Cell wall modification by the xyloglucan endotransglucosylase/hydrolase XTH19 influences freezing tolerance after cold and sub-zero acclimation

New insights into plant cell wall functions

The plant cell wall is an extremely complicated natural nanoscale structure composed of cellulose microfibrils embedded in a matrix of noncellulosic polysaccharides, further reinforced by the phenolic compound lignins in some cell types. Such network formed by the interactions of multiscale polymers actually reflects functional form of cell wall to meet the requirements of plant cell functionalization. Therefore, how plants assemble cell wall functional structure is fundamental in plant biology and critical for crop trait formation and domestication as well. Due to the lack of effective analytical techniques to characterize this fundamental but complex network, it remains difficult to establish direct links between cell-wall genes and phenotypes. The roles of plant cell walls are often underestimated as indirect. Over the past decades, many genes involved in cell wall biosynthesis, modification, and remodeling have been identified. The application of a variety of state-of-the-art techniques has made it possible to reveal the fine cell wall networks and polymer interactions. Hence, many exciting advances in cell wall biology have been achieved in recent years. This review provides an updated overview of the mechanistic and conceptual insights in cell wall functionality, and prospects the opportunities and challenges in this field.

Genome-Wide Characterization of Gibberellin Oxidase Genes (GbGAoxs) and Illustration of Their Molecular Responses to Exogenous GA3 in Gossypium barbadense

As key enzymes in the gibberellin (GA) biosynthesis pathway, GAoxs function as regulators of bioactive GA levels and plant architecture, yet little is understood about GAoxs in Gossypium. In this study, 78 GAox genes identified in four cotton species were divided into three subgroups: GA2ox, GA3ox, and GA20ox. Syntenic relationships of GAoxs in Gossypium suggested that divergencies in gene function may be attributed to whole-genome duplication during evolution. Cis-acting element analysis suggested that the GbGAox genes might participate in plant growth, development, and hormone responses. Moreover, transcriptome analysis was performed to characterize the molecular response of the exogenous GA3 application. It was found that DEGs (differentially expressed genes) are widely involved in cell division and cell wall modification, in which the most XTH (xyloglucan endotransglucosylase/hydrolase) and GAox genes responded actively to the exogenous GA3 treatment. Some transcription factors and protein kinases cooperated with those GbGAoxs in response to GA3. These findings underlie the biological function of GAox genes and their responses to GA3 in regulating plant growth in Gossypium barbadense.

The banana MaFLA27 confers cold tolerance partially through modulating cell wall remodeling

Fasciclin-like arabinogalactan proteins (FLAs) have been shown to improve plant tolerance to salt stress. However, their role in cold tolerance (CT) remains unclear. Here, we report that banana MaFLA27 positively regulates CT in Arabidopsis. MaFLA27-overexpression (OE) caused the upregulation of differentially expressed arabinogalactan proteins (AGPs) and genes involved in the biosynthesis of cellulose, lignin, and xylan, as well as the degradation of pectin and xyloglucan. Correspondingly, MaFLA27-OE plants exhibited increased cell wall thickness, enhanced cellulose lignin and starch granule content, elevated levels of partially homogalacturonans recognized by JIM5 and JIM7 antibodies, xyloglucan components recognized by CCRC-M39/104 and LM15 antibodies, LM14 antibody binding AGPs. In contrast, transgenic plants showed a decreased degree of pectin methyl-esterification and accumulated less reactive oxygen species after cold acclimation when compared to wild-type plants. A higher number of pectin methylesterases and cellulose and xylan biosynthesis genes were elevated after cold acclimation. Additionally, both Arabidopsis mutant cesa8 and cellulose inhibitor-treated plants displayed decreased freezing tolerance. Our data suggested that MaFLA27-OE in Arabidopsis may perceive and transmit low-temperature stress signals to the cellulose synthase complexes, activating cellulose synthesis and enhancing cold tolerance. These findings reveal a previously unreported cold-tolerance function of FLAs and highlight associated cell wall-mediated tolerance mechanisms.

Genome-Wide Identification of Xyloglucan Endotransglucosylase/Hydrolase Multigene Family in Chinese Jujube (Ziziphus jujuba) and Their Expression Patterns Under Different Environmental Stresses

The Xyloglucan endotransglucosylase/hydrolase (XTH) family, a group of cell wall-modifying enzymes, plays crucial roles in plant growth, development, and stress adaptation. The quality and yield of Chinese jujube (Ziziphus jujuba) fruit are significantly impacted by environmental stresses, including excessive salinity, drought, freezing, and disease. However, there has been no report of the XTH encoding genes present in the Chinese jujube genome and their response transcription level under various stresses. This study provides an in-depth analysis of ZjXTH genes in the genome of Chinese jujube and elucidates their structural motifs, regulatory networks, and expression patterns under various stresses. A total of 29 ZjXTH genes were identified from the Ziziphus jujuba genome. Phylogenetic analysis classifies ZjXTH genes into four distinct groups, while conserved motifs and domain analyses reveal coordinated xyloglucan modifications, highlighting key shared motifs and domains. Interaction network predictions suggest that ZjXTHs may interact with proteins such as Expansin-B1 (EXPB1) and Pectin Methylesterase 22 (PME22). Additionally, cis-regulatory element analysis enhances our understanding of Chinese jujube plant's defensive systems, where TCA- and TGACG-motifs process environmental cues and orchestrate stress responses. Expression profiling revealed that ZjXTH1 and ZjXTH5 were significantly upregulated under salt, drought, freezing, and phytoplasma infection, indicating their involvement in biotic and abiotic stress responses. Collectively, these findings deepen our understanding of the functional roles of Chinese jujube XTHs, emphasizing their regulatory function in adaptive responses in Chinese jujube plants.

Evaluation of Cold Resistance in Alfalfa Varieties Based on Root Traits and Winter Survival in Horqin Sandy Land

The Horqin Sandy Land in China is a key alfalfa production base, challenged by low winter temperatures and large diurnal temperature shifts, affecting alfalfa's winter survival. Alfalfa roots are the primary organs responsible for winter adaptability; consequently, by investigating the changes in the root physiology and nutritional components of alfalfa during the overwintering period, we can enhance our understanding of its mechanisms for cold resistance. Over the course of two years (2022-2023), field trials were conducted on 40 alfalfa varieties selected from both domestic and international sources for their potential cold resistance. This study assessed winter survival rates and analyzed root contents, including soluble sugars, starch, soluble proteins, and the concentrations of carbon (C), nitrogen (N), phosphorus (P) and their stoichiometric ratios. Principal component analysis, subordinate function analysis, and cluster analysis were employed for comprehensive evaluation. Biochemical markers varied significantly across varieties. The C, N, and starch contents in the roots were the main factors determining cold resistance. The varieties were categorized into four groups: Category I included five highly resistant varieties ('Baimu 202', 'WL168HQ', 'Zhongmu No. 1', 'Gongnong No. 1', and 'Legacy'); Category II consisted of six moderately resistant varieties; Category III included twenty-eight slightly resistant varieties; and Category IV contained one non-resistant variety ('3010'). This study recommends the adoption of the five varieties in Category I to enhance alfalfa cultivation in the Horqin region. This research provides valuable theoretical and practical guidance for improving the cultivation of alfalfa in the cold regions of northeastern China, supporting the development of the local livestock industry.

Xyloglucan endotransglucosylase-hydrolase 1 is a negative regulator of drought tolerance in barley via modulating lignin biosynthesis and stomatal closure

The projected increase in drought severity and duration worldwide poses a significant threat to crop growth and sustainable food production. Xyloglucan endotransglucosylase/hydrolases (XTHs) family is essential in cell wall modification through the construction and restructuring of xyloglucan cross-links, but their role in drought tolerance and stomatal regulation is still illusive. We cloned and functionally characterized HvXTH1 using genetic, physiological, biochemical, transcriptomic and metabolomic approaches in barley. Evolutionary bioinformatics showed that orthologues of XTH1 was originated from Streptophyte algae (e.g. some species in the Zygnematales) the closest clade to land plants based on OneKP database. HvXTH1 is highly expressed in leaves and HvXTH1 is localized to the plasma membrane. Under drought conditions, silencing HvXTH1 in drought-tolerant Tibetan wild barley XZ5 induced a significant reduction in water loss rate and increase in biomass, however overexpressing HvXTH1 exhibited drought sensitivity with significantly less drought-responsive stomata, lower lignin content and a thicker cell wall. Transcriptome profile of the wild type Golden Promise and HvXTH1-OX demonstrated that drought-induced differentially expressed genes in leaves are related to cell wall biosynthesis, abscisic acid and stomatal signaling, and stress response. Furthermore, overexpressing HvXTH1 suppressed both genes and metabolites in the phenylpropanoid pathway for lignin biosynthesis, leading to drought sensitivity of HvXTH1-OX. We provide new insight by deciphering the function of a novel protein HvXTH1 for drought tolerance in cell wall modification, stomatal regulation, and phenylpropanoid pathway for lignin biosynthesis in barley. The function of HvXTH1 in drought response will be beneficial to develop crop varieties adapted to drought.

Seasonal Variations in the Toughness of Leaves: A Case Study Using Griselinia littoralis

Potential effects of climate change include greater extremes of temperature and increased severity of storms. Many plants have evolved to resist the challenges of winter (freezing, dehydration, and wind) in a process known as cold hardening. Sensing reducing temperatures, they make structural changes at the cellular level to increase their mechanical resistance and prevent damage. Previous work on this topic, though extensive, has been conducted under laboratory conditions rather than in the field, and while many workers have observed changes to cell wall thickness and composition, which imply increased mechanical strength, few have actually measured strength or any other parameter describing structural integrity. This paper describes experiments on a model system designed to measure the structural integrity of leaf laminae from plants growing naturally in the field over extended periods, allowing seasonal variations to be captured. Standard engineering properties-tensile strength and fracture toughness-were measured for leaves of Griselinia littoralis on 19 separate occasions over a 12-month period. Toughness (rather than strength) was found to be the controlling mechanical property. Toughness values were found to change significantly during the year, by more than a factor of 2. Toughness correlated strongly with average daily soil temperature, but with a lag of about 1-2 weeks, suggesting that this is the time needed for structural adjustments to take place. Highest toughness values occurred in winter, confirming cold hardening. Increasing temperature in the spring was associated with decreasing toughness, but in the summer, when highest temperatures occurred, toughness increased again. This apparent "hot hardening" may be a response to dehydration. Results imply that a given leaf is able to both increase and decrease its toughness in response to temperature changes, demonstrating excellent plasticity of response. This case study of a single species establishes a method of reliably measuring changes in a plant's structural integrity due to cold hardening and other seasonal variations, which may be used to investigate the effects of climate change and other variables.

Mutation of tomato xyloglucan transglucosylase/hydrolase5 increases fruit firmness and contributes to prolonged shelf life

Fruit ripening in tomato is a highly coordinated developmental process accompanied with fruit softening, which is closely associated with cell wall degradation and remodeling. Xyloglucan endotransglucosylase/hydrolases (XTHs) are known to play an essential role in cell wall xyloglucan metabolism. Tomato XTH5 exhibits xyloglucan endotransglucosylase (XET) activity in vitro, but the understanding of its biological role in fruit ripening remains unclear. In this study, we revealed that SlXTH5 is highly expressed in mature fruits. Knockout mutant plants of SlXTH5 were generated by CRISPR/Cas9 gene editing strategy in tomato cultivar Micro-Tom. The mutant fruits showed accelerated transition from unripe to ripe process and earlier ethylene accumulation compared to wild type fruits. Although the mutation of SlXTH5 did not affect the size, weight and number of fruits, it indeed increased fruit firmness and extended shelf life, which is probably attributed to the increased cell layer and cell wall thickness of pericarp tissue. Pathogen infection experiment showed the enhanced resistance of mutant fruits to Botrytis cinerea. These results revealed the role of SlXTH5 in fruit ripening process, and provide new insight into how cell wall metabolism and remodeling regulate fruit softening and shelf life.

Comparative Metabolome and Transcriptome Analysis of Rapeseed (Brassica napus L.) Cotyledons in Response to Cold Stress

Cold stress affects the seed germination and early growth of winter rapeseed, leading to yield losses. We employed transmission electron microscopy, physiological analyses, metabolome profiling, and transcriptome sequencing to understand the effect of cold stress (0 °C, LW) on the cotyledons of cold-tolerant (GX74) and -sensitive (XY15) rapeseeds. The mesophyll cells in cold-treated XY15 were severely damaged compared to slightly damaged cells in GX74. The fructose, glucose, malondialdehyde, and proline contents increased after cold stress in both genotypes; however, GX74 had significantly higher content than XY15. The pyruvic acid content increased after cold stress in GX74, but decreased in XY15. Metabolome analysis detected 590 compounds, of which 32 and 74 were differentially accumulated in GX74 (CK vs. cold stress) and XY15 (CK vs. cold stressed). Arachidonic acid and magnoflorine were the most up-accumulated metabolites in GX74 subjected to cold stress compared to CK. There were 461 and 1481 differentially expressed genes (DEGs) specific to XY15 and GX74 rapeseeds, respectively. Generally, the commonly expressed genes had higher expressions in GX74 compared to XY15 in CK and cold stress conditions. The expression changes in DEGs related to photosynthesis-antenna proteins, chlorophyll biosynthesis, and sugar biosynthesis-related pathways were consistent with the fructose and glucose levels in cotyledons. Compared to XY15, GX74 showed upregulation of a higher number of genes/transcripts related to arachidonic acid, pyruvic acid, arginine and proline biosynthesis, cell wall changes, reactive oxygen species scavenging, cold-responsive pathways, and phytohormone-related pathways. Taken together, our results provide a detailed overview of the cold stress responses in rapeseed cotyledons.

Freezing treatment under light conditions leads to a dramatic enhancement of freezing tolerance in cold-acclimated Arabidopsis

Overwintering plants survive subzero temperatures by cold acclimation (CA), wherein they acquire freezing tolerance through short-term exposure to low temperatures above 0°C. The freezing tolerance of CA plants increases when they are subsequently exposed to mild subzero temperatures, a phenomenon known as second-phase cold hardening (2PH). Here, we explored the molecular mechanism and physiological conditions of 2PH. The results show that, compared with supercooling, a freezing treatment during 2PH after CA enhanced the freezing tolerance of Arabidopsis. This required CA as a pretreatment, and was designated as second-phase freezing acclimation (2PFA). Light increased the effect of 2PFA to enhance freezing tolerance after CA. C-repeat binding factor and cold-regulated genes were downregulated by light during the 2PFA treatment, a different transcription profile from that during CA. The freezing tolerance of 2PFA plants was decreased by the presence of the photosynthetic electron transfer inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea during the 2PFA treatment. Compared with wild-type plants, phototropin1,2 and phyb mutants showed lower freezing tolerance after 2PFA treatment. These results show that exposure to freezing after CA increases freezing tolerance as a secondary process, and that freezing under light conditions further increases freezing tolerance via pathways involving photoreceptors and photosynthetic electron transfer.

Genome-wide analysis of wheat xyloglucan endotransglucosylase/hydrolase (XTH) gene family revealed TaXTH17 involved in abiotic stress responses

BACKGROUND

Environmental stresses, including high salinity and drought, severely diminish wheat yield and quality globally. The xyloglucan endotransglucosylase/hydrolase (XTH) family represents a class of cell wall-modifying enzymes and plays important roles in plants growth, development and stress adaptation. However, systematic analyses of XTH family genes and their functions under salt and drought stresses have not been undertaken in wheat.

RESULTS

In this study, we identified a total of 135 XTH genes in wheat, which were clustered into three evolutionary groups. These TaXTHs were unevenly distributed on 21 chromosomes of wheat with a majority of TaXTHs located on homelogous groups 2, 3 and 7. Gene duplication analysis revealed that segmental and tandem duplication were the main reasons for the expansion of XTH family in wheat. Interaction network predictions indicated that TaXTHs could interact with multiple proteins, including three kinases, one methyltransferase and one gibberellin-regulated protein. The promoters of the TaXTH genes harbored various cis-acting elements related to stress and hormone responses. RNA-seq data analyses showed that some TaXTH genes were induced by salt and drought stresses. Furthermore, we verified that TaXTH17 was induced by abiotic stresses and phytohormone treatments, and demonstrated that TaXTH17 was localized in the secretory pathway and cell wall. Functional analyses conducted in heterologous expression systems and in wheat established that TaXTH17 plays a negative role in plant resistance to salt and drought.

CONCLUSIONS

We identified 135 XTH genes in wheat and conducted comprehensive analyses of their phylogenetic relationships, gene structures, conserved motifs, gene duplication events, chromosome locations, interaction networks, cis-acting elements and gene expression patterns. Furthermore, we provided solid evidence supporting the notion that TaXTH17 plays a negative role in plant resistance to salt and drought stresses. Collectively, our results provide valuable insights into understanding wheat XTHs, particularly their involvement in plant stress responses, and establish a foundation for further functional and mechanistic studies of TaXTHs.

Identification and expression analysis of the Xyloglucan transglycosylase/hydrolase (XTH) gene family under abiotic stress in oilseed (Brassica napus L.)

XTH genes are key genes that regulate the hydrolysis and recombination of XG components and plays role in the structure and composition of plant cell walls. Therefore, clarifying the changes that occur in XTHs during plant defense against abiotic stresses is informative for the study of the plant stress regulatory mechanism mediated by plant cell wall signals. XTH proteins in Arabidopsis thaliana was selected as the seed sequences in combination with its protein structural domains, 80 members of the BnXTH gene family were jointly identified from the whole genome of the Brassica napus ZS11, and analyzed for their encoded protein physicochemical properties, phylogenetic relationships, covariance relationships, and interoperating miRNAs. Based on the transcriptome data, the expression patterns of BnXTHs were analyzed in response to different abiotic stress treatments. The relative expression levels of some BnXTH genes under Al, alkali, salt, and drought treatments after 0, 6, 12 and 24 h were analyzed by using qRT-PCR to explore their roles in abiotic stress tolerance in B. napus. BnXTHs showed different expression patterns in response to different abiotic stress signals, indicating that the response mechanisms of oilseed rape against different abiotic stresses are also different. This paper provides a theoretical basis for clarifying the function and molecular genetic mechanism of the BnXTH gene family in abiotic stress tolerance in rapeseed.

Spaceflight impacts xyloglucan oligosaccharide abundance in Arabidopsis thaliana root cell walls

Over the course of more than a decade, space biology investigations have consistently indicated that cell wall remodeling occurs in a variety of spaceflight-grown plants. Here, we describe a mass spectrometric method to study the fundamental composition of xyloglucan, the most abundant hemicellulose in dicot cell walls, in space-grown plants. Four representative Arabidopsis root samples, from a previously conducted spaceflight experiment - Advanced Plant EXperiment - 04 (APEX-04), were used to investigate changes in xyloglucan oligosaccharides abundances in spaceflight-grown plants compared to ground controls. In situ localized enzymatic digestions and surface sampling mass spectrometry analysis provided spatial resolution of the changes in xyloglucan oligosaccharides abundances. Overall, the results showed that oligosaccharide XXLG/XLXG and XXFG branching patterns were more abundant in the lateral roots of spaceflight-grown plants, while XXXG, XLFG, and XLFG/XLFG were more abundant in the lateral roots of ground control plants. In the primary roots, XXFG had a higher abundance in ground controls than in spaceflight plants. This methodology of analyzing the basic components of the cell wall in this paper highlights two important findings. First, that are differences in the composition of xyloglucan oligosaccharides in spaceflight root cell walls compared to ground controls and, second, most of these differences are observed in the lateral roots. Thus, the methodology described in this paper provides insights into spaceflight cell wall modifications for future investigations.

Proteomics reveals the significance of vacuole Pi transporter in the adaptability of Brassica napus to Pi deprivation

Vacuolar Pi transporters (VPTs) have recently been identified as important regulators of cellular Pi status in Arabidopsis thaliana and Oryza sativa. In the oil crop Brassica napus, BnA09PHT5;1a and BnC09PHT5;1a are two homologs of AtPHT5;1, the vacuolar Pi influx transporter in Arabidopsis. Here, we show that Pi deficiency induces the transcription of both homologs of PHT5;1a genes in B. napus leaves. Brassica PHT5;1a double mutants (DM) had smaller shoots and higher cellular Pi concentrations than wild-type (WT, Westar 10), suggesting the potential role of BnPHT5;1a in modulating cellular Pi status in B. napus. A proteomic analysis was performed to estimate the role of BnPHT5;1a in Pi fluctuation. Results show that Pi deprivation disturbs the abundance of proteins in the physiological processes involved in carbohydrate metabolism, response to stimulus and stress in B. napus, while disruption of BnPHT5;1a genes may exacerbate these processes. Besides, the processes of cell redox homeostasis, lipid metabolic and proton transmembrane transport are supposed to be unbalanced in BnPHT5;1a DM under the -Pi condition. Noteworthy, disruption of BnPHT5;1a genes severely alters the abundance of proteins related to ATP biosynthesis, and proton/inorganic cation transmembrane under normal Pi condition, which might contribute to B. napus growth limitations. Additionally, seven new protein markers of Pi homeostasis are identified in B. napus. Taken together, this study characterizes the important regulatory role of BnPHT5;1a genes as vacuolar Pi influx transporters in Pi homeostasis in B. napus.

Meta-analysis of public RNA sequencing data of abscisic acid-related abiotic stresses in Arabidopsis thaliana

Abiotic stresses such as drought, salinity, and cold negatively affect plant growth and crop productivity. Understanding the molecular mechanisms underlying plant responses to these stressors is essential for stress tolerance in crops. The plant hormone abscisic acid (ABA) is significantly increased upon abiotic stressors, inducing physiological responses to adapt to stress and regulate gene expression. Although many studies have examined the components of established stress signaling pathways, few have explored other unknown elements. This study aimed to identify novel stress-responsive genes in plants by performing a meta-analysis of public RNA sequencing (RNA-Seq) data in Arabidopsis thaliana, focusing on five ABA-related stress conditions (ABA, Salt, Dehydration, Osmotic, and Cold). The meta-analysis of 216 paired datasets from five stress conditions was conducted, and differentially expressed genes were identified by introducing a new metric, called TN [stress-treated (T) and non-treated (N)] score. We revealed that 14 genes were commonly upregulated and 8 genes were commonly downregulated across all five treatments, including some that were not previously associated with these stress responses. On the other hand, some genes regulated by salt, dehydration, and osmotic treatments were not regulated by exogenous ABA or cold stress, suggesting that they may be involved in the plant response to dehydration independent of ABA. Our meta-analysis revealed a list of candidate genes with unknown molecular mechanisms in ABA-dependent and ABA-independent stress responses. These genes could be valuable resources for selecting genome editing targets and potentially contribute to the discovery of novel stress tolerance mechanisms and pathways in plants.

Structural changes in cell wall pectic polymers contribute to freezing tolerance induced by cold acclimation in plants

Subzero temperatures are often lethal to plants. Many temperate herbaceous plants have a cold acclimation mechanism that allows them to sense a drop in temperature and prepare for freezing stress through accumulation of soluble sugars and cryoprotective proteins. As ice formation primarily occurs in the apoplast (the cell wall space), cell wall functional properties are important for plant freezing tolerance. Although previous studies have shown that the amounts of constituent sugars of the cell wall, in particular those of pectic polysaccharides, are altered by cold acclimation, the significance of this change during cold acclimation has not been clarified. We found that β-1,4-galactan, which forms neutral side chains of the acidic pectic rhamnogalacturonan-I, accumulates in the cell walls of Arabidopsis and various freezing-tolerant vegetables during cold acclimation. The gals1 gals2 gals3 triple mutant, which has reduced β-1,4-galactan in the cell wall, exhibited impaired freezing tolerance compared with wild-type Arabidopsis during initial stages of cold acclimation. Expression of genes involved in the galactan biosynthesis pathway, such as galactan synthases and UDP-glucose 4-epimerases, was induced during cold acclimation in Arabidopsis, explaining the galactan accumulation. Cold acclimation resulted in a decrease in extensibility and an increase in rigidity of the cell wall in the wild type, whereas these changes were not observed in the gals1 gals2 gals3 triple mutant. These results indicate that the accumulation of pectic β-1,4-galactan contributes to acquired freezing tolerance by cold acclimation, likely via changes in cell wall mechanical properties.

Genome-wide identification of XTH gene family in Musa acuminata and response analyses of MaXTHs and xyloglucan to low temperature

Banana (Musa spp.) production is seriously threatened by low temperature (LT) in tropical and subtropical regions. Xyloglucan endotransglycosylase/hydrolases (XTHs) are considered chief enzymes in cell wall remodelling and play a central role in stress responses. However, whether MaXTHs are involved in the low temperature stress tolerance in banana is not clear. Here, the identification and characterization of MaXTHs were carried out, followed by prediction of their cis-acting elements and protein-protein interactions. In addition, candidate MaXTHs involved in banana tolerance to LT were screened through a comparison of their responses to LT between tolerant and sensitive cultivars using RNA-Seq analysis. Moreover, immunofluorescence (IF) labelling was employed to compare changes in the temporal and spatial distribution of different types of xyloglucan components between these two cultivars upon stress. In total, 53 MaXTHs have been identified, and all were predicted to be located in the cell wall, 14 of them also in the cytoplasm. Only 11 MaXTHs have been found to interact with other proteins. Among 16 MaXTHs with LT responsiveness elements, MaXTH26/29/32/35/50 (Group I/II members) and MaXTH7/8 (Group IIIB members) might be involved in banana tolerance to LT stress. IF results suggested that the content of xyloglucan components recognized by CCRC-M87/103/104/106 antibodies might be negatively related to banana chilling tolerance. In conclusion, we have identified the MaXTH gene family and assessed cell wall re-modelling under LT stress. These results will be beneficial for banana breeding against stresses and enrich the cell wall-mediated resistance mechanism in plants to stresses.

Striking differences in frost hardiness and inability to cold acclimate in two Mougeotia species (Zygnematophyceae) from alpine and lowland habitats

Zygnematophyceae, a class of freshwater green algae, exhibit distinctive seasonal dynamics. The increasing frequency of cold snaps during the growing season might challenge the persistence of some populations. The present study explored the frost hardiness of two Mougeotia species isolated from different elevations and habitats. Additionally, a phylogenetic (rbc L sequence), ultrastructural and physiological characterization was performed. Both species, grown under standard culture conditions and cold acclimated cultures (+4°C), were exposed to freezing temperatures down to −9°C. Furthermore, ultrastructural‐, hydrogen peroxide (H2O2)‐ and photosynthetic pigment analysis were performed on cells exposed to −2°C, with and without induced ice nucleation. The alpine M. disjuncta showed a higher frost hardiness (LT50 = −5.8°C), whereas the lowland M. scalaris was susceptible to ice. However, frost hardiness did not improve after cold acclimation in either species but rather decreased significantly in M. disjuncta (LT50 = −4.7°C). Despite darkness, prolonged sub‐zero temperatures or freezing induced the activation of the xanthophyll (VAZ) cycle in M. scalaris . Our results demonstrate that frost hardiness varies within the genus Mougeotia and that the VAZ cycle can be activated in the dark under subzero temperature‐ and freezing stress but does not necessarily increase frost hardiness. As highly frost hardy cell types are usually formed at the end of the growing season, the ability of young cells to survive ice formation in the upper subzero temperature range represents a crucial survival strategy in populations exposed to late spring frosts.

Integrative transcriptomic analysis unveils lncRNA-miRNA-mRNA interplay in tomato plants responding to Ralstonia solanacearum

Ralstonia solanacearum, a bacterial plant pathogen, poses a significant threat to tomato (Solanum lycopersicum) production through destructive wilt disease. While noncoding RNA has emerged as a crucial regulator in plant disease, its specific involvement in tomato bacterial wilt remains limited. Here, we conducted a comprehensive analysis of the transcriptional landscape, encompassing both mRNAs and noncoding RNAs, in a tomato resistant line ('ZRS_7') and a susceptible line ('HTY_9') upon R. solanacearum inoculation using high-throughput RNA sequencing. Differential expression (DE) analysis revealed significant alterations in 7506 mRNAs, 997 lncRNAs, and 69 miRNAs between 'ZRS_7' and 'HTY_9' after pathogen exposure. Notably, 4548 mRNAs, 367 lncRNAs, and 26 miRNAs exhibited genotype-specific responses to R. solanacearum inoculation. GO and KEGG pathway analyses unveiled the potential involvement of noncoding RNAs in the response to bacterial wilt disease, targeting receptor-like kinases, cell wall-related genes, glutamate decarboxylases, and other key pathways. Furthermore, we constructed a comprehensive competing endogenous RNA (ceRNA) network incorporating 13 DE-miRNAs, 30 DE-lncRNAs, and 127 DEGs, providing insights into their potential contributions to the response against bacterial inoculation. Importantly, the characterization of possible endogenous target mimics (eTMs) of Sly-miR482e-3p via VIGS technology demonstrated the significant impact of eTM482e-3p-1 silencing on tomato's sensitivity to R. solanacearum. These findings support the existence of an eTM482e-3p-1-Sly-miR482e-3p-NBS-LRRs network in regulating tomato's response to the pathogen. Collectively, our findings shed light on the intricate interactions among lncRNAs, miRNAs, and mRNAs as underlying factors in conferring resistance to R. solanacearum in tomato.

Comparative transcriptomes reveal molecular mechanisms of apple blossoms of different tolerance genotypes to chilling injury

Apple (Malus domestica, Borkh.) is one of the four largest fruits in the world. Freezing damage during the flowering period of apples is one of the main factors leading to the reduction or even extinction of apple production. Molecular breeding of hardy apples is a good solution to these problems. However, the current screening of cold tolerance genes still needs to be resolved. Therefore, in this article, the transcriptome detection and cold tolerance gene screening during the cold adaptation process of apple were studied in order to obtain potential cold-resistant genes. Herein, two high-quality apple tree species (Malus robusta Rehd and M. domestica) were used for cold adaptation experiments and studied under different low-temperature stress conditions (0, -2 and -4°C). The antioxidant levels of two apple flower tissues were tested, and the transcriptome of the flowers after cold culture was tested by next-generation sequencing technology. Antioxidant test results show that the elimination of peroxides in M. robusta Rehd and the adjustment of the expression of antioxidant enzymes promote the cold resistance of this variety of apples. Functional enrichment found that the expression of enzyme activity, cell wall and cell membrane structure, glucose metabolism/gluconeogenesis, and signal transmission are the main biological processes that affect the differences in the cold resistance characteristics of the two apples. In addition, three potential cold-resistant genes AtERF4, RuBisCO activase 1, and an unknown gene (ID: MD09G1075000) were screened. In this study, three potential cold-resistant genes (AtERF4, RuBisCO activase 1, and an unknown gene [ID: MD09G1075000]) and three cold-repressed differential genes (AtDTX29, XTH1, and TLP) were screened.

Label-Free Quantitative Proteomics Reveal the Mechanisms of Young Wheat (Triticum aestivum L.) Ears' Response to Spring Freezing

Late spring frost is an important meteorological factor threatening the safe production of winter wheat in China. The young ear is the most vulnerable organ of the wheat plant to spring frost. To gain an insight into the mechanisms underpinning young wheat ears' tolerance to freezing, we performed a comparative proteome analysis of wheat varieties Xumai33 (XM33, freezing-sensitive) and Jimai22 (JM22, freezing-tolerant) under normal and freezing conditions using label-free quantitative proteomic techniques during the anther connective tissue formation phase (ACFP). Under freezing stress, 392 and 103 differently expressed proteins (DEPs) were identified in the young ears of XM33 and JM22, respectively, and among these, 30 proteins were common in both varieties. A functional characterization analysis revealed that these DEPs were associated with antioxidant capacity, cell wall modification, protein folding, dehydration response, and plant-pathogen interactions. The young ears of JM22 showed significantly higher expression levels of antioxidant enzymes, heat shock proteins, and dehydrin under normal conditions compared to those of XM33, which might help to prepare the young ears of JM22 for freezing stress. Our results lead to new insights into understanding the mechanisms in young wheat ears' response to freezing stress and provide pivotal potential candidate proteins required for improving young wheat ears' tolerance to spring frost.

Abiotic Stress in Crop Production

The vast majority of agricultural land undergoes abiotic stress that can significantly reduce agricultural yields. Understanding the mechanisms of plant defenses against stresses and putting this knowledge into practice is, therefore, an integral part of sustainable agriculture. In this review, we focus on current findings in plant resistance to four cardinal abiotic stressors—drought, heat, salinity, and low temperatures. Apart from the description of the newly discovered mechanisms of signaling and resistance to abiotic stress, this review also focuses on the importance of primary and secondary metabolites, including carbohydrates, amino acids, phenolics, and phytohormones. A meta-analysis of transcriptomic studies concerning the model plant Arabidopsis demonstrates the long-observed phenomenon that abiotic stressors induce different signals and effects at the level of gene expression, but genes whose regulation is similar under most stressors can still be traced. The analysis further reveals the transcriptional modulation of Golgi-targeted proteins in response to heat stress. Our analysis also highlights several genes that are similarly regulated under all stress conditions. These genes support the central role of phytohormones in the abiotic stress response, and the importance of some of these in plant resistance has not yet been studied. Finally, this review provides information about the response to abiotic stress in major European crop plants—wheat, sugar beet, maize, potatoes, barley, sunflowers, grapes, rapeseed, tomatoes, and apples.

Infrared thermography of in situ natural freezing and mechanism of winter-thermonasty in Rhododendron maximum

Evergreen leaves of Rhododendron species inhabiting temperate/montane climates are typically exposed to both high radiation and freezing temperatures during winter when photosynthetic biochemistry is severely inhibited. Cold-induced "thermonasty," that is, lamina rolling and petiole curling, can reduce the amount of leaf area exposed to solar radiation and has been associated with photoprotection in overwintering rhododendrons. The present study was conducted on natural, mature plantings of a cold-hardy and large-leaved thermonastic North American species (Rhododendron maximum) during winter freezes. Infrared thermography was used to determine initial sites of ice formation, patterns of ice propagation, and dynamics of the freezing process in leaves to understand the temporal and mechanistic relationship between freezing and thermonasty. Results indicated that ice formation in whole plants is initiated in the stem, predominantly in the upper portions, and propagates in both directions from the original site. Ice formation in leaves initially occurred in the vascular tissue of the midrib and then propagated into other portions of the vascular system/venation. Ice was never observed to initiate or propagate into palisade, spongy mesophyll, or epidermal tissues. These observations, together with the leaf- and petiole-histology, and a simulation of the rolling effect of dehydrated leaves using a cellulose-based, paper-bilayer system, suggest that thermonasty occurs due to anisotropic contraction of cell wall cellulose fibers of adaxial versus abaxial surface as the cells lose water to ice present in vascular tissues.

Physiological and transcriptomic responses to cold waves of the most cold-tolerant mangrove, Kandelia obovata

Mangrove forests inhabit tropical or subtropical intertidal zones and have remarkable abilities in coastline protection. Kandelia obovata is considered the most cold-tolerant mangrove species and has been widely transplanted to the north subtropical zone of China for ecological restoration. However, the physiological and molecular mechanisms of K. obovata under colder climate was still unclear. Here, we manipulated the typical climate of cold waves in the north subtropical zone with cycles of cold/recovery and analyzed the physiological and transcriptomic responses of seedlings. We found that both physiological traits and gene expression profiles differed between the first and later cold waves, indicating K. obovata seedlings were acclimated by the first cold experience and prepared for latter cold waves. 1,135 cold acclimation-related genes (CARGs) were revealed, related to calcium signaling, cell wall modification, and post-translational modifications of ubiquitination pathways. We identified the roles of CBFs and CBF-independent transcription factors (ZATs and CZF1s) in regulating the expression of CARGs, suggesting both CBF-dependent and CBF- independent pathways functioned in the cold acclimation of K. obovata. Finally, we proposed a molecular mechanism of K. obovata cold acclimation with several key CARGs and transcriptional factors involved. Our experiments reveal strategies of K. obovata coping with cold environments and provide prospects for mangrove rehabilitation and management.

Transcriptomic profiling of wheat stem during meiosis in response to freezing stress

Low temperature injury in spring has seriously destabilized the production and grain quality of common wheat. However, the molecular mechanisms underlying spring frost tolerance remain elusive. In this study, we investigated the response of a frost-tolerant wheat variety Zhongmai8444 to freezing stress at the meiotic stage. Transcriptome profiles over a time course were subsequently generated by high-throughput sequencing. Our results revealed that the prolonged freezing temperature led to the significant reductions in plant height and seed setting rate. Cell wall thickening in the vascular tissue was also observed in the stems. RNA-seq analyses demonstrated the identification of 1010 up-regulated and 230 down-regulated genes shared by all time points of freezing treatment. Enrichment analysis revealed that gene activity related to hormone signal transduction and cell wall biosynthesis was significantly modulated under freezing. In addition, among the identified differentially expressed genes, 111 transcription factors belonging to multiple gene families exhibited dynamic expression pattern. This study provided valuable gene resources beneficial for the breeding of wheat varieties with improved spring frost tolerance.

Temporal cell wall changes during cold acclimation and deacclimation and their potential involvement in freezing tolerance and growth

Plants adapt to freezing stress through cold acclimation, which is induced by nonfreezing low temperatures and accompanied by growth arrest. A later increase in temperature after cold acclimation leads to rapid loss of freezing tolerance and growth resumption, a process called deacclimation. Appropriate regulation of the trade-off between freezing tolerance and growth is necessary for efficient plant development in a changing environment. The cell wall, which mainly consists of polysaccharide polymers, is involved in both freezing tolerance and growth. Still, it is unclear how the balance between freezing tolerance and growth is affected during cold acclimation and deacclimation by the changes in cell wall structure and what role is played by its monosaccharide composition. Therefore, to elucidate the regulatory mechanisms controlling freezing tolerance and growth during cold acclimation and deacclimation, we investigated cell wall changes in detail by sequential fractionation and monosaccharide composition analysis in the model plant Arabidopsis thaliana, for which a plethora of information and mutant lines are available. We found that arabinogalactan proteins and pectic galactan changed in close coordination with changes in freezing tolerance and growth during cold acclimation and deacclimation. On the other hand, arabinan and xyloglucan did not return to nonacclimation levels after deacclimation but stabilized at cold acclimation levels. This indicates that deacclimation does not completely restore cell wall composition to the nonacclimated state but rather changes it to a specific novel composition that is probably a consequence of the loss of freezing tolerance and provides conditions for growth resumption.

Frozen mountain pine needles: The endodermis discriminates between the ice-containing central tissue and the ice-free fully functional mesophyll

Conifer (Pinaceae) needles are the most frost-hardy leaves. During needle freezing, the exceptional leaf anatomy, where an endodermis separates the mesophyll from the vascular tissue, could have consequences for ice management and photosynthesis. The eco-physiological importance of needle freezing behaviour was evaluated based on the measured natural freezing strain at the alpine treeline. Ice localisation and cellular responses to ice were investigated in mountain pine needles by cryo-microscopic techniques. Their consequences for photosynthetic activity were assessed by gas exchange measurements. The freezing response was related to the microchemistry of cell walls investigated by Raman microscopy. In frozen needles, ice was confined to the central vascular cylinder bordered by the endodermis. The endodermal cell walls were lignified. In the ice-free mesophyll, cells showed no freeze-dehydration and were found photosynthetically active. Mesophyll cells had lignified tangential cell walls, which adds rigidity. Ice barriers in mountain pine needles seem to be realised by a specific lignification patterning of cell walls. This, additionally, impedes freeze-dehydration of mesophyll cells and enables gas exchange of frozen needles. At the treeline, where freezing is a dominant environmental factor, the elaborate needle freezing pattern appears of ecological importance.

Identification and functional characterization of the xyloglucan endotransglucosylase/hydrolase 32 (AhXTH32) in peanut during aluminum-induced programmed cell death

The toxicity of aluminum (Al) in acidic soil is a prevalent problem and causes reduced crop yields. In the plant response to Al toxicity, programmed cell death (PCD) appears to be an important mechanism. The plant cell wall of crop roots is the predominant site targeted by Al. Here, studies of the capacities of different cell wall constituents (pectin, hemicellulose 1 {HC1} and HC2) to adsorb Al indicated that HC1 has the greater ability to bind Al. The activity of xyloglucan endotransglucosylase (XET) was significantly inhibited by Al in the Al-tolerant peanut cultivar '99-1507' compared to that in 'ZH 2' (Al-sensitive). Results from qPCR analysis suggested that the suppression of XET activity by Al was transcriptionally regulated and that xyloglucan endotransglucosylase/hydrolase 32 (AhXTH32) was the major contributor to these changes. The overexpression of AhXTH32 in Arabidopsis strongly inhibited root growth with a loss of viability in root cells and the occurrence of typical hallmarks of PCD, while largely opposite effects were observed after xth32 suppression. AhXTH32 contributed to the modulation XET and xyloglucan endohydrolase (XEH) activity in vivo. Taken together, our results demonstrate that Al-tolerant peanut cultivar root tips cell walls bind Al predominantly in the HC1 fraction, which results in the inhibition of AhXTH32, with consequences to root growth, Al sensitivity, the occurrence of PCD and the XET/XEH activity ratio.

Genome-Wide Identification and Expression Analysis of the Xyloglucan Endotransglucosylase/Hydrolase Gene Family in Sweet Potato [Ipomoea batatas (L.) Lam]

The xyloglucan endotransglucosylase/hydrolase (XET/XEH, also named XTH) family is a multigene family, the function of which plays a significant role in cell-wall rebuilding and stress tolerance in plants. However, the specific traits of the XTH gene family members and their expression pattern in different tissues and under stress have not been carried out in sweet potato. Thirty-six XTH genes were identified in I. batatas, all of which had conserved structures (Glyco_hydro_16). Based on Neighbor-Joining phylogenetic analysis the IbXTHs can be divided into three subfamilies-the I/II, IIIA, and IIIB subfamilies, which were unevenly distributed on 13 chromosomes, with the exception of Chr9 and Chr15. Multiple cis-acting regions related to growth and development, as well as stress responses, may be found in the IbXTH gene promoters. The segmental duplication occurrences greatly aided the evolution of IbXTHs. The results of a collinearity analysis showed that the XTH genes of sweet potato shared evolutionary history with three additional species, including A. thaliana, G. max, and O. sativa. Additionally, based on the transcriptome sequencing data, the results revealed that the IbXTHs have different expression patterns in leaves, stems, the root body (RB), the distal end (DE), the root stock (RS), the proximal end (PE), the initiative storage root (ISR), and the fibrous root (FR), and many of them are well expressed in the roots. Differentially expressed gene (DEG) analysis of FRs after hormone treatment of the roots indicated that IbXTH28 and IbXTH30 are up-regulated under salicylic acid (SA) treatment but down-regulated under methyl jasmonate (MeJA) treatment. Attentionally, there were only two genes showing down-regulation under the cold and drought treatment. Collectively, all of the findings suggested that genes from the XTH family are crucial for root specificity. This study could provide a theoretical basis for further research on the molecular function of sweet potato XTH genes.

Identification of the Xyloglucan Endotransglycosylase/Hydrolase (XTH) Gene Family Members Expressed in Boehmeria nivea in Response to Cadmium Stress

Xyloglucan endotransglycosylase/hydrolase (XTH) genes play an important role in plant resistance to abiotic stress. However, systematic studies of the response of Boehmeria nivea (ramie) XTH genes (BnXTHs) to cadmium (Cd) stress are lacking. We sought to identify the BnXTH-family genes in ramie through bioinformatics analyses and to investigate their responses to Cd stress. We identified 19 members of the BnXTH gene family from the ramie genome, referred to as BnXTH1-19, among which BnXTH18 and BnXTH19 were located on no chromosomes and the remaining genes were unevenly distributed across 11 chromosomes. The 19 members were divided into four groups, Groups I/II/IIIA/IIIB, according to their phylogenetic relationships, and these groups were supported by analyses of intron-exon structure and conserved motif composition. A highly conserved catalytic site (HDEIDFEFLG) was observed in all BnXTH proteins. Additionally, three gene pairs (BnXTH6-BnXTH16, BnXTH8-BnXTH9, and BnXTH17-BnXTH18) were obtained with a fragment and tandem-repeat event analysis of the ramie genome. An analysis of cisregulatory elements revealed that BnXTH expression might be regulated by multiple hormones and abiotic and biotic stress responses. In particular, 17 cisregulatory elements related to abiotic and biotic stress responses and 11 cisregulatory elements related to hormone responses were identified. We also found that most BnXTH genes responded to Cd stress, and BnXTH1, BnXTH3, BnXTH6, and BnXTH15 were most likely to contribute to the Cd tolerance of ramie, as evidenced by the substantial increases in expression under Cd treatment. Heterologous expression of BnXTH1, BnXTH6, and BnXTH15 significantly enhanced the Cd tolerance of transgenic yeast cells. These results suggest that the BnXTH gene family is involved in Cd stress responses, laying a theoretical foundation for functional studies of BnXTH genes and the innovative breeding of Cd-tolerant ramie.

Freeze dehydration vs. supercooling of mesophyll cells: Impact of cell wall, cellular and tissue traits on the extent of water displacement

The extent of freeze dehydration of mesophyll cells in response to extracellular ice varies from supercooling to severe freezing cytorrhysis. The structural factors involved are poorly understood. In a comparison of mesophyll cells of 11 species, the factors "cell wall", "cellular" and "tissue" traits were investigated. The extent of freeze dehydration was quantified as reduction in the sectional area during controlled freezing in the presence of ice. The cell wall thickness, cell size, cell area and the relative area of intercellular spaces were determined. The modulus of elasticity was determined by psychrometry. To grasp the relationships between factors and with freeze dehydration, we applied a principal component analysis. The first two components explain 84% of the variance in the dataset. The first principal component correlated negatively with the extent of freeze dehydration and relative area of intercellular spaces, and positively with the squared cell wall thickness to cell size ratio, elasticity and cell wall thickness. The cell size parameters determined the second principal component. Supercooling appeared preferable in cells with a high squared cell wall thickness to cell size ratio and a low relative area of intercellular spaces. Such factors are hypothesised to affect the magnitude of negative turgor pressure being built up below the turgor loss point. Negative turgor pressure slows dehydration by reducing the water potential gradient to the extracellular ice. With high levels of freeze dehydration, sufficient intercellular spaces for extracellular ice accommodation are needed. The low relative area of intercellular spaces increases cell-to-cell contact area and could support tissue stability.

Plasma membrane proteomic changes of Arabidopsis DRP1E during cold acclimation in association with the enhancement of freezing tolerance

The freezing tolerance of plants that live in cold regions increases after exposure to low temperature, a process termed cold acclimation (CA). During CA, restructuring of the plasma membrane (PM) is important to enhance freezing tolerance. We have previously shown that the function of DYNAMIN-RELATED PROTEIN 1 E (DRP1E), which regulates endocytosis by pinching vesicles from the PM, is associated with the enhancement of freezing tolerance during CA in Arabidopsis. DRP1E is predicted to play a role in reconstituting the PM composition during CA. In this study, to test the validity of this hypothesis, we studied the changes in PM proteome patterns induced by drp1e mutation. In a detailed physiological analysis, after 3 days of CA, only young leaves showed significantly less increase in freezing tolerance in the mutant than in the wild type (WT). Using nano-liquid chromatography-tandem mass spectrometry, 496 PM proteins were identified. Among these proteins, 81 or 71 proteins were specifically altered in the WT or the mutant, respectively, in response to CA. Principal component analysis showed that the proteomic pattern differed between the WT and the mutant upon cold acclimation (CA), suggesting that DRP1E contributes to reconstruction of the PM during CA. Cluster analysis revealed that proteins that were significantly increased in the mutant after CA were biased toward glycosylphosphatidylinositol-anchored proteins, such as fasciclin-like arabinogalactan proteins. Thus, a primary target of DRP1E-associated PM reconstruction during CA is considered to be glycosylphosphatidylinositol-anchored proteins, which may be removed from the PM by DRP1E in young leaves after 3 days of CA.

Fine mapping of the reduced height gene Rht22 in tetraploid wheat landrace Jianyangailanmai (Triticum turgidum L.)

Rht22 was fine mapped in the interval of 0.53-1.48 Mb on 7AS, which reduces cell number of internode to cause semi-dwarfism in Jianyangailanmai. As a valuable germplasm resource for wheat genetic improvement, tetraploid wheat has several reduced height (Rht) and enhanced harvest index genes. Rht22, discovered in Jianyangailanmai (JAM, Triticum turgidum L., 2n = 4x = 28, AABB), significantly increases the spikelet number per spike, but its accurate chromosomal position is still unknown. In this study, a high-density genetic map was constructed using specific-length amplified fragment sequencing in an F₇ RIL_DJ population, which was derived from a cross between dwarf Polish wheat (T. polonicum L., 2n = 4x = 28, AABB) and JAM. Two plant height loci, Qph.sicau-4B and Qph.sicau-7A, were mapped on chromosomes 4BS and 7AS, respectively. Qph.sicau-7A was mapped to the 0.33-4.46 Mb interval on 7AS and likely represents the candidate region of Rht22. Fine mapping confirmed and narrowed Rht22 on chromosome arm 7AS between Xbag295.s53 and Xb295.191 in three different populations. The physical region ranged from 0.53 to 1.48 Mb and included 18 candidate genes. Transcriptome analysis of two pairs of near-isogenic lines revealed that 135 differentially expressed genes (DEGs) were associated with semi-dwarfism. Of these, the expression of 83 annotated DEGs involved in hormones synthesis and signal transduction, cell wall composition, DNA replication, microtubule and phragmoplast arrays was significantly down-regulated in the semi-dwarf line. Therefore, Rht22 causes semi-dwarfism in JAM by disrupting these cellular processes, which impairs cell proliferation and reduces internode cell number.

DNA methylation affects freezing tolerance in winter rapeseed by mediating the expression of genes related to JA and CK pathways

Winter rapeseed is the largest source of edible oil in China and is especially sensitive to low temperature, which causes tremendous agricultural yield reduction and economic losses. It is still unclear how DNA methylation regulates the formation of freezing tolerance in winter rapeseed under freezing stress. Therefore, in this study, the whole-genome DNA methylation map and transcriptome expression profiles of freezing-resistant cultivar NTS57 (NS) under freezing stress were obtained. The genome-wide methylation assay exhibited lower levels of methylation in gene-rich regions. DNA methylation was identified in three genomic sequence contexts including CG, CHG and CHH, of which CG contexts exhibited the highest methylation levels (66.8%), followed by CHG (28.6%) and CHH (9.5%). Higher levels of the methylation were found in upstream 2 k and downstream 2 k of gene regions, whereas lowest levels were in the gene body regions. In addition, 331, 437, and 1720 unique differentially methylated genes (DMGs) were identified in three genomic sequence contexts in 17NS under freezing stress compared to the control. Function enrichment analysis suggested that most of enriched DMGs were involved in plant hormones signal transduction, phenylpropanoid biosynthesis and protein processing pathways. Changes of genes expression in signal transduction pathways for cytokinin (CK) and jasmonic acid (JA) implied their involvement in freezing stress responses. Collectively, these results suggested a critical role of DNA methylation in their transcriptional regulation in winter rapeseed under freezing stress.

Specific CBF transcription factors and cold-responsive genes fine-tune the early triggering response after acquisition of cold priming and memory

Plants need to adapt to fluctuating temperatures throughout their lifetime. Previous research showed that Arabidopsis memorizes a first cold stress (priming) and improves its primed freezing tolerance further when subjected to a second similar stress after a lag phase. This study investigates primary metabolomic and transcriptomic changes during early cold priming or triggering after 3 days at 4°C interrupted by a memory phase. DREB1 family transcription factors DREB1C/CBF2, DREB1D/CBF4, DREB1E/DDF2, and DREB1F/DDF1 were strongly significantly induced throughout the entire triggering. During triggering, genes encoding Late Embryogenesis Abundant (LEA), antifreeze proteins or detoxifiers of reactive oxygen species (ROS) were higher expressed compared with priming. Examples of early triggering responders were xyloglucan endotransglucosylase/hydrolase genes encoding proteins involved in cell wall remodeling, while late responders were identified to act in fine-tuning the stress response and developmental regulation. Induction of non-typical members of the DREB subfamily of ERF/AP2 transcription factors, the relatively small number of induced CBF regulon genes and a slower accumulation of selected cold stress associated metabolites indicate that a cold triggering stimulus might be sensed as milder stress in plants compared with priming. Further, strong induction of CBF4 throughout triggering suggests a unique function of this gene for the response to alternating temperatures.

Identifying plant genes shaping microbiota composition in the barley rhizosphere

A prerequisite to exploiting soil microbes for sustainable crop production is the identification of the plant genes shaping microbiota composition in the rhizosphere, the interface between roots and soil. Here, we use metagenomics information as an external quantitative phenotype to map the host genetic determinants of the rhizosphere microbiota in wild and domesticated genotypes of barley, the fourth most cultivated cereal globally. We identify a small number of loci with a major effect on the composition of rhizosphere communities. One of those, designated the QRMC-3HS, emerges as a major determinant of microbiota composition. We subject soil-grown sibling lines harbouring contrasting alleles at QRMC-3HS and hosting contrasting microbiotas to comparative root RNA-seq profiling. This allows us to identify three primary candidate genes, including a Nucleotide-Binding-Leucine-Rich-Repeat (NLR) gene in a region of structural variation of the barley genome. Our results provide insights into the footprint of crop improvement on the plant's capacity of shaping rhizosphere microbes.

Identification and expression analysis of xyloglucan endotransglucosylase/hydrolase (XTH) family in grapevine (Vitis vinifera L.)

Xyloglucan endotransglucosylases/hydrolases (XTH) are key enzymes in cell wall reformulation. They have the dual functions of catalyzing xyloglucan endotransglucosylase (XET) and xyloglucan endonuclease (XEH) activity and play a crucial role in the responses against abiotic stresses, such as drought, salinity, and freezing. However, a comprehensive analysis of the XTH family and its functions in grapevine (Vitis vinifera L.) has not yet been completed. In this study, 34 XTHs were identified in the whole grapevine genome and then named according to their distribution on chromosomes. Based on a phylogenetic analysis including Arabidopsis XTHs, the VvXTHs were classified into three groups. Cis-element analysis indicated that these family members are related to most abiotic stresses. We further selected 14 VvXTHs from different groups and then examined their transcription levels under drought and salt stress. The results indicated that the transcription levels of selected VvXTHs in the leaves and roots presented the largest changes, suggesting that VvXTHs are likely to take part in the responses to drought and salt stress in grapevines. These results provide useful evidence for the further investigation of VvXTHs function in response to abiotic stresses in grapevine.

Cold and exogenous calcium alter Allium fistulosum cell wall pectin to depress intracellular freezing temperatures

De-methyl esterification of homogalacturonan and subsequent cross-linking with Ca2+ is hypothesized to enhance the freezing survival of cold acclimated plants by reducing the porosity of primary cell walls. To test this theory, we collected leaf epidermal peels from non- (23/18 °C) and cold acclimated (2 weeks at 12/4 °C) Japanese bunching onion (Allium fistulosum L.). Cold acclimation enhanced the temperature at which half the cells survived freezing injury by 8 °C (LT50 =-20 °C), and reduced tissue permeability by 70-fold compared with non-acclimated epidermal cells. These effects were associated with greater activity of pectin methylesterase (PME) and a reduction in the methyl esterification of homogalacturonan. Non-acclimated plants treated with 50 mM CaCl2 accumulated higher concentrations of galacturonic acid, Ca2+ in the cell wall, and a lower number of visible cell wall pores compared with that observed in cold acclimated plants. Using cryo-microscopy, we observed that 50 mM CaCl2 treatment did not lower the LT50 of non-acclimated cells, but reduced the lethal intracellular ice nucleation to temperatures observed in cold acclimated epidermal cells. We postulate that the PME-homogalacturonan-mediated reduction in cell wall porosity is integral to intracellular freezing avoidance strategies in cold acclimated herbaceous cells.

Genome-Wide Identification and Expression Analysis of XTH Gene Family during Flower-Opening Stages in Osmanthus fragrans

Osmanthus fragrans is an aromatic plant which is widely used in landscaping and garden greening in China. However, the process of flower opening is significantly affected by ambient temperature changes. Cell expansion in petals is the primary factor responsible for flower opening. Xyloglucan endoglycolase/hydrolase (XTH) is a cell-wall-loosening protein involved in cell expansion or cell-wall weakening. Through whole-genome analysis, 38 OfXTH genes were identified in O. fragrans which belong to the four main phylogenetic groups. The gene structure, chromosomal location, synteny relationship, and cis-acting elements prediction and expression patterns were analyzed on a genome-wide scale. The expression patterns showed that most OfXTHs were closely associated with the flower-opening period of O. fragrans. At the early flower-opening stage (S1 and S2), transcriptome and qRT-PCR analysis revealed the expression of OfXTH24, 27, 32, 35, and 36 significantly increased under low ambient temperature (19 °C). It is speculated that the five genes might be involved in the regulation of flower opening by responding to ambient temperature changes. Our results provide solid foundation for the functional analysis of OfXTH genes and help to explore the mechanism of flower opening responding to ambient temperature in O. fragrans.

Cellulose synthase-like protein OsCSLD4 plays an important role in the response of rice to salt stress by mediating abscisic acid biosynthesis to regulate osmotic stress tolerance

Cell wall polysaccharide biosynthesis enzymes play important roles in plant growth, development and stress responses. The functions of cell wall polysaccharide synthesis enzymes in plant growth and development have been well studied. In contrast, their roles in plant responses to environmental stress are poorly understood. Previous studies have demonstrated that the rice cell wall cellulose synthase-like D4 protein (OsCSLD4) is involved in cell wall polysaccharide synthesis and is important for rice growth and development. This study demonstrated that the OsCSLD4 function-disrupted mutant nd1 was sensitive to salt stress, but insensitive to abscisic acid (ABA). The expression of some ABA synthesis and response genes was repressed in nd1 under both normal and salt stress conditions. Exogenous ABA can restore nd1-impaired salt stress tolerance. Moreover, overexpression of OsCSLD4 can enhance rice ABA synthesis gene expression, increase ABA content and improve rice salt tolerance, thus implying that OsCSLD4-regulated rice salt stress tolerance is mediated by ABA synthesis. Additionally, nd1 decreased rice tolerance to osmotic stress, but not ion toxic tolerance. The results from the transcriptome analysis showed that more osmotic stress-responsive genes were impaired in nd1 than salt stress-responsive genes, thus indicating that OsCSLD4 is involved in rice salt stress response through an ABA-induced osmotic response pathway. Intriguingly, the disruption of OsCSLD4 function decreased grain width and weight, while overexpression of OsCSLD4 increased grain width and weight. Taken together, this study demonstrates a novel plant salt stress adaptation mechanism by which crops can coordinate salt stress tolerance and yield.

Reconsidering the function of the xyloglucan endotransglucosylase/hydrolase family

Plants possess an outer cell layer called the cell wall. This matrix comprises various molecules, such as polysaccharides and proteins, and serves a wide array of physiologically important functions. This structure is not static but rather flexible in response to the environment. One of the factors responsible for this plasticity is the xyloglucan endotransglucosylase/hydrolase (XTH) family, which cleaves and reconnects xyloglucan molecules. Since xyloglucan molecules have been hypothesised to tether cellulose microfibrils forming the main load-bearing network in the primary cell wall, XTHs have been thought to play a central role in cell wall loosening for plant cell expansion. However, multiple lines of recent evidence have questioned this classic model. Nevertheless, reverse genetic analyses have proven the biological importance of XTHs; therefore, a major challenge at present is to reconsider the role of XTHs in planta. Recent advances in analytical techniques have allowed for gathering rich information on the structure of the primary cell wall. Thus, the integration of accumulated knowledge in current XTH studies may offer a turning point for unveiling the precise functions of XTHs. In the present review, we redefine the biological function of the XTH family based on the recent architectural model of the cell wall. We highlight three key findings regarding this enzyme family: (1) XTHs are not strictly required for cell wall loosening during plant cell expansion but play vital roles in response to specific biotic or abiotic stresses; (2) in addition to their transglycosylase activity, the hydrolase activity of XTHs is involved in physiological benefits; and (3) XTHs can recognise a wide range of polysaccharides other than xyloglucans.

Broad Specific Xyloglucan:Xyloglucosyl Transferases Are Formidable Players in the Re-Modelling of Plant Cell Wall Structures

Plant xyloglucan:xyloglucosyl transferases, known as xyloglucan endo-transglycosylases (XETs) are the key players that underlie plant cell wall dynamics and mechanics. These fundamental roles are central for the assembly and modifications of cell walls during embryogenesis, vegetative and reproductive growth, and adaptations to living environments under biotic and abiotic (environmental) stresses. XET enzymes (EC 2.4.1.207) have the β-sandwich architecture and the β-jelly-roll topology, and are classified in the glycoside hydrolase family 16 based on their evolutionary history. XET enzymes catalyse transglycosylation reactions with xyloglucan (XG)-derived and other than XG-derived donors and acceptors, and this poly-specificity originates from the structural plasticity and evolutionary diversification that has evolved through expansion and duplication. In phyletic groups, XETs form the gene families that are differentially expressed in organs and tissues in time- and space-dependent manners, and in response to environmental conditions. Here, we examine higher plant XET enzymes and dissect how their exclusively carbohydrate-linked transglycosylation catalytic function inter-connects complex plant cell wall components. Further, we discuss progress in technologies that advance the knowledge of plant cell walls and how this knowledge defines the roles of XETs. We construe that the broad specificity of the plant XETs underscores their roles in continuous cell wall restructuring and re-modelling.

The XTH Gene Family in Schima superba: Genome-Wide Identification, Expression Profiles, and Functional Interaction Network Analysis

Xyloglucan endotransglucosylase/hydrolase (XTH), belonging to glycoside hydrolase family 16, is one of the key enzymes in plant cell wall remodeling. Schima superba is an important timber and fireproof tree species in southern China. However, little is known about XTHs in S. superba. In the present study, a total of 34 SsuXTHs were obtained, which were classified into three subfamilies based on the phylogenetic relationship and unevenly distributed on 18 chromosomes. Furthermore, the intron-exon structure and conserved motif composition of them supported the classification and the members belonging to the same subfamily shared similar gene structures. Segmental and tandem duplication events did not lead to SsuXTH gene family expansion, and strong purifying selection pressures during evolution led to similar structure and function of SsuXTH gene family. The interaction network and cis-acting regulatory elements analysis revealed the SsuXTH expression might be regulated by multiple hormones, abiotic stresses and transcription factors. Finally, expression profiles and GO enrichment analysis showed most of the tandem repeat genes were mainly expressed in the phloem and xylem and they mainly participated in glycoside metabolic processes through the transfer and hydrolysis of xyloglucan in the cell wall and then regulated fiber elongation.

Responses of the Plant Cell Wall to Sub-Zero Temperatures: A Brief Update

Our general understanding of plant responses to sub-zero temperatures focuses on mechanisms that mitigate stress to the plasma membrane. The plant cell wall receives comparatively less attention, and questions surrounding its role in mitigating freezing injury remain unresolved. Despite recent molecular discoveries that provide insight into acclimation responses, the goal of reducing freezing injury in herbaceous and woody crops remains elusive. This is likely due to the complexity associated with adaptations to low temperatures. Understanding how leaf cell walls of herbaceous annuals promote tissue tolerance to ice does not necessarily lead to understanding how meristematic tissues are protected from freezing by tissue-level barriers formed by cell walls in overwintering tree buds. In this mini-review, we provide an overview of biological ice nucleation and explain how plants control the spatiotemporal location of ice formation. We discuss how sugars and pectin side chains alleviate adhesive injury that develops at sub-zero temperatures between the matrix polysaccharides and ice. The importance of site-specific cell-wall elasticity to promote tissue expansion for ice accommodation and control of porosity to impede ice growth and promote supercooling will be presented. How specific cold-induced proteins modify plant cell walls to mitigate freezing injury will also be discussed. The opinions presented in this report emphasize the importance of a plant's developmental physiology when characterizing mechanisms of freezing survival.

A Novel Banana Mutant "RF 1" (Musa spp. ABB, Pisang Awak Subgroup) for Improved Agronomic Traits and Enhanced Cold Tolerance and Disease Resistance

Banana is a major fruit crop grown in tropical and subtropical regions worldwide. Among cultivars, "FenJiao, FJ" (Musa spp. ABB, Pisang Awak subgroup) is a popular variety of bananas, due to its better sugar-acid blend and relatively small fruit shape. However, because the traditional FJ variety grows relatively high in height, it is vulnerable to lodging and unsuitable for harvesting. In this study, we sought desirable banana mutants by carrying out ethyl methanesulfonate (EMS) mutagenesis with the FJ cultivar. After the FJ shoot tips had been treated with 0.8% (v/v) EMS for 4 h, we obtained a stably inherited mutant, here called "ReFen 1" (RF1), and also observed a semi-dwarfing phenotype. Compared with the wild type (FJ), this RF1 mutant featured consistently improved agronomic traits during 5-year field experiments conducted in three distinct locations in China. Notably, the RF1 plants showed significantly enhanced cold tolerance and Sigatoka disease resistance, mainly due to a substantially increased soluble content of sugar and greater starch accumulation along with reduced cellulose deposition. Therefore, this study not only demonstrated how a powerful genetic strategy can be used in fruit crop breeding but also provided insight into the identification of novel genes for agronomic trait improvement in bananas and beyond.

Recent Advances in Understanding the Roles of Pectin as an Active Participant in Plant Signaling Networks

Pectin is an abundant cell wall polysaccharide with essential roles in various biological processes. The structural diversity of pectins, along with the numerous combinations of the enzymes responsible for pectin biosynthesis and modification, plays key roles in ensuring the specificity and plasticity of cell wall remodeling in different cell types and under different environmental conditions. This review focuses on recent progress in understanding various aspects of pectin, from its biosynthetic and modification processes to its biological roles in different cell types. In particular, we describe recent findings that cell wall modifications serve not only as final outputs of internally determined pathways, but also as key components of intercellular communication, with pectin as a major contributor to this process. The comprehensive view of the diverse roles of pectin presented here provides an important basis for understanding how cell wall-enclosed plant cells develop, differentiate, and interact.

Genome-Wide Identification of Banana Csl Gene Family and Their Different Responses to Low Temperature between Chilling-Sensitive and Tolerant Cultivars

The cell wall plays an important role in responses to various stresses. The cellulose synthase-like gene (Csl) family has been reported to be involved in the biosynthesis of the hemicellulose backbone. However, little information is available on their involvement in plant tolerance to low-temperature (LT) stress. In this study, a total of 42 Csls were identified in Musa acuminata and clustered into six subfamilies (CslA, CslC, CslD, CslE, CslG, and CslH) according to phylogenetic relationships. The genomic features of MaCsl genes were characterized to identify gene structures, conserved motifs and the distribution among chromosomes. A phylogenetic tree was constructed to show the diversity in these genes. Different changes in hemicellulose content between chilling-tolerant and chilling-sensitive banana cultivars under LT were observed, suggesting that certain types of hemicellulose are involved in LT stress tolerance in banana. Thus, the expression patterns of MaCsl genes in both cultivars after LT treatment were investigated by RNA sequencing (RNA-Seq) technique followed by quantitative real-time PCR (qPCR) validation. The results indicated that MaCslA4/12, MaCslD4 and MaCslE2 are promising candidates determining the chilling tolerance of banana. Our results provide the first genome-wide characterization of the MaCsls in banana, and open the door for further functional studies.

Primary Cell Wall Modifying Proteins Regulate Wall Mechanics to Steer Plant Morphogenesis

Plant morphogenesis involves multiple biochemical and physical processes inside the cell wall. With the continuous progress in biomechanics field, extensive studies have elucidated that mechanical forces may be the most direct physical signals that control the morphology of cells and organs. The extensibility of the cell wall is the main restrictive parameter of cell expansion. The control of cell wall mechanical properties largely determines plant cell morphogenesis. Here, we summarize how cell wall modifying proteins modulate the mechanical properties of cell walls and consequently influence plant morphogenesis.