Extensin, an underestimated key component of cell wall defence?

Demethylating drugs alter protoplast development, regeneration, and the genome stability of protoplast-derived regenerants of cabbage

BACKGROUND

Methylation is a major DNA modification contributing to the epigenetic regulation of nuclear gene expression and genome stability. DNA methyltransferases (DNMT) inhibitors are widely used in epigenetic and cancer research, but their biological effects and the mechanisms of their action are not well recognized in plants. This research focuses on comparing the effects of two DNMT inhibitors, namely 5-azacytidine (AZA) and zebularine (ZEB), on cellular processes, including organogenesis in vitro. Protoplasts are a unique single-cell system to analyze biological processes in plants; therefore in our study, both inhibitors were applied to protoplast culture medium or the medium used for the regeneration of protoplast-derived calluses.

RESULTS

AZA induced a dose-dependent reduction in protoplast viability, delayed cell wall reconstruction, and reduced mitotic activity, while ZEB in low concentration (2.5 µM) promoted mitoses and stimulated protoplast-derived callus development. The higher effectiveness of shoot regeneration was observed when drugs were applied directly to protoplasts compared to protoplast-derived callus treatments. Our findings reveal that both drugs affected the genome stability of the obtained regenerants by inducing polyploidization. Both drugs induced hypomethylation and modulated the distribution patterns of methylated DNA in the protoplast-derived callus.

CONCLUSION

AZA was more toxic to plant protoplasts compared to ZEB. Both inhibitors affect the ploidy status of protoplast-derived regenerants. A comparison of the data on global methylation levels with the regeneration efficiency suggests that organogenesis in cabbage is partially controlled by variations in DNA methylation levels.

Differential cell wall composition in eriococcid-induced galls: unraveling the influence of sexual dimorphism

Eriococcid-induced galls exhibit many unusual features, such as sexual dimorphism and differences in life cycle duration, which are reflected in the morphology, anatomy, and metabolism of female- and male-induced galls. These sex-based distinctions between the galls result from differential developmental processes related to the time female and male insects remain inside the gall, their feeding activity, and the degree of stress imposed on the host plant cellular machinery. We assessed the immunocytochemical and Raman spectroscopy profiles of two host plant-gall inducer systems: Annona dolabripetala (Annonaceae)-Pseudotectococccus rolliniae Hodgson and Gonçalves, and Pseudobombax grandiflorum (Malvaceae)-Eriogallococcus isaias Hodgson and Magalhães. We expected differences in the dynamics of cell wall chemical components between male and female galls, with particular effects on gall structural and functional profiles, also regarding specific constraints in each system. The epitopes of xylogalacturonans and homogalacturonans, as well as their degree of methylesterification, are affected by the sex of the gall inducers, conferring a rigid structure to the cell walls of female-induced galls in comparison with those of the male-induced galls. Raman spectroscopy detected cellulose peaks in both female- and male-induced galls, with pronounced lignin bands in female-induced galls. The sex-based chemical distinctions between female- and male-induced galls imply in differences in terms of higher rigid cell walls in the female-induced galls, which was similar for both host plants.

Meloidogyne incognita-Induced Giant Cells in Tomato and the Impact of Acetic Acid

The plant parasitic root-knot nematodes of the species Meloidogyne incognita infect many cultivated plants, one of which is the tomato (Solanum lycopersicum). To be fed, M. incognita selects unique feeding sites inside the root and induces the formation of large galls (knots) encompassing the so-called giant cells (GCs). In the present study, a comparative analysis of the GCs/root cell and cell wall components between M. incognita-infected and uninfected tomato plants and plants pre-treated with the plant biostimulant and nematicide acetic acid (AA) was carried out. Pectin, hemicellulose and extensin epitopes were detected in tomato root sections. M. incognita-induced GCs in tomato roots had cell walls with arabinans, unesterified/methylesterified homogalacturonans and xyloglucans, but were devoid of mannans and extensins. Interestingly, the above epitope distribution also differed in root sections made near the formed root knot, proximal to the root cap. Moreover, it seemed that AA was able to induce the deposition of extensins in AA-treated, M. incognita-uninfected roots and hamper the GC development in AA-treated, M. incognita-infected roots. According to the above the AA, stimulates natural defense mechanisms in tomato, thus protecting it from nematode infestation.

Dynamic transcriptome and GWAS uncover a hydroxyproline-rich glycoprotein that suppresses Agrobacterium-mediated transformation in maize

Genetic transformation is a crucial tool for investigating gene function and advancing molecular breeding in crops, with Agrobacterium tumefaciens-mediated transformation being the primary method for plant genetic modification. However, this approach exhibits significant genotypic dependence in maize. Therefore, to overcome these limitations, we combined dynamic transcriptome analysis and genome-wide association study (GWAS) to identify the key genes controlling Agrobacterium infection frequency (AIF) in immature maize embryos. Transcriptome analysis of Agrobacterium-infected embryos uncovered 8483 and 1580 genotype-specific response genes in the maize line 18-599R with low AIF and A188 with high AIF, respectively. A weighted gene co-expression network analysis (WGCNA) revealed five and seven stage-specific co-expression modules in each corresponding line. Based on a self-developed AIF quantitation method, the GWAS revealed 30 AIF-associated single-nucleotide polymorphisms and 315 candidate genes under multiple environments. Integration of GWAS and WGCNA further identified 12 key genes associated with high AIF in A188. ZmHRGP, encoding a hydroxyproline-rich glycoprotein, was functionally validated as a key factor of AIF in immature embryos. Knockout of ZmHRGP enabled us to establish a high-efficiency genetic transformation system for the 18-599R line, with the transformation frequency being approximately 80%. Moreover, the transient reduction of ZmHRGP expression significantly enhanced the AIF of maize calluses and leaves. Collectively, these findings advance our understanding of plant factors controlling Agrobacterium infection and contribute to developing more efficient Agrobacterium-mediated transformation systems in crops.

Characterization of the Helicase activity of Pif-like Helicases from Arabidopsis thaliana and role in gene regulation and abiotic stress response

Regulation in gene expression is rendered by the dynamic structural transitions between classical B-form DNA and non-canonical DNA folds such as G-Quadruplexes (GQS). Several proteins interact with GQSes and regulate a range of biological processes including regulation of DNA replication, transcription, and translation. Several GQS helicases resolve these structures and contribute to maintaining genomic stability. Arabidopsis has 3 homologs of Pif-Like-Helicases (AtPLHs) which have been characterized in this study. Cellular localization and tissue-specific profiling of these homologs revealed nuclear localization and expression in seedling, leaves, and flower tissues suggesting they might play a role in gene regulation and plant development. Molecular characterization of recombinant AtPLH2 and AtPLH3 suggested DNA binding, ATPase, and helicase activity of these two proteins. Further, phenotyping of Atplh mutants revealed alterated seedling root and hypocotyl growth and pollen germination. Transcriptomic profiling of mutants revealed the upregulation of genes associated with processes like systemic acquired resistance, glutathione metabolic process, cellular oxidant detoxification, β-glucosidase activity, SAM-dependent methyltransferase activity, heme binding, calmodulin binding, etc, with these genes harboring WRKY and AP2/EREB TF binding sites in their promoters. These mutants showed susceptibility to different abiotic stresses like salinity and ABA stress as well as to hydroxyurea. Further, AtPLH2 and AtPLH3 mutants showed no root elongation under low pH suggesting root elongation is affected in the absence of these genes. Our study highlights the role of these GQS helicases in regulating genes involved in root development and stress tolerance.

Understanding pectin cross-linking in plant cell walls

Pectin is a major component of plant cells walls. The extent to which pectin chains crosslink with one another determines crucial properties including cell wall strength, porosity, and the ability of small, biologically significant molecules to access the cell. Despite its importance, significant gaps remain in our comprehension, at the molecular level, of how pectin cross-links influence the mechanical and physical properties of cell walls. This study employs a multidisciplinary approach, combining molecular dynamics simulations, experimental investigations, and mathematical modelling, to elucidate the mechanism of pectin cross-linking and its effect on cell wall porosity. The computational aspects of this work challenge the prevailing egg-box model, favoring instead a zipper model for pectin cross-linking, whilst our experimental work highlights the significant impact of pectin cross-linking on cell wall porosity. This work advances our fundamental understanding of the biochemistry underpinning the structure and function of the plant cell wall. This knowledge has important implications for agricultural biotechnology, informing us about the chemical properties of plant pectins that are best suited for improving crop resilience and amenability to biofuel extraction by modifying the cell wall.

The AP2/ERF Transcription Factor ERF56 Negatively Regulating Nitrate-Dependent Plant Growth in Arabidopsis

ERF56, a member of the APETALA2/ETHYLENE-RESPONSIVE FACTOR (AP2/ERF) transcription factor (TF) family, was reported to be an early nitrate-responsive TF in Arabidopsis. But the function of ERF56 in nitrate signaling remains not entirely clear. This study aimed to investigate the role of ERF56 in nitrate-dependent plant growth and nitrate signaling. We confirmed with reverse transcription quantitative PCR (RT-qPCR) that the transcription of ERF56 is quickly induced by nitrate. ERF56 overexpressors displayed decreased nitrate-dependent plant growth, while erf56 mutants exhibited increased plant growth. Confocal imaging demonstrated that ERF56 is localized into nuclei. Assays with the glucuronidase (GUS) reporter showed that ERF56 is mainly expressed at the region of maturation of roots and in anthers. The dual-luciferase assay manifested that the transcription of ERF56 is not directly regulated by NIN-LIKE PROTEIN 7 (NLP7). The transcriptome analysis identified 1038 candidate genes regulated by ERF56 directly. A gene ontology (GO) over-representation analysis showed that ERF56 is involved in the processes of water transport, inorganic molecule transmembrane transport, secondary metabolite biosynthesis, and cell wall organization. We revealed that ERF56 represses nitrate-dependent growth through regulating the processes of inorganic molecule transmembrane transport, the secondary metabolite biosynthesis, and cell wall organization.

Immunolocalization of Extensin and Pectin Epitopes in Liparis loeselii Protocorm and Protocorm-like Bodies

Liparis loeselii (L.) Rich, an endangered member of the Orchidaceae family, is found in alkaline fens. With the declining populations of L. loeselii, there is a pressing need to reintroduce this species in Central Europe. As in vitro germination is a crucial tool for obtaining plants for introduction into the environment, we looked at the morphological changes occurring during the early stages of L. loeselii development in vitro. As the early stages of orchid development, especially the protocorm stage, are thought to be responsible for SAM formation and the initiation of symbiotic association, we focused on cell wall elements whose epitopes have been found in similar processes in other species: the extensin and pectin rhamnogalacturonan I (RG-I) side chain epitopes. We addressed the following questions: Does the cell wall of L. loeselii change its composition during the early stages of development, as noted in other species? Are there noticeable similarities in the cell wall to organs of different species whose function is to contact microorganisms? Are there regularities that allow the recognition of individual structures on this basis? Immunolocalization revealed changes in the distribution of certain extensins (JIM11 and JIM20) and RG-I (LM5 and LM6) side chain epitopes. Extensins, a type of cell wall protein, were observed during the initial stages of the formation of PLB and the shoot apical meristem of protocorms and PLBs. RG-I, on the other hand, was found to play a significant role in the development of the protocorm and PLB. In pseudobulbs, which appeared on the protocorms, extensins occurred in their storage part. However, RG-I side chains (1→4)-β-galactans (LM5), and (1→5)-α-L-arabinans (LM6) were not found in pseudobulbs. We revealed that a common feature of protocorms and PLBs was an increased amount of extensins, which were detected with the JIM11 antibody, and pectins, which were detected with the LM5 antibody, that were present together, which may prove helpful in determining the identity of the induced structures and distinguishing them from pseudobulbs. Thus, our study unveiled the role of extensins and RG-I during the growth of protocorms and PLBs. We suggest that PLBs may mimic the wall remodelling that occurs in protocorms, which indicates that using cell wall components is an invitation to be colonised by a fungal partner. However, this needs to be tested in future research. The findings of this research can help interpret future studies on the propagation, acclimatisation, and introduction of L. loeselii into the natural environment.

Comparative transcriptomic analyses of diploid and tetraploid citrus reveal how ploidy level influences salt stress tolerance

Introduction

Citrus is an important fruit crop for human health. The sensitivity of citrus trees to a wide range of abiotic stresses is a major challenge for their overall growth and productivity. Among these abiotic stresses, salinity results in a significant loss of global citrus yield. In order to find straightforward and sustainable solutions for the future and to ensure citrus productivity, it is of paramount importance to decipher the mechanisms responsible for salinity stress tolerance. Thisstudy aimed to investigate how ploidy levels influence salt stress tolerance in citrus by comparing the transcriptomic responses of diploid and tetraploid genotypes. In a previous article we investigated the physiological and biochemical response of four genotypes with different ploidy levels: diploid trifoliate orange (Poncirus trifoliata [L.] Raf.) (PO2x) and Cleopatra mandarin (Citrus reshni Hort. Ex Tan.) (CL2x) and their respective tetraploids (PO4x, CL4x).

Methods

In this study, we useda multifactorial gene selection and gene clustering approach to finely dissect the influence of ploidy level on the salt stress response of each genotype. Following transcriptome sequencing, differentially expressed genes (DEGs) were identified in response to salt stress in leaves and roots of the different citrus genotypes.

Result and discussion

Gene expression profiles and functional characterization of genes involved in the response to salt stress, as a function of ploidy level and the interaction between stress response and ploidy level, have enabled us to highlight the mechanisms involved in the varieties tested. Saltstress induced overexpression of carbohydrate biosynthesis and cell wall remodelling- related genes specifically in CL4x Ploidy level enhanced oxidative stress response in PO and ion management capacity in both genotypes. Results further highlighted that under stress conditions, only the CL4x genotype up- regulated genes involved in sugar biosynthesis, transport management, cell wall remodelling, hormone signalling, enzyme regulation and antioxidant metabolism. These findings provide crucial insights that could inform breeding strategies for developing salt-tolerant citrus varieties.

Pectic-AGP is a major form of Arabidopsis AGPs

As a key component in cell walls of numerous organisms ranging from green algae to higher plants, AGPs play principal roles in many biological processes such as cell-cell adhesion and regulating Ca2+ signaling pathway as a Ca2+-capacitor. Consistently, AGP structures vary from species to species and from tissue to tissue. To understand the functions of AGPs, it is vital to know their structural differences relative to their location in the plant. Thus, AGPs were purified from different Arabidopsis tissues. Analyses of these AGPs demonstrated that the AGPs comprised covalently linked pectin and AGP, referred to as pectic-AGPs. Importantly, these pectic-AGPs were glycosylated with a remarkable variety of polysaccharides including homogalacturonan, rhamnogalacturonan-I, and type II arabinogalactan at different ratios and lengths. This result not only suggests that pectic-AGP is a major form of Arabidopsis AGPs, but also supports AGPs serve as crosslinkers covalently connecting pectins with structures tailored for tissue-specific functions.

Vegetative cell wall protein OsGP1 regulates cell wall mediated soda saline-alkali stress in rice

Plant growth and development are inhibited by the high levels of ions and pH due to soda saline-alkali soil, and the cell wall serves as a crucial barrier against external stresses in plant cells. Proteins in the cell wall play important roles in plant cell growth, morphogenesis, pathogen infection and environmental response. In the current study, the full-length coding sequence of the vegetative cell wall protein gene OsGP1 was characterized from Lj11 (Oryza sativa longjing11), it contained 660 bp nucleotides encoding 219 amino acids. Protein-protein interaction network analysis revealed possible interaction between CESA1, TUBB8, and OsJ_01535 proteins, which are related to plant growth and cell wall synthesis. OsGP1 was found to be localized in the cell membrane and cell wall. Furthermore, overexpression of OsGP1 leads to increase in plant height and fresh weight, showing enhanced resistance to saline-alkali stress. The ROS (reactive oxygen species) scavengers were regulated by OsGP1 protein, peroxidase and superoxide dismutase activities were significantly higher, while malondialdehyde was lower in the overexpression line under stress. These results suggest that OsGP1 improves saline-alkali stress tolerance of rice possibly through cell wall-mediated intracellular environmental homeostasis.

The extracellular matrix of green algae

Green algae display a wide range of extracellular matrix (ECM) components that include various types of cell walls (CW), scales, crystalline glycoprotein coverings, hydrophobic compounds, and complex gels or mucilage. Recently, new information derived from genomic/transcriptomic screening, advanced biochemical analyses, immunocytochemical studies, and ecophysiology has significantly enhanced and refined our understanding of the green algal ECM. In the later diverging charophyte group of green algae, the CW and other ECM components provide insight into the evolution of plants and the ways the ECM modulates during environmental stress. Chlorophytes produce diverse ECM components, many of which have been exploited for various uses in medicine, food, and biofuel production. This review highlights major advances in ECM studies of green algae.

Extensin and senescence: a cell wall connection

This article comments on:

Lu H, Niu X, Fan Y, Yuan Y, Huang L, Zhao B, Liu Y, Xiao F. 2023. The extensin protein SAE1 plays a role in leaf senescence and is targeted by the ubiquitin ligase SINA4 in tomato. Journal of Experimental Botany 74, 5635–5652.

Mechanistic basis for mitigating drought tolerance by selenium application in tobacco (Nicotiana tabacum L.): a multi-omics approach

The lack of irrigation water in agricultural soils poses a significant constraint on global crop production. In-depth investigation into microRNAs (miRNAs) has been widely used to achieve a comprehensive understanding of plant defense mechanisms. However, there is limited knowledge on the association of miRNAs with drought tolerance in cigar tobacco. In this study, a hydroponic experiment was carried out to identify changes in plant physiological characteristics, miRNA expression and metabolite profile under drought stress, and examine the mitigating effects of selenium (Se) application. The shoot dry weight of drought-stressed plants was approximately half (50.3%) of that in non-stressed (control) conditions. However, plants supplied with Se attained 38.8% greater shoot dry weight as compared to plants with no Se supply under drought stress. Thirteen miRNAs were identified to be associated with drought tolerance. These included 7 known (such as nta-miR156b and nta-miR166a) and 6 novel miRNAs (such as novel-nta-miR156-5p and novel-nta-miR209-5p) with the target genes of squamosa promoter-binding-like protein 4 (SPL4), serine/threonine protein phosphatase 2A (PPP2A), cation/calcium exchanger 4-like (CCX4), extensin-1-like (EXT1) and reduced wall acetylation 2 (RWA2). Further investigation revealed that the expression levels of Ext1 and RWA2 were significantly decreased under drought stress but increased with Se addition. Moreover, key metabolites such as catechin and N-acetylneuraminic acid were identified, which may play a role in the regulation of drought tolerance. The integrated analysis of miRNA sequencing and metabolome highlighted the significance of the novel-nta-miR97-5p- LRR-RLK- catechin pathway in regulating drought tolerance. Our findings provide valuable insights into the molecular mechanisms underlying drought tolerance and Se-induced stress alleviation in cigar tobacco.

Transcriptome analysis provides insights into the cell wall and aluminum toxicity related to rusty root syndrome of Panax ginseng

Rusty root syndrome is a common and serious disease in the process of Panax ginseng cultivation. This disease greatly decreases the production and quality of P. ginseng and causes a severe threat to the healthy development of the ginseng industry. However, its pathogenic mechanism remains unclear. In this study, Illumina high-throughput sequencing (RNA-seq) technology was used for comparative transcriptome analysis of healthy and rusty root-affected ginseng. The roots of rusty ginseng showed 672 upregulated genes and 526 downregulated genes compared with the healthy ginseng roots. There were significant differences in the expression of genes involved in the biosynthesis of secondary metabolites, plant hormone signal transduction, and plant-pathogen interaction. Further analysis showed that the cell wall synthesis and modification of ginseng has a strong response to rusty root syndrome. Furthermore, the rusty ginseng increased aluminum tolerance by inhibiting Al entering cells through external chelating Al and cell wall-binding Al. The present study establishes a molecular model of the ginseng response to rusty roots. Our findings provide new insights into the occurrence of rusty root syndrome, which will reveal the underlying molecular mechanisms of ginseng response to this disease.

Phosphatidylinositol-phospholipase C4 suppresses the hypersensitive response of Nicotiana benthamiana

Phospholipid signaling plays an important role in plant immune responses. Here, we isolated two phospholipase C4 (PLC4) orthologs in the Nicotiana benthamiana genome, designated as N. benthamiana PLC4-1 and PLC4-2 (NbPLC4-1 and NbPLC4-2). We created NbPLC4-1- and NbPLC4-2- silenced plants. Induction of the hypersensitive response (HR), including HR cell death and bacterial population reduction, was accelerated in both NbPLC4-1- and NbPLC4-2-silenced plants challenged with N. benthamiana-incompatible Ralstonia solanacearum 8107. The NbPLC4-1- and NbPLC4-2-silenced plants also showed enhanced expression of Nbhin1, a HR marker gene. Expressions of genes for salicylic acid (SA) and jasmonic acid (JA) signaling were drastically increased in NbPLC4-1- and NbPLC4-2-silenced plants by R. solanacearum inoculation. In addition, NbPLC4-1 and NbPLC4-2 silencing triggered reactive oxygen species (ROS) hyper-production. These results suggest that NbPLC4s are closely associated with JA, SA, and ROS signaling and act as negative regulators of the HR in N. benthamiana.

COG2 negatively regulates chilling tolerance through cell wall components altered in rice

Chilling-tolerant QTL gene COG2 encoded an extensin and repressed chilling tolerance by affecting the compositions of cell wall. Rice as a major crop is susceptible to chilling stress. Chilling tolerance is a complex trait controlled by multiple quantitative trait loci (QTLs). Here, we identify a QTL gene, COG2, that negatively regulates cold tolerance at seedling stage in rice. COG2 overexpression transgenic plants are sensitive to cold, whereas knockout transgenic lines enhance chilling tolerance. Natural variation analysis shows that Hap1 is a specific haplotype in japonica/Geng rice and correlates with chilling tolerance. The SNP1 in COG2 promoter is a specific divergency and leads to the difference in the expression level of COG2 between japonica/Geng and indica/Xian cultivars. COG2 encodes a cell wall-localized extensin and affects the compositions of cell wall, including pectin and cellulose, to defense the chilling stress. The results extend the understanding of the adaptation to the environment and provide an editing target for molecular design breeding of cold tolerance in rice.

Morphogenesis and cell wall composition of trichomes and their function in response to salt in halophyte Salsola ferganica

BACKGROUND

To survive harsh environmental conditions, desert plants show various adaptions, such as the evolution of trichomes, which are protective epidermal protrusions. Currently, the morphogenesis and function of trichomes in desert plants are not well understood. Salsola ferganica is an annual halophyte distributed in cold deserts; at the seedling stage, its rod-shaped true leaves are covered with long and thick trichomes and are affected by habitat conditions. Therefore, we evaluated the trichomes on morphogenesis and cell wall composition of S. ferganica compared to Arabidopsis thaliana and cotton, related gene expression, and preliminary function in salt accumulation of the leaves.

RESULTS

The trichomes of S. ferganica were initiated from the epidermal primordium, followed by two to three rounds of cell division to form a multicellular trichome, while some genes associated with them were positively involved. Cell wall composition analysis showed that different polysaccharides including heavily methyl-esterified and fully de-esterified pectins (before maturation, probably in the primary wall), xyloglucans (in the mid-early and middle stages, probably in the secondary wall), and extensin (during the whole developmental period) were detected, which were different from those found in trichomes of Arabidopsis and cotton. Moreover, trichome development was affected by abiotic stress, and might accumulate salt from the mesophyll cells and secrete outside.

CONCLUSIONS

S. ferganica has multicellular, non-branched trichomes that undergo two to three rounds of cell division and are affected by abiotic stress. They have a unique cell wall composition which is different from that of Arabidopsis and cotton. Furthermore, several genes positively or negatively regulate trichome development. Our findings should contribute to our further understanding of the biogenesis and adaptation of plant accessory structures in desert plant species.

The root apoplastic pH as an integrator of plant signaling

Plant nutrition, growth, and response to environmental stresses are pH-dependent processes that are regulated at the apoplastic and subcellular levels. The root apoplastic pH is especially sensitive to external cues and can also be modified by intracellular inputs, such as hormonal signaling. Optimal crosstalk of the mechanisms involved in the extent and span of the apoplast pH fluctuations promotes plant resilience to detrimental biotic and abiotic factors. The fact that variations in local pHs are a standard mechanism in different signaling pathways indicates that the pH itself can be the pivotal element to provide a physiological context to plant cell regions, allowing a proportional reaction to different situations. This review brings a collective vision of the causes that initiate root apoplastic pHs variations, their interaction, and how they influence root response outcomes.

Elicitation of Roots and AC-DC with PEP-13 Peptide Shows Differential Defense Responses in Multi-Omics

The root extracellular trap (RET) has emerged as a specialized compartment consisting of root AC-DC and mucilage. However, the RET’s contribution to plant defense is still poorly understood. While the roles of polysaccharides and glycoproteins secreted by root AC-DC have started to be elucidated, how the low-molecular-weight exudates of the RET contribute to root defense is poorly known. In order to better understand the RET and its defense response, the transcriptomes, proteomes and metabolomes of roots, root AC-DC and mucilage of soybean (Glycine max (L.) Merr, var. Castetis) upon elicitation with the peptide PEP-13 were investigated. This peptide is derived from the pathogenic oomycete Phytophthora sojae. In this study, the root and the RET responses to elicitation were dissected and sequenced using transcriptional, proteomic and metabolomic approaches. The major finding is increased synthesis and secretion of specialized metabolites upon induced defense activation following PEP-13 peptide elicitation. This study provides novel findings related to the pivotal role of the root extracellular trap in root defense.

Comparative Transcriptomics Analysis of the Symbiotic Germination of D. officinale (Orchidaceae) With Emphasis on Plant Cell Wall Modification and Cell Wall-Degrading Enzymes

Orchid seed germination in nature is an extremely complex physiological and ecological process involving seed development and mutualistic interactions with a restricted range of compatible mycorrhizal fungi. The impact of the fungal species' partner on the orchids' transcriptomic and metabolic response is still unknown. In this study, we performed a comparative transcriptomic analysis between symbiotic and asymbiotic germination at three developmental stages based on two distinct fungi (Tulasnella sp. and Serendipita sp.) inoculated to the same host plant, Dendrobium officinale. Differentially expressed genes (DEGs) encoding important structural proteins of the host plant cell wall were identified, such as epidermis-specific secreted glycoprotein, proline-rich receptor-like protein, and leucine-rich repeat (LRR) extensin-like protein. These DEGs were significantly upregulated in the symbiotic germination stages and especially in the protocorm stage (stage 3) and seedling stage (stage 4). Differentially expressed carbohydrate-active enzymes (CAZymes) in symbiotic fungal mycelium were observed, they represented 66 out of the 266 and 99 out of the 270 CAZymes annotated in Tulasnella sp. and Serendipita sp., respectively. These genes were speculated to be involved in the reduction of plant immune response, successful colonization by fungi, or recognition of mycorrhizal fungi during symbiotic germination of orchid seed. Our study provides important data to further explore the molecular mechanism of symbiotic germination and orchid mycorrhiza and contribute to a better understanding of orchid seed biology.

Plant-microbe interactions in the apoplast: Communication at the plant cell wall

The apoplast is a continuous plant compartment that connects cells between tissues and organs and is one of the first sites of interaction between plants and microbes. The plant cell wall occupies most of the apoplast and is composed of polysaccharides and associated proteins and ions. This dynamic part of the cell constitutes an essential physical barrier and a source of nutrients for the microbe. At the same time, the plant cell wall serves important functions in the interkingdom detection, recognition, and response to other organisms. Thus, both plant and microbe modify the plant cell wall and its environment in versatile ways to benefit from the interaction. We discuss here crucial processes occurring at the plant cell wall during the contact and communication between microbe and plant. Finally, we argue that these local and dynamic changes need to be considered to fully understand plant-microbe interactions.

Remodeling of Cell Wall Components in Root Nodules and Flower Abscission Zone under Drought in Yellow Lupine

We recently showed that yellow lupine is highly sensitive to soil water deficits since this stressor disrupts nodule structure and functioning, and at the same time triggers flower separation through abscission zone (AZ) activation in the upper part of the plant. Both processes require specific transformations including cell wall remodeling. However, knowledge about the involvement of particular cell wall elements in nodulation and abscission in agronomically important, nitrogen-fixing crops, especially under stressful conditions, is still scarce. Here, we used immuno-fluorescence techniques to visualize dynamic changes in cell wall compounds taking place in the root nodules and flower AZ of Lupinus luteus following drought. The reaction of nodules and the flower AZ to drought includes the upregulation of extensins, galactans, arabinans, xylogalacturonan, and xyloglucans. Additionally, modifications in the localization of high- and low-methylated homogalacturonans and arabinogalactan proteins were detected in nodules. Collectively, we determined for the first time the drought-associated modification of cell wall components responsible for their remodeling in root nodules and the flower AZ of L. luteus. The involvement of these particular molecules and their possible interaction in response to stress is also deeply discussed herein.

Immune Priming Triggers Cell Wall Remodeling and Increased Resistance to Halo Blight Disease in Common Bean

The cell wall (CW) is a dynamic structure extensively remodeled during plant growth and under stress conditions, however little is known about its roles during the immune system priming, especially in crops. In order to shed light on such a process, we used the Phaseolus vulgaris-Pseudomonas syringae (Pph) pathosystem and the immune priming capacity of 2,6-dichloroisonicotinic acid (INA). In the first instance we confirmed that INA-pretreated plants were more resistant to Pph, which was in line with the enhanced production of H₂O₂ of the primed plants after elicitation with the peptide flg22. Thereafter, CWs from plants subjected to the different treatments (non- or Pph-inoculated on non- or INA-pretreated plants) were isolated to study their composition and properties. As a result, the Pph inoculation modified the bean CW to some extent, mostly the pectic component, but the CW was as vulnerable to enzymatic hydrolysis as in the case of non-inoculated plants. By contrast, the INA priming triggered a pronounced CW remodeling, both on the cellulosic and non-cellulosic polysaccharides, and CW proteins, which resulted in a CW that was more resistant to enzymatic hydrolysis. In conclusion, the increased bean resistance against Pph produced by INA priming can be explained, at least partially, by a drastic CW remodeling.