Implications of cell wall immunocytochemical profiles on the structural and functional traits of root and stem galls induced by Eriosoma lanigerum on Malus domestica

Alterations in cell wall composition imply in new structural and functional traits in gall developmental sites, even when the inducer is a sucking exophytophagous insect with strict feeding sites as the aphid associated to Malus domestica Borkh. This host plant is an economically important, fruit-bearing species, susceptible to gall induction by the sucking aphid Eriosoma lanigerum Hausmann, 1802. Herein, the immunocytochemical detection of arabinogalactan-proteins (AGPs), pectins, and hemicelluloses using monoclonal antibodies was performed in samples of non-galled roots and stems, and of root and stem galls on M. domestica. The dynamics of these cell wall components was discussed under the structural and functional traits of the galls proximal, median, and distal regions, according to the proximity of E. lanigerum colony feeding site. In the proximal region, the epitopes of AGPs and homogalacturonans (HGs) are related to cell growth and divisions, which result in the overproduction of parenchyma cells both in root and stem galls. In the proximal and median regions, the co-occurrence of HGs and arabinans in the cell walls of parenchyma and secondary tissues favors the nutrient flow and water-holding capacity, while the xylogalacturonans and hemicelluloses may function as additional carbohydrate resources to E. lanigerum. The immunocytochemical profile of the cell walls support the feeding activity of E. lanigerum mainly in the gall proximal region. The similarity of the cell wall components of the gall distal region and the non-galled portions, both in roots and stems, relates to the decrease of the cecidogenetic field the more distant the E. lanigerum colony is.

Maturation of demineralized arabinogalactan-proteins from Acacia seyal gum in dry state: Aggregation kinetics and structural properties of aggregates

The aggregation in dry state of mineral-loaded arabinogalactan-proteins (AGPs) from Acacia seyal gum (GA) generally occurs above 70 °C. This study focuses on the aggregation sensitivity of AGPs after their demineralization. The dry incubation in mild temperature (25 °C to 70 °C) of demineralized AGPs induced the formation of aggregates, not observed with GA. AGPs aggregated following a self-assembly mechanism for which temperature only modulated the aggregation rate. The activation energy was around 90-100 kJ·mol^-1 that could correspond to chemical condensation reactions induced by the AGPs surface dehydration. The aggregation kinetics were characterized by the formation of soluble aggregates during the first times of incubation, whose molar mass increased from 1 · 10⁶ g·mol^-1 to 6.7 · 10⁶ g·mol^-1 (SEC MALS) or 12 · 10⁶ g·mol^-1 (AF4 MALS) after 1.66 days of dry heating at 40 °C. These soluble aggregates revealed they adopted a similar conformation to that of not aggregated AGPs with a ν_h value around 0.45. Above 1.66 days at 40 °C, the soluble aggregates grew up to form microparticles with sizes ranging from 10 to around 200 μm. This study highlighted the protective role of cations from AGPs whose demineralization increased their sensibility to dry heating and their chemical reactivity for aggregation.

Functional characterization of hydroxyproline-O-galactosyltransferases for Arabidopsis arabinogalactan-protein synthesis

BACKGROUND

Arabinogalactan-proteins (AGPs) are structurally complex hydroxyproline-rich cell wall glycoproteins ubiquitous in the plant kingdom. AGPs biosynthesis involves a series of post-translational modifications including the addition of type II arabinogalactans to non-contiguous Hyp residues. To date, eight Hyp-galactosyltransferases (Hyp-GALTs; GALT2-GALT9) belonging to CAZy GT31, are known to catalyze the addition of the first galactose residues to AGP protein backbones and enable subsequent AGP glycosylation. The extent of genetic redundancy, however, remains to be elucidated for the Hyp-GALT gene family.

RESULTS

To examine their gene redundancy and functions, we generated various multiple gene knock-outs, including a triple mutant (galt5 galt8 galt9), two quadruple mutants (galt2 galt5 galt7 galt8, galt2 galt5 galt7 galt9), and one quintuple mutant (galt2 galt5 galt7 galt8 galt9), and comprehensively examined their biochemical and physiological phenotypes. The key findings include: AGP precipitations with β-Yariv reagent showed that GALT2, GALT5, GALT7, GALT8 and GALT9 act redundantly with respect to AGP glycosylation in cauline and rosette leaves, while the activity of GALT7, GALT8 and GALT9 dominate in the stem, silique and flowers. Monosaccharide composition analysis showed that galactose was decreased in the silique and root AGPs of the Hyp-GALT mutants. TEM analysis of 25789 quintuple mutant stems indicated cell wall defects coincident with the observed developmental and growth impairment in these Hyp-GALT mutants. Correlated with expression patterns, galt2, galt5, galt7, galt8, and galt9 display equal additive effects on insensitivity to β-Yariv-induced growth inhibition, silique length, plant height, and pollen viability. Interestingly, galt7, galt8, and galt9 contributed more to primary root growth and root tip swelling under salt stress, whereas galt2 and galt5 played more important roles in seed morphology, germination defects and seed set. Pollen defects likely contributed to the reduced seed set in these mutants.

CONCLUSION

Additive and pleiotropic effects of GALT2, GALT5, GALT7, GALT8 and GALT9 on vegetative and reproductive growth phenotypes were teased apart via generation of different combinations of Hyp-GALT knock-out mutants. Taken together, the generation of higher order Hyp-GALT mutants demonstrate the functional importance of AG polysaccharides decorating the AGPs with respect to various aspects of plant growth and development.

Bioinformatic Identification of Plant Hydroxyproline-Rich Glycoproteins

Hydroxyproline-rich glycoproteins (HRGPs) are a superfamily of plant cell wall proteins that function in diverse aspects of plant growth and development. This superfamily consists of three members: arabinogalactan-proteins (AGPs), extensins (EXTs), and proline-rich proteins (PRPs). Hybrid and chimeric HRGPs also exist. A bioinformatic software program, BIO OHIO 2.0, was developed to expedite the genome-wide identification and classification of AGPs, EXTs, and PRPs based on characteristic HRGP motifs and biased amino acid compositions. This chapter explains the principles of identifying HRGPs and provides a stepwise tutorial for using the BIO OHIO 2.0 program with genomic/proteomic data. Here, as an example, the genome/proteome of the common bean (Phaseolus vulgaris) is analyzed using the BIO OHIO 2.0 program to identify and characterize its set of HRGPs.

CRISPR-Cas9 multiplex genome editing of the hydroxyproline-O-galactosyltransferase gene family alters arabinogalactan-protein glycosylation and function in Arabidopsis

BACKGROUND

Arabinogalactan-proteins (AGPs) are a class of hydroxyproline-rich proteins (HRGPs) that are heavily glycosylated (> 90%) with type II arabinogalactans (AGs). AGPs are implicated in various plant growth and development processes including cell expansion, somatic embryogenesis, root and stem growth, salt tolerance, hormone signaling, male and female gametophyte development, and defense. To date, eight Hyp-O-galactosyltransferases (GALT2-6, HPGT1-3) have been identified; these enzymes are responsible for adding the first sugar, galactose, onto AGPs. Due to gene redundancy among the GALTs, single or double galt genetic knockout mutants are often not sufficient to fully reveal their biological functions.

RESULTS

Here, we report the successful application of CRISPR-Cas9 gene editing/multiplexing technology to generate higher-order knockout mutants of five members of the GALT gene family (GALT2-6). AGPs analysis of higher-order galt mutants (galt2 galt5, galt3 galt4 galt6, and galt2 galt3 galt4 galt5 gal6) demonstrated significantly less glycosylated AGPs in rosette leaves, stems, and siliques compared to the corresponding wild-type organs. Monosaccharide composition analysis of AGPs isolated from rosette leaves revealed significant decreases in arabinose and galactose in all the higher-order galt mutants. Phenotypic analyses revealed that mutation of two or more GALT genes was able to overcome the growth inhibitory effect of β-D-Gal-Yariv reagent, which specifically binds to β-1,3-galactan backbones on AGPs. In addition, the galt2 galt3 galt4 galt5 gal6 mutant exhibited reduced overall growth, impaired root growth, abnormal pollen, shorter siliques, and reduced seed set. Reciprocal crossing experiments demonstrated that galt2 galt3 galt4 galt5 gal6 mutants had defects in the female gametophyte which were responsible for reduced seed set.

CONCLUSIONS

Our CRISPR/Cas9 gene editing/multiplexing approach provides a simpler and faster way to generate higher-order mutants for functional characterization compared to conventional genetic crossing of T-DNA mutant lines. Higher-order galt mutants produced and characterized in this study provide insight into the relationship between sugar decorations and the various biological functions attributed to AGPs in plants.

Elucidating the roles of three β-glucuronosyltransferases (GLCATs) acting on arabinogalactan-proteins using a CRISPR-Cas9 multiplexing approach in Arabidopsis

BACKGROUND

Arabinogalactan-proteins (AGPs) are one of the most complex protein families in the plant kingdom and are present in the cell walls of all land plants. AGPs are implicated in diverse biological processes such as plant growth, development, reproduction, and stress responses. AGPs are extensively glycosylated by the addition of type II arabinogalactan (AG) polysaccharides to hydroxyproline residues in their protein cores. Glucuronic acid (GlcA) is the only negatively charged sugar added to AGPs and the functions of GlcA residues on AGPs remain to be elucidated.

RESULTS

Three members of the CAZy GT14 family (GLCAT14A-At5g39990, GLCAT14B-At5g15050, and GLCAT14C-At2g37585), which are responsible for transferring glucuronic acid (GlcA) to AGPs, were functionally characterized using a CRISPR/Cas9 gene editing approach in Arabidopsis. RNA seq and qRT-PCR data showed all three GLCAT genes were broadly expressed in different plant tissues, with GLCAT14A and GLCAT14B showing particularly high expression in the micropylar endosperm. Biochemical analysis of the AGPs from knock-out mutants of various glcat single, double, and triple mutants revealed that double and triple mutants generally had small increases of Ara and Gal and concomitant reductions of GlcA, particularly in the glcat14a glcat14b and glcat14a glcat14b glcat14c mutants. Moreover, AGPs isolated from all the glcat mutants displayed significant reductions in calcium binding compared to WT. Further phenotypic analyses found that the glcat14a glcat14b and glcat14a glcat14b glcat14c mutants exhibited significant delays in seed germination, reductions in root hair length, reductions in trichome branching, and accumulation of defective pollen grains. Additionally, both glcat14b glcat14c and glcat14a glcat14b glcat14c displayed significantly shorter siliques and reduced seed set. Finally, all higher-order mutants exhibited significant reductions in adherent seed coat mucilage.

CONCLUSIONS

This research provides genetic evidence that GLCAT14A-C function in the transfer of GlcA to AGPs, which in turn play a role in a variety of biochemical and physiological phenotypes including calcium binding by AGPs, seed germination, root hair growth, trichome branching, pollen development, silique development, seed set, and adherent seed coat mucilage accumulation.

Intermolecular interactions between glycomodules of plant cell wall arabinogalactan-proteins and extensins

Hydroxyproline-rich glycoproteins (HRGPs) are a unique component of plant cell walls, undergoing extensive posttranslational modification such as proline hydroxylation and hydroxyproline-O-glycosylation. Arabinogalactan proteins (AGPs) and extensins are major members of the HRGP superfamily. AGPs have repetitive AlaHyp, SerHyp, and ThrHyp peptides, the Hyp residues being glycosylated with large type II arabinogalactan polysaccharides, while extensins contain characteristic SerHyp4 and SerHyp2 motifs with arabinosylated (1-4 residues) Hyp. Although they are less than ten percent in all wall materials, AGPs and extensins play important roles in all aspects of plant growth and development. The detailed mechanisms of their functions are still under investigation. However, many of the functions may be attributed to their adhesive properties. Here, we used a forced unbinding technique to measure relative adhesive potential of the well characterized (AlaHyp)51 and (SerHyp4)18 glycomodules representing AGPs and extensins, respectively. In the presence of different wall ions such as protons, Ca2+, and boron, the glycomodules exhibited different adhesive patterns, suggesting that the wall ion-regulated intermolecular interactions/adhesions between AGPs and/or extensins may be involved in maintaining wall-plasma membrane integrity during wall loosening processes such as wall elongation or expansion. This research applies a biophysical approach to understand the biological function of plant cell wall glycoproteins.

Pyridine 2,4-Dicarboxylic Acid Suppresses Tomato Seedling Growth

Pyridine 2,4-dicarboxylic acid is a structural analog of 2-oxoglutarate and is known to inhibit 2-oxoglutare-dependent dioxygenases. The effect of this inhibitor in tomato seedlings grown in MS media supplied with various concentrations of PDCA was investigated, resulting in shorter roots and hypocotyls in a dose-dependent manner. The partial inhibition of growth in roots was more drastic compared to hypocotyls and was attributed to a decrease in the elongation of root and hypocotyl cells. Concentrations of 100 and 250 μM of PDCA decreased hydroxyproline content in roots while only the 250 μM treatment reduced the hydroxyproline content in shoots. Seedlings treated with 100 μM PDCA exhibited enhanced growth of hypocotyl and cotyledon cells and higher hydroxyproline content resulting in cotyledons with greater surface area. However, no alterations in hypocotyl length were observed. Prolyl 4 hydroxylases (P4Hs) are involved in the O-glycosylation of AGPs and were also highly expressed during seedling growth. Moreover PDCA induced a decrease in the accumulation of HRGPs and particularly in AGPs-bound epitopes in a dose dependent-manner while more drastic reduction were observed in roots compared to shoots. In addition, bulged root epidermal cells were observed at the high concentration of 250 μM which is characteristic of root tissues with glycosylation defects. These results indicate that PDCA induced pleiotropic effects during seedling growth while further studies are required to better investigate the physiological significance of this 2-oxoglutarate analog. This pharmacological approach might be used as a tool to better understand the physiological significance of HRGPs and probably P4Hs in various growth and developmental programs in plants.

Xyloglucan endotransglucosylase/hydrolases (XTHs) are inactivated by binding to glass and cellulosic surfaces, and released in active form by a heat-stable polymer from cauliflower florets

Xyloglucan endotransglucosylase (XET) activity, which cuts and re-joins hemicellulose chains in the plant cell wall, contributing to wall assembly and growth regulation, is the major activity of XTH proteins. During purification, XTHs often lose XET activity which, however, is restored by treatment with certain cold-water-extractable, heat-stable polymers (CHPs), e.g. from cauliflower florets. It was not known whether the XTH-activating factor (XAF) present in CHPs works by promoting (e.g. allosterically) XET activity or by re-solubilising sequestered XTH proteins. We now show that XTHs in dilute solution bind to diverse surfaces (e.g. glass and cellulose), and that CHPs can re-solubilise the bound enzyme, re-activating it. Cell walls prepared from cauliflower florets, mung bean shoots and Arabidopsis cell-suspension cultures each contained endogenous, tightly bound, inactive XTHs, which were likewise rapidly solubilised (within 0.5h) and thus activated by cauliflower XAF. We present a convenient quantitative assay for XAF acting on the native sequestered XTHs of Arabidopsis cell walls; using this assay, we show that CHPs from all plants tested possess XAF activity. The XAF activity of diverse CHPs does not correlate with their conductivity, showing that this activity is not a simple ionic effect. The XAF action of cauliflower CHPs was augmented by NaCl, although NaCl alone was much less effective than a CHP solution of similar conductivity, confirming that the cauliflower polymers did not simply exert a salt effect. We suggest that XAF is an endogenous regulator of XET action, modulating cell-wall loosening and/or assembly in vivo.

Monoclonal antibodies indicate low-abundance links between heteroxylan and other glycans of plant cell walls

The derivation of two sensitive monoclonal antibodies directed to heteroxylan cell wall polysaccharide preparations has allowed the identification of potential inter-linkages between xylan and pectin in potato tuber cell walls and also between xylan and arabinogalactan-proteins in oat grain cell walls. Plant cell walls are complex composites of structurally distinct glycans that are poorly understood in terms of both in muro inter-linkages and developmental functions. Monoclonal antibodies (MAbs) are versatile tools that can detect cell wall glycans with high sensitivity through the specific recognition of oligosaccharide structures. The isolation of two novel MAbs, LM27 and LM28, directed to heteroxylan, subsequent to immunisation with a potato cell wall fraction enriched in rhamnogalacturonan-I (RG-I) oligosaccharides, is described. LM27 binds strongly to heteroxylan preparations from grass cell walls and LM28 binds to a glucuronosyl-containing epitope widely present in heteroxylans. Evidence is presented suggesting that in potato tuber cell walls, some glucuronoxylan may be linked to pectic macromolecules. Evidence is also presented that suggests in oat spelt xylan both the LM27 and LM28 epitopes are linked to arabinogalactan-proteins as tracked by the LM2 arabinogalactan-protein epitope. This work extends knowledge of the potential occurrence of inter-glycan links within plant cell walls and describes molecular tools for the further analysis of such links.

Is Ca2+ involved in the signal transduction pathway of boron deficiency? New hypotheses for sensing boron deprivation

Plants sense and transmit nutrient-deprivation signals to the nucleus. This increasingly interesting research field advances knowledge of signal transduction pathways for mineral deficiencies. The understanding of this topic for most micronutrients, especially boron (B), is more limited. Several hypotheses have been proposed to explain how a B deprivation signal would be conveyed to the nucleus, which are briefly summarized in this review. These hypotheses do not explain how so many metabolic and physiological processes quickly respond to B deficiency. Short-term B deficiency affects the cytosolic Ca(2+) levels as well as root expression of genes involved in Ca(2+) signaling. We propose and discuss that Ca(2+) and Ca(2+)-related proteins - channels/transporters, sensor relays, and sensor responders - might have major roles as intermediates in a transduction pathway triggered by B deprivation. This hypothesis may explain how plants sense and convey the B-deprivation signal to the nucleus and modulate physiological responses. The possible role of arabinogalactan-proteins in the B deficiency signaling pathway is also taken into account.