Disrupting cell wall integrity impacts endomembrane trafficking to promote secretion over endocytic trafficking

The plant cell wall provides a strong yet flexible barrier to protect cells from the external environment. Modifications of the cell wall, either during development or under stress conditions, can induce cell wall integrity responses and ultimately lead to alterations in gene expression, hormone production, and cell wall composition. These changes in cell wall composition presumably require remodelling of the secretory pathway to facilitate synthesis and secretion of cell wall components and cell wall synthesis enzymes from the Golgi apparatus. Here, we used a combination of live-cell confocal imaging and transmission electron microscopy to examine the short-term and constitutive impact of isoxaben, which reduces cellulose biosynthesis, and Driselase, a cocktail of cell-wall-degrading fungal enzymes, on cellular processes during cell wall integrity responses. We show that both treatments altered organelle morphology and triggered rebalancing of the secretory pathway to promote secretion while reducing endocytic trafficking. The actin cytoskeleton was less dynamic following cell wall modification, and organelle movement was reduced. These results demonstrate active remodelling of the endomembrane system and actin cytoskeleton following changes to the cell wall.

The Cell Wall Proteome of Craterostigma plantagineum Cell Cultures Habituated to Dichlobenil and Isoxaben

The remarkable desiccation tolerance of the vegetative tissues in the resurrection species Craterostigma plantagineum (Hochst.) is favored by its unique cell wall folding mechanism that allows the ordered and reversible shrinking of the cells without damaging neither the cell wall nor the underlying plasma membrane. The ability to withstand extreme drought is also maintained in abscisic acid pre-treated calli, which can be cultured both on solid and in liquid culture media. Cell wall research has greatly advanced, thanks to the use of inhibitors affecting the biosynthesis of e.g., cellulose, since they allowed the identification of the compensatory mechanisms underlying habituation. Considering the innate cell wall plasticity of C. plantagineum, the goal of this investigation was to understand whether habituation to the cellulose biosynthesis inhibitors dichlobenil and isoxaben entailed or not identical mechanisms as known for non-resurrection species and to decipher the cell wall proteome of habituated cells. The results showed that exposure of C. plantagineum calli/cells triggered abnormal phenotypes, as reported in non-resurrection species. Additionally, the data demonstrated that it was possible to habituate Craterostigma cells to dichlobenil and isoxaben and that gene expression and protein abundance did not follow the same trend. Shotgun and gel-based proteomics revealed a common set of proteins induced upon habituation, but also identified candidates solely induced by habituation to one of the two inhibitors. Finally, it is hypothesized that alterations in auxin levels are responsible for the increased abundance of cell wall-related proteins upon habituation.

Corrigendum: Loss of the Arabidopsis Protein Kinases ANPs Affects Root Cell Wall Composition, and Triggers the Cell Wall Damage Syndrome

[This corrects the article DOI: 10.3389/fpls.2017.02234.].

Gain-of-function mutation of AtDICE1, encoding a putative endoplasmic reticulum-localized membrane protein, causes defects in anisotropic cell elongation by disturbing cell wall integrity in Arabidopsis

Background and Aims

Anisotropic cell elongation depends on cell wall relaxation and cellulose microfibril arrangement. The aim of this study was to characterize the molecular function of AtDICE1 encoding a novel transmembrane protein involved in anisotropic cell elongation in Arabidopsis.

Methods

Phenotypic characterizations of transgenic Arabidopsis plants mis-regulating AtDICE1 expression with different pharmacological treatments were made, and biochemical, cell biological and transcriptome analyses were performed.

Key Results

Upregulation of AtDICE1 in Arabidopsis (35S::AtDICE1) resulted in severe dwarfism, probably caused by defects in anisotropic cell elongation. Epidermal cell swelling was evident in all tissues, and abnormal secondary wall thickenings were observed in pith cells of stems. These phenotypes were reproduced not only by inducible expression of AtDICE1 but also by overexpression of its poplar homologue in Arabidopsis. RNA interference suppression lines of AtDICE1 resulted in no observable phenotypic changes. Interestingly, wild-type plants treated with isoxaben, a cellulose biosynthesis inhibitor, phenocopied the 35S::AtDICE1 plants, suggesting that cellulose biosynthesis was compromised in the 35S::AtDICE1 plants. Indeed, disturbed cortical microtubule arrangements in 35S::AtDICE1/GFP-TuA6 plants were observed, and the cellulose content was significantly reduced in 35S::AtDICE1 plants. A promoter::GUS analysis showed that AtDICE1 is mainly expressed in vascular tissue, and transient expression of GFP:AtDICE1 in tobacco suggests that AtDICE1 is probably localized in the endoplasmic reticulum (ER). In addition, the external N-terminal conserved domain of AtDICE1 was found to be necessary for AtDICE1 function. Whole transcriptome analyses of 35S::AtDICE1 revealed that many genes involved in cell wall modification and stress/defence responses were mis-regulated.

Conclusions

AtDICE1, a novel ER-localized transmembrane protein, may contribute to anisotropic cell elongation in the formation of vascular tissue by affecting cellulose biosynthesis.

Grass cell walls have a role in the inherent tolerance of grasses to the cellulose biosynthesis inhibitor isoxaben

BACKGROUND

Cellulose biosynthesis inhibitors (CBIs) are pre-emergence herbicides that inhibit anisotropic cell expansion resulting in a severely swollen and stunted growth phenotype. Resistance to group 21 CBIs, such as isoxaben, is conferred by missense mutations in CELLOSE SYNTHASE A (CesA) genes required for primary cell wall synthesis, concluding that this is their in vivo target.

RESULTS

Herein, we show that grasses exhibit tolerance to group 21 CBIs and explore the mechanism of tolerance to isoxaben in the grass Brachypodium distachyon (L.). Comparative genomics failed to identify synonymous point mutations that have been found to confer isoxaben resistance in the dicot Arabidopsis thaliana (L.). Brachypodium did not metabolize ¹⁴ C-isoxaben. We next explored the role of grass-specific non-cellulosic cell wall components, specifically the hemicellulose polysaccharide mix linkage glucans (MLG), as a potential tolerance mechanism by compensating for the loss of cellulose during cell elongation. A partial-transcriptional knockdown T-DNA insertion was found in a key MLG synthesis gene, Cellulose synthase-like F6 (CslF6) and this mutant was found to be 2.1 times more sensitive to isoxaben than wild-type plants.

CONCLUSION

These data suggest that the composition and compensatory response of grass cell walls may be a factor in conferring tolerance to group 21 CBIs. © 2017 Society of Chemical Industry.

Alleles Causing Resistance to Isoxaben and Flupoxam Highlight the Significance of Transmembrane Domains for CESA Protein Function

The cellulose synthase (CESA) proteins in Arabidopsis play an essential role in the production of cellulose in the cell walls. Herbicides such as isoxaben and flupoxam specifically target this production process and are prominent cellulose biosynthesis inhibitors (CBIs). Forward genetic screens in Arabidopsis revealed that mutations that can result in varying degrees of resistance to either isoxaben or flupoxam CBI can be attributed to single amino acid substitutions in primary wall CESAs. Missense mutations were almost exclusively present in the predicted transmembrane regions of CESA1, CESA3, and CESA6. Resistance to isoxaben was also conferred by modification to the catalytic residues of CESA3. This resulted in cellulose deficient phenotypes characterized by reduced crystallinity and dwarfism. However, mapping of mutations to the transmembrane regions also lead to growth phenotypes and altered cellulose crystallinity phenotypes. These results provide further genetic evidence supporting the involvement of CESA transmembrane regions in cellulose biosynthesis.

T-DNA alleles of the receptor kinase THESEUS1 with opposing effects on cell wall integrity signaling

Perturbation of cellulose synthesis in plants triggers stress responses, including growth retardation, mediated by the cell wall integrity-sensing receptor-like kinase (RLK) THESEUS1 (THE1). The analysis of two alleles carrying T-DNA insertions at comparable positions has led to conflicting conclusions concerning the impact of THE1 signaling on growth. Here we confirm that, unlike the1-3 and other the1 alleles in which cellular responses to genetic or pharmacological inhibition of cellulose synthesis are attenuated, the1-4 showed enhanced responses, including growth inhibition, ectopic lignification, and stress gene expression. Both the1-3 and the1-4 express a transcript encoding a predicted membrane-associated truncated protein lacking the kinase domain. However, the1-3, in contrast to the1-4, strongly expresses antisense transcripts, which are expected to prevent the expression of the truncated protein as suggested by the genetic interactions between the two alleles. Seedlings overexpressing such a truncated protein react to isoxaben treatment similarly to the1-4 and the full-length THE overexpressor. We conclude that the1-4 is a hypermorphic allele; that THE1 signaling upon cell wall damage has a negative impact on cell expansion; and that caution is required when interpreting the phenotypic effects of T-DNA insertions in RLK genes.

The cell wall of Arabidopsis thaliana influences actin network dynamics

In plant cells, molecular connections link the cell wall-plasma membrane-actin cytoskeleton to form a continuum. It is hypothesized that the cell wall provides stable anchor points around which the actin cytoskeleton remodels. Here we use live cell imaging of fluorescently labelled marker proteins to quantify the organization and dynamics of the actin cytoskeleton and to determine the impact of disrupting connections within the continuum. Labelling of the actin cytoskeleton with green fluorescent protein (GFP)-fimbrin actin-binding domain 2 (FABD2) resulted in a network composed of fine filaments and thicker bundles that appeared as a highly dynamic remodelling meshwork. This differed substantially from the GFP-Lifeact-labelled network that appeared much more sparse with thick bundles that underwent 'simple movement', in which the bundles slightly change position, but in such a manner that the structure of the network was not substantially altered during the time of observation. Label-dependent differences in actin network morphology and remodelling necessitated development of two new image analysis techniques. The first of these, 'pairwise image subtraction', was applied to measurement of the more rapidly remodelling actin network labelled with GFP-FABD2, while the second, 'cumulative fluorescence intensity', was used to measure bulk remodelling of the actin cytoskeleton when labelled with GFP-Lifeact. In each case, these analysis techniques show that the actin cytoskeleton has a decreased rate of bulk remodelling when the cell wall-plasma membrane-actin continuum is disrupted either by plasmolysis or with isoxaben, a drug that specifically inhibits cellulose deposition. Changes in the rate of actin remodelling also affect its functionality, as observed by alteration in Golgi body motility.

Loss of the Arabidopsis Protein Kinases ANPs Affects Root Cell Wall Composition, and Triggers the Cell Wall Damage Syndrome

The Arabidopsis NPK1-related Protein kinases ANP1, ANP2 and ANP3 belong to the MAP kinase kinase kinase (MAPKKK) superfamily and were previously described to be crucial for cytokinesis, elicitor-induced immunity and development. Here we investigate the basis of their role in development by using conditional β-estradiol-inducible triple mutants to overcome lethality. In seedlings, lack of ANPs causes root cell bulging, with the transition zone being the most sensitive region. We uncover a role of ANPs in the regulation of cell wall composition and suggest that developmental defects of the triple mutants, observed at the cellular level, might be a consequence of the alterations of the pectic and cellulosic cell wall components. Lack of ANPs also induced a typical cell wall damage syndrome (CWDS) similar to that observed in plants treated with the cellulose biosynthesis inhibitor isoxaben (ISX). Moreover, anp double mutants and plants overexpressing single ANPs (ANP1 or ANP3) respectively showed increased and reduced accumulation of jasmonic acid and PDF1.2 transcripts upon ISX treatment, suggesting that ANPs are part of the pathway targeted by this inhibitor and play a role in cell wall integrity surveillance. Highlights: The loss of ANP function affects cell wall composition and leads to typical cell wall damage-induced phenotypes, such as ectopic lignification and jasmonic acid accumulation.

Methiozolin sorption and mobility in sand-based root zones

BACKGROUND

Methiozolin is a herbicide currently used for annual bluegrass control in golf course putting greens. Previous research indicates that maximum weed control efficacy requires root exposure; however, soil sorption and mobility of methiozolin have not been established. Research was conducted to investigate soil sorption and subsequent desorption by dilution of methiozolin, as well as soil mobility using batch equilibrium experiments and thin-layer chromatography in nine root zones. Evaluations focused on sand-based systems typical of many golf course putting greens.

RESULTS

Sorption coefficients (Kd values) ranged from 0.4 to 29.4 mL g(-1) and averaged 13.8 mL g(-1) . Sorption was most influenced by organic matter content; conversely, soil pH had a negligible effect. Methiozolin desorption did not occur with a 0.01 M CaCl2 dilution. Methiozolin mobility was low; retardation factors (Rf values) were <0.05 for all media with ≥0.3% organic matter. Sand (0.1% organic matter) resulted in an Rf value of 0.46.

CONCLUSION

Approximately 24% of applied methiozolin is available for root uptake, and mobility is limited, suggesting resistance to loss through leaching displacement.

Enhanced resistance to the cellulose biosynthetic inhibitors, thaxtomin A and isoxaben in Arabidopsis thaliana mutants, also provides specific co-resistance to the auxin transport inhibitor, 1-NPA

BACKGROUND

Thaxtomin A (TA) is a phytotoxin produced by plant pathogenic Streptomyces spp. responsible for potato common scab. TA inhibits cellulose biosynthesis in expanding plant tissues and is essential for disease induction. Auxin treatment of various plant tissues has been repeatedly demonstrated to inhibit TA toxicity and to reduce common scab. This work utilises Arabidopsis thaliana mutants with resistance to cellulose biosynthesis inhibitors (CBIs) to investigate the interaction between TA, other CBIs and auxins.

RESULTS

Three CBI resistant A. thaliana mutants; txr1-1 (tolerance to TA), ixr1-1 (tolerance to isoxaben - IXB) and KOR1 (cellulose deficiency), showed no altered root growth response to treatment with natural or synthetic auxins, nor with the auxin efflux transport inhibitor 2,3,5-Triiodobenzoic acid (TIBA). However, all mutants had significantly enhanced tolerance to 1-napthylphthalamic acid (NPA), another auxin efflux transport inhibitor, which blocks polar auxin transport at a site distinct from TIBA. NPA tolerance of txr1-1 and ixr1-1 was further supported by electrophysiological analysis of net H+ fluxes in the mature, but not elongation zone of roots. All three mutants showed increased tolerance to IXB, but only txr1-1 showed tolerance to TA. No mutant showed enhanced tolerance to a third CBI, dichlobenil (DCB).

CONCLUSIONS

We have demonstrated that plant tolerance to TA and IXB, as well as cell wall synthesis modifications in roots, have resulted in specific co-resistance to NPA but not TIBA. This suggests that CBI resistance has an impact on polar auxin efflux transport processes associated with the NPA binding protein. We also show that NPA inhibitory response in roots occurs in the mature root zone but not the elongation zone. Responses of mutants to CBIs indicate a similar, but not identical mode of action of TA and IXB, in contrast to DCB.

Chemical genetics to examine cellulose biosynthesis

Long-term efforts to decode plant cellulose biosynthesis via molecular genetics and biochemical strategies are being enhanced by the ever-expanding scale of omics technologies. An alternative approach to consider are the prospects for inducing change in plant metabolism using exogenously supplied chemical ligands. Cellulose biosynthesis inhibitors (CBIs) have been identified among known herbicides, during diverse combinatorial chemical libraries screens, and natural chemical screens from microbial agents. In this review, we summarize the current knowledge of the inhibitory effects of CBIs and further group them by how they influence fluorescently tagged cellulose synthase A proteins. Additional attention is paid to the continuing development of the CBI toolbox to explore the cell biology and genetic mechanisms underpinning effector molecule activity.

Thaxtomin A affects CESA-complex density, expression of cell wall genes, cell wall composition, and causes ectopic lignification in Arabidopsis thaliana seedlings

Thaxtomin A, a phytotoxin produced by Streptomyces eubacteria, is suspected to act as a natural cellulose synthesis inhibitor. This view is confirmed by the results obtained from new chemical, molecular, and microscopic analyses of Arabidopsis thaliana seedlings treated with thaxtomin A. Cell wall analysis shows that thaxtomin A reduces crystalline cellulose, and increases pectins and hemicellulose in the cell wall. Treatment with thaxtomin A also changes the expression of genes involved in primary and secondary cellulose synthesis as well as genes associated with pectin metabolism and cell wall remodelling, in a manner nearly identical to isoxaben. In addition, it induces the expression of several defence-related genes and leads to callose deposition. Defects in cellulose synthesis cause ectopic lignification phenotypes in A. thaliana, and it is shown that lignification is also triggered by thaxtomin A, although in a pattern different from isoxaben. Spinning disc confocal microscopy further reveals that thaxtomin A depletes cellulose synthase complexes from the plasma membrane and results in the accumulation of these particles in a small microtubule-associated compartment. The results provide new and clear evidence for thaxtomin A having a strong impact on cellulose synthesis, thus suggesting that this is its primary mode of action.

A novel, cellulose synthesis inhibitory action of ancymidol impairs plant cell expansion

The co-ordination of cell wall synthesis with plant cell expansion is an important topic of contemporary plant biology research. In studies of cell wall synthesis pathways, cellulose synthesis inhibitors are broadly used. It is demonstrated here that ancymidol, known as a plant growth retardant primarily affecting gibberellin biosynthesis, is also capable of inhibiting cellulose synthesis. Its ability to inhibit cellulose synthesis is not related to its anti-gibberellin action and possesses some unique features never previously observed when conventional cellulose synthesis inhibitors were used. It is suggested that ancymidol targets the cell wall synthesis pathway at a regulatory step where cell wall synthesis and cell expansion are coupled. The elucidation of the ancymidol target in plant cells could potentially contribute to our understanding of cell wall synthesis and cell expansion control.