The authors described the xylem regulatory network, and how changes in both salinity and iron can introduce perturbations, which in turn produced phenotypic changes in the secondary cell wall. There are a great deal of data available from previous studies on abiotic stress that detail the global response of gene transcription, and in some few cases an assessment of changes in cell wall phenotypes as a response.
Similar datasets are available for biotic stresses applied to diverse species and tissues. It is evident from the genetic studies reviewed above that similar gene family members e. However, the inherent complexity of the wall and the large number of genes involved in its synthesis and modification means that many details remain unclear in regards to the genetic and biochemical basis for cell wall responses to stress. Despite the obvious difficulties involved in comparing experiments between different species, stresses and tissues, in the final section of this review we have revisited public transcriptome datasets to highlight broad similarities between different stress types, and consider whether more attention might be focussed on putative cell wall-related genes that have been overlooked previously.
The previous sections of this review summarized research conducted on various aspects of cell wall reinforcement and modification during pathogen infection and abiotic stress.
Cell wall reinforcement in the form of papillae is a relatively common mechanism that determines the outcomes of infection. However, given the diversity of biotic stresses that can be exposed to a plant, any commonalities in papillae formation would likely be accompanied by a range of distinct cell wall-related defense responses.
The same might be expected for different abiotic stresses such as extreme temperature, salinity and flooding. Whilst it is currently not possible to perform a detailed review of all cell wall changes induced during the response to a range of different biotic and abiotic stresses, it is possible to perform a meta-analysis using publically available transcript expression data of plant-pathogen and plant-stress interactions in order to highlight overlaps in the responses of the cell wall machinery.
Protein family Pfam domains associated with the CAZy database annotations were used to identify barley carbohydrate-related genes present on the 22K Barley1 GeneChip. The normalized transcript levels for each carbohydrate-related gene from Arabidopsis and barley were compared following each stress relative to untreated controls within each experiment and represented as a log 2 -fold induction.
As might be expected from the previous sections of this review, many cell wall genes showed pronounced responses to the different stresses. In order to test whether these responses might be more generally conserved on a CAZy gene family level, the average fold induction observed across all family members was calculated and analyzed using the TIGR Multiexperiment Viewer MeV.
Hierarchical clustering was used to arrange gene families according to similarity in pattern of gene expression Figures 1A,B Eisen et al. Figures 1A,B clearly demonstrate that most CAZy gene families are upregulated in response to an abiotic or biotic stress in Arabidopsis and barley. Although not all CAZy families contain members that act on the same substrate, and the likelihood of all specialized family members responding in the same way is remote, this approach was targeted toward providing a simple means of identifying key carbohydrate-related activities that are shared between different stresses.
Similar behaviors of well-characterized and poorly-characterized CAZy families may provide useful insight into novel stress-related cell wall and carbohydrate-related changes.
To identify trends conserved in response to the stresses between Arabidopsis and barley, the fold induction for each gene family was averaged for all abiotic and all biotic stresses and presented in Figure 1C.
The comparative responses of these genes families to abiotic and biotic stress are shown in Figure 2 in both species. Figure 1. Analysis of cell wall-related transcripts following abiotic and biotic stresses in Arabidopsis A and barley B. Hierarchical clustering was performed based on the Pearson correlation coefficients across each dataset and CAZy family.
Trends conserved in response to the stresses between Arabidopsis and barley are observed in C which shows the average fold induction for each gene family for all abiotic and all biotic stresses in Arabidopsis and barley. Asterisks indicate gene families for which expression is upregulated by both abiotic and biotic stresses in Arabidopsis and barley. Figure 2. Graphical representation the average log 2 -fold induction for each gene family presented in Figure 1C , which shows the average abiotic x axis and average biotic y axis stress response in Arabidopsis A and barley B.
CAZy families that are upregulated in response to abiotic stresses, but not biotic stresses are colored red, CAZy families that are upregulated in response to biotic stresses, but not abiotic stresses are colored yellow, and CAZy families that are upregulated in response to both abiotic and biotic stresses are colored orange.
Two different clustering methods have been considered. Within each species the gene families can be clustered based on the correlation of their transcript profile across each experiment. Second, the experiment datasets can also be clustered based on the correlation of the gene family transcript profiles.
It is interesting to note that the abiotic stresses generally form a cluster together, as do the biotic stresses, even though there is a vast difference in the type of stress within each cluster Figures 1A,B. The dogma that the standard cell wall defense response is primarily driven by callose and the glucan synthase-like genes of the GT48 CAZy family is hard to support, given the large number of gene families that appear to be upregulated across most stresses. Even though the individual genes induced within each experiment are different, the clustering of CAZy families across the experiments suggests that there is a similar defense response mounted irrespective of the exact stress type.
We can see examples of biotrophic fungi inducing similar responses to necrotrophic fungi, drought stress inducing similar responses to cold stress and even examples across different tissues with nematodes in roots compared to whitefly infested leaves. Clustering the response of CAZy families identifies activities that appear to be generally upregulated across most of the experiments, and therefore cell wall or carbohydrate components may be altered similarly during the interaction.
Figure 1C depicts the average fold induction for each CAZy gene family across all of the Arabidopsis abiotic stresses, the Arabidopsis biotic stresses, the barley abiotic stresses and the barley biotic stresses. Twelve CAZy gene families are up-regulated on average across abiotic and biotic stresses in Arabidopsis and barley.
Some of these CAZy families have already been implicated in stress-responses as pathogenesis-related PR proteins. Pectin methylesterases modify the esterification status of pectin in the wall affecting the susceptibility of the cell wall barrier to fungal and bacterial CWDEs Collmer and Keen, De-esterification of pectin also influences the porosity of the plasmodesmata, which can alter the spread of signaling molecules during the defense response Chen et al.
The role of the CAZy glycosyltransferase families during the defense response is less characterized than the hydrolytic enzymes. The GT1 family includes a large number of genes with a wide range of putative functions including UDP-glucuronosyltransferase activity. By transferring sugars to a wide range of secondary metabolites, UGTs increase the stability and solubility of aglycones and therefore modify their bioactivity and effectiveness as regulators of the defense response Lim and Bowles, ; Langlois-Meurinne et al.
The GT8 family catalyse the transfer of diverse sugars Glc, Gal, GlcNAC, GalA onto lipo-oligosaccharide, protein, inositol, oligosaccharide or polysaccharide acceptors using nucleotide sugar substrates Yin et al.
Members of the family have been implicated in several different functions including the synthesis of pectins and xylan, and the raffinose family of oligosaccharides that play a role in stress response Kim et al. With the recent finding of arabinoxylan in the papillae produced by barley in response to the attempted penetration of Blumeria graminis f. Therefore, although this broad-brush meta-analysis of CAZy families during abiotic and biotic stress does not take into account differences in individual family member activity, gene family copy number or tissue-specific expression patterns, it identifies a set of CAZy families that are well-characterized in terms of stress response e.
Whether members of these CAZy families have specific or similar effects on cell-wall targets can be addressed by characterizing the function of the underlying genes. For example, up-regulated expression of the GT8 and GT61 families highlights a potential role of pectin and xylan synthesis in the plant stress response across both species.
However, these families contain members that are involved in many different processes and it is important to assess the expression and function of each gene in more detail. The majority of the Arabidopsis GT8 genes are up-regulated in response to at least one stress, but there appears to be subgroups that respond to specific stresses. Conversely the barley GT8 family is split into two groups, one containing genes that are unchanged or down-regulated in response to stress and the other containing genes which are up-regulated by most of the stresses.
Comparison of the stress responsive barley GT8 genes to characterized GT8 family members from Arabidopsis Figure 3C suggests that the general stress responsive barley genes are not restricted to clades with a single putative function, but are spread between the galactinol synthase GolS , xylan glucuronosyltransferase GUX and galacturonosyltransferase GAUT and GATL activities. There is no clear clustering of stress responsive and non-responsive genes as observed for the GT8 family, with GT61 genes upregulated in a number of different stresses for both Arabidopsis and barley.
Comparison of the stress responsive barley and Arabidopsis GT61 genes to other GT61 family members that have been functionally characterized Figure 4C indicates that the stress responsive genes are not restricted to clades with a single putative function i.
Once again this highlights the need for further characterization of cell-wall related genes in stress responses. Conserved changes in CAZy gene expression in different species may indicate that related genes are recruited to act on similar substrates during stress responses.
Alternatively, genes from the same family may be recruited to modify different substrates but in a similar way. Figure 3.
C Phylogenetic tree of GT8 family members from Arabidopsis and barley with putative functions assigned for each clade. Red dots highlight barley genes that are upregulated in response to stress B. Figure 4. C Phylogenetic tree of GT61 family members from Arabidopsis, barley, and rice with putative functions assigned for each clade. The basis for this review was to consider cell wall and polysaccharide-related activities that influence biotic and abiotic stress responses, and highlight those that might fulfill a common function in promoting remodeling of the cell wall as a direct response to abiotic stress or pathogen attack.
Genetic and transgenic evidence suggests that modification of specific cell wall activities has a pronounced effect on stress tolerance. In several cases, similar gene families appear to modulate the effect of distinct biotic and abiotic stresses within and across different species, implying that common mechanisms may have been recruited to target seemingly disparate stress types. This is supported by broader whole-transcriptome analyses, which indicate similar responses of individual cell-wall related genes and even CAZy families to different abiotic and biotic stresses.
Whether these overlaps in gene expression lead to similar changes in cell wall structure has yet to be confirmed in most cases, particularly in the case of pectins and xylans which show distinct differences in abundance between monocot and dicot models.
Indeed, the functions of many cell wall-related genes have yet to be reported during normal growth and development, let alone during stress responses. This highlights a need to further extend genome editing technologies toward entire CAZy families, and to develop methodologies for chemical cell wall analysis that are high-throughput and capable of being targeted toward single cell-types. KH, AL, MT, NS, and JC conceived this review, drafted and revised the manuscript, approved the final version prior to publication and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Abebe, T. Drought response in the spikes of barley: gene expression in the lemma, palea, awn, and seed.
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Genomics , — Arabidopsis wat1 walls are thin1 -mediated resistance to the bacterial vascular pathogen, Ralstonia solanacearum , is accompanied by cross-regulation of salicylic acid and tryptophan metabolism. Deubert, K. Zuckerman, W. Mai, and R. Google Scholar. More commonly, however, additional substances, especially lignin , are found in the secondary wall. Lignin is the general name for a group of polymers of aromatic alcohols that are hard and impart considerable strength to the structure of the secondary wall.
Lignin is what provides the favorable characteristics of wood to the fiber cells of woody tissues and is also common in the secondary walls of xylem vessels, which are central in providing structural support to plants. Lignin also makes plant cell walls less vulnerable to attack by fungi or bacteria, as do cutin , suberin , and other waxy materials that are sometimes found in plant cell walls.
A specialized region associated with the cell walls of plants, and sometimes considered an additional component of them, is the middle lamella see Figure 1. Rich in pectins, the middle lamella is shared by neighboring cells and cements them firmly together.
Positioned in such a manner, cells are able to communicate with one another and share their contents through special conduits. Termed plasmodesmata , these small passages penetrate the middle lamella as well as the primary and secondary cell walls, providing pathways for transporting cytoplasmic molecules from one cell to another. License Info. Image Use. Custom Photos. Site Info. Contact Us. The Galleries:. Photo Gallery.
Previous approaches to modeling plant cell walls were focused either at a scale too large to incorporate the behavior of individual cell components or at a scale too small — the atomic level — to incorporate actual mechanics of the wall.
In this study, the researchers used a coarse-grained computer model at the level of the polymers that make up the cell wall — the strings of cellulose and other sugar molecules that are linked together in long chains.
Instead of modeling individual atoms, the researchers represented cellulose microfibers and other components with chains of beads that behave like sticky springs, in order to replicate these components' physical properties. The team specifically modeled layers of an onion cell wall so that they could compare their modeled values of mechanical characteristics to experiments they conducted with actual onion skins. New research by Penn State biologists models the plant cell wall and reveal's the molecular basis for its unique ability to expand without weakening or breaking.
Credit: Penn State. The researchers determined that individual cellulose fibers align with and stick to each other, forming a network of bundles. In this study, we clarified the roles of the various components in the plant cell wall and provide a quantitative framework for interpreting experiments used in plant research.
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