The gene for a xyloglucan endotransglucosylase/hydrolase from Cicer arietinum is strongly expressed in elongating tissues

Combined transcriptome and proteome analysis reveal the key physiological processes in seed germination stimulated by decreased salinity in the seagrass Zostera marina L

BACKGROUND

Zostera marina L., or eelgrass, is the most widespread seagrass species throughout the temperate northern hemisphere. Unlike the dry seeds of terrestrial plants, eelgrass seeds must survive in water, and salinity is the key factor influencing eelgrass seed germination. In the present study, transcriptome and proteome analysis were combined to investigate the mechanisms via which eelgrass seed germination was stimulated by low salinity, in addition to the dynamics of key metabolic pathways under germination.

RESULTS

According to the results, low salinity stimulated the activation of Ca2+ signaling and phosphatidylinositol signaling, and further initiated various germination-related physiological processes through the MAPK transduction cascade. Starch, lipids, and storage proteins were mobilized actively to provide the energy and material basis for germination; abscisic acid synthesis and signal transduction were inhibited whereas gibberellin synthesis and signal transduction were activated, weakening seed dormancy and preparing for germination; cell wall weakening and remodeling processes were activated to provide protection for cotyledon protrusion; in addition, multiple antioxidant systems were activated to alleviate oxidative stress generated during the germination process; ERF transcription factor has the highest number in both stages suggested an active role in eelgrass seed germination.

CONCLUSION

In summary, for the first time, the present study investigated the mechanisms by which eelgrass seed germination was stimulated by low salinity and analyzed the transcriptomic and proteomic features during eelgrass seed germination comprehensively. The results of the present study enhanced our understanding of seagrass seed germination, especially the molecular ecology of seagrass seeds.

Genome-wide identification of xyloglucan endotransglucosylase/hydrolase gene family members in peanut and their expression profiles during seed germination

Seed germination marks the beginning of a new plant life cycle. Improving the germination rate of seeds and the consistency of seedling emergence in the field could improve crop yields. Many genes are involved in the regulation of seed germination. Our previous study found that some peanut XTHs (xyloglucan endotransglucosylases/hydrolases) were expressed at higher levels at the newly germinated stage. However, studies of the XTH gene family in peanut have not been reported. In this study, a total of 58 AhXTH genes were identified in the peanut genome. Phylogenetic analysis showed that these AhXTHs, along with 33 AtXTHs from Arabidopsis and 61 GmXTHs from soybean, were classified into three subgroups: the I/II, IIIA and IIIB subclades. All AhXTH genes were unevenly distributed on the 18 peanut chromosomes, with the exception of chr. 07 and 17, and they had relatively conserved exon-intron patterns, most with three to four introns. Through chromosomal distribution pattern and synteny analysis, it was found that the AhXTH family experienced many replication events, including 42 pairs of segmental duplications and 23 pairs of tandem duplications, during genome evolution. Conserved motif analysis indicated that their encoded proteins contained the conserved ExDxE domain and N-linked glycosylation sites and displayed the conserved secondary structural loops 1-3 in members of the same group. Expression profile analysis of freshly harvested seeds, dried seeds, and newly germinated seeds using transcriptome data revealed that 26 AhXTH genes, which account for 45% of the gene family, had relatively higher expression levels at the seed germination stage, implying the important roles of AhXTHs in regulating seed germination. The results of quantitative real-time PCR also confirmed that some AhXTHs were upregulated during seed germination. The results of GUS histochemical staining showed that AhXTH4 was mainly expressed in germinated seeds and etiolated seedlings and had higher expression levels in elongated hypocotyls. AhXTH4 was also verified to play a crucial role in the cell elongation of hypocotyls during seed germination.

Distinct transcriptional profiles of ozone stress in soybean (Glycine max) flowers and pods

BACKGROUND

Tropospheric ozone (O3) is a secondary air pollutant and anthropogenic greenhouse gas. Concentrations of tropospheric O3 ([O3] have more than doubled since the Industrial Revolution, and are high enough to damage plant productivity. Soybean (Glycine max L. Merr.) is the world's most important legume crop and is sensitive to O3. Current ground-level [O3] are estimated to reduce global soybean yields by 6% to 16%. In order to understand transcriptional mechanisms of yield loss in soybean, we examined the transcriptome of soybean flower and pod tissues exposed to elevated [O3] using RNA-Sequencing.

RESULTS

Elevated [O3] elicited a strong transcriptional response in flower and pod tissues, with increased expression of genes involved in signaling in both tissues. Flower tissues also responded to elevated [O3] by increasing expression of genes encoding matrix metalloproteinases (MMPs). MMPs are zinc- and calcium-dependent endopeptidases that have roles in programmed cell death, senescence and stress response in plants. Pod tissues responded to elevated [O3] by increasing expression of xyloglucan endotransglucosylase/hydrolase genes, which may be involved with increased pod dehiscence in elevated [O3].

CONCLUSIONS

This study established that gene expression in reproductive tissues of soybean are impacted by elevated [O3], and flowers and pods have distinct transcriptomic responses to elevated [O3].

Evaluation of the hormonal state of columnar apple trees (Malus x domestica) based on high throughput gene expression studies

The columnar phenotype of apple trees (Malus x domestica) is characterized by a compact growth habit with fruit spurs instead of lateral branches. These properties provide significant economic advantages by enabling high density plantings. The columnar growth results from the presence of a dominant allele of the gene Columnar (Co) located on chromosome 10 which can appear in a heterozygous (Co/co) or homozygous (Co/Co) state. Although two deep sequencing approaches could shed some light on the transcriptome of columnar shoot apical meristems (SAMs), the molecular mechanisms of columnar growth are not yet elaborated. Since the influence of phytohormones is believed to have a pivotal role in the establishment of the phenotype, we performed RNA-Seq experiments to study genes associated with hormone homeostasis and clearly affected by the presence of Co. Our results provide a molecular explanation for earlier findings on the hormonal state of columnar apple trees. Additionally, they allow hypotheses on how the columnar phenotype might develop. Furthermore, we show a statistically approved enrichment of differentially regulated genes on chromosome 10 in the course of validating RNA-Seq results using additional gene expression studies.

Physiological and molecular analysis of polyethylene glycol-induced reduction of aluminium accumulation in the root tips of common bean (Phaseolus vulgaris)

• Aluminium (Al) toxicity and drought are two major stress factors limiting common bean (Phaseolus vulgaris) production on tropical acid soils. Polyethylene glycol (PEG) treatment reduces Al uptake and Al toxicity. • The effect of PEG 6000-induced osmotic stress on the expression of genes was studied using SuperSAGE combined with next-generation sequencing and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) for selected genes. • Less Al stress in PEG-treated roots was confirmed by decreased Al-induced up-regulation of MATE and ACCO genes. The withdrawal of PEG from the Al treatment solution restored the Al accumulation and reversed the expression of MATE and ACCO genes to the level of the treatment with Al alone. Using SuperSAGE, we identified 611 up- and 728 down-regulated genes in PEG-treated root tips, and the results were confirmed by qRT-PCR using 46 differentially expressed genes. Among the 12 genes studied in more detail, XTHa and BEG (down-regulated by PEG) and HRGP, bZIP, MYB and P5CS (up-regulated by PEG) recovered completely within 2 h after removal of PEG stress. • The results suggest that genes related to cell wall assembly and modification, such as XTHs, BEG and HRGP, play important roles in the PEG-induced decrease in cell wall porosity, leading to reduced Al accumulation in root tips.

Xyloglucan endo-transglycosylase-mediated xyloglucan rearrangements in developing wood of hybrid aspen

Xyloglucan endo-transglycosylases (XETs) encoded by xyloglucan endo-transglycosylases/hydrolase (XTH) genes modify the xyloglucan-cellulose framework of plant cell walls, thereby regulating their expansion and strength. To evaluate the importance of XET in wood development, we studied xyloglucan dynamics and XTH gene expression in developing wood and modified XET activity in hybrid aspen (Populus tremula × tremuloides) by overexpressing PtxtXET16-34. We show that developmental modifications during xylem differentiation include changes from loosely to tightly bound forms of xyloglucan and increases in the abundance of fucosylated xyloglucan epitope recognized by the CCRC-M1 antibody. We found that at least 16 Populus XTH genes, all likely encoding XETs, are expressed in developing wood. Five genes were highly and ubiquitously expressed, whereas PtxtXET16-34 was expressed more weakly but specifically in developing wood. Transgenic up-regulation of XET activity induced changes in cell wall xyloglucan, but its effects were dependent on developmental stage. For instance, XET overexpression increased abundance of the CCRC-M1 epitope in cambial cells and xylem cells in early stages of differentiation but not in mature xylem. Correspondingly, an increase in tightly bound xyloglucan content was observed in primary-walled xylem but a decrease was seen in secondary-walled xylem. Thus, in young xylem cells, XET activity limits xyloglucan incorporation into the tightly bound wall network but removes it from cell walls in older cells. XET overexpression promoted vessel element growth but not fiber expansion. We suggest that the amount of nascent xyloglucan relative to XET is an important determinant of whether XET strengthens or loosens the cell wall.

Differences in enzymic properties of five recombinant xyloglucan endotransglucosylase/hydrolase (XTH) proteins of Arabidopsis thaliana

Xyloglucan endotransglucosylase/hydrolases (XTHs) are cell wall enzymes that are able to graft xyloglucan chains to oligosaccharides or to other available xyloglucan chains and/or to hydrolyse xyloglucan chains. As they are involved in the modification of the load-bearing cell-wall components, they are believed to be very important in the regulation of growth and development. Given the large number (33) of XTH genes in Arabidopsis and the overlapping expression patterns, specific enzymic properties may be expected. Five predominantly root-expressed Arabidopsis thaliana XTHs belonging to subgroup I/II were analysed here. These represent two sets of closely related genes: AtXTH12 and 13 on the one hand (trichoblast-enriched) and AtXTH17, 18, and 19 on the other (expressed in nearly all cell types in the root). They were all recombinantly produced in the yeast Pichia pastoris and partially purified by ammonium sulphate precipitation before they were subsequently all subjected to a series of identical in vitro tests. The kinetic properties of purified AtXTH13 were investigated in greater detail to rule out interference with the assays by contaminating yeast proteins. All five proteins were found to exhibit only the endotransglucosylase (XET; EC 2.4.1.207) activity towards xyloglucan and non-detectable endohydrolytic (XEH; EC 3.2.1.151) activity. Their endotransglucosylase activity was preferentially directed towards xyloglucan and, in some cases, water-soluble cellulose acetate, rather than to mixed-linkage β-glucan. Isoforms differed in optimum pH (5.0-7.5), in temperature dependence and in acceptor substrate preferences.

The immunolocation of XTH1 in embryonic axes during chickpea germination and seedling growth confirms its function in cell elongation and vascular differentiation

In a previous work, the immunolocation of the chickpea XTH1 (xyloglucan endotransglucosylase/hydrolase 1) protein in the cell walls of epicotyls, radicles, and stems was studied, and a role for this protein in the elongation of vascular cells was suggested. In the present work, the presence and the location of the XTH1 protein in embryonic axes during the first 48 h of seed imbibition, including radicle emergence, were studied. The presence of the XTH1 protein in the cell wall of embryonic axes as early as 3 h after imbibition, before radicle emergence, supports its involvement in germination, and the fact that the protein level increased until 24 h, when the radicle had already emerged, also suggests its participation in the elongation of embryonic axes. The localization of XTH1 clearly indicates that the protein is related to the development of vascular tissue in embryonic axes during the period studied, suggesting that the role of this protein in embryonic axes is the same as that proposed for seedlings and plants.

Inducible expression of Pisum sativum xyloglucan fucosyltransferase in the pea root cap meristem, and effects of antisense mRNA expression on root cap cell wall structural integrity

Mitosis and cell wall synthesis in the legume root cap meristem can be induced and synchronized by the nondestructive removal of border cells from the cap periphery. Newly synthesized cells can be examined microscopically as they differentiate progressively during cap development, and ultimately detach as a new population of border cells. This system was used to demonstrate that Pisum sativum L. fucosyl transferase (PsFut1) mRNA expression is strongly expressed in root meristematic tissues, and is induced >2-fold during a 5-h period when mitosis in the root cap meristem is increased. Expression of PsFut1 antisense mRNA in pea hairy roots under the control of the CaMV35S promoter, which exhibits meristem localized expression in pea root caps, resulted in a 50-60% reduction in meristem localized endogenous PsFut1 mRNA expression measured using whole mount in situ hybridization. Changes in gross levels of cell wall fucosylated xyloglucan were not detected, but altered surface localization patterns were detected using whole mount immunolocalization with CCRC-M1, an antibody that recognizes fucosylated xyloglucan. Emerging hairy roots expressing antisense PsFut1 mRNA appeared normal macroscopically but scanning electron microscopy of tissues with altered CCRC-M1 localization patterns revealed wrinkled, collapsed cell surfaces. As individual border cells separated from the cap periphery, cell death occurred in correlation with extrusion of cellular contents through breaks in the wall.

Involvement of the xyloglucan endotransglycosylase/hydrolases encoded by celery XTH1 and Arabidopsis XTH33 in the phloem response to aphids

During infestation, phloem-feeding insects induce transcriptional reprogramming in plants that may lead to protection. Transcripts of the celery XTH1 gene, encoding a xyloglucan endotransglycosylase/hydrolase (XTH), were previously found to accumulate systemically in celery (Apium graveolens) phloem, following infestation with the generalist aphid Myzus persicae. XTH1 induction was specific to the phloem but was not correlated with an increase in xyloglucan endotransglycosylase (XET) activity in the phloem. XTH1 is homologous to the Arabidopsis thaliana XTH33 gene. XTH33 expression was investigated following M. persicae infestation. The pattern of XTH33 expression is tightly controlled during development and indicates a possible role in cell expansion. An xth33 mutant was assayed for preference assay with M. persicae. Aphids settled preferentially on the mutant rather than on the wild type. This suggests that XTH33 is involved in protecting plants against aphids; therefore, that cell wall modification can alter the preference of aphids for a particular plant. Nevertheless, the ectopic expression of XTH33 in phloem tissue was not sufficient to confer protection, demonstrating that modifying the expression of this single gene does not readily alter plant-aphid interactions.

Transcriptional profiling of cell wall protein genes in chickpea embryonic axes during germination and growth

Cell wall hydrolases have been assumed to be involved in the regulation of seed germination, mostly through their contribution to the cell wall disassembly associated with endosperm cap weakening. In Cicer arietinum (a non-endospermic leguminosae seed), we have focused our research directly on the elongation process of the embryonic axes themselves during germination. The genes encoding cell wall proteins, previously implicated in the elongation of chickpea epicotyls, might also be involved in the expansion of embryonic axis cells, and the modulation of their expression could be part of the control of the germinative process. Thus, chickpea alpha-expansins and xyloglucan endotransglycosylase/hydrolase (XTH) acting on the cellulose/xyloglucan network seem to be involved in the elongation of both chickpea epicotyls and embryonic axes, although the products of different genes perform their actions on each organ. Among the four known cDNAs encoding chickpea alpha-expansins, Ca-EXPA1 was the only isoform highly expressed in embryonic axes during germination. In contrast to epicotyl elongation, the genes encoding cell wall beta-galactosidases, involved in pectin degradation, were not expressed during germination, suggesting no role in embryonic axis elongation, mainly due to the different metabolism of pectins during cell wall loosening in embryonic axis or epicotyl cells. The results concerning CanST-1 and -2, encoding two growth-related cell wall proteins, suggested that these genes were not involved in elongation of embryonic axes during germination. The transcription pattern of Cap28, which encodes a glutamic acid rich cell wall protein of unknown function, indicated a role in the development of the embryonic axes during germination.

Differential expression of litchi XET genes in relation to fruit growth

Xyloglucan endotransglycosylase (XET) catalyses the transglycosylation of xyloglucan, the major hemicellulose polymer, which has been thought to mediate the cross-linking of cellulose microfibrils in cellular walls and proposed to be involved in the control of cell wall relaxation. To understand the relationship between litchi fruit cracking and gene expression patterns, three XET genes from litchi fruit were identified and then examined for their expression profiles in pericarp and aril tissues at different development stages, using a cracking-resistant cultivar, 'Huaizhi', and a cracking-susceptible cultivar, 'Nuomici'. Three full-length cDNAs of 1267, 1095 and 1156 bp encoding XETs, named LcXET1, LcXET2 and LcXET3, respectively, were isolated from expanding fruit using RT-PCR and RACE-PCR (rapid amplification of cDNA ends) methods. Northern blotting analysis showed that LcXET1 mRNA accumulation occurred much earlier in aril tissues at 59 days after anthesis (DAA) than in pericarp tissues at 73 DAA in 'Nuomici'. However, it appeared at almost the same time (66 DAA) in pericarp and aril tissues in 'Huaizhi', which suggested that differential accumulation of LcXET1 in pericarp and aril tissues in 'Nuomici' and 'Huaizhi' was closely associated with fruit cracking. LcXET2 mRNA accumulation could be detected in pericarp and aril tissues throughout fruit development but exhibited a differential accumulation pattern between pericarp and aril tissues. In the aril of 'Nuomici', intensive signal bands were detectable at 59-73 DAA in rapidly expanding fruits of 'Nuomici' but only weak bands could be found in the pericarp tissues. In contrast, moderate signal bands were detectable both in pericarp and aril tissues of 'Huaizhi' fruits. Furthermore, LcXET3 showed constitutive expression in both pericarp and aril tissues of developing 'Nuomici' and 'Huaizhi' litchi fruit. In addition, differential expression patterns of three XETs genes were observed in different tissues of litchi, with only LcXET1 being fruit-specific. To further address the role of LcXET in fruit cracking, alpha-naphthalene acetic acid (NAA) was used to treat 'Nuomoci' to reduce fruit cracking. Enhanced LcXET1 mRNA accumulation appeared in pericarp while LcXET2 and LcXET3 mRNA accumulation enhanced in aril tissues in the NAA-treated fruits. Thus, LcXET1 is more likely to play a role in reducing litchi fruit cracking than LcXET2 and LcXET3.

BcXTH1, a Brassica campestris homologue of Arabidopsis XTH9, is associated with cell expansion

Xyloglucan endotransglucosylase/hydrolases (XTHs) are a group of the enzymes that are responsible for reorganization of the cellulose-xyloglucan framework by catalyzing cleavage and religation of the xyloglucan chains in the plant cell wall. In this study, we report the isolation and characterization of a XTH gene from a pistil cDNA library of Brassica campestris. Sequence analysis of the gene, designated BcXTH1, revealed that it is homologous to the XTH9 gene of Arabidopsis. The highly conserved domain (DEIDFEFLG) found among all XTHs was also present in BcXTH1 but with the two amino acid substitutions (NEFDFEFLG) also found in Arabidopsis XTH9. These results suggest that BcXTH1 is the B. campestris homologue of XTH9. Expression analysis of BcXTH1 revealed that it was expressed in most of the plant organs. In situ hybridization showed that the gene is highly expressed in the floral primodia, especially in the epidermal cell layer. Southern blot analysis indicated that the BcXTH1 gene exists as a multi-copy gene in the B. campestris genome. The function of the BcXTH1 gene was deduced from using an overexpression strategy in Arabidopsis. Interestingly, the transgenic plants showed a pronounced cell expansion phenotype. Immunoelectron microscopy shows that BcXTH1 is localized almost exclusively to the cell wall, supporting our conclusion that it participates in the regulation of cell expansion in B. campestris.