The Arabidopsis Class III Peroxidase AtPRX71 Negatively Regulates Growth under Physiological Conditions and in Response to Cell Wall Damage

The Class III Peroxidase gene TaPRX-2A controls grain number per spike in common wheat (Triticum aestivum L.)

Some peroxidases (PRXs) are involved in abiotic stress response. However, to the best of our knowledge, the effects of PRXs on agronomic traits including grain number per spike (GNS), spikelet number per spike (SNS) and spike length (SL) are also largely unknown. In our study, we cloned a wheat PRX gene TaPRX-2A and identified its function in controlling GNS by generating transgenic overexpression lines. The results showed that TaPRX-2A overexpression displayed lower GNS and shorter SL, compared with the wild-type plants. RNA-seq analysis indicated alterations in various pathways including flavonoid biosynthesis, lignin biosynthesis, phytohormone signaling, as well as sucrose and starch biosynthesis. Co-expression analysis showed that transcription factors, such as bHLH, WRKY, and bZIP may be involved in the regulation of various genes associated with these pathways. Our findings provide insights into the mechanisms by which PRXs regulate agronomic traits, illustrating potential applicability in crop improvement programs.

Comprehensive analysis of class III peroxidase genes revealed PePRX2 enhanced lignin biosynthesis and drought tolerance in Phyllostachys edulis

Class III peroxidase (PRX) is the key enzyme in lignin biosynthesis and critical for maintaining the redox balance in plants to respond to stress. In moso bamboo (Phyllostachys edulis), a globally significant non-timber forestry species, the potential roles of PRX genes remain largely unknown. In this research, a total of 179 PePRXs was identified on a genome-wide scale in moso bamboo. Phylogenic relationship, conserved motifs, gene structure, collinearity and cis-acting elements were investigated. Analysis of gene expression indicated that PePRXs exhibited tissue-specific expression and different response patterns to hormones and abiotic stresses. Based on the transcriptome data, 10 PePRXs with positive correlations between expression levels and lignification degree were screened out. Among them, PePRX2 was selected as a candidate gene according to the co-expression network. Y1H and Dual-Luc assays demonstrated that PeMYB61 could bind to the promoter of PePRX2 and enhance its transcription. The result of in situ hybridization showed that PePRX2 was specifically expressed in the vascular bundle sheath cells of bamboo shoot. As a secreted protein, PePRX2 was located on the cell wall. Overexpression of PePRX2 led to a significant increase in lignin content in transgenic poplar, indicating that PePRX2 could promote lignin polymerization. In comparison with the WT, the PePRX2-OE poplar lines exhibited increased peroxidase activity and decreased levels of MDA, O2- and H2O2 under drought stress, indicating enhanced drought resistance. This thorough analysis of the PRX family in moso bamboo provided new insight into the roles of PePRXs in lignin biosynthesis and drought adaptation.

Cell wall integrity modulates HOOKLESS1 and PHYTOCHROME INTERACTING FACTOR4 expression controlling apical hook formation

Formation of the apical hook in etiolated dicot seedlings results from differential growth in the hypocotyl apex and is tightly controlled by environmental cues and hormones, among which auxin and gibberellins (GAs) play an important role. Cell expansion is tightly regulated by the cell wall, but whether and how feedback from this structure contributes to hook development are still unclear. Here, we show that etiolated seedlings of the Arabidopsis (Arabidopsis thaliana) quasimodo2-1 (qua2) mutant, defective in pectin biosynthesis, display severe defects in apical hook formation and maintenance, accompanied by loss of asymmetric auxin maxima and differential cell expansion. Moreover, qua2 seedlings show reduced expression of HOOKLESS1 (HLS1) and PHYTOCHROME INTERACTING FACTOR4 (PIF4), which are positive regulators of hook formation. Treatment of wild-type seedlings with the cellulose inhibitor isoxaben (isx) also prevents hook development and represses HLS1 and PIF4 expression. Exogenous GAs, loss of DELLA proteins, or HLS1 overexpression partially restore hook development in qua2 and isx-treated seedlings. Interestingly, increased agar concentration in the medium restores, both in qua2 and isx-treated seedlings, hook formation, asymmetric auxin maxima, and PIF4 and HLS1 expression. Analyses of plants expressing a Förster resonance energy transfer-based GA sensor indicate that isx reduces accumulation of GAs in the apical hook region in a turgor-dependent manner. Lack of the cell wall integrity sensor THESEUS 1, which modulates turgor loss point, restores hook formation in qua2 and isx-treated seedlings. We propose that turgor-dependent signals link changes in cell wall integrity to the PIF4-HLS1 signaling module to control differential cell elongation during hook formation.

Identification of systemic nitrogen signaling in foxtail millet (Setaria italica) roots based on split-root system and transcriptome analysis

KEY MESSAGE

The study established split-root system (SRS) in foxtail millet, and identified the molecular regulatory mechanisms and metabolic pathways related to systemic nitrogen signaling based on this system and transcriptome analysis. The growth of crops is primarily constrained by the availability of nitrogen (N), an essential nutrient. Foxtail millet (Setaria italica L.) is a significant orphan crop known for its strong tolerance to barren conditions. Despite this, the signaling pathway of nitrogen in foxtail millet remains largely unexplored. Identifying the candidate genes responsible for nitrogen response in foxtail millet is crucial for enhancing its agricultural productivity. This study utilized the split-root system (SRS) in foxtail millet to uncover genes associated with Systemic Nitrogen Signaling (SNS). Transcriptome analysis of the SRS revealed 2158 differentially expressed genes (DEGs) implicated in SNS, including those involved in cytokinin synthesis, transcription factors, E3 ubiquitin ligase, and ROS metabolism. Silencing of SiIPT5 and SiATL31 genes through RNAi in transgenic plants resulted in reduced SNS response, indicating their role in the nitrogen signaling pathway of foxtail millet. Furthermore, the induction of ROS metabolism-related genes in response to KNO3 of the split-root System (Sp.KNO3) suggests a potential involvement of ROS signaling in the SNS of foxtail millet. Overall, this study sheds light on the molecular regulatory mechanisms and metabolic pathways of foxtail millet in relation to SNS.

Comparative analysis of POD genes and their expression under multiple hormones in Pyrus bretschenedri

BACKGROUND

Class III peroxidase (POD) enzymes play vital roles in plant development, hormone signaling, and stress responses. Despite extensive research on POD families in various plant species, the knowledge regarding the POD family in Chinese pear (Pyrus bretschenedri) is notably limited.

RESULTS

We systematically characterized 113 POD family genes, designated as PbPOD1 to PbPOD113 based on their chromosomal locations. Phylogenetic analysis categorized these genes into seven distinct subfamilies (I to VII). The segmental duplication events were identified as a prevalent mechanism driving the expansion of the POD gene family. Microsynteny analysis, involving comparisons with Pyrus bretschenedri, Fragaria vesca, Prunus avium, Prunus mume and Prunus persica, highlighted the conservation of duplicated POD regions and their persistence through purifying selection during the evolutionary process. The expression patterns of PbPOD genes were performed across various plant organs and diverse fruit development stages using transcriptomic data. Furthermore, we identified stress-related cis-acting elements within the promoters of PbPOD genes, underscoring their involvement in hormonal and environmental stress responses. Notably, qRT-PCR analyses revealed distinctive expression patterns of PbPOD genes in response to melatonin (MEL), salicylic acid (SA), abscisic acid (ABA), and methyl jasmonate (MeJA), reflecting their responsiveness to abiotic stress and their role in fruit growth and development.

CONCLUSIONS

In this study, we investigated the potential functions and evolutionary dynamics of PbPOD genes in Pyrus bretschenedri, positioning them as promising candidates for further research and valuable indicators for enhancing fruit quality through molecular breeding strategies.

An effector SsCVNH promotes the virulence of Sclerotinia sclerotiorum through targeting class III peroxidase AtPRX71

Many plant pathogens secrete effector proteins into the host plant to suppress host immunity and facilitate pathogen colonization. The necrotrophic pathogen Sclerotinia sclerotiorum causes severe plant diseases and results in enormous economic losses, in which secreted proteins play a crucial role. SsCVNH was previously reported as a secreted protein, and its expression is significantly upregulated at 3 h after inoculation on the host plant. Here, we further demonstrated that deletion of SsCVNH leads to attenuated virulence. Heterologous expression of SsCVNH in Arabidopsis enhanced pathogen infection, inhibited the host PAMP-triggered immunity (PTI) response and increased plant susceptibility to S. sclerotiorum. SsCVNH interacted with class III peroxidase AtPRX71, a positive regulator of innate immunity against plant pathogens. SsCVNH could also interact with other class III peroxidases, thus reducing peroxidase activity and suppressing plant immunity. Our results reveal a new infection strategy employed by S. sclerotiorum in which the fungus suppresses the function of class III peroxidases, the major component of PTI to promote its own infection.

The MADS-box genes SOC1 and AGL24 antagonize XAL2 functions in Arabidopsis thaliana root development

MADS-domain transcription factors play pivotal roles in numerous developmental processes in Arabidopsis thaliana. While their involvement in flowering transition and floral development has been extensively examined, their functions in root development remain relatively unexplored. Here, we explored the function and genetic interaction of three MADS-box genes (XAL2, SOC1 and AGL24) in primary root development. By analyzing loss-of-function and overexpression lines, we found that SOC1 and AGL24, both critical components in flowering transition, redundantly act as repressors of primary root growth as the loss of function of either SOC1 or AGL24 partially recovers the primary root growth, meristem cell number, cell production rate, and the length of fully elongated cells of the short-root mutant xal2-2. Furthermore, we observed that the simultaneous overexpression of AGL24 and SOC1 leads to short-root phenotypes, affecting meristem cell number and fully elongated cell size, whereas SOC1 overexpression is sufficient to affect columella stem cell differentiation. Additionally, qPCR analyses revealed that these genes exhibit distinct modes of transcriptional regulation in roots compared to what has been previously reported for aerial tissues. We identified 100 differentially expressed genes in xal2-2 roots by RNA-seq. Moreover, our findings revealed that the expression of certain genes involved in cell differentiation, as well as stress responses, which are either upregulated or downregulated in the xal2-2 mutant, reverted to WT levels in the absence of SOC1 or AGL24.

Key candidate genes for male sterility in peppers unveiled via transcriptomic and proteomic analyses

This study aimed to enhance the use of male sterility in pepper to select superior hybrid generations. Transcriptomic and proteomic analyses of fertile line 1933A and nucleic male sterility line 1933B of Capsicum annuum L. were performed to identify male sterility-related proteins and genes. The phylogenetic tree, physical and chemical characteristics, gene structure characteristics, collinearity and expression characteristics of candidate genes were analyzed. The study identified 2,357 differentially expressed genes, of which 1,145 and 229 were enriched in the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes databases, respectively. A total of 7,628 quantifiable proteins were identified and 29 important proteins and genes were identified. It is worth noting that the existence of CaPRX genes has been found in both proteomics and transcriptomics, and 3 CaPRX genes have been identified through association analysis. A total of 66 CaPRX genes have been identified at the genome level, which are divided into 13 subfamilies, all containing typical CaPRX gene conformal domains. It is unevenly distributed across 12 chromosomes (including the virtual chromosome Chr00). Salt stress and co-expression analysis show that male sterility genes are expressed to varying degrees, and multiple transcription factors are co-expressed with CaPRXs, suggesting that they are involved in the induction of pepper salt stress. The study findings provide a theoretical foundation for genetic breeding by identifying genes, metabolic pathways, and molecular mechanisms involved in male sterility in pepper.

PRX102 Participates in Root Hairs Tip Growth of Rice

Root hairs are extensions of epidermal cells on the root tips that increase the root contract surface area with the soil. For polar tip growth, newly synthesized proteins and other materials must be incorporated into the tips of root hairs. Here, we report the characterization of PRX102, a root hair preferential endoplasmic reticulum peroxidase. During root hair growth, PRX102 has a polar localization pattern within the tip regions of root hairs but it loses this polarity after growth termination. Moreover, PRX102 participates in root hair outgrowth by regulating dense cytoplasmic streaming toward the tip. This role is distinct from those of other peroxidases playing roles in the root hairs and regulating reactive oxygen species homeostasis. RNA-seq analysis using prx102 root hairs revealed that 87 genes including glutathione S-transferase were downregulated. Our results therefore suggest a new function of peroxidase as a player in the delivery of substances to the tips of growing root hairs.

H2 O2 , NO, and H2 S networks during root development and signalling under physiological and challenging environments: Beneficial or toxic?

Hydrogen peroxide (H₂ O₂ ) is a reactive oxygen species (ROS) and a key modulator of the development and architecture of the root system under physiological and adverse environmental conditions. Nitric oxide (NO) and hydrogen sulphide (H₂ S) also exert myriad functions on plant development and signalling. Accumulating pieces of evidence show that depending upon the dose and mode of applications, NO and H₂ S can have synergistic or antagonistic actions in mediating H₂ O₂ signalling during root development. Thus, H₂ O₂ -NO-H₂ S crosstalk might essentially impart tolerance to elude oxidative stress in roots. Growth and proliferation of root apex involve crucial orchestration of NO and H₂ S-mediated ROS signalling which also comprise other components including mitogen-activated protein kinase, cyclins, cyclin-dependent kinases, respiratory burst oxidase homolog (RBOH), and Ca²⁺ flux. This assessment provides a comprehensive update on the cooperative roles of NO and H₂ S in modulating H₂ O₂ homoeostasis during root development, abiotic stress tolerance, and root-microbe interaction. Furthermore, it also analyses the scopes of some fascinating future investigations associated with strigolactone and karrikins concerning H₂ O₂ -NO-H₂ S crosstalk in plant roots.

Comparative transcriptomic analysis and functional characterization reveals that the class III peroxidase gene TaPRX-2A regulates drought stress tolerance in transgenic wheat

Drought is a major abiotic stress that reduces crop yields and quality worldwide. Although some genes involved in the response to drought stress have been identified, a more in-depth understanding of the mechanisms underlying wheat tolerance to drought is needed for the control of drought tolerance. Here, we evaluated the drought tolerance of 15 wheat cultivars and measured their physiological-biochemical parameters. Our data showed that the drought tolerance of the resistant wheat cultivars was significantly higher than that of drought-sensitive cultivars, which was associated with a greater antioxidant capacity of the former. Transcriptomic analysis revealed that different mechanisms of drought tolerance exist between the wheat cultivars Ziyou 5 and Liangxing 66. Transcriptomic analysis also revealed a large number of DEGs, including those involved in flavonoid biosynthesis, phytohormone signalling, phenolamides and antioxidants. qRT-PCR was performed, and the results showed that the expression levels of TaPRX-2A were significantly different among the various wheat cultivars under drought stress. Further study revealed that overexpression of TaPRX-2A enhanced tolerance to drought stress through the maintenance of increased antioxidase activities and reductions in ROS contents. Overexpression of TaPRX-2A also increased the expression levels of stress-related genes and ABA-related genes. Taken together, our findings show that flavonoids, phytohormones, phenolamides and antioxidants are involved in the plant response to drought stress and that TaPRX-2A is a positive regulator of this response. Our study provides insights into tolerance mechanisms and highlights the potential of TaPRX-2A overexpression in enhancing drought tolerance in crop improvement programmes.

Reduced Expression of PRX2/ATPRX1, PRX8, PRX35, and PRX73 Affects Cell Elongation, Vegetative Growth, and Vasculature Structures in Arabidopsis thaliana

Class III peroxidases (PRXs) are involved in a broad spectrum of physiological and developmental processes throughout the life cycle of plants. However, the specific function of each PRX member in the family remains largely unknown. In this study, we selected four class III peroxidase genes (PRX2/ATPRX1, PRX8, PRX35, and PRX73) from a previous genome-wide transcriptome analysis, and performed phenotypic and morphological analyses, including histochemical staining, in PRX2RNAi, PRX8RNAi, PRX35RNAi, and PRX73RNAi plants. The reduced mRNA levels of corresponding PRX genes in PRX2RNAi, PRX8RNAi, PRX35RNAi, and PRX73RNAi seedlings resulted in elongated hypocotyls and roots, and slightly faster vegetative growth. To investigate internal structural changes in the vasculature, we performed histochemical staining, which revealed alterations in cell wall structures in the main vasculature of hypocotyls, stems, and roots of each PRXRNAi plant compared to wild-type (Col-0) plants. Furthermore, we found that PRX35RNAi plants displayed the decrease in the cell wall in vascular regions, which are involved in downregulation of lignin biosynthesis and biosynthesis-regulated genes' expression. Taken together, these results indicated that the reduced expression levels of PRX2/ATPRX1, PRX8, PRX35, and PRX73 affected hypocotyl and root elongation, vegetative growth, and the vasculature structures in hypocotyl, stem, and root tissues, suggesting that the four class III PRX genes play roles in plant developmental processes.

GhTCE1-GhTCEE1 dimers regulate transcriptional reprogramming during wound-induced callus formation in cotton

Wounded plant cells can form callus to seal the wound site. Alternatively, wounding can cause adventitious organogenesis or somatic embryogenesis. These distinct developmental pathways require specific cell fate decisions. Here, we identify GhTCE1, a basic helix-loop-helix family transcription factor, and its interacting partners as a central regulatory module of early cell fate transition during in vitro dedifferentiation of cotton (Gossypium hirsutum). RNAi- or CRISPR/Cas9-mediated loss of GhTCE1 function resulted in excessive accumulation of reactive oxygen species (ROS), arrested callus cell elongation, and increased adventitious organogenesis. In contrast, GhTCE1-overexpressing tissues underwent callus cell growth, but organogenesis was repressed. Transcriptome analysis revealed that several pathways depend on proper regulation of GhTCE1 expression, including lipid transfer pathway components, ROS homeostasis, and cell expansion. GhTCE1 bound to the promoters of the target genes GhLTP2 and GhLTP3, activating their expression synergistically, and the heterodimer TCE1-TCEE1 enhances this activity. GhLTP2- and GhLTP3-deficient tissues accumulated ROS and had arrested callus cell elongation, which was restored by ROS scavengers. These results reveal a unique regulatory network involving ROS and lipid transfer proteins, which act as potential ROS scavengers. This network acts as a switch between unorganized callus growth and organized development during in vitro dedifferentiation of cotton cells.

Lectin affinity-based glycoproteome analysis of the developing xylem in poplar

Glycosylation is a significant post-translational modification of proteins, and some glycoproteins serve as players in plant cell wall synthesis and modification. Wood is a highly developed cell wall organization, and protein glycosylation as a regulatory mechanism may be involved in wood formation. Here, a lectin affinity-based glycoproteome was performed in stem developing xylem of poplar. After enrichment, trypsin digestion, LC-MS/MS analysis and peptide identification, we identified 154 glycoproteins from poplar developing xylem, which were classified into nine functional groups mainly including protein acting on carbohydrates, oxido-reductase, proteases, and protein kinases. Further, N- and/or O-glycosylation sites of the identified proteins were analyzed using bioinformatic tools, and deglycosylation experiments in the selected PtSOD and PtHAD proteins verified the reliability of the identified glycoproteins. Analysis of protein subcellular localization showed that a total of 63% of the identified glycoproteins were extracellular proteins or located in the plasma membrane. Poplar eFP and RT-qPCR data showed that a number of the genes encoding these glycoproteins such as laccase, peroxidase and cysteine protease, have highly preferential expression profiles in the developing xylem. Together with previously published research, most identified glycoproteins could be involved in wood cell wall synthesis and modification in poplar. Thus, our study provides some potential wood formation-related glycoproteins to be determined during tree stem development.

Transcriptomics Using the Enriched Arabidopsis Shoot Apex Reveals Developmental Priming Genes Involved in Plastic Plant Growth under Salt Stress Conditions

In the shoot apical meristem (SAM), the homeostasis of the stem cell population supplying new cells for organ formation is likely a key mechanism of multicellular plant growth and development. As plants are sessile organisms and constantly encounter environmental abiotic stresses, postembryonic development from the shoot stem cell population must be considered with surrounding abiotic stresses for plant adaptation. However, the underlying molecular mechanisms for plant adaptation remain unclear. Previous studies found that the stem-cell-related mutant clv3-2 has the property of salt tolerance without the differential response of typical stress-responsive genes compared to those in WT Ler. Based on these facts, we hypothesized that shoot meristems contain developmental priming genes having comprehensively converged functions involved in abiotic stress response and development. To better understand the biological process of developmental priming genes in the SAM, we performed RNA sequencing (RNA-seq) and transcriptome analysis through comparing genome-wide gene expression profiles between enriched shoot apex and leaf tissues. As a result, 121 putative developmental priming genes differentially expressed in the shoot apex compared to the leaf were identified under normal and salt stress conditions. RNA-seq experiments also revealed the shoot apex-specific responsive genes for salt stress conditions. Based on combinatorial comparisons, 19 developmental priming genes were finally identified, including developmental genes related to cell division and abiotic/biotic-stress-responsive genes. Moreover, some priming genes showed CLV3-dependent responses under salt stress conditions in the clv3-2. These results presumably provide insight into how shoot meristem tissues have relatively high viability against stressful environmental conditions for the developmental plasticity of plants.

Brittle Culm 15 mutation alters carbohydrate composition, degradation and methanogenesis of rice straw during in vitro ruminal fermentation

Brittle Culm 15 (BC15) gene encodes a membrane-associated chitinase-like protein that participates in cellulose synthesis, and BC15 gene mutation affects cell wall composition in plant, such as cellulose or hemicellulose. The present study was designed to investigate the changes of carbohydrates composition in bc15 mutant straw, and the resulting consequence on rumen fermentation, methanogenesis, and microbial populations (qPCR) during in vitro ruminal fermentation process. Two substrates, bc15 mutant and wild-type (WT) rice straws, were selected for in vitro rumen batch culture. The first experiment was designed to investigate the kinetics of total gas and CH₄ production through 48-h in vitro ruminal fermentation, while the second experiment selected incubation time of 12 and 48 h to represent the early and late stage of in vitro ruminal incubation, respectively, and then investigated changes in biodegradation, fermentation end products, and selected representative microbial populations. The bc15 mutant straw had lower contents of cellulose, neutral detergent fiber (NDF) and acid detergent fiber (ADF), and higher contents of water-soluble carbohydrates, neutral detergent solubles (NDS) and monosaccharides. The bc15 mutant straw exhibited a distinct kinetics of 48-h total gas and CH₄ production with faster increases in early incubation when compared with WT straw. The bc15 mutant straw had higher DM degradation, NDF degradation and total volatile fatty acid concentration at 12 h of incubation, and lower NDF degradation and CH₄ production at 48 h of incubation, together with lower acetate to propionate ratio and ADF degradation and higher butyrate molar percentage and NDS degradation at both incubation times. Furthermore, the bc15 mutant straw resulted in greater 16S gene copies of F. succinogenes, with lower 18S gene copies of fungi at both incubation times. These results indicated that the BC15 gene mutation decreased fibrosis of cell wall of rice straw, enhanced degradation at the early stage of rumen fermentation, and shifts fermentation pattern from acetate to propionate and butyrate production, leading to the decreased volume and fractional rate of CH₄ production. However, BC15 gene mutation may enhance hardenability of cell wall structure of rice straw, which is more resistant for microbial colonization with decreased fiber degradation. Thus, this study modified rice straw by manipulating a cell wall biosynthesis gene and provides a potential strategy to alter degradation and CH₄ production during in vitro ruminal fermentation process.

Ceratocystis cacaofunesta differentially modulates the proteome in xylem-enriched tissue of cocoa genotypes with contrasting resistance to Ceratocystis wilt

Decreased accumulation of polyphenol oxidase, H₂O₂ accumulation, effective regulation of programmed cell death, and a protein predicted as allergenic can play key roles in cacao defense against Ceratocystis cacaofunesta. Ceratocystis wilt, caused by the fungus Ceratocystis cacaofunesta, has destroyed millions of Theobroma cacao trees in several countries of the Americas. Through proteomics, systems biology, and enzymatic analyses of infected stems, it was possible to infer mechanisms used by resistant (TSH1188) and susceptible (CCN51) cacao genotypes during infection. Protein extraction from xylem-enriched tissue of stems inoculated with the fungus and their controls 1 day after inoculation was carried out, followed by separation through two-dimensional gel electrophoresis and identification by mass spectrometry. Enzyme activity was determined at 1, 3, 7 and 15 days after inoculation. A total of 50 differentially accumulated distinct proteins were identified in the treatments of both genotypes and were classified into 10 different categories. An interaction network between homologous proteins from Arabidospsis thaliana was generated for each genotype, using the STRING database and Cytoscape software. Primary metabolism processes were apparently repressed in both genotypes. The resistance factors suggested for genotype TSH1188 were: H₂O₂ accumulation, effective regulation of programmed cell death, production of phytoalexins derived from tryptophan and furanocoumarins, and participation of a predicted allergenic protein with probable ribonuclease function inhibiting the germination and propagation of the fungus. In the susceptible genotype, it is possible that its recognition and signaling mechanism through proteins from the SEC14 family is easily overcome by the pathogen. Our results will help to better understand the interaction between cacao and one of its most aggressive pathogens, to create disease control strategies.

Narrow-Band 311 nm Ultraviolet-B Radiation Evokes Different Antioxidant Responses from Broad-Band Ultraviolet

Supplemental narrow-band 311 nm UV-B radiation was applied in order to study the effect of this specific wavelength on tobacco as a model plant. UV-B at photon fluxes varying between 2.9 and 9.9 μmol m⁻² s⁻¹ was applied to supplement 150 μmol m⁻² s⁻¹ photosynthetically active radiation (PAR) for four hours in the middle of the light period for four days. Narrow-band UV-B increased leaf flavonoid and phenolic acid contents. In leaves exposed to 311 nm radiation, superoxide dismutase activity increased, but phenolic peroxidase activity decreased, and the changes were proportional to the UV flux. Ascorbate peroxidase activities were not significantly affected. Narrow-band UV-B caused a dose-dependent linear decrease in the quantum efficiency of photosystem II, up to approximately 10% loss. A parallel decrease in non-regulated non-photochemical quenching indicates potential electron transfer to oxygen in UV-treated leaves. In addition to a flux-dependent increase in the imbalance between enzymatic H₂O₂ production and neutralization, this resulted in an approximately 50% increase in leaf H₂O₂ content under 2.9–6 μmol m⁻² s⁻¹ UV-B. Leaf H₂O₂ decreased to control levels under higher UV-B fluxes due to the onset of increased non-enzymatic H₂O₂- and superoxide-neutralizing capacities, which were not observed under lower fluxes. These antioxidant responses to 311 nm UV-B were different from our previous findings in plants exposed to broad-band UV-B. The results suggest that signaling pathways activated by 311 nm radiation are distinct from those stimulated by other wavelengths and support the heterogeneous regulation of plant UV responses.

Effects of Arabidopsis wall associated kinase mutations on ESMERALDA1 and elicitor induced ROS

Angiosperm cell adhesion is dependent on interactions between pectin polysaccharides which make up a significant portion of the plant cell wall. Cell adhesion in Arabidopsis may also be regulated through a pectin-related signaling cascade mediated by a putative O-fucosyltransferase ESMERALDA1 (ESMD1), and the Epidermal Growth Factor (EGF) domains of the pectin binding Wall associated Kinases (WAKs) are a primary candidate substrate for ESMD1 activity. Genetic interactions between WAKs and ESMD1 were examined using a dominant hyperactive allele of WAK2, WAK2cTAP, and a mutant of the putative O-fucosyltransferase ESMD1. WAK2cTAP expression results in a dwarf phenotype and activation of the stress response and reactive oxygen species (ROS) production, while esmd1 is a suppressor of a pectin deficiency induced loss of adhesion. Here we find that esmd1 suppresses the WAK2cTAP dwarf and stress response phenotype, including ROS accumulation and gene expression. Additional analysis suggests that mutations of the potential WAK EGF O-fucosylation site also abate the WAK2cTAP phenotype, yet only evidence for an N-linked but not O-linked sugar addition can be found. Moreover, a WAK locus deletion allele has no effect on the ability of esmd1 to suppress an adhesion deficiency, indicating WAKs and their modification are not a required component of the potential ESMD1 signaling mechanism involved in the control of cell adhesion. The WAK locus deletion does however affect the induction of ROS but not the transcriptional response induced by the elicitors Flagellin, Chitin and oligogalacturonides (OGs).

Extensins: Self-Assembly, Crosslinking, and the Role of Peroxidases

The extensin (EXT) network is elaborated by the covalent intermolecular crosslinking of EXT glycoprotein monomers, and its proper assembly is important for numerous aspects of basic wall architecture and cellular defense. In this review, we discuss new advances in the secretion of EXT monomers and the molecular drivers of EXT network self-assembly. Many of the functions of EXTs are conferred through covalent crosslinking into the wall, so we also discuss the different types of known intermolecular crosslinks, the enzymes that are involved, as well as the potential for additional crosslinks that are yet to be identified. EXTs also function in wall architecture independent of crosslinking status, and therefore, we explore the role of non-crosslinking EXTs. As EXT crosslinking is upregulated in response to wounding and pathogen infection, we discuss a potential regulatory mechanism to control covalent crosslinking and its relationship to the subcellular localization of the crosslinking enzymes.

Maintenance of Cell Wall Integrity under High Salinity

Cell wall biosynthesis is a complex biological process in plants. In the rapidly growing cells or in the plants that encounter a variety of environmental stresses, the compositions and the structure of cell wall can be dynamically changed. To constantly monitor cell wall status, plants have evolved cell wall integrity (CWI) maintenance system, which allows rapid cell growth and improved adaptation of plants to adverse environmental conditions without the perturbation of cell wall organization. Salt stress is one of the abiotic stresses that can severely disrupt CWI, and studies have shown that the ability of plants to sense and maintain CWI is important for salt tolerance. In this review, we highlight the roles of CWI in salt tolerance and the mechanisms underlying the maintenance of CWI under salt stress. The unsolved questions regarding the association between the CWI and salt tolerance are discussed.

Genome-wide analysis of spatiotemporal expression patterns during rice leaf development

BACKGROUND

Rice leaves consist of three distinct regions along a proximal-distal axis, namely the leaf blade, sheath, and blade-sheath boundary region. Each region has a unique morphology and function, but the genetic programs underlying the development of each region are poorly understood. To fully elucidate rice leaf development and discover genes with unique functions in rice and grasses, it is crucial to explore genome-wide transcriptional profiles during the development of the three regions.

RESULTS

In this study, we performed microarray analysis to profile the spatial and temporal patterns of gene expression in the rice leaf using dissected parts of leaves sampled in broad developmental stages. The dynamics in each region revealed that the transcriptomes changed dramatically throughout the progress of tissue differentiation, and those of the leaf blade and sheath differed greatly at the mature stage. Cluster analysis of expression patterns among leaf parts revealed groups of genes that may be involved in specific biological processes related to rice leaf development. Moreover, we found novel genes potentially involved in rice leaf development using a combination of transcriptome data and in situ hybridization, and analyzed their spatial expression patterns at high resolution. We successfully identified multiple genes that exhibit localized expression in tissues characteristic of rice or grass leaves.

CONCLUSIONS

Although the genetic mechanisms of leaf development have been elucidated in several eudicots, direct application of that information to rice and grasses is not appropriate due to the morphological and developmental differences between them. Our analysis provides not only insights into the development of rice leaves but also expression profiles that serve as a valuable resource for gene discovery. The genes and gene clusters identified in this study may facilitate future research on the unique developmental mechanisms of rice leaves.

Host Cell Wall Damage during Pathogen Infection: Mechanisms of Perception and Role in Plant-Pathogen Interactions

The plant cell wall (CW) is a complex structure that acts as a mechanical barrier, restricting the access to most microbes. Phytopathogenic microorganisms can deploy an arsenal of CW-degrading enzymes (CWDEs) that are required for virulence. In turn, plants have evolved proteins able to inhibit the activity of specific microbial CWDEs, reducing CW damage and favoring the accumulation of CW-derived fragments that act as damage-associated molecular patterns (DAMPs) and trigger an immune response in the host. CW-derived DAMPs might be a component of the complex system of surveillance of CW integrity (CWI), that plants have evolved to detect changes in CW properties. Microbial CWDEs can activate the plant CWI maintenance system and induce compensatory responses to reinforce CWs during infection. Recent evidence indicates that the CWI surveillance system interacts in a complex way with the innate immune system to fine-tune downstream responses and strike a balance between defense and growth.

The FLA4-FEI Pathway: A Unique and Mysterious Signaling Module Related to Cell Wall Structure and Stress Signaling

Cell wall integrity control in plants involves multiple signaling modules that are mostly defined by genetic interactions. The putative co-receptors FEI1 and FEI2 and the extracellular glycoprotein FLA4 present the core components of a signaling pathway that acts in response to environmental conditions and insults to cell wall structure to modulate the balance of various growth regulators and, ultimately, to regulate the performance of the primary cell wall. Although the previously established genetic interactions are presently not matched by intermolecular binding studies, numerous receptor-like molecules that were identified in genome-wide interaction studies potentially contribute to the signaling machinery around the FLA4-FEI core. Apart from its function throughout the model plant Arabidopsis thaliana for the homeostasis of growth and stress responses, the FLA4-FEI pathway might support important agronomic traits in crop plants.

Impaired Cuticle Functionality and Robust Resistance to Botrytis cinerea in Arabidopsis thaliana Plants With Altered Homogalacturonan Integrity Are Dependent on the Class III Peroxidase AtPRX71

Pectin is a major cell wall component that plays important roles in plant development and response to environmental stresses. Arabidopsis thaliana plants expressing a fungal polygalacturonase (PG plants) that degrades homogalacturonan (HG), a major pectin component, as well as loss-of-function mutants for QUASIMODO2 (QUA2), encoding a putative pectin methyltransferase important for HG biosynthesis, show accumulation of reactive oxygen species (ROS), reduced growth and almost complete resistance to the fungal pathogen Botrytis cinerea. Both PG and qua2 plants show increased expression of the class III peroxidase AtPRX71 that contributes to their elevated ROS levels and reduced growth. In this work, we show that leaves of PG and qua2 plants display greatly increased cuticle permeability. Both increased cuticle permeability and resistance to B. cinerea in qua2 are suppressed by loss of AtPRX71. Increased cuticle permeability in qua2, rather than on defects in cuticle ultrastructure or cutin composition, appears to be dependent on reduced epidermal cell adhesion, which is exacerbated by AtPRX71, and is suppressed by the esmeralda1 mutation, which also reverts the adhesion defect and the resistant phenotype. Increased cuticle permeability, accumulation of ROS, and resistance to B. cinerea are also observed in mutants lacking a functional FERONIA, a receptor-like kinase thought to monitor pectin integrity. In contrast, mutants with defects in other structural components of primary cell wall do not have a defective cuticle and are normally susceptible to the fungus. Our results suggest that disrupted cuticle integrity, mediated by peroxidase-dependent ROS accumulation, plays a major role in the robust resistance to B. cinerea of plants with altered HG integrity.

Prior exposure of Arabidopsis seedlings to mechanical stress heightens jasmonic acid-mediated defense against necrotrophic pathogens

BACKGROUND

Prolonged mechanical stress (MS) causes thigmomorphogenesis, a stress acclimation response associated with increased disease resistance. What remains unclear is if; 1) plants pre-exposed to a short period of repetitive MS can prime defence responses upon subsequent challenge with necrotrophic pathogens, 2) MS mediates plant immunity via jasmonic acid (JA) signalling, and 3) a short period of repetitive MS can cause long-term changes in gene expression resembling a stress-induced memory. To address these points, 10-days old juvenile Arabidopsis seedlings were mechanically stressed for 7-days using a soft brush and subsequently challenged with the necrotrophic pathogens, Alternaria brassicicola, and Botrytis cinerea. Here we assessed how MS impacted structural cell wall appositions, disease symptoms and altered gene expression in response to infection.

RESULTS

The MS-treated plants exhibited enhanced cell wall appositions and jasmonic acid (JA) accumulation that correlated with a reduction in disease progression compared to unstressed plants. The expression of genes involved in JA signalling, callose deposition, peroxidase and phytoalexin biosynthesis and reactive oxygen species detoxification were hyper-induced 4-days post-infection in MS-treated plants. The loss-of-function in JA signalling mediated by the JA-insensitive coronatine-insensitive 1 (coi1) mutant impaired the hyper-induction of defense gene expression and promoted pathogen proliferation in MS-treated plants subject to infection. The basal expression level of PATHOGENESIS-RELATED GENE 1 and PLANT DEFENSIN 1.2 defense marker genes were constitutively upregulated in rosette leaves for 5-days post-MS, as well as in naïve cauline leaves that differentiated from the inflorescence meristem well after ceasing MS.

CONCLUSION

This study reveals that exposure of juvenile Arabidopsis plants to a short repetitive period of MS can alter gene expression and prime plant resistance upon subsequent challenge with necrotrophic pathogens via the JA-mediated COI1 signalling pathway. MS may facilitate a stress-induced memory to modulate the plant's response to future stress encounters. These data advance our understanding of how MS primes plant immunity against necrotrophic pathogens and how that could be utilised in sustainable agricultural practices.

The Class III Peroxidase Encoding Gene AtPrx62 Positively and Spatiotemporally Regulates the Low pH-Induced Cell Death in Arabidopsis thaliana Roots

Exogenous low pH stress causes cell death in root cells, limiting root development, and agricultural production. Different lines of evidence suggested a relationship with cell wall (CW) remodeling players. We investigated whether class III peroxidase (CIII Prx) total activity, CIII Prx candidate gene expression, and reactive oxygen species (ROS) could modify CW structure during low pH-induced cell death in Arabidopsis thaliana roots. Wild-type roots displayed a good spatio-temporal correlation between the low pH-induced cell death and total CIII Prx activity in the early elongation (EZs), transition (TZs), and meristematic (MZs) zones. In situ mRNA hybridization showed that AtPrx62 transcripts accumulated only in roots treated at pH 4.6 in the same zones where cell death was induced. Furthermore, roots of the atprx62-1 knockout mutant showed decreased cell mortality under low pH compared to wild-type roots. Among the ROS, there was a drastic decrease in O₂^●− levels in the MZs of wild-type and atprx62-1 roots upon low pH stress. Together, our data demonstrate that AtPrx62 expression is induced by low pH and that the produced protein could positively regulate cell death. Whether the decrease in O₂^●− level is related to cell death induced upon low pH treatment remains to be elucidated.

Comparative Transcriptomics and Co-Expression Networks Reveal Tissue- and Genotype-Specific Responses of qDTYs to Reproductive-Stage Drought Stress in Rice (Oryza sativa L.)

Rice (Oryza sativa L.) is more sensitive to drought stress than other cereals. To dissect molecular mechanisms underlying drought-tolerant yield in rice, we applied differential expression and co-expression network approaches to transcriptomes from flag-leaf and emerging panicle tissues of a drought-tolerant yield introgression line, DTY-IL, and the recurrent parent Swarna, under moderate reproductive-stage drought stress. Protein turnover and efficient reactive oxygen species scavenging were found to be the driving factors in both tissues. In the flag-leaf, the responses further included maintenance of photosynthesis and cell wall reorganization, while in the panicle biosynthesis of secondary metabolites was found to play additional roles. Hub genes of importance in differential drought responses included an expansin in the flag-leaf and two peroxidases in the panicle. Overlaying differential expression data with allelic variation in DTY-IL quantitative trait loci allowed for the prioritization of candidate genes. They included a differentially regulated auxin-responsive protein, with DTY-IL-specific amino acid changes in conserved domains, as well as a protein kinase with a DTY-IL-specific frameshift in the C-terminal region. The approach highlights how the integration of differential expression and allelic variation can aid in the discovery of mechanism and putative causal contribution underlying quantitative trait loci for drought-tolerant yield.

Enzymes contributing to the hydrogen peroxide signal dynamics that regulate wall labyrinth formation in transfer cells

Transfer cells are characterized by an amplified plasma membrane area supported on a wall labyrinth composed of a uniform wall layer (UWL) from which wall ingrowth (WI) papillae arise. Adaxial epidermal cells of developing Vicia faba cotyledons, when placed in culture, undergo a rapid (hours) trans-differentiation to a functional epidermal transfer cell (ETC) phenotype. The trans-differentiation event is controlled by a signalling cascade comprising auxin, ethylene, apoplasmic reactive oxygen species (apoROS), and cytosolic Ca2+. Apoplasmic hydrogen peroxide (apoH2O2) was confirmed as the apoROS regulating UWL and WI papillae formation. Informed by an ETC-specific transcriptome, a pharmacological approach identified a temporally changing cohort of H2O2 biosynthetic enzymes. The cohort contained a respiratory burst oxidase homologue, polyamine oxidase, copper amine oxidase, and a suite of class III peroxidases. Collectively these generated two consecutive bursts in apoH2O2 production. Spatial organization of biosynthetic/catabolic enzymes was deduced from responses to pharmacologically blocking their activities on the cellular and subcellular distribution of apoH2O2. The findings were consistent with catalase activity constraining the apoH2O2 signal to the outer periclinal wall of the ETCs. Strategic positioning of class III peroxidases in this outer domain shaped subcellular apoH2O2 signatures that differed during assembly of the UWL and WI papillae.

An Integrated Analysis of the Rice Transcriptome and Metabolome Reveals Root Growth Regulation Mechanisms in Response to Nitrogen Availability

Nitrogen is an essential nutrient for plant growth and basic metabolic processes. Root systems play an important role in the ability of plants to obtain nutrients from the soil, and are closely related to the growth and development of above-ground plants. Root morphology analysis showed that root growth was induced under low-nitrogen conditions and inhibited under high-nitrogen conditions. To better understand the molecular mechanisms and metabolic basis underlying the rice root response to nitrogen availability, an integrated analysis of the rice root transcriptome and metabolome under three environmental conditions (low-, control, and high-nitrogen conditions) was conducted. A total of 262 and 262 differentially level metabolites were identified under low- and high-nitrogen conditions, respectively. A total of 696 and 808 differentially expressed genes were identified under low- and high-nitrogen conditions, respectively. For both the differentially expressed genes and metabolites, KEGG pathway analysis indicated that amino acid metabolism, carbon and nitrogen metabolism, phenylpropanoid metabolism, and phytohormones’ signal transduction were significantly affected by nitrogen availability. Additionally, variable levels of 65 transcription factors (TFs) were identified in rice leaves exposed to high and low nitrogen, covering 22 TF families. These results also indicate that there is a significant difference in the transcriptional regulation mechanisms of rice roots between low and high nitrogen. In summary, our study provides new information for a further understanding of the response of rice roots to low-nitrogen and high-nitrogen conditions.

Genome-wide and evolutionary analysis of the class III peroxidase gene family in wheat and Aegilops tauschii reveals that some members are involved in stress responses

Background

The class III peroxidase (PRX) gene family is a plant-specific member of the PRX superfamily that is closely related to various physiological processes, such as cell wall loosening, lignification, and abiotic and biotic stress responses. However, its classification, evolutionary history and gene expression patterns are unclear in wheat and Aegilops tauschii.

Results

Here, we identified 374, 159 and 169 PRXs in Triticum aestivum, Triticum urartu and Ae. tauschii, respectively. Together with PRXs detected from eight other plants, they were classified into 18 subfamilies. Among subfamilies V to XVIII, a conserved exon-intron structure within the “001” exon phases was detected in the PRX domain. Based on the analysis, we proposed a phylogenetic model to infer the evolutionary history of the exon-intron structures of PRX subfamilies. A comparative genomics analysis showed that subfamily VII could be the ancient subfamily that originated from green algae (Chlamydomonas reinhardtii). Further integrated analysis of chromosome locations and collinearity events of PRX genes suggested that both whole genome duplication (WGD) and tandem duplication (TD) events contributed to the expansion of T. aestivum PRXs (TaePRXs) during wheat evolution. To validate functions of these genes in the regulation of various physiological processes, the expression patterns of PRXs in different tissues and under various stresses were studied using public microarray datasets. The results suggested that there were distinct expression patterns among different tissues and PRXs could be involved in biotic and abiotic responses in wheat. qRT-PCR was performed on samples exposed to drought, phytohormone treatments and Fusarium graminearum infection to validate the microarray predictions. The predicted subcellular localizations of some TaePRXs were consistent with the confocal microscopy results. We predicted that some TaePRXs had hormone-responsive cis-elements in their promoter regions and validated these predicted cis-acting elements by sequencing promoters.

Conclusion

In this study, identification, classification, evolution, and expression patterns of PRXs in wheat and relative plants were performed. Our results will provide information for further studies on the evolution and molecular mechanisms of wheat PRXs.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6006-5) contains supplementary material, which is available to authorized users.

Expression of a novel PSK-encoding gene from soybean improves seed growth and yield in transgenic plants

MAIN CONCLUSION

Expression of GmPSKγ1 , a novel PSK-encoding gene from soybean, increases seed size and yield in transgenic plants by promoting cell expansion. Phytosulfokine-α (PSK-α), a sulfated pentapeptide hormone with the sequence YIYTQ, plays important roles in many aspects of plant growth and development. In this study, we identified a pair of putative precursor genes in soybean, GmPSKγ1 and -2, encoding a PSK-like peptide: PSK-γ. Similar to PSK-α in amino acid composition, the sequence of PSK-γ is YVYTQ, and the tyrosines undergo sulfonylation. Treatment of Arabidopsis seedlings with synthetic sulfated PSK-γ significantly enhanced root elongation, indicating that PSK-γ might be a functional analog of PSK-α. Expression pattern analysis revealed that the two GmPSKγ genes, especially GmPSKγ1, are primarily expressed in developing soybean seeds. Heterologous expression of GmPSKγ1 under the control of a seed-specific promoter markedly increased seed size and weight in Arabidopsis, and this promoting effect of PSK-γ on seed growth was further confirmed in transgenic tobacco constitutively expressing GmPSKγ1. Cytological analysis of transgenic Arabidopsis seeds revealed that PSK-γ promotes seed growth by inducing embryo cell expansion. In addition, expression analysis of downstream candidate genes suggested that PSK-γ signaling might regulate cell wall loosening to promote cell expansion in Arabidopsis seeds. Overall, our results shed light on the mechanism by which PSK-γ promotes seed growth, paving the way for the use of this new peptide for biotechnological improvement of crop seed/grain size and yield.

Adaption of Roots to Nitrogen Deficiency Revealed by 3D Quantification and Proteomic Analysis

Rapeseed (Brassica napus) is an important oil crop worldwide. However, severe inhibition of rapeseed production often occurs in the field due to nitrogen (N) deficiency. The root system is the main organ to acquire N for plant growth, but little is known about the mechanisms underlying rapeseed root adaptions to N deficiency. Here, dynamic changes in root architectural traits of N-deficient rapeseed plants were evaluated by 3D in situ quantification. Root proteome responses to N deficiency were analyzed by the tandem mass tag-based proteomics method, and related proteins were characterized further. Under N deficiency, rapeseed roots become longer, with denser cells in the meristematic zone and larger cells in the elongation zone of root tips, and also become softer with reduced solidity. A total of 171 and 755 differentially expressed proteins were identified in short- and long-term N-deficient roots, respectively. The abundance of proteins involved in cell wall organization or biogenesis was highly enhanced, but most identified peroxidases were reduced in the N-deficient roots. Notably, peroxidase activities also were decreased, which might promote root elongation while lowering the solidity of N-deficient roots. These results were consistent with the cell wall components measured in the N-deficient roots. Further functional analysis using transgenic Arabidopsis (Arabidopsis thaliana) plants demonstrated that the two root-related differentially expressed proteins contribute to the enhanced root growth under N deficiency conditions. These results provide insights into the global changes of rapeseed root responses to N deficiency and may facilitate the development of rapeseed cultivars with high N use efficiency through root-based genetic improvements.

Hitting the Wall-Sensing and Signaling Pathways Involved in Plant Cell Wall Remodeling in Response to Abiotic Stress

Plant cells are surrounded by highly dynamic cell walls that play important roles regulating aspects of plant development. Recent advances in visualization and measurement of cell wall properties have enabled accumulation of new data about wall architecture and biomechanics. This has resulted in greater understanding of the dynamics of cell wall deposition and remodeling. The cell wall is the first line of defense against different adverse abiotic and biotic environmental influences. Different abiotic stress conditions such as salinity, drought, and frost trigger production of Reactive Oxygen Species (ROS) which act as important signaling molecules in stress activated cellular responses. Detection of ROS by still-elusive receptors triggers numerous signaling events that result in production of different protective compounds or even cell death, but most notably in stress-induced cell wall remodeling. This is mediated by different plant hormones, of which the most studied are jasmonic acid and brassinosteroids. In this review we highlight key factors involved in sensing, signal transduction, and response(s) to abiotic stress and how these mechanisms are related to cell wall-associated stress acclimatization. ROS, plant hormones, cell wall remodeling enzymes and different wall mechanosensors act coordinately during abiotic stress, resulting in abiotic stress wall acclimatization, enabling plants to survive adverse environmental conditions.

Reactive oxygen species in plant development

Reactive oxygen species (ROS) are produced by metabolic pathways in almost all cells. As signaling components, ROS are best known for their roles in abiotic and biotic stress-related events. However, recent studies have revealed that they are also involved in numerous processes throughout the plant life cycle, from seed development and germination, through to root, shoot and flower development. Here, we provide an overview of ROS production and signaling in the context of plant growth and development, highlighting the key functions of ROS and their interactions with plant phytohormonal networks.

The Role of Auxin in Cell Wall Expansion

Plant cells are surrounded by cell walls, which are dynamic structures displaying a strictly regulated balance between rigidity and flexibility. Walls are fairly rigid to provide support and protection, but also extensible, to allow cell growth, which is triggered by a high intracellular turgor pressure. Wall properties regulate the differential growth of the cell, resulting in a diversity of cell sizes and shapes. The plant hormone auxin is well known to stimulate cell elongation via increasing wall extensibility. Auxin participates in the regulation of cell wall properties by inducing wall loosening. Here, we review what is known on cell wall property regulation by auxin. We focus particularly on the auxin role during cell expansion linked directly to cell wall modifications. We also analyze downstream targets of transcriptional auxin signaling, which are related to the cell wall and could be linked to acid growth and the action of wall-loosening proteins. All together, this update elucidates the connection between hormonal signaling and cell wall synthesis and deposition.

Comparative morphology and transcriptome analysis reveals distinct functions of the primary and secondary laticifer cells in the rubber tree

Laticifers are highly specialized cells that synthesize and store natural rubber. Rubber trees (Hevea brasiliensis Muell. Arg.) contain both primary and secondary laticifers. Morphological and functional differences between the two types of laticifers are largely unknown, but such information is important for breeding and cultivation practices. Morphological comparison using paraffin sections revealed only distribution differences: the primary laticifers were distributed randomly, while the secondary laticifers were distributed in concentric rings. Using isolated laticifer networks, the primary laticifers were shown to develop via intrusive "budding" and formed necklace-like morphology, while the secondary laticifers developed straight and smooth cell walls. Comparative transcriptome analysis indicated that genes involved in cell wall modification, such as pectin esterase, lignin metabolic enzymes, and expansins, were highly up-regulated in the primary laticifers and correspond to its necklace-like morphology. Genes involved in defense against biotic stresses and rubber biosynthesis were highly up-regulated in the primary laticifers, whereas genes involved in abiotic stresses and dormancy were up-regulated in the secondary laticifers, suggesting that the primary laticifers are more adequately prepared to defend against biotic stresses, while the secondary laticifers are more adequately prepared to defend against abiotic stresses. Therefore, the two types of laticifers are morphologically and functionally distinct.

Defense Responses in Aspen with Altered Pectin Methylesterase Activity Reveal the Hormonal Inducers of Tyloses

Tyloses are ingrowths of parenchyma cells into the lumen of embolized xylem vessels, thereby protecting the remaining xylem from pathogens. They are found in heartwood, sapwood, and in abscission zones and can be induced by various stresses, but their molecular triggers are unknown. Here, we report that down-regulation of PECTIN METHYLESTERASE1 (PtxtPME1) in aspen (Populus tremula × tremuloides) triggers the formation of tyloses and activation of oxidative stress. We tested whether any of the oxidative stress-related hormones could induce tyloses in intact plantlets grown in sterile culture. Jasmonates, including jasmonic acid (JA) and methyl jasmonate, induced the formation of tyloses, whereas treatments with salicylic acid (SA) and 1-aminocyclopropane-1-carboxylic acid (ACC) were ineffective. SA abolished the induction of tyloses by JA, whereas ACC was synergistic with JA. The ability of ACC to stimulate tyloses formation when combined with JA depended on ethylene (ET) signaling, as shown by a decrease in the response in ET-insensitive plants. Measurements of internal ACC and JA concentrations in wild-type and ET-insensitive plants treated simultaneously with these two compounds indicated that ACC and JA regulate each other's concentration in an ET-dependent manner. The findings indicate that jasmonates acting synergistically with ethylene are the key molecular triggers of tyloses.

Extra-Cellular But Extra-Ordinarily Important for Cells: Apoplastic Reactive Oxygen Species Metabolism

Reactive oxygen species (ROS), by their very nature, are highly reactive, and it is no surprise that they can cause damage to organic molecules. In cells, ROS are produced as byproducts of many metabolic reactions, but plants are prepared for this ROS output. Even though extracellular ROS generation constitutes only a minor part of a cell's total ROS level, this fraction is of extraordinary importance. In an active apoplastic ROS burst, it is mainly the respiratory burst oxidases and peroxidases that are engaged, and defects of these enzymes can affect plant development and stress responses. It must be highlighted that there are also other less well-known enzymatic or non-enzymatic ROS sources. There is a need for ROS detoxification in the apoplast, and almost all cellular antioxidants are present in this space, but the activity of antioxidant enzymes and the concentration of low-mass antioxidants is very low. The low antioxidant efficiency in the apoplast allows ROS to accumulate easily, which is a condition for ROS signaling. Therefore, the apoplastic ROS/antioxidant homeostasis is actively engaged in the reception and reaction to many biotic and abiotic stresses.

Role of Reactive Oxygen Species during Cell Expansion in Leaves

Reactive oxygen species as potent regulators of leaf development poses special interest for cell expansion.

Modification of plant cell wall structure accompanied by enhancement of saccharification efficiency using a chemical, lasalocid sodium

The cell wall is one major determinant of plant cell morphology, and is an attractive bioresource. Here, we report a novel strategy to modify plant cell wall property by small molecules. Lasalocid sodium (LS) was isolated by chemical screening to identify molecules that affect the cell morphology of tobacco BY-2 cells. LS treatment led to an increase in cell wall thickness, whilst the quantity and sugar composition of the cell wall remained unchanged in BY-2 cells. The chemical also disordered the cellular arrangement of hypocotyls of Arabidopsis plants, resulting in a decrease in hypocotyl length. LS treatment enhanced enzymatic saccharification efficiency in both BY-2 cells and Arabidopsis plants. Microarray analysis on Arabidopsis showed that exposure to LS upregulated type III peroxidase genes, of which some are involved in lignin biogenesis, and jasmonic acid response genes, and phloroglucinol staining supported the activation of lignification by the LS treatment. As jasmonic acid-mediated lignification is a typical reaction to cell wall damage, it is possible that LS induces cell wall loosening, which can trigger cell wall damage response. Thus, LS is a unique chemical for modification of cell wall and morphology through changes in cell wall architecture.

Plastid osmotic stress influences cell differentiation at the plant shoot apex

The balance between proliferation and differentiation in the plant shoot apical meristem is controlled by regulatory loops involving the phytohormone cytokinin and stem cell identity genes. Concurrently, cellular differentiation in the developing shoot is coordinated with the environmental and developmental status of plastids within those cells. Here, we employ an Arabidopsis thaliana mutant exhibiting constitutive plastid osmotic stress to investigate the molecular and genetic pathways connecting plastid osmotic stress with cell differentiation at the shoot apex. msl2 msl3 mutants exhibit dramatically enlarged and deformed plastids in the shoot apical meristem, and develop a mass of callus tissue at the shoot apex. Callus production in this mutant requires the cytokinin receptor AHK2 and is characterized by increased cytokinin levels, downregulation of cytokinin signaling inhibitors ARR7 and ARR15, and induction of the stem cell identity gene WUSCHEL Furthermore, plastid stress-induced apical callus production requires elevated plastidic reactive oxygen species, ABA biosynthesis, the retrograde signaling protein GUN1, and ABI4. These results are consistent with a model wherein the cytokinin/WUS pathway and retrograde signaling control cell differentiation at the shoot apex.