Cold acclimation is accompanied by complex responses of glycosylphosphatidylinositol (GPI)-anchored proteins in Arabidopsis

Characterization of the FLA Gene Family in Tomato (Solanum lycopersicum L.) and the Expression Analysis of SlFLAs in Response to Hormone and Abiotic Stresses

Fasciclin-like arabinogalactan proteins (FLAs), a subclass of arabinogalactan proteins (AGPs), participate in mediating plant growth, development, and response to abiotic stress. However, the characterization and function of FLAs in tomato are currently unknown. In this study, members of the tomato FLA family are characterized and analyzed in relation to their response to phytohormonal and abiotic stresses. The results show that a total of 24 FLA members were characterized in tomato. The structural domain analysis showed that these members have a high protein similarity. The expression profiles of different tissues indicated that the genes of most members of the tomato FLA gene family are highly expressed in roots, but to a lower extent in fruits. qRT-PCR analysis revealed that all 24 tomato FLA genes are responsive to ABA and MeJA. SlFLAs showed a positive response to salt and cold stress. SlFLA1, SlFLA12, and SlFLA14 are significantly induced under darkness. SlFLA1 and SlFLA3 are significantly induced under drought stress. This study provides a basis for a further understanding of the role of tomato FLA homologous genes in plant response to abiotic stress and lays the foundation for further research on the function of FLAs in tomato.

Single-filament imaging mass spectrometry lipidomics in Arthrospira platensis

RATIONALE

Elucidating intra-organismal biochemical and lipid organization in photosynthetic biological cell factories of filamentous cyanobacteria, such as Arthrospira platensis (Spirulina), is important for tracking physiological response mechanisms during growth. Little is known about the filaments' biochemical organization and cellular structure and no label-free imaging techniques exist that provide molecular mapping.

METHODS

We applied ultrahigh-resolution mass spectrometry (MS) with matrix-assisted laser desorption ionization (MALDI) imaging to immobilized Spirulina filaments to investigate the localization of lipids across distinct physiological regions. We optimized matrix selection and deposition methods with the goal of facilitating high spatial, and intra-filament, resolution using untargeted multivariate statistical spectral deconvolution across MS pixels.

RESULTS

Our results demonstrate an improved two-step matrix application with an optimized procedure for intra-organismal lipid profiling to improve analyte sensitivity and achieve higher spatial resolution. We evaluate several conventional matrices, namely 2,5-dihydroxybenzoic acid (DHB), superDHB (sDHB), 1,5-diaminonaphthalene (DAN), and a 50:50 mix of DHB and sDHB, and compare delineation and pixel-based elucidation of intra-filament lipidomics. We identified a total of 1626 features that could be putatively assigned a lipid-like formula based on database query and 46 unique features, with associated lipid assignments that were significantly distinct in their intra-filament location.

CONCLUSIONS

MALDI imaging MS with untargeted statistical spectral deconvolution was used to visualize intra-filament lipidomics organization in Spirulina filaments. Improvements in matrix deposition, including sequential sublimation and pneumatic spraying, increased signal abundance at high spatial resolution and allowed for identification of distinct lipid composition regions. This work outlines a methodology that may be used for micro-ecological untargeted molecular phenotyping.

Pexophagy suppresses ROS-induced damage in leaf cells under high-intensity light

Although light is essential for photosynthesis, it has the potential to elevate intracellular levels of reactive oxygen species (ROS). Since high ROS levels are cytotoxic, plants must alleviate such damage. However, the cellular mechanism underlying ROS-induced leaf damage alleviation in peroxisomes was not fully explored. Here, we show that autophagy plays a pivotal role in the selective removal of ROS-generating peroxisomes, which protects plants from oxidative damage during photosynthesis. We present evidence that autophagy-deficient mutants show light intensity-dependent leaf damage and excess aggregation of ROS-accumulating peroxisomes. The peroxisome aggregates are specifically engulfed by pre-autophagosomal structures and vacuolar membranes in both leaf cells and isolated vacuoles, but they are not degraded in mutants. ATG18a-GFP and GFP-2×FYVE, which bind to phosphatidylinositol 3-phosphate, preferentially target the peroxisomal membranes and pre-autophagosomal structures near peroxisomes in ROS-accumulating cells under high-intensity light. Our findings provide deeper insights into the plant stress response caused by light irradiation.

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.

The Application of Auxin-like Compounds Promotes Cold Acclimation in the Oilseed Rape Plant

Cold is a major environmental key factor influencing plant growth, development, and productivity. Responses and adaption processes depend on plant physiological and biochemical modifications, first of all via the hormonal system. Indole-3-acetic acid (IAA) plays a critical role in the processes of plant functioning. To assess the influence of the auxin-like compounds 1-[2-chloroethoxycarbonylmethyl]-4-naphthalenesulfonic acid calcium salt (TA-12) and 1-[2-dimethylaminoethoxycarbonylmethyl]naphthalene chloromethylate (TA-14) in the process of cold acclimation, long-term field trials over four years were performed with two rapeseed (Brassica napus L.) plant cultivars with different wintering resistance in temperate-zone countries. In these two rapeseed cultivars, namely ‘Casino’ (less resistant) and ‘Valesca’ (more resistant), investigations were conducted in the terminal buds and root collars. The application of auxin-like compounds revealed a close interlinkage between the composition of dehydrins and the participation of the phytohormone IAA in the adaptation processes. By applying TA-12 and TA-14, the importance of the proteins, especially the composition of the dehydrins, the IAA amount, and the status of the oilseed rape cultivars at the end of the cold acclimation period were confirmed. Following on from this, when introducing oilseed rape cultivars from foreign countries, it may also be of value to assess their suitability for cultivation in temperate-zone countries.

Nitric oxide mediated alleviation of abiotic challenges in plants

Agriculture and ecosystem are negatively influenced by the abiotic stresses which create solemn pressures on plants as they are sessile in nature leading to excessive losses in economy. For maintenance of sustainable agriculture and to fulfil the cumulative call of food for rapidly growing population worldwide, it becomes crucial to protects the crop plants from climate fluctuations. Plants fight back against these challenges by generation of redox molecules comprising reactive oxygen species (ROS) and reactive nitrogen species (RNS) and cause modulation at cellular, physiological and molecular levels. Nitric oxide (NO) deliver tolerance to several biotic and abiotic stresses in plants by acting as signalling molecule or free radicals. It is also intricated in several developmental processes in plants using different mechanisms. Supplementation of exogenous NO reduce toxicity of abiotic stresses and provide resistance. In this review article, we summarize the recent research studies (five years) depicting the functional role of NO in alleviation of abiotic stresses such as drought, cold, heat, heavy metals and flooding. Moreover, by investigating studies found that among heavy metals works associated with Hg, Pb, and Cr is limited comparatively. Additionally, role of NO in abiotic stress resistance such as cold, freezing and heat stress less/poorly investigated. Consequently, further emphasis should be diverted towards how NO can facilitate protection against these stresses. In recent studies mostly beneficial role of NO against abiotic challenges have been elucidated by observing physiological/biochemical parameters but relatively inadequate research done at the transcripts level or gene regulation subsequently researchers should include it in future. Lastly, brief outline and an evaluative discussion on the present information and future prospective provided. Altogether, these inclusive experimental agendas could facilitate in future to produce climate tolerant plants. This will help to confront the constant fluctuations in the environment and to reduce the challenges in way of agriculture productivity and global food demands.

The regulation of the cell wall by glycosylphosphatidylinositol-anchored proteins in Arabidopsis

A polysaccharides-based cell wall covers the plant cell, shaping it and protecting it from the harsh environment. Cellulose microfibrils constitute the cell wall backbone and are embedded in a matrix of pectic and hemicellulosic polysaccharides and glycoproteins. Various environmental and developmental cues can regulate the plant cell wall, and diverse glycosylphosphatidylinositol (GPI)-anchored proteins participate in these regulations. GPI is a common lipid modification on eukaryotic proteins, which covalently tethers the proteins to the membrane lipid bilayer. Catalyzed by a series of enzymic complexes, protein precursors are post-translationally modified at their hydrophobic carboxyl-terminus in the endomembrane system and anchored to the lipid bilayer through an oligosaccharidic GPI modification. Ultimately, mature proteins reach the plasma membrane via the secretory pathway facing toward the apoplast and cell wall in plants. In Arabidopsis, more than three hundred GPI-anchored proteins (GPI-APs) have been predicted, and many are reported to be involved in diverse regulations of the cell wall. In this review, we summarize GPI-APs involved in cell wall regulation. GPI-APs are proposed to act as structural components of the cell wall, organize cellulose microfibrils at the cell surface, and during cell wall integrity signaling transduction. Besides regulating protein trafficking, the GPI modification is potentially governed by a GPI shedding system that cleaves and releases the GPI-anchored proteins from the plasma membrane into the cell wall.

Different responses of banana classical AGP genes and cell wall AGP components to low-temperature between chilling sensitive and tolerant cultivars

Seventeen classical MaAGPs and 9 MbAGPs were identified and analyzed. MaAGP1/2/6/9/16/17, the antigens of JIM13 and LM2 antibodies are likely to be involved in banana chilling tolerance. Classical arabinogalactan proteins (AGPs) belong to glycosylphosphatidylinositol-anchored proteins, which are proved to be involved in signaling and cell wall metabolism upon stresses. However, rare information is available on the roles of classical AGPs in low temperature (LT) tolerance. Cultivation of banana in tropical and subtropical region is seriously threatened by LT stress. In the present study, 17 classical MaAGPs and nine MbAGPs in banana A and B genome were identified and characterized, respectively. Great diversity was present among different classical MaAGP/MbAGP members while five members (AGP3/6/11/13/14) showed 100% identity between these two gene families. We further investigated different responses of classical AGPs to LT between a chilling sensitive (CS) and tolerant (CT) banana cultivars. In addition, different changes in the temporal and spatial distribution of cell wall AGP components under LTs between these two cultivars were compared using immunofluorescence labeling. Seven classical MbAGPs were upregulated by LT(s) in the CT cultivar. Classical MaAGP4/6 was induced by LT(s) in both cultivars while MaAGP1/2/9/16/17 only in the CT cultivar. Moreover, these genes showed significantly higher transcription abundance in the CT cultivar than the CS one under LT(s) except classical MaAGP4. Similar results were observed with the epitopes of JIM13 and LM2 antibodies. The antigens of these antibodies and classical MaAGP1/2/6/9/16/17 might be related to LT tolerance of banana. These results provide additional information about plant classical AGPs and their involvement in LT tolerance, as well as their potential as candidate genes to be targeted when breeding CT banana.

Increasing Expression of PnGAP and PnEXPA4 Provides Insights Into the Enlargement of Panax notoginseng Root Size From Qing Dynasty to Cultivation Era

Root size is a key trait in plant cultivation and can be influenced by the cultivation environment. However, physical evidence of root size change in a secular context is scarce due to the difficulty in preserving ancient root samples, and how they were modified during the domestication and cultivation stays unclear. About 100 ancient root samples of Panax notoginseng, preserved as tribute in the Palace Museum (A.D. 1636 to 1912, Qing dynasty), provided an opportunity to investigate the root size changes during the last 100 years of cultivation. The dry weight of ancient root samples (~120 tou samples, tou represents number of roots per 500 g dry weight) is 0.22-fold of the modern samples with the biggest size (20 tou samples). Transcriptome analysis revealed that PnGAP and PnEXPA4 were highly expressed in 20 tou samples, compared with the 120 tou samples, which might contribute to the thicker cell wall and a higher content of lignin, cellulose, and callose in 20 tou samples. A relatively lower content of dencichine and higher content of ginsenoside Rb1 in 20 tou samples are also consistent with higher expression of ginsenoside biosynthesis-related genes. PnPHL8 was filtrated through transcriptome analysis, which could specifically bind the promoters of PnGAP, PnCYP716A47, and PnGGPPS3, respectively. The results in this study represent the first physical evidence of root size changes in P. notoginseng in the last 100 years of cultivation and contribute to a comprehensive understanding of how the cultivation environment affected root size, chemical composition, and clinical application.

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.

The Brachypodium distachyon cold-acclimated plasma membrane proteome is primed for stress resistance

In order to survive subzero temperatures, some plants undergo cold acclimation (CA) where low, nonfreezing temperatures, and/or shortened day lengths allow cold-hardening and survival during subsequent freeze events. Central to this response is the plasma membrane (PM), where low temperature is perceived and cellular homeostasis must be preserved by maintaining membrane integrity. Here, we present the first PM proteome of cold-acclimated Brachypodium distachyon, a model species for the study of monocot crops. A time-course experiment investigated CA-induced changes in the proteome following two-phase partitioning PM enrichment and label-free quantification by nano-liquid chromatography-mass spectrophotometry. Two days of CA were sufficient for membrane protection as well as an initial increase in sugar levels and coincided with a significant change in the abundance of 154 proteins. Prolonged CA resulted in further increases in soluble sugars and abundance changes in more than 680 proteins, suggesting both a necessary early response to low-temperature treatment, as well as a sustained CA response elicited over several days. A meta-analysis revealed that the identified PM proteins have known roles in low-temperature tolerance, metabolism, transport, and pathogen defense as well as drought, osmotic stress, and salt resistance suggesting crosstalk between stress responses, such that CA may prime plants for other abiotic and biotic stresses. The PM proteins identified here present keys to an understanding of cold tolerance in monocot crops and the hope of addressing economic losses associated with modern climate-mediated increases in frost events.

Comparison of mass spectrometry data and bioinformatics predictions to assess the bona fide localization of proteins identified in cell wall proteomics studies

Plant cell walls have complex architectures made of polysaccharides among which cellulose, hemicelluloses, pectins and cell wall proteins (CWPs). Some CWPs are anchored in the plasma membrane through a glycosylphosphatidylinositol (GPI)-anchor. The secretion pathway is the classical route to reach the extracellular space. Based on experimental data, a canonical signal peptide (SP) has been defined, and bioinformatics tools allowing the prediction of the sub-cellular localization of proteins have been designed. In the same way, the presence of GPI-anchor attachment sites can be predicted using bioinformatics programs. This article aims at comparing the bioinformatics predictions of the sub-cellular localization of proteins assumed to be CWPs to mass spectrometry (MS) data. The sub-cellular localization of a few CWPs exhibiting particular features has been checked by cell biology approaches. Although the prediction of SP length is confirmed in most cases, it is less conclusive for GPI-anchors. Three main observations were done: (i) the variability observed at the N-terminus of a few mature CWPs could play a role in the regulation of their biological activity; (ii) one protein was shown to have a double sub-cellular localization in the cell wall and the chloroplasts; and (iii) peptides were found to be located at the C-terminus of several CWPs previously identified in GPI-anchored proteomes, thus raising the issue of their actual anchoring to the plasma membrane.

Drought Responsive Putative Marker-Trait Association in Tall Fescue as Influenced by the Presence of a Novel Endophyte

Tall fescue (Festuca arundinacea Schreb.) is one of the most important cool-season perennial obligatory outcrossing forage grasses in the United States. The production and persistence of tall fescue is significantly affected by drought in the south-central United States. Shoot-specific endophyte (Epichloë coenophiala)-infected tall fescue showed superior performance under both biotic and abiotic stress conditions. We performed a genome-wide association analysis using clonal pairs of novel endophyte AR584-positive (EP) and endophyte-free (EF) tall fescue populations consisting of 205 genotypes to identify marker-trait associations (MTAs) that contribute to drought tolerance. The experiment was performed through November 2014 to June 2018 in the field, and phenotypic data were taken on plant height, plant spread, plant vigor, and dry biomass weight under natural summer conditions of sporadic drought. Genotyping-by-sequencing of the population generated 3,597 high quality single nucleotide polymorphisms (SNPs) for further analysis. We identified 26 putative drought responsive MTAs (17 specific to EP, eight specific to EF, and one in both EP and EF populations) and nine of them (i.e., V.ep_10, S.ef_12, V.ep_27, HSV.ef_31, S.ep_30, SV.ef_32, V.ep_68, V.ef_56, and H.ef_57) were identified within 0.5 Mb region in the tall fescue genome (44.5-44.7, 75.3-75.8, 77.5-77.9 and 143.7-144.2 Mb). Using 26 MTAs, 11 tall fescue genotypes were selected for subsequent study to develop EP and EF drought tolerant tall fescue populations. Ten orthologous genes (six for EP and four for EF population) were identified in Brachypodium genome as potential candidates for drought tolerance in tall fescue, which were also earlier reported for their involvement in abiotic stress tolerance. The MTAs and candidate genes identified in this study will be useful for marker-assisted selection in improving drought tolerance of tall fescue as well opening avenue for further drought study in tall fescue.

Glycosylphosphatidylinositol-Anchor Synthesis in Plants: A Glycobiology Perspective

More than 200 diverse secretory proteins from Arabidopsis thaliana carry a glycosylphosphatidylinositol (GPI) lipid anchor covalently attached to their carboxyl-terminus. The GPI-anchor contains a lipid-linked glycan backbone that is preassembled in the endoplasmic reticulum (ER) of plants and subsequently transferred to distinct proteins, which provides them with specific features. The GPI-anchored proteins exit the ER and are transported through the Golgi apparatus to the plasma membrane. In the Golgi, the glycan moiety can be further modified by the specific attachment of sugar residues. While these biosynthetic steps are already quite well understood in mammals and yeast, comparatively little is known in plants. In this perspective, we discuss the current knowledge about the biosynthesis of the GPI-anchor glycan moiety in the light of recent findings for mammalian GPI-anchor glycan modifications.

Large-Scale Phosphoproteomic Study of Arabidopsis Membrane Proteins Reveals Early Signaling Events in Response to Cold

Cold stress is one of the major factors limiting global crop production. For survival at low temperatures, plants need to sense temperature changes in the surrounding environment. How plants sense and respond to the earliest drop in temperature is still not clearly understood. The plasma membrane and its adjacent extracellular and cytoplasmic sites are the first checkpoints for sensing temperature changes and the subsequent events, such as signal generation and solute transport. To understand how plants respond to early cold exposure, we used a mass spectrometry-based phosphoproteomic method to study the temporal changes in protein phosphorylation events in Arabidopsis membranes during 5 to 60 min of cold exposure. The results revealed that brief cold exposures led to rapid phosphorylation changes in the proteins involved in cellular ion homeostasis, solute and protein transport, cytoskeleton organization, vesical trafficking, protein modification, and signal transduction processes. The phosphorylation motif and kinase-substrate network analysis also revealed that multiple protein kinases, including RLKs, MAPKs, CDPKs, and their substrates, could be involved in early cold signaling. Taken together, our results provide a first look at the cold-responsive phosphoproteome changes of Arabidopsis membrane proteins that can be a significant resource to understand how plants respond to an early temperature drop.

Fasciclin-like arabinogalactan gene family in Nicotiana benthamiana: genome-wide identification, classification and expression in response to pathogens

BACKGROUND

Nicotiana benthamiana is widely used as a model plant to study plant-pathogen interactions. Fasciclin-like arabinogalactan proteins (FLAs), a subclass of arabinogalactan proteins (AGPs), participate in mediating plant growth, development and response to abiotic stress. However, the members of FLAs in N. benthamiana and their response to plant pathogens are unknown.

RESULTS

38 NbFLAs were identified from a genome-wide study. NbFLAs could be divided into four subclasses, and their gene structure and motif composition were conserved in each subclass. NbFLAs may be regulated by cis-acting elements such as STRE and MBS, and may be the targets of transcription factors like C2H2. Quantitative real time polymerase chain reaction (RT-qPCR) results showed that selected NbFLAs were differentially expressed in different tissues. All of the selected NbFLAs were significantly downregulated following infection by turnip mosaic virus (TuMV) and most of them also by Pseudomonas syringae pv tomato strain DC3000 (Pst DC3000), suggesting possible roles in response to pathogenic infection.

CONCLUSIONS

This study systematically identified FLAs in N. benthamiana, and indicates their potential roles in response to biotic stress. The identification of NbFLAs will facilitate further studies of their role in plant immunity in N. benthamiana.

Genome-wide analyses of banana fasciclin-like AGP genes and their differential expression under low-temperature stress in chilling sensitive and tolerant cultivars

Thirty MaFLAs vary in their molecular features. MaFLA14/18/27/29 are likely to be involved in banana chilling tolerance by facilitating the cold signaling pathway and enhancing the cell wall biosynthesis. Although several studies have identified the molecular functions of individual fasciclin-like arabinogalactan protein (FLA) genes in plant growth and development, little information is available on their involvement in plant tolerance to low-temperature (LT) stress, and the related underlying mechanism is far from clear. In this study, the different expression of FLAs of banana (Musa acuminata) (MaFLAs) in the chilling-sensitive (CS) and chilling-tolerant (CT) banana cultivars under natural LT was investigated. Based on the latest banana genome database, a genome-wide identification of this gene family was done and the molecular features were analyzed. Thirty MaFLAs were distributed in 10 out of 11 chromosomes and these clustered into four major phylogenetic groups based on shared gene structure. Twenty-four MaFLAs contained N-terminal signal, 19 possessed predicted glycosylphosphatidylinositol (GPI), while 16 had both. Most MaFLAs were downregulated by LT stress. However, MaFLA14/18/29 were upregulated by LT in both cultivars with higher expression level recorded in the CT cultivar. Interestingly, MaFLA27 was significantly upregulated in the CT cultivar, but the opposite occurred for the CS cultivar. MaFLA27 possessed only N-terminal signal, MaFLA18 contained only GPI anchor, MaFLA29 possessed both, while MaFLA14 had neither. Thus, it was suggested that the accumulation of these FLAs in banana under LT could improve banana chilling tolerance through facilitating cold signal pathway and thereafter enhancing biosynthesis of plant cell wall components. The results provide background information of MaFLAs, suggest their involvement in plant chilling tolerance and their potential as candidate genes to be targeted when breeding CT banana.

Root Skewing-Associated Genes Impact the Spaceflight Response of Arabidopsis thaliana

The observation that plant roots skew in microgravity recently refuted the long-held conviction that skewing was a gravity-dependent phenomenon. Further, spaceflight root skewing suggests that specific root morphologies and cell wall remodeling systems may be important aspects of spaceflight physiological adaptation. However, connections between skewing, cell wall modification and spaceflight physiology are currently based on inferences rather than direct tests. Therefore, the Advanced Plant Experiments-03-2 (APEX-03-2) spaceflight study was designed to elucidate the contribution of two skewing- and cell wall-associated genes in Arabidopsis to root behavior and gene expression patterns in spaceflight, to assess whether interruptions of different skewing pathways affect the overall spaceflight-associated process. SPIRAL1 is a skewing-related protein implicated in directional cell expansion, and functions by regulating cortical microtubule dynamics. SKU5 is skewing-related glycosylphosphatidylinositol-anchored protein of the plasma membrane and cell wall implicated in stress response signaling. These two genes function in different cellular pathways that affect skewing on the Earth, and enable a test of the relevance of skewing pathways to spaceflight physiological adaptation. In this study, both sku5 and spr1 mutants showed different skewing behavior and markedly different patterns of gene expression in the spaceflight environment. The spr1 mutant showed fewer differentially expressed genes than its Col-0 wild-type, whereas sku5 showed considerably more than its WS wild-type. Developmental age played a substantial role in spaceflight acclimation in all genotypes, but particularly in sku5 plants, where spaceflight 4d seedlings had almost 10-times as many highly differentially expressed genes as the 8d seedlings. These differences demonstrated that the two skewing pathways represented by SKU5 and SPR1 have unique and opposite contributions to physiological adaptation to spaceflight. The spr1 response is less intense than wild type, suggesting that the loss of SPR1 positively impacts spaceflight adaptation. Conversely, the intensity of the sku5 responses suggests that the loss of SKU5 initiates a much more complex, deeper and more stress related response to spaceflight. This suggests that proper SKU5 function is important to spaceflight adaptation.

Xyloglucan exoglycosidases in the monocot model Brachypodium distachyon and the conservation of xyloglucan disassembly in angiosperms

Brachypodium distachyon has a full set of exoglycosidases active on xyloglucan, including α-xylosidase, β-galactosidase, soluble and membrane-bound β-glucosidases and two α-fucosidases. However, unlike in Arabidopsis, both fucosidases are likely cytosolic. Xyloglucan is present in primary walls of all angiosperms. While in most groups it regulates cell wall extension, in Poaceae its role is still unclear. Five exoglycosidases participate in xyloglucan hydrolysis in Arabidopsis: α-xylosidase, β-galactosidase, α-fucosidase, soluble β-glucosidase and GPI-anchored β-glucosidase. Mutants in the corresponding genes show alterations in xyloglucan composition. In this work putative orthologs in the model grass Brachypodium distachyon were tested for their ability to complement Arabidopsis mutants. Xylosidase and galactosidase mutants were complemented, respectively, by BdXYL1 (Bd2g02070) and BdBGAL1 (Bd2g56607). BdBGAL1, unlike other xyloglucan β-galactosidases, is able to remove both galactoses from XLLG oligosaccharides. In addition, soluble β-glucosidase BdBGLC1 (Bd1g08550) complemented a glucosidase mutant. Closely related BdBGLC2 (Bd2g51280), which has a putative GPI-anchor sequence, was found associated with the plasma membrane and only a truncated version without GPI-anchor complemented the mutant, proving that Brachypodium also has soluble and membrane-bound xyloglucan glucosidases. Both BdXFUC1 (Bd3g25226) and BdXFUC2 (Bd1g28366) can hydrolyze fucose from xyloglucan oligosaccharides but were unable to complement a fucosidase mutant. Fluorescent protein fusions of BdXFUC1 localized to the cytosol and both proteins lack a signal peptide. Signal peptides appear to have evolved only in some eudicot lineages of this family, like the one leading to Arabidopsis. These results could be explained if cytosolic xyloglucan α-fucosidases are the ancestral state in angiosperms, with fucosylated oligosaccharides transported across the plasma membrane.

Nitric oxide deficiency decreases C-repeat binding factor-dependent and -independent induction of cold acclimation

Plant tolerance to freezing temperatures is governed by endogenous components and environmental factors. Exposure to low non-freezing temperatures is a key factor in the induction of freezing tolerance in the process called cold acclimation. The role of nitric oxide (NO) in cold acclimation was explored in Arabidopsis using triple nia1nia2noa1-2 mutants that are impaired in the nitrate-dependent and nitrate-independent pathways of NO production, and are thus NO deficient. Here, we demonstrate that cold-induced NO accumulation is required to promote the full cold acclimation response through C-repeat Binding Factor (CBF)-dependent gene expression, as well as the CBF-independent expression of other cold-responsive genes such as Oxidation-Related Zinc Finger 2 (ZF/OZF2). NO deficiency also altered abscisic acid perception and signaling and the cold-induced production of anthocyanins, which are additional factors involved in cold acclimation.

Glycosylphosphatidylinositol-Anchored Proteins in Arabidopsis and One of Their Common Roles in Signaling Transduction

Diverse proteins are found modified with glycosylphosphatidylinositol (GPI) at their carboxyl terminus in eukaryotes, which allows them to associate with membrane lipid bilayers and anchor on the external surface of the plasma membrane. GPI-anchored proteins (GPI-APs) play crucial roles in various processes, and more and more GPI-APs have been identified and studied. In this review, previous genomic and proteomic predictions of GPI-APs in Arabidopsis have been updated, which reveal their high abundance and complexity. From studies of individual GPI-APs in Arabidopsis, certain GPI-APs have been found associated with partner receptor-like kinases (RLKs), targeting RLKs to their subcellular localization and helping to recognize extracellular signaling polypeptide ligands. Interestingly, the association might also be involved in ligand selection. The analyses suggest that GPI-APs are essential and widely involved in signal transduction through association with RLKs.

Temporal proteomics of Arabidopsis plasma membrane during cold- and de-acclimation

Freezing stress is one of the most important limiting factors of plant survival. Plants have developed a freezing adaptation mechanism upon sensing low temperatures (cold acclimation). Compositional changes in the plasma membrane, one of the initial sites of freezing injury, is prerequisite of achieving cold acclimation and have been investigated in several plant species. Conversely, the cold dehardening process at elevated temperatures (de-acclimation) has not yet been fully characterized and few studies have addressed the importance of the plasma membrane in the de-acclimation process. In the present study, we conducted shotgun proteomics with label-free semiquantification on plasma membrane fractions of Arabidopsis leaves during cold acclimation and de-acclimation. We consequently obtained a list of 873 proteins with significantly changed proteins in response to the two processes. Although the cold-acclimation-responsive proteins were globally returned to non-acclimated levels by de-acclimation, several representative cold-acclimation-responsive proteins tended to remain at higher abundance during de-acclimation process. Taken together, our results suggest plants deharden right after cold acclimation to restart growth and development but some cold-acclimation-induced changes of the plasma membrane may be maintained under de-acclimation to cope with the threat of sudden freezing during de-acclimation process. SIGNIFICANCE: Plant freezing tolerance can be enhanced by low temperature treatment (cold acclimation), while elevated temperatures right after cold acclimation can result in the dehardening of freezing tolerance (de-acclimation). However, the de-acclimation process, particularly its relevance to the plasma membrane as the primary site of freezing injury, has not been elucidated. In the present study, a comprehensive proteomic analysis of the plasma membrane during cold acclimation and de-acclimation was carried out as a first step to elucidating how plants respond to rising temperatures. Cold acclimation induced a number of proteomic changes as reported in previous studies, but most proteins, in general, immediately returned to NA levels during de-acclimation treatment for two days. However, the abundances of stress-related proteins (e.g. LTI29, COR78 and TIL) decreased slower than other functional proteins during de-acclimation. Therefore, plants harden during cold acclimation by aborting growth and development and accumulating stress-responsive proteins but seem to deharden quickly under subsequent elevated temperature to resume these processes while guarding against the threat of sudden temperature drops.

Nitric Oxide Controls Constitutive Freezing Tolerance in Arabidopsis by Attenuating the Levels of Osmoprotectants, Stress-Related Hormones and Anthocyanins

Plant tolerance to freezing temperatures is governed by endogenous constitutive components and environmental inducing factors. Nitric oxide (NO) is one of the endogenous components that participate in freezing tolerance regulation. A combined metabolomic and transcriptomic characterization of NO-deficient nia1,2noa1-2 mutant plants suggests that NO acts attenuating the production and accumulation of osmoprotective and regulatory metabolites, such as sugars and polyamines, stress-related hormones, such as ABA and jasmonates, and antioxidants, such as anthocyanins and flavonoids. Accordingly, NO-deficient plants are constitutively more freezing tolerant than wild type plants.

Tissue-specific changes in apoplastic proteins and cell wall structure during cold acclimation of winter wheat crowns

The wheat (Triticum aestivum L.) crown is the critical organ of low temperature stress survival over winter. In cold-acclimated crowns, ice formation in the apoplast causes severe tissue disruption as it grows at the expense of intracellular water. While previous crown studies have shown the vascular transition zone (VTZ) to have a higher freezing sensitivity than the shoot apical meristem (SAM), the mechanism behind the differential freezing response is not fully understood. Cooling cold-acclimated crowns to -10 °C resulted in an absence of VTZ tetrazolium chloride staining, whereas the temperatures at which 50% of the SAM stained positive and 50% of plants recovered (LT50) were similar after cold acclimation for 21 (-16 °C) and 42 d (-20 °C) at 4 °C. Proteomic analysis of the apoplastic fluids identified dehydrins, vernalization-responsive proteins, and cold shock proteins preferentially accumulated in the SAM. In contrast, modifications to the VTZ centered on increases in pathogenesis-related proteins, anti-freeze proteins, and sugar hydrolyzing enzymes. Fourier transform infrared spectroscopy focal plane array analysis identified the biochemical modification of the cell wall to enhance methyl-esterified cross-linking of glucuronoarabinoxylans in the VTZ. These findings indicate that the SAM and VTZ express two distinct tissue-specific apoplastic responses during cold acclimation.

Freezing Tolerance of Plant Cells: From the Aspect of Plasma Membrane and Microdomain

Freezing stress is accompanied by a state change from water to ice and has multiple facets causing dehydration; consequently, hyperosmotic and mechanical stresses coupled with unfavorable chilling stress act in a parallel way. Freezing tolerance varies widely among plant species, and, for example, most temperate plants can overcome deleterious effects caused by freezing temperatures in winter. Destabilization and dysfunction of the plasma membrane are tightly linked to freezing injury of plant cells. Plant freezing tolerance increases upon exposure to nonfreezing low temperatures (cold acclimation). Recent studies have unveiled pleiotropic responses of plasma membrane lipids and proteins to cold acclimation. In addition, advanced techniques have given new insights into plasma membrane structural non-homogeneity, namely, microdomains. This chapter describes physiological implications of plasma membrane responses enhancing freezing tolerance during cold acclimation, with a focus on microdomains.

Plant Lectins and Lectin Receptor-Like Kinases: How Do They Sense the Outside?

Lectins are fundamental to plant life and have important roles in cell-to-cell communication; development and defence strategies. At the cell surface; lectins are present both as soluble proteins (LecPs) and as chimeric proteins: lectins are then the extracellular domains of receptor-like kinases (LecRLKs) and receptor-like proteins (LecRLPs). In this review; we first describe the domain architectures of proteins harbouring G-type; L-type; LysM and malectin carbohydrate-binding domains. We then focus on the functions of LecPs; LecRLKs and LecRLPs referring to the biological processes they are involved in and to the ligands they recognize. Together; LecPs; LecRLKs and LecRLPs constitute versatile recognition systems at the cell surface contributing to the detection of symbionts and pathogens; and/or involved in monitoring of the cell wall structure and cell growth.

Soluble and Membrane-Bound β-Glucosidases Are Involved in Trimming the Xyloglucan Backbone

In many flowering plants, xyloglucan is a major component of primary cell walls, where it plays an important role in growth regulation. Xyloglucan can be degraded by a suite of exoglycosidases that remove specific sugars. In this work, we show that the xyloglucan backbone, formed by (1→4)-linked β-d-glucopyranosyl residues, can be attacked by two different Arabidopsis (Arabidopsis thaliana) β-glucosidases from glycoside hydrolase family 3. While BGLC1 (At5g20950; for β-glucosidase active against xyloglucan 1) is responsible for all or most of the soluble activity, BGLC3 (At5g04885) is usually a membrane-anchored protein. Mutations in these two genes, whether on their own or combined with mutations in other exoglycosidase genes, resulted in the accumulation of partially digested xyloglucan subunits, such as GXXG, GXLG, or GXFG. While a mutation in BGLC1 had significant effects on its own, lack of BGLC3 had only minor effects. On the other hand, double bglc1 bglc3 mutants revealed a synergistic interaction that supports a role for membrane-bound BGLC3 in xyloglucan metabolism. In addition, bglc1 bglc3 was complemented by overexpression of either BGLC1 or BGLC3 In overexpression lines, BGLC3 activity was concentrated in a microsome-enriched fraction but also was present in soluble form. Finally, both genes were generally expressed in the same cell types, although, in some cases, BGLC3 was expressed at earlier stages than BGLC1 We propose that functional specialization could explain the separate localization of both enzymes, as a membrane-bound β-glucosidase could specifically digest soluble xyloglucan without affecting the wall-bound polymer.