCBL and 89 SsCIPK genes in Saccharum spontaneum, and 90 ScCBL and 367 ScCIPK genes in the sugarcane cultivar ZZ1. Phylogenetic analysis classified CBL genes into three groups and CIPK genes into twenty-five groups, with whole-genome duplication events promoting their expansion in sugarcane. RNA-seq analysis revealed their involvement in abiotic stress responses through ABA, JA, and SA pathways. Four ScCBLs and eight ScCIPKs were cloned from ZZ1. Three CBL-CIPK interactions were detected using a yeast two-hybrid system and Firefly luciferase complementation imaging, showing CBLs as membrane proteins and CIPKs as nuclear proteins. Spatial expression profiles indicate these genes are expressed in various tissues, with the highest expression in roots. Gene expression analyses suggested that CBL-CIPK signaling networks are involved in responses to drought, salt, and reactive oxygen species, possibly through Ca2+-induced hormone pathways. These findings establish three CBL-CIPK signaling networks responding to abiotic stress, providing a molecular basis for improving sugarcane stress resistance.

Evolution and subfunctionalization of CIPK6 homologous genes in regulating cotton drought resistance

The occurrence of whole-genome duplication or polyploidy may promote plant adaptability to harsh environments. Here, we clarify the evolutionary relationship of eight GhCIPK6 homologous genes in upland cotton (Gossypium hirsutum). Gene expression and interaction analyses indicate that GhCIPK6 homologous genes show significant functional changes after polyploidy. Among these, GhCIPK6D1 and GhCIPK6D3 are significantly up-regulated by drought stress. Functional studies reveal that high GhCIPK6D1 expression promotes cotton drought sensitivity, while GhCIPK6D3 expression promotes drought tolerance, indicating clear functional differentiation. Genetic and biochemical analyses confirm the synergistic negative and positive regulation of cotton drought resistance through GhCBL1A1-GhCIPK6D1 and GhCBL2A1-GhCIPK6D3, respectively, to regulate stomatal movement by controlling the directional flow of K+ in guard cells. These results reveal differentiated roles of GhCIPK6 homologous genes in response to drought stress in upland cotton following polyploidy. The work provides a different perspective for exploring the functionalization and subfunctionalization of duplicated genes in response to polyploidization.

Integrative gene duplication and genome-wide analysis as an approach to facilitate wheat reverse genetics: An example in the TaCIPK family

INTRODUCTION

Reverse genetic studies conducted in the plant with a complex or polyploidy genome enriched with large gene families (like wheat) often meet challenges in identifying the key candidate genes related to important traits and prioritizing the genes for functional experiments.

OBJECTIVE

To overcome the above-mentioned challenges of reverse genetics, this work aims to establish an efficient multi-species strategy for genome-wide gene identification and prioritization of the key candidate genes.

METHODS

We established the integrative gene duplication and genome-wide analysis (iGG analysis) as a strategy for pinpointing key candidate genes deserving functional research. The iGG captures the evolution, and the expansion/contraction of large gene families across phylogeny-related species and integrates spatial-temporal expression information for gene function inference. Transgenic approaches were also employed to functional validation.

RESULTS

As a proof-of-concept for the iGG analysis, we took the wheat calcineurin B-like protein-interacting protein kinases (CIPKs) family as an example. We identified CIPKs from seven monocot species, established the orthologous relationship of CIPKs between rice and wheat, and characterized Triticeae-specific CIPK duplicates (e.g., CIPK4 and CIPK17). Integrated with our analysis of CBLs and CBL-CIPK interaction, we revealed that divergent expressions of TaCBLs and TaCIPKs could play an important role in keeping the stoichiometric balance of CBL-CIPK. Furthermore, we validated the function of TaCIPK17-A2 in the regulation of drought tolerance by using transgenic approaches. Overexpression of TaCIPK17 enhanced antioxidant capacity and improved drought tolerance in wheat.

CONCLUSION

The iGG analysis leverages evolutionary and comparative genomics of crops with large genomes to rapidly highlight the duplicated genes potentially associated with speciation, domestication and/or particular traits that deserve reverse-genetic functional studies. Through the identification of Triticeae-specific TaCIPK17 duplicates and functional validation, we demonstrated the effectiveness of the iGG analysis and provided a new target gene for improving drought tolerance in wheat.

Interplay between phosphorylation and oligomerization tunes the conformational ensemble of SWEET transporters

SWEET sugar transporters are desirable biotechnological targets for improving plant growth. One engineering strategy includes modulating how SWEET transporters are regulated. Phosphorylation and oligomerization have been shown to positively regulate SWEET function, leading to increased sugar transport activity. However, constitutive phosphorylation may not be beneficial to plant health under basal conditions. Structural and mechanistic understanding of the interplay between phosphorylation and oligomerization in functional regulation of SWEETs remains limited. Using extensive molecular dynamics simulations coupled with Markov state models, we demonstrate the thermodynamic and kinetic effects of SWEET phosphorylation and oligomerization using OsSWEET2b as a model. We report that the beneficial effects of these SWEET regulatory mechanisms bias outward-facing states and improved extracellular gating, which complement published experimental findings. Our results offer molecular insights to SWEET regulation and may guide engineering strategies throughout the SWEET transport family.

Glucose uptake from the rhizosphere mediated by MdDOF3-MdHT1.2 regulates drought resistance in apple

In plants under drought stress, sugar content in roots increases, which is important for drought resistance. However, the molecular mechanisms for controlling the sugar content in roots during response to drought remain elusive. Here, we found that the MdDOF3-MdHT1.2 module-mediated glucose influx into the root is essential for drought resistance in apple (Malus × domestica). Drought induced glucose uptake from the rhizosphere and up-regulated the transcription of hexose transporter MdHT1.2. Compared with the wild-type plants, overexpression of MdHT1.2 promoted glucose uptake from the rhizosphere, thereby facilitating sugar accumulation in root and enhancing drought resistance, whereas silenced plants showed the opposite phenotype. Furthermore, ATAC-seq, RNA-seq and biochemical analysis demonstrated that MdDOF3 directly bound to the promoter of MdHT1.2 and was strongly up-regulated under drought. Overexpression of MdDOF3 in roots improved MdHT1.2-mediated glucose transport capacity and enhanced plant resistance to drought, but MdDOF3-RNAihr apple plants showed the opposite phenotype. Moreover, overexpression of MdDOF3 in roots did not attenuate drought sensitivity in MdHT1.2-RNAi plants, which was correlated with a lower glucose uptake capacity and glucose content in root. Collectively, our findings deciphered the molecular mechanism through which glucose uptake from the rhizosphere is mediated by MdDOF3-MdHT1.2, which acts to modulate sugar content in root and promote drought resistance.

Phosphorylation of sugar transporter TST2 by protein kinase CPK27 enhances drought tolerance in tomato

Drought is a major environmental stress threatening plant growth and productivity. Calcium-dependent protein kinases (CPKs) are plant-specific Ca2+ sensors with multifaceted roles in signaling drought responses. Nonetheless, the mechanisms underpinning how CPKs transmit downstream drought signaling remain unresolved. Through genetic investigations, our study unveiled that knocking out CPK27 reduced drought tolerance in tomato (Solanum lycopersicum) plants and impaired abscisic acid (ABA)-orchestrated plant response to drought stress. Proteomics and phosphoproteomics revealed that CPK27-dependent drought-induced proteins were highly associated with the sugar metabolism pathway, which was further verified by reduced soluble sugar content in the cpk27 mutant under drought conditions. Using protein-protein interaction assays and phosphorylation assessments, we demonstrated that CPK27 directly interacted with and phosphorylated tonoplast sugar transporter 2 (TST2), promoting intercellular soluble sugar accumulation during drought stress. Furthermore, Ca2+ and ABA enhanced CPK27-mediated interaction and phosphorylation of TST2, thus revealing a role of TST2 in tomato plant drought tolerance. These findings extend the toolbox of potential interventions for enhancing plant drought stress tolerance and provide a target to improve drought tolerance by manipulating CPK27-mediated soluble sugar accumulation for rendering drought tolerance in a changing climate.

Grape SnRK2.7 Positively Regulates Drought Tolerance in Transgenic Arabidopsis

In this study, we obtained and cloned VvSnRK2.7 by screening transcriptomic data to investigate the function of the grape sucrose non-fermenting kinase 2 (SnRK2) gene under stress conditions. A yeast two-hybrid (Y2H) assay was used to further screen for interaction proteins of VvSnRK2.7. Ultimately, VvSnRK2.7 was heterologously expressed in Arabidopsis thaliana, and the relative conductivity, MDA content, antioxidant enzyme activity, and sugar content of the transgenic plants were determined under drought treatment. In addition, the expression levels of VvSnRK2.7 in Arabidopsis were analyzed. The results showed that the VvSnRK2.7-EGFP fusion protein was mainly located in the cell membrane and nucleus of tobacco leaves. In addition, the VvSnRK2.7 protein had an interactive relationship with the VvbZIP protein during the Y2H assay. The expression levels of VvSnRK2.7 and the antioxidant enzyme activities and sugar contents of the transgenic lines were higher than those of the wild type under drought treatment. Moreover, the relative conductivity and MDA content were lower than those of the wild type. The results indicate that VvSnRK2.7 may activate the enzyme activity of the antioxidant enzyme system, maintain normal cellular physiological metabolism, stabilize the berry sugar metabolism pathway under drought stress, and promote sugar accumulation to improve plant resistance.

Phosphorylation of birch BpNAC90 improves the activation of gene expression to confer drought tolerance

The NAC transcription factors (TFs) play important roles in mediating abiotic stress tolerance; however, the mechanism is still not fully known. Here, an NAC gene (BpNAC90) from a gene regulatory network of Betula platyphylla (birch) that responded to drought was characterized. Overexpression and knockout of BpNAC90 displayed increased and reduced drought tolerance, respectively, relative to wild-type (WT) birch. BpNAC90 binds to different DNA motifs to regulate target genes in conferring drought tolerance, such as Eomes2, ABRE and Tgif2. BpNAC90 is phosphorylated by drought stress at Ser 205 by birch SNF1-related protein kinase 2 (BpSRK2A). Mutated BpNAC90 (termed S205A) with abolished phosphorylation, was transformed into birch for overexpression. The transgenic S205A plants displayed significantly reduced drought tolerance compared with plants overexpressing BpNAC90, but still showed increased drought tolerance relative to WT birch. At the same time, S205A showed a decreased capability to bind to motifs and reduced activation of target gene expression, which contributed to the reduced drought tolerance. Additionally, BpSRK2A and BpNAC90 can be induced by drought stress and form a complex to phosphorylate BpNAC90. The results together indicated that phosphorylation of BpNAC90 is necessary in conferring drought tolerance in birch.

Optimization of apple fruit flavor by MdVHP1-2 via modulation of soluble sugar and organic acid accumulation

For fleshy fruits, the content and ratio of organic acids and soluble sugars are key factors for their flavor. Therefore, a better understanding of soluble sugar and organic acid accumulation in vacuoles is essential to the improvement of fruit quality. Vacuolar-type inorganic pyrophosphatase (V-PPase) has been found in various plants with crucial functions based on the hydrolysis of PPi. However, the effects of V-PPase on the soluble sugar and organic acid accumulation in apple fruit remain unclear. In this study, MdVHP1-2, a V-PPase protein in the vacuolar membrane, was identified. The results showed a positive correlation between the expression of MdVHP1-2 and the sugar/acid ratio during ripening of apple fruits. A series of transgenic analyses showed that overexpression of MdVHP1-2 significantly elevated the contents of soluble sugars and organic acids as well as the sugar/acid ratio in apple fruits and calli. Additionally, transient interference induced by MdVHP1-2 expression inhibited the accumulation of soluble sugars and organic acids in apple fruits. In summary, this study provides insight into the mechanisms by which MdVHP1-2 modulates fruit flavor through mediation of soluble sugar and organic acid accumulation, thereby facilitating improvement of the overall quality of apple and other fruits.

Comparative Proteomic Analysis of Two Wild Soybean (Glycine soja) Genotypes Reveals Positive Regulation of Saline-Alkaline Stress Tolerance by Tonoplast Transporters

Soil saline-alkalization is a significant constraint for soybean production. Owing to higher genetic diversity of wild soybean, we compared the proteomic landscape of saline-alkaline stress-tolerant (SWBY032) and stress-sensitive (SWLJ092) wild soybean (Glycine soja) strains under saline and saline-alkaline stress. Out of 346 differentially expressed proteins (DEPs) specifically involved in saline-alkaline stress, 159 and 133 DEPs were identified in only SWLJ092 and SWBY032, respectively. Functional annotations revealed that more ribosome proteins were downregulated in SWLJ092, whereas more membrane transporters were upregulated in SWBY032. Moreover, protein-protein interaction analysis of 133 DEPs revealed that 14 protein-synthesis- and 2 TCA-cycle-related DEPs might alter saline-alkaline tolerance by affecting protein synthesis and amino acid metabolism. Furthermore, we confirmed G. soja tonoplast intrinsic protein (GsTIP2-1 and GsTIP2-2), inositol transporter (GsINT1), sucrose transport protein (GsSUC4), and autoinhibited Ca2+-ATPase (GsACA11) as tonoplast transporters can synergistically improve saline-alkaline tolerance in soybean, possibly by relieving the inhibition of protein synthesis and amino acid metabolism. Overall, our findings provided a foundation for molecular breeding of a saline-alkaline stress-tolerant soybean.

A straight-forward gene mining strategy to identify TaCIPK19 as a new regulator of drought tolerance in wheat

Drought stress is one of the most impactful abiotic stresses to global wheat production. Therefore, identifying key regulators such as the calcineurin B-like protein interacting protein kinase (CIPK) in the signaling cascades known to coordinate developmental cues and environmental stimuli represents a useful approach to improve drought tolerance. However, functional studies have been very limited partly due to the difficulties in prioritizing candidate genes from the large TaCIPK family. To address this issue, we demonstrate a straight-forward strategy by analyzing gene expression patterns in response to phytohormones or stresses and identified TaCIPK19 as a new regulator to improve drought tolerance. The effects of TaCIPK19 on drought tolerance were evaluated in both tobacco and wheat through transgenic approach. Ectopic expression of TaCIPK19 in tobacco greatly improves drought tolerance with enhanced ABA biosynthesis/signaling and ROS scavenging capacity. TaCIPK19 overexpression in wheat also confers the drought tolerance at both seedling and mature stages with enhanced ROS scavenging capacity. Additionally, potential CBL partners interacting with TaCIPK19 were investigated. Collectively, our finding exemplifies a straight-forward approach to facilitate reverse genetics related to abiotic stress improvement and demonstrates TaCIPK19 as a new candidate gene to improve ROS scavenging capacity and drought tolerance, which is useful for genetic improvement and breeding application in wheat.

The CBL-CIPK network is involved in the physiological crosstalk between plant growth and stress adaptation

Plants have evolved to deal with different stresses during plant growth, relying on complex interactions or crosstalk between multiple signalling pathways in plant cells. In this sophisticated regulatory network, Ca2+ transients in the cytosol ([Ca2+ ]cyt ) act as major physiological signals to initiate appropriate responses. The CALCINEURIN B-LIKE PROTEIN (CBL)-CBL-INTERACTING PROTEIN KINASE (CIPK) network relays physiological signals characterised by [Ca2+ ]cyt transients during plant development and in response to environmental changes. Many studies are aimed at elucidating the role of the CBL-CIPK network in plant growth and stress responses. This review discusses the involvement of the CBL-CIPK pathways in two levels of crosstalk between plant development and stress adaptation: direct crosstalk through interaction with regulatory proteins, and indirect crosstalk through adaptation of correlated physiological processes that affect both plant development and stress responses. This review thus provides novel insights into the physiological roles of the CBL-CIPK network in plant growth and stress adaptation.

Insights into the molecular mechanisms underlying responses of apple trees to abiotic stresses

Apple (Malus[Formula: see text]domestica) is a popular temperate fruit crop worldwide. However, its growth, productivity, and quality are often adversely affected by abiotic stresses such as drought, extreme temperature, and high salinity. Due to the long juvenile phase and highly heterozygous genome, the conventional breeding approaches for stress-tolerant cultivars are time-consuming and resource-intensive. These issues may be resolved by feasible molecular breeding techniques for apples, such as gene editing and marker-assisted selection. Therefore, it is necessary to acquire a more comprehensive comprehension of the molecular mechanisms underpinning apples' response to abiotic stress. In this review, we summarize the latest research progress in the molecular response of apples to abiotic stressors, including the gene expression regulation, protein modifications, and epigenetic modifications. We also provide updates on new approaches for improving apple abiotic stress tolerance, while discussing current challenges and future perspectives for apple molecular breeding.

The transcription factor CitZAT5 modifies sugar accumulation and hexose proportion in citrus fruit

Sugars are fundamental to plant developmental processes. For fruits, the accumulation and proportion of sugars play crucial roles in the development of quality and attractiveness. In citrus (Citrus reticulata Blanco.), we found that the difference in sweetness between mature fruits of "Gongchuan" and its bud sport "Youliang" is related to hexose contents. Expression of a SuS (sucrose synthase) gene CitSUS5 and a SWEET (sugars will eventually be exported transporter) gene CitSWEET6, characterized by transcriptome analysis at different developmental stages of these 2 varieties, revealed higher expression levels in "Youliang" fruit. The roles of CitSUS5 and CitSWEET6 were investigated by enzyme activity and transient assays. CitSUS5 promoted the cleavage of sucrose to hexoses, and CitSWEET6 was identified as a fructose transporter. Further investigation identified the transcription factor CitZAT5 (ZINC FINGER OF ARABIDOPSIS THALIANA) that contributes to sucrose metabolism and fructose transportation by positively regulating CitSUS5 and CitSWEET6. The role of CitZAT5 in fruit sugar accumulation and hexose proportion was investigated by homologous transient CitZAT5 overexpression, -VIGS, and -RNAi. CitZAT5 modulates the hexose proportion in citrus by mediating CitSUS5 and CitSWEET6 expression, and the molecular mechanism explained the differences in sugar composition of "Youliang" and "Gongchuan" fruit.

The TabHLH35-TaWAK20-TaSPL5 pathway positively regulates Cd stress in wheat

KEY MESSAGE

Cadmium-induced TaWAK20 regulates the cadmium stress response by phosphorylating TaSPL5 in wheat. Receptor-like kinases (RLKs) are thought to play important roles in responses to abiotic stresses in plants. In this study, we identified a cadmium (Cd)-induced RLK in wheat, TaWAK20, which is a positive regulator of the Cd stress response. TaWAK20 is specifically expressed in root tissue. Overexpression of TaWAK20 significantly improved the tolerance of Cd stress in wheat and decreased Cd accumulation in wheat plants by regulating reactive oxygen species production and scavenging. Yeast one-hybrid assays, electrophoretic mobility shift assays, and firefly luciferase activity analyses demonstrated that the TaWAK20 promoter was bound by the TabHLH35 transcription factor. TaWAK20 interacted with and phosphorylated squamosa promoter binding protein-like 5 (TaSPL5). Furthermore, phosphorylation of TaSPL5 increased its DNA-binding activity. In addition, Arabidopsis-expressing phosphorylated TaSPL5 exhibited greater Cd tolerance than Arabidopsis-expressing unphosphorylated TaSPL5. Taken together, these data identify a TabHLH35-TaWAK20-TaSPL5 module that regulates Cd stress.

1H NMR-Based Metabolic Profiling to Follow Changes in Pomelo Cultivars during Postharvest Senescence

This study investigated metabolite changes in three pomelo cultivars during postharvest senescence using ¹H NMR-based metabolic profiling. Three pomelo cultivars, 'Hongroumiyou', 'Bairoumiyou' and 'Huangroumiyou', abbreviated as "R", "W" and "Y" according to the color of their juice sacs, were stored at 25 °C for 90 days, and NMR was applied to determine the metabolite changes in juice sacs during storage. Fifteen metabolites were identified, including organic acids, sugars, amino acids, fatty acids, phenols and naringin. Partial least squares discriminant analysis (PLS-DA) was used to screen the significant metabolites according to the variable importance for the projection (VIP) scores in three pomelo cultivars during 90 days of storage. Additionally, eight metabolites, naringin, alanine, asparagine, choline, citric acid, malic acid, phosphocholine and β-D-glucose, were screened to be the crucial biomarkers with VIP > 1. The undesirable flavor of "bitter and sour" during the 60 days of storage was mainly attributed to the naringin, citric acid and sugars. According to the correlation analysis, the citric acid content determined by NMR showed a significantly positive relationship with that analyzed by HPLC. These findings suggested that NMR technology was accurate and efficient for metabolomic analysis of pomelo fruit, and the ¹H NMR-based metabolic profiling can be efficient during quality evaluation and useful for improving the fruit flavor quality during postharvest storage.

Identification and molecular characterization of novel sucrose transporters in the hexaploid wheat (Triticum aestivum L.)

Common wheat (Triticum aestivum) is a major cereal crop grown and consumed globally. Recent advances in sequencing technology have facilitated the exploration of large and repetitive genomes. Plant sucrose transporter (SUT) genes are vital components of energy transport systems that play prominent roles in various plant functions, such as signaling and stress regulation. In this study, we identified and analyzed five novel sucrose transporter genes in wheat. The wheat sucrose transporter genes were divided into five clades based on their phylogenetic relationships. Synteny analysis revealed that synteny in the genome is highly conserved between wheat and rye, barley, and Brachypodium. Furthermore, the cis-element analysis indicated that sucrose transporter genes might be regulated by light and some phytohormone-related transcriptional factors. Overall, plant tissue-specific gene expression revealed enhanced expression of the transporter genes in the root and stem, whereas they were differentially expressed under abiotic stress treatments (cold, heat, NaCl, PEG-6000, and sucrose). These results indicate that each TaSUT gene may play a crucial role in stabilizing plants under stress by actively regulating the energy demands of cells. The findings of this study may provide a basis for further research on sucrose transporters and their significant roles in plant energy metabolism as well as in abiotic stress response, signaling, and regulation.

Overexpression of the FERONIA receptor kinase MdMRLK2 enhances apple cold tolerance

Cold is one of the main abiotic stresses in temperate fruit crops, affecting the yield and fruit quality of apple in China and European countries. The plant receptor-like kinase FERONIA is widely reported to be involved in abiotic stresses. However, its function in apple cold resistance remains unknown. Modification of cell wall components and accumulation of soluble sugars and amino acids are important strategies by which plants cope with cold. In this study, expression of the apple FERONIA receptor-like kinase gene MdMRLK2 was rapidly induced by cold. Apple plants overexpressing MdMRLK2 (35S:MdMRLK2) showed enhanced cold resistance relative to the wild type. Under cold conditions, 35S:MdMRLK2 apple plants had higher amounts of water insoluble pectin, lignin, cellulose, and hemicellulose, which may have resulted from reduced activities of polygalacturonase, pectinate lyase, pectinesterase, and cellulase. More soluble sugars and free amino acids and less photosystem damage were also observed in 35S:MdMRLK2 apple plants. Intriguingly, MdMRLK2 interacted with the transcription factor MdMYBPA1 and promoted its binding to MdANS and MdUFGT promoters, leading to more anthocyanin biosynthesis, particularly under cold conditions. These findings complemented the function of apple FERONIA MdMRLK2 responding to cold resistance.

MdbZIP74 negatively regulates osmotic tolerance and adaptability to moderate drought conditions of apple plants

Drought is the most prominent threat to global agricultural production. The basic leucine zipper (bZIP) family is related to the response to a series of abiotic stress. In this case, apple calli and the seedlings of MdbZIP74-RNAi transgenic lines were obtained. Under osmotic stress and moderate drought conditions, the content of malondialdehyde, relative water content and other stress-related assays were measured. MdbZIP74 was found to negatively regulate the osmotic tolerance of apple callus. The growth of MdbZIP74-RNAi calli enhanced resistance without significant production loss. The silencing of MdbZIP74 contributes to redox balance and the adaptability of apple seedlings to moderate drought conditions. Four related differentially expressed genes in the biosynthesis of cytokinin and catabolic pathway were identified through a transcriptome analysis of MdbZIP74-RNAi seedlings under moderate drought conditions. MdLOG8 was further identified as the target of MdbZIP74 involved in the drought adaptability of apple plants using a dual experiment. Further confirmation showed MdLOG8 was maintained in the MdbZIP74-RNAi seedlings presumably acting as the growth regulator to enhance drought adaptability. It was concluded that the correct regulation of cytokinin level under moderate drought conditions maintains the redox balance and avoids the situation of plants surviving with the minimal resources.

Physicochemical properties and transcriptional changes underlying the quality of 'Gala' apples (Malus × domestica Borkh.) under atmosphere manipulation in long-term storage

BACKGROUND

The year-round availability of apples (Malus × domestica Borkh.) depends on post-harvest technologies, which are essential for the retention of fruit sensory and chemical properties by delaying senescence. The effectiveness of strategies for preserving the quality of apples depends on complex interactions between the storage environment and endogenous biological factors. In the current work, we integrated instrumental, sensory, and transcriptional data to determine the role of conservation technologies cold storage, controlled atmosphere, and 1-methylcyclopropene-mediated ethylene blockage on the long-term conservation of apples.

RESULTS

The results demonstrated that inhibition of the consumer's perception of the apples' ethylene content is essential for long-term cold storage, and such quality conservation can be achieved by reducing oxygen pressure. Overall appreciation of apples after storage was determined mainly by their texture, with crispness and juiciness contributing favorably, and mealiness contributing negatively. Reduced oxygen pressure and inhibition of ethylene perception exerted distinct effects on the transcription of candidate genes associated with ripening in apple. Hexose and cell-wall carbohydrate metabolism genes exhibit distinct expression patterns under storage.

CONCLUSION

Inhibition of ethylene perception and reduction of relative oxygen pressure under cold storage both promote similar conservation of apple sensory traits under long-term cold storage. Texture was the main contributor to global appreciation of apples subjected to long-term storage. The conditions that were investigated were able to delay, but not fully prevent, senescence, as evidenced by physicochemical and gene expression analyses. The expression of gene-encoding enzymes involved in hexose metabolism was mainly developmentally regulated, whereas storage conditions exerted a stronger effect on the expression of genes associated with cell-wall metabolism. © 2022 Society of Chemical Industry.

Halophyte Nitraria billardieri CIPK25 promotes photosynthesis in Arabidopsis under salt stress

The calcineurin B-like (CBL)-interacting protein kinases (CIPKs), a type of plant-specific genes in the calcium signaling pathway, function in response to adverse environments. However, few halophyte derived CIPKs have been studied for their role in plant physiological and developmental adaptation during abiotic stresses, which inhibits the potential application of these genes to improve environmental adaptability of glycophytes. In this study, we constructed Nitraria billardieri CIPK25 overexpressing Arabidopsis and analyzed the seedling development under salt treatment. Our results show that Arabidopsis with NbCIPK25 expression exhibits more vigorous growth than wild type plants under salt condition. To gain insight into the molecular mechanisms underlying salt tolerance, we profiled the transcriptome of WT and transgenic plants via RNA-seq. GO and KEGG analyses revealed that upregulated genes in NbCIPK25 overexpressing seedlings under salt stress are enriched in photosynthesis related terms; Calvin-cycle genes including glyceraldehyde-3-phosphate dehydrogenases (GAPDHs) are significantly upregulated in transgenic plants, which is consistent with a decreased level of NADPH (GAPDH substrate) and increased level of NADP⁺. Accordingly, NbCIPK25 overexpressing plants exhibited more efficient photosynthesis; soluble sugar and proteins, as photosynthesis products, showed a higher accumulation in transgenic plants. These results provide molecular insight into how NbCIPK25 promotes the expression of genes involved in photosynthesis, thereby maintaining plant growth under salt stress. Our finding supports the potential application of halophyte-derived NbCIPK25 in genetic modification for better salt adaptation.

Evolution of the CBL and CIPK gene families in Medicago: genome-wide characterization, pervasive duplication, and expression pattern under salt and drought stress

BACKGROUND

Calcineurin B-like proteins (CBLs) are ubiquitous Ca²⁺ sensors that mediate plant responses to various stress and developmental processes by interacting with CBL-interacting protein kinases (CIPKs). CBLs and CIPKs play essential roles in acclimatization of crop plants. However, evolution of these two gene families in the genus Medicago is poorly understood.

RESULTS

A total of 68 CBL and 135 CIPK genes have been identified in five genomes from Medicago. Among these genomes, the gene number of CBLs and CIPKs shows no significant difference at the haploid genome level. Phylogenetic and comprehensive characteristic analyses reveal that CBLs and CIPKs are classified into four clades respectively, which is validated by distribution of conserved motifs. The synteny analysis indicates that the whole genome duplication events (WGDs) have contributed to the expansion of both families. Expression analysis demonstrates that two MsCBLs and three MsCIPKs are specifically expressed in roots, mature leaves, developing flowers and nitrogen fixing nodules of Medicago sativa spp. sativa, the widely grown tetraploid species. In particular, the expression of these five genes was highly up-regulated in roots when exposed to salt and drought stress, indicating crucial roles in stress responses.

CONCLUSIONS

Our study leads to a comprehensive understanding of evolution of CBL and CIPK gene families in Medicago, but also provides a rich resource to further address the functions of CBL-CIPK complexes in cultivated species and their closely related wild relatives.

MdTCP46 interacts with MdABI5 to negatively regulate ABA signalling and drought response in apple

TEOSINTE BRANCHED 1/CYCLOIDEA/PCF (TCP) transcription factors play crucial roles in plant abiotic stresses. However, little is known about the role of TCP genes in the drought stress tolerance of apple. Here, we found that abscisic acid (ABA) and drought treatment reduced the expression of MdTCP46, and overexpression of MdTCP46 reduced ABA sensitivity and drought stress resistance. MdTCP46 was found to interact with MdABI5 both in vitro and in vivo, and this interaction was essential for drought resistance via the ABA-dependent pathway. Overexpression of MdABI5 enhanced ABA sensitivity and drought stress resistance by directly activating the expression of MdEM6 and MdRD29A. MdTCP46 significantly suppressed the transcriptional activity of MdABI5, thereby negatively regulating MdABI5-mediated ABA signalling and drought response. Overall, our results demonstrate that the MdTCP46-MdABI5-MdEM6/MdRD29A regulatory module plays a key role in the modulation of ABA signalling and the drought stress response. These findings provide new insight into the role of MdTCP46 in ABA signalling and abiotic stress responses.

Identification and characteristics of SnRK genes and cold stress-induced expression profiles in Liriodendron chinense

Background

The sucrose non-fermenting 1 (SNF1)-related protein kinases (SnRKs) play a vivid role in regulating plant metabolism and stress response, providing a pathway for regulation between metabolism and stress signals. Conducting identification and stress response studies on SnRKs in plants contributes to the development of strategies for tree species that are more tolerant to stress conditions.

Results

In the present study, a total of 30 LcSnRKs were identified in Liriodendron chinense (L. chinense) genome, which was distributed across 15 chromosomes and 4 scaffolds. It could be divided into three subfamilies: SnRK1, SnRK2, and SnRK3 based on phylogenetic analysis and domain types. The LcSnRK of the three subfamilies shared the same Ser/Thr kinase structure in gene structure and motif composition, while the functional domains, except for the kinase domain, showed significant differences. A total of 13 collinear gene pairs were detected in L. chinense and Arabidopsis thaliana (A. thaliana), and 18 pairs were detected in L. chinense and rice, suggesting that the LcSnRK family genes may be evolutionarily more closely related to rice. Cis-regulation element analysis showed that LcSnRKs were LTR and TC-rich, which could respond to different environmental stresses. Furthermore, the expression patterns of LcSnRKs are different at different times under low-temperature stress. LcSnRK1s expression tended to be down-regulated under low-temperature stress. The expression of LcSnRK2s tended to be up-regulated under low-temperature stress. The expression trend of LcSnRK3s under low-temperature stress was mainly up-or down-regulated.

Conclusion

The results of this study will provide valuable information for the functional identification of the LcSnRK gene in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08902-0.

Changes and response mechanism of sugar and organic acids in fruits under water deficit stress

The content and the ratio of soluble sugars and organic acids in fruits are significant indicators for fruit quality. They are affected by multiple environmental factors, in which water-deficient is the most concern. Previous studies found that the content of soluble sugars and organic acids in fruit displayed great differences under varied water stress. It is important to clarify the mechanism of such difference and to provide researchers with systematic knowledge about the response to drought stress and the mechanism of sugar and acid changes in fruits, so that they can better carry out the study of fruit quality under drought stress. Therefore, the researchers studied dozens of research articles about the content of soluble sugar and organic acid, the activity of related metabolic enzymes, and the expression of related metabolic genes in fruits under water stress, and the stress response of plants to water stress. We found that after plants perceived and transmitted the signal of water deficit, the expression of genes related to the metabolism of soluble sugars and organic acids changed. It was then affected the synthesis of metabolic enzymes and changed their metabolic rate, ultimately leading to changes in soluble sugar and organic acid content. Based on the literature review, we described the pathway diagrams of sugar metabolism, organic acid metabolism, mainly malic acid, tartaric acid, and citric acid metabolism, and of the response to drought stress. From many aspects including plants' perception of water stress signal, signal conversion and transmission, induced gene expression, the changes in soluble sugar and the enzyme activities of organic acids, as well as the final sugar and acid content in fruits, this thesis summarized previous studies on the influence of water stress on soluble sugars and the metabolism of organic acids in fruits.

GmCIPK21, a CBL-interacting protein kinase confers salt tolerance in soybean (Glycine max. L)

Salt stress severely affects plant development and yield. Calcineurin B-like protein interacting protein kinases (CIPKs) play a crucial role in plant adaptation to environmental challenges. However, the biological functions of CIPKs in soybean remain poorly understood. Here, we identified GmCIPK21, a salt-responsive CIPK gene from soybean. Overexpression of GmCIPK21 in Arabidopsis and soybean hairy roots led to increased salt tolerance. The hairy roots with GmCIPK21 suppression by RNA interference exhibited salt-sensitive phenotypes. Further physiological analysis revealed that GmCIPK21 reduced the content of hydrogen peroxide (H2O2) and malondialdehyde (MDA) and increased the activity of the antioxidant enzymes under salt stress. Additionally, GmCIPK21 was found to enhance the ABA sensitivity of transgenic plants. GmCIPK21 was also implicated in increasing the activation of antioxidant-, salt-, and ABA-related genes upon salt stress. Interestingly, GmCIPK21 interacted with GmCBL4, promoting the scavenging salt-induced reactive oxygen species (ROS). These results collectively suggested that GmCIPK21 affects ROS homeostasis and ABA response to improve salt tolerance in soybean.

Resource Translocation Modelling Highlights Density-Dependence Effects in Fruit Production at Various Levels of Organisation

The size of fruit cells, seeds and fruits depends on their number. Could this density-dependence effect result from sugar resource sharing and, if so, does it involve phloem sugar flow or the intensity of sugar unloading to the sink? A density-dependence model (DDM) describing these processes was designed and parameterised for six species at five levels of organisation: cells and seeds within fruits, fruits within clusters, fruits within plants and plants within plots. Sugar flow was driven by phloem conductance, determined by parameters α, governing the shape of its relationship to population size, and κ, its value for a population size of one. Sugar unloading followed Michaelis-Menten kinetics with parameters V_m (maximal unloading rate) and K_m (Michaelis constant). The DDM effectively reproduced the observed individual mass dynamics, the undercompensating density dependence observed in most species at all sub-plant levels and the undercompensating, exact and overcompensating density dependence observed at the plant level. Conductance (κ) was a scaling factor varying with the level of organisation. V_m was positively correlated with density dependence, and α was negatively correlated with density dependence only if the plant-within-plot level was not considered. Analysis of the model's behaviour indicates that density dependence of fruit growth could be a result of sugar sharing, and that both phloem sugar flow and sugar unloading contribute to these effects.

Alkali salt stress causes fast leaf apoplastic alkalinization together with shifts in ion and metabolite composition and transcription of key genes during the early adaptive response of Vicia faba L

The mechanisms by which plants respond to alkali salt stress are still obscure, and the relevance of alkaline pH under combined alkali salt stress. Early stress responses can indicate mechanisms leading to damage and plant resistance. The apoplast contains essential determinants for plant growth, specifically early apoplastic pH fluctuations are induced by many stressors and hypothesized to be involved in stress signalling. Hence, this study aims to identify fast responses specific to alkaline pH and alkali salt stress by exposing the root of hydroponically grown Vicia faba L. plants to 150 min of either 50 mM NaHCO₃ (pH 9) treatment or alkaline pH 9 alone. Apoplastic pH was monitored in real-time by ratiometric fluorescence microscopy simultaneously with SWIR transmission-based measurements of leaf water content (LWC). Moreover, we examined the effect of these stresses on apoplastic, symplastic and xylem ion and metabolite composition together with transcriptions of certain stress-responsive genes. Physiological and transcriptional changes were observed in response to NaHCO₃ but not to alkaline pH alone. NaHCO₃ elicited a transient reduction in LWC, followed by a transient alkalinization of the apoplast and stomatal closure. Simultaneously, organic acids and sugars accumulated. Fast upregulation of stress-responsive genes showed the significance of gene regulation for early plant adaptation to alkali salt stress.

Phloem Loading and Unloading of Sucrose: What a Long, Strange Trip from Source to Sink

Sucrose is transported from sources (mature leaves) to sinks (importing tissues such as roots, stems, fruits, and seeds) through the phloem tissues in veins. In many herbaceous crop species, sucrose must first be effluxed to the cell wall by a sugar transporter of the SWEET family prior to being taken up into phloem companion cells or sieve elements by a different sugar transporter, called SUT or SUC. The import of sucrose into these cells is termed apoplasmic phloem loading. In sinks, sucrose can similarly exit the phloem apoplasmically or, alternatively, symplasmically through plasmodesmata into connecting parenchyma storage cells. Recent advances describing the regulation and manipulation of sugar transporter expression and activities provide stimulating new insights into sucrose phloem loading in sources and unloading processes in sink tissues. Additionally, new breakthroughs have revealed distinct subpopulations of cells in leaves with different functions pertaining to phloem loading. These and other discoveries in sucrose transport are discussed.

The SUMO E3 Ligase MdSIZ1 Sumoylates a Cell Number Regulator MdCNR8 to Control Organ Size

Plant growth and organ size putatively associated with crop yield are regulated by a complex network of genes including ones for controlling cell proliferation. The gene fw2.2 was first identified in tomatoes and reported to govern fruit size variation through controlling cell division. In this study, we isolated a putative ortholog of the tomato fw2.2 gene from apple, Cell Number Regulator 8 (MdCNR8). Our functional analysis showed that MdCNR8 may control fruit size and root growth. MdCNR8 was mediated by the SUMO E3 ligase MdSIZ1, and SUMOylation of MdCNR8 at residue-Lys39 promoted the translocation of MdCNR8 from plasma membrane to the nucleus. The effect of MdCNR8 in inhibiting root elongation could be completely counteracted by the coexpression of MdSIZ1. Moreover, the lower cell proliferation of apple calli due to silencing MdSIZ1 could be rescued by silencing MdCNR8. Collectively, our results showed that the MdSIZ1-mediated SUMOylation is required for the fulfillment of MdCNR8 in regulating cell proliferation to control plant organ size. This regulatory interaction between MdSIZ1 and MdCNR8 will facilitate understanding the mechanism underlying the regulation of organ size.

Coordination Between Phloem Loading and Structure Maintains Carbon Transport Under Drought

Maintaining phloem transport under water stress is expected to be crucial to whole-plant drought tolerance, but the traits that benefit phloem function under drought are poorly understood. Nearly half of surveyed angiosperm species, including important crops, use sucrose transporter proteins to actively load sugar into the phloem. Plants can alter transporter abundance in response to stress, providing a potential mechanism for active-loading species to closely regulate phloem loading rates to avoid drought-induced reductions or failures in phloem transport. We developed an integrated xylem-phloem-stomatal model to test this hypothesis by quantifying the joint impacts of transporter kinetics, phloem anatomy, and plant water status on sucrose export to sinks. We parameterized the model with phloem hydraulic resistances and sucrose transporter kinetic parameters compiled from the literature, and simulated loading regulation by allowing loading rates to decline exponentially with phloem pressure to prevent excessive sucrose concentrations from inducing viscosity limitations. In the absence of loading regulation, where loading rates were independent of phloem pressure, most resistance values produced unrealistic phloem pressures owing to viscosity effects, even under well-watered conditions. Conversely, pressure-regulated loading helped to control viscosity buildup and improved export to sinks for both lower and higher resistant phloem pathways, while maintaining realistic phloem pressures. Regulation also allowed for rapid loading and export in wet conditions while maintaining export and viable phloem pressures during drought. Therefore, we expect feedbacks between phloem pressure and loading to be critical to carbon transport in active-loading species, especially under drought, and for transporter kinetics to be strongly coordinated with phloem architecture and plant water status. This work provides an important and underexplored physiological framework to understand the ecophysiology of phloem transport under drought and to enhance the genetic engineering of crop plants.

Zinc-finger protein MdBBX7/MdCOL9, a target of MdMIEL1 E3 ligase, confers drought tolerance in apple

Water deficit is one of the main challenges for apple (Malus × domestica) growth and productivity. Breeding drought-tolerant cultivars depends on a thorough understanding of the drought responses of apple trees. Here, we identified the zinc-finger protein B-BOX 7/CONSTANS-LIKE 9 (MdBBX7/MdCOL9), which plays a positive role in apple drought tolerance. The overexpression of MdBBX7 enhanced drought tolerance, whereas knocking down MdBBX7 expression reduced it. Chromatin immunoprecipitation-sequencing (ChIP-seq) analysis identified one cis-element of MdBBX7, CCTTG, as well as its known binding motif, the T/G box. ChIP-seq and RNA-seq identified 1,197 direct targets of MdBBX7, including ETHYLENE RESPONSE FACTOR (ERF1), EARLY RESPONSIVE TO DEHYDRATION 15 (ERD15), and GOLDEN2-LIKE 1 (GLK1) and these were further verified by ChIP-qPCR and electronic mobility shift assays. Yeast two-hybrid screen identified an interacting protein of MdBBX7, RING-type E3 ligase MYB30-INTERACTING E3 LIGASE 1 (MIEL1). Further examination revealed that MdMIEL1 could mediate the ubiquitination and degradation of MdBBX7 by the 26S proteasome pathway. Genetic interaction analysis suggested that MdMIEL1 acts as an upstream factor of MdBBX7. In addition, MdMIEL1 was a negative regulator of the apple drought stress response. Taken together, our results illustrate the molecular mechanisms by which the MdMIEL1-MdBBX7 module influences the response of apple to drought stress.

Sugar Metabolism and Transcriptome Analysis Reveal Key Sugar Transporters during Camellia oleifera Fruit Development

Camellia oleifera is a widely planted woody oil crop with economic significance because it does not occupy cultivated land. The sugar-derived acetyl-CoA is the basic building block in fatty acid synthesis and oil synthesis in C. oleifera fruit; however, sugar metabolism in this species is uncharacterized. Herein, the changes in sugar content and metabolic enzyme activity and the transcriptomic changes during C. oleifera fruit development were determined in four developmental stages (CR6: young fruit formation; CR7: expansion; CR9: oil transformation; CR10: ripening). CR7 was the key period of sugar metabolism since it had the highest amount of soluble sugar, sucrose, and glucose with a high expression of genes related to sugar transport (four sucrose transporters (SUTs) or and one SWEET-like gene, also known as a sugar, will eventually be exported transporters) and metabolism. The significant positive correlation between their expression and sucrose content suggests that they may be the key genes responsible for sucrose transport and content maintenance. Significantly differentially expressed genes enriched in the starch and sucrose metabolism pathway were observed in the CR6 versus CR10 stages according to KEGG annotation. The 26 enriched candidate genes related to sucrose metabolism provide a molecular basis for further sugar metabolism studies in C. oleifera fruit.

Xylella fastidiosa and Drought Stress in Olive Trees: A Complex Relationship Mediated by Soluble Sugars

Simple Summary

Carbohydrates play important roles in tolerance to both biotic and abiotic stressors. Xylella fastidiosa, the causal agent of “Olive Quick Decline Syndrome”, is a quarantine pathogen that induces drought stress in the host, aggravated by eventual water shortage, which is a frequent environmental condition in Mediterranean olive groves. At present, the resistance mechanisms shown by few resistant olive cultivars (e.g., cv Leccino) are not completely known; therefore, the aim of this research is to understand whether sugar metabolism is involved in the cross-talk mechanisms of biotic and abiotic responses. The results show that drought stress response induces effects beneficial to resistance of Xylella fastidiosa in cv Leccino. In the current context of global climate change, this study supports the importance of investigating the complex drought–disease interaction to detect resistance traits and thus find ways to counter the threat of this pathogen in the future.

Abstract

Xylella fastidiosa (Xf) subsp. pauca “De Donno” is the etiological agent of “Olive Quick Decline Syndrome” (OQDS) on olive trees (Olea europaea L.); the presence of the bacterium causes xylem vessel occlusions inducing a drought stress and the development of leaf scorch symptoms, which may be worsened by water shortage in summer. In order to evaluate how the two stress factors overlap each other, the carbohydrate content and the expression patterns of genes related to carbohydrate metabolism have been evaluated in two olive cvs trees (Cellina di Nardò, susceptible to Xf, and Leccino, resistant to Xf) reporting transcriptional dynamics elicited by Xf infection, drought, or combined stress (drought/Xf). In the Xf-susceptible Cellina di Nardò plants, Xf and its combination with drought significantly decrease total sugars compared to control (−27.0% and −25.7%, respectively). In contrast, the Xf-resistant Leccino plants show a more limited reduction in sugar content in Xf-positive conditions (−20.1%) and combined stresses (−11.1%). Furthermore, while the amount of glucose decreases significantly in stressed Cellina di Nardò plants (≈18%), an increase was observed in Leccino plants under drought/Xf combined stresses (+11.2%). An opposite behavior among cvs was also observed for sucrose, as an accumulation of the disaccharide was recorded in stressed Leccino plants (≈37%). The different response to combined stress by Xf-resistant plants was confirmed considering genes coding for the sucrose or monosaccharide transporter (OeSUT1, OeMST2), the cell wall or vacuolar invertase (OeINV-CW, OeINV-V), the granule-bound starch synthase I (OeGBSSI) and sucrose synthase (OeSUSY), with a higher expression than at least one single stress (e.g., ≈1-fold higher or more than Xf for OeMST2, OeINV-CW, OeINV-V, OeGBSSI). It is probable that the pathways involved in drought stress response induce positive effects useful for pathogen resistance in cv Leccino, confirming the importance of investigating the mechanisms of cross-talk of biotic and abiotic responses.

Genome-wide identification and expression analysis of sucrose nonfermenting-1-related protein kinase (SnRK) genes in Triticum aestivum in response to abiotic stress

The SnRK gene family is a key regulator that plays an important role in plant stress response by phosphorylating the target protein to regulate subsequent signaling pathways. This study was aimed to perform a genome-wide analysis of the SnRK gene family in wheat and the expression profiling of SnRKs in response to abiotic stresses. An in silico analysis identified 174 SnRK genes, which were then categorized into three subgroups (SnRK1/2/3) on the basis of phylogenetic analyses and domain types. The gene intron-exon structure and protein-motif composition of SnRKs were similar within each subgroup but different amongst the groups. Gene duplication and synteny between the wheat and Arabidopsis genomes was also investigated in order to get insight into the evolutionary aspects of the TaSnRK family genes. The result of cis-acting element analysis showed that there were abundant stress- and hormone-related cis-elements in the promoter regions of 129 SnRK genes. Furthermore, quantitative real-time PCR data revealed that heat, salt and drought treatments enhanced TaSnRK2.11 expression, suggesting that it might be a candidate gene for abiotic stress tolerance. We also identified eight microRNAs targeting 16 TaSnRK genes which are playing important role across abiotic stresses and regulation in different pathways. These findings will aid in the functional characterization of TaSnRK genes for further research.

Ectopic Expression of VvSUC27 Induces Stenospermocarpy and Sugar Accumulation in Tomato Fruits

Seedless fruits are favorable in the market because of their ease of manipulation. Sucrose transporters (SUTs or SUCs) are essential for carbohydrate metabolism in plants. Whether SUTs participate directly in causing stenospermocarpy, thereby increasing fruit quality, remains unclear. Three SUTs, namely, VvSUC11, VvSUC12, and VvSUC27 from Vitis vinifera, were characterized and ectopic expression in tomatoes. VvSUC11- and VvSUC12-overexpressing lines had similar flower and fruit phenotypes compared with those of the wild type. VvSUC27-overexpressing lines produced longer petals and pistils, an abnormal stigma, much less and shrunken pollen, and firmer seedless fruits. Moreover, produced fruits from all VvSUC-overexpressing lines had a higher soluble solid content and sugar concentration. Transcriptomic analysis revealed more genes associated with carbohydrate metabolism and sugar transport and showed downregulation of auxin- and ethylene-related signaling pathways during early fruit development in VvSUC27-overexpressing lines relative to that of the wild type. Our findings demonstrated that stenospermocarpy can be induced by overexpression of VvSUC27 through a consequential reduction in nutrient delivery to pollen at anthesis, with a subsequent downregulation of the genes involved in carbohydrate metabolism and hormone signaling. These commercially desirable results provide a new strategy for bioengineering stenospermocarpy in tomatoes and in other fruit plants.

The soybean CBL-interacting protein kinase, GmCIPK2, positively regulates drought tolerance and ABA signaling

Calcineurin B-like protein (CBL) and CBL-interacting protein kinase (CIPK) play important roles in plant environmental stress responses. However, the biological functions of the CBL-CIPK signaling pathway in the tolerance of soybean (Glycine max) to drought stress remain elusive. Here, we characterized the GmCIPK2 gene in soybean, and its expression was induced by drought stress and exogenous abscisic acid (ABA) treatments. The overexpression of GmCIPK2 enhanced drought tolerance in transgenic Arabidopsis and soybean hairy roots, whereas downregulation of GmCIPK2 expression in soybean hairy roots by RNA interference resulted in increased drought sensitivity. Further analysis showed that GmCIPK2 was involved in ABA-mediated stomatal closure in plants under drought stress conditions. GmCIPK2 increased the expression of ABA- and drought-responsive genes during drought stress. Additionally, yeast two-hybrid, pull-down, and bimolecular fluorescence complementation assays demonstrated that a positive regulator of drought stress, GmCBL1, physically interacted with GmCIPK2 on the plasma membrane. Collectively, our results demonstrated that GmCIPK2 positively regulates drought tolerance and ABA signaling in plants, providing new insights into the underlying mechanisms of how the CBL-CIPK signaling pathway contributes to drought tolerance in soybean.

Cold Stress in Citrus: A Molecular, Physiological and Biochemical Perspective

Due to climate change, we are forced to face new abiotic stress challenges like cold and heat waves that currently result from global warming. Losses due to frost and low temperatures force us to better understand the physiological, hormonal, and molecular mechanisms of response to such stress to face losses, especially in tropical and subtropical crops like citrus fruit, which are well adapted to certain weather conditions. Many of the responses to cold stress that are found are also conserved in citrus. Hence, this review also intends to show the latest work on citrus. In addition to basic research, there is a great need to employ and cultivate new citrus rootstocks to better adapt to environmental conditions.