GDSL-domain proteins have key roles in suberin polymerization and degradation

Striga hermonthica induces lignin deposition at the root tip to facilitate prehaustorium formation and obligate parasitism

Striga hermonthica, an obligate parasitic plant that causes severe agricultural damage, recognizes its hosts by sensing haustorium-inducing factors (HIFs). Perception of HIFs induces the rapid transformation of S. hermonthica radicles into prehaustoria, structures that enable host invasion and mature into haustoria. HIFs consist of various aromatic compounds, including quinones, lignin monomers, and flavonoids. However, the downstream molecular pathways that orchestrate these developmental events are largely unknown. Here, we report that S. hermonthica root-tip cells rapidly deposit lignin, a major cell wall component, in response to HIFs. In addition to enhancing lignin levels, HIFs strongly induce genes involved in lignin monomer biosynthesis and polymerization, including several respiratory burst oxidase homologs (RBOHs) and class III peroxidases. Disruption of lignin monomer biosynthesis compromises prehaustorium formation, whereas HIF-induced class III peroxidases facilitate the process by promoting lignification. Our study demonstrates that cell wall lignification is a converged cellular process downstream of various HIFs that guides root meristematic cells in prehaustorium development.

A single-nuclei transcriptome census of the Arabidopsis maturing root identifies that MYB67 controls phellem cell maturation

The periderm provides a protective barrier in many seed plant species. The development of the suberized phellem, which forms the outermost layer of this important tissue, has become a trait of interest for enhancing both plant resilience to stresses and plant-mediated CO2 sequestration in soils. Despite its importance, very few genes driving phellem development are known. Employing single-nuclei sequencing, we have generated an expression census capturing the complete developmental progression of Arabidopsis root phellem cells, from their progenitor cell type, the pericycle, through to their maturation. With this, we identify a whole suite of genes underlying this process, including MYB67, which we show has a role in phellem cell maturation. Our expression census and functional discoveries represent a resource, expanding our comprehension of secondary growth in plants. These data can be used to fuel discoveries and engineering efforts relevant to plant resilience and climate change.

Abscisic Acid and Ethylene Antagonistically Regulate Root Endodermal Suberization to Mitigate Nonuniform Salt Stress in Cotton

The heterogeneity of soil salinity is a critical attribute of saline agricultural environments, particularly for the physiological adaptability of cotton (Gossypium hirsutum L.) plants. However, the mechanisms by which cotton plants acclimate to heterogenous salinity remain poorly understood. To investigate the responses of cotton seedlings to nonuniform salinity, a split-root system using germination paper was employed to replicate spatially variable salinity conditions within the root zone. The root endodermal barriers, consisting of the suberin lamellae and Casparian strip, were found to be enhanced in the roots on the saline side of this system relative to the nonsaline side, playing a crucial role in maintaining ion balance for cotton seedlings under heterogeneous salt environment. Ethylene levels were higher in roots on the nonsaline side, but significantly lower in roots on the saline side. Notably, abscisic acid (ABA) levels increased in roots on both sides. The delicate balance between ABA and ethylene can modify the root endodermal suberization, thereby regulating the adaptability of cotton seedlings to diverse salt environments.

Multiple layers of regulators emerge in the network controlling lateral root organogenesis

Lateral root (LR) formation is a postembryonic organogenesis process that is crucial for plant root system development and adaptation to heterogenous soil environments. Since the early 1990s, a wealth of experimental data on arabidopsis (Arabidopsis thaliana) has helped reveal the LR formation regulatory network, in which dynamic auxin distribution and transcriptional cascades direct root cells through their organogenesis pathway. Some parts of this network appear conserved across diverse plant species or distinct developmental contexts. Recently, our knowledge of this process dramatically expanded thanks to technical advances, from single cell profiling to whole-root system phenotyping. Interestingly, new players are now emerging in this network, such as fatty acids and reactive oxygen species (ROS), transforming our knowledge of this hidden half of plant biology.

Natural variation in salt-induced changes in root:shoot ratio reveals SR3G as a negative regulator of root suberization and salt resilience in Arabidopsis

Soil salinity is one of the major threats to agricultural productivity worldwide. Salt stress exposure alters root and shoots growth rates, thereby affecting overall plant performance. While past studies have extensively documented the effect of salt stress on root elongation and shoot development separately, here we take an innovative approach by examining the coordination of root and shoot growth under salt stress conditions. Utilizing a newly developed tool for quantifying the root:shoot ratio in agar-grown Arabidopsis seedlings, we found that salt stress results in a loss of coordination between root and shoot growth rates. We identify a specific gene cluster encoding domain-of-unknown-function 247 (DUF247), and characterize one of these genes as Salt Root:shoot Ratio Regulator Gene (SR3G). Further analysis elucidates the role of SR3G as a negative regulator of salt stress tolerance, revealing its function in regulating shoot growth, root suberization, and sodium accumulation. We further characterize that SR3G expression is modulated by WRKY75 transcription factor, known as a positive regulator of salt stress tolerance. Finally, we show that the salt stress sensitivity of wrky75 mutant is completely diminished when it is combined with sr3g mutation. Together, our results demonstrate that utilizing root:shoot ratio as an architectural feature leads to the discovery of a new stress resilience gene. The study's innovative approach and findings not only contribute to our understanding of plant stress tolerance mechanisms but also open new avenues for genetic and agronomic strategies to enhance crop environmental resilience.

Cell wall dynamic changes and signaling during plant lateral root development

Lateral roots (LRs), are an important component of plant roots, playing a crucial role in anchoring the plant in the soil and facilitating the uptake of water and nutrients. As post-embryonic organs, LRs originate from the pericycle cells of the primary root, and their formation is characterized by precise regulation of cell division and complex intercellular interactions, both of which are closely tied to cell wall regulation. Considering the rapid advances in molecular techniques over the past three decades, we reframe the understanding of the dynamic change in cell wall during LR development by summarizing the factors that precipitate these changes and their effects, as well as the regulated signals involved. Additionally, we discuss current challenges in this field and propose potential solutions.

Rhizobacteria protective hydrogel to promote plant growth and adaption to acidic soil

Endophytic plant growth promoting rhizobacteria (PGPRs) could replace chemical fertilizers in sustainable agriculture. Unfortunately, they are susceptible to harsh environmental conditions. Here, we proposed a polymeric hydrogel (PMH) consisting of carboxymethyl chitosan, sodium alginate, and calcium chloride for loading and protecting endophytic PGPR. This hydrogel can load endophytic PGPRs to not only boost its growth-promoting efficiency, but also help them adapt more effectively to environments. Using endophytic PGPR Ensifer C5 as model bacteria and Brasscia napus as host, we demonstrate that the PMH facilitate the colonization of endophytic PGPRs in the apical and lateral root primordia regions. Further analysis indicates that the PMH modulate suberin deposition of the endodermal cell layers and regulate the accumulation of auxin at the root tip. Meanwhile, PMH enhances the antioxidant capacity and disease resistance properties of plants by increasing the content of arachidonic acid metabolism intermediates in the plant. Importantly, the combination of PMH and endophytic PGPRs increases the yields of B. napus by approximately 30% in the field. Furthermore, PMH attenuates the loss of endophytic PGPR activity in the acidic environments. Overall, this microbial encapsulation strategy is a promising way to protect fragile endophytic microorganisms, providing attractive avenues in sustainable agriculture.

Update on the structure and regulated biosynthesis of the apoplastic polymers cutin and suberin

The plant lipid polymers cutin and suberin play a critical role in many aspects of plant growth, development, and physiology. The mechanisms of cutin and suberin biosynthesis are relatively well understood thanks to just over 2 decades of work with primarily Arabidopsis (Arabidopsis thaliana) mutants. Recent advances in our understanding of cutin and suberin structure have arisen through the application of novel chemistries targeted at quantitative comprehension of intermolecular linkages, isolating intact suberins and cutins, and the application of advanced analytical techniques. The advent of high-throughput transcription factor binding assays and next-generation sequencing has facilitated the discovery of numerous cutin and suberin-regulating transcription factors and their gene promoter targets. Herein we provide an overview of aspects of cutin and suberin structure, biosynthesis, and transcriptional regulation of their synthesis highlighting recent developments in our understanding of these facets of cutin and suberin biology. We further identify outstanding questions in these respective areas and provide perspectives on how to advance the field to address these questions.

Modification of peroxidase activity and proteome in maize exposed to cadmium in the presence of galactoglucomannan oligosaccharides

We tested the effects of galactoglucomannan oligosaccharides (GGMOs) and/or cadmium (Cd) on peroxidase activity and the proteome in maize (Zea mays L.) roots and leaves. Our previous work confirmed that GGMOs ameliorate the symptoms of Cd stress in seedlings. Here, the plants were hydroponically cultivated for 7 days, and the protein content and peroxidase activity were estimated in intracellular, neutral cell wall, and acidic cell wall protein fractions. The peroxidase activity varied between the plant organs as well as among the fractions and treatments. The GGMOs in the presence of Cd did not significantly influence content of peroxidases but modulated their activity, which implies posttranslational regulation. The changes in the content of various proteins (e.g., related to the defence reactions, cell wall structure/metabolism, and activation of plant hormones) caused by GGMOs and Cd indicate possible protective mechanisms that improve the vitality of maize seedlings exposed to metal stress. GGMOs partially reverted Cd-induced protein disbalance, which was a reoccurring phenomenon of mitigation in leaves.

Building the physiological barrier: Suberin plasticity in response to environmental stimuli

In response to environmental changes, plant roots undergo two major differentiations: the formation of the Casparian strip and the suberin lamella, both of them are widely recognized as an apoplastic diffusion barrier for nutrient and water exchange between the soil and the root vascular bundle. Suberin is a complex biopolyester composed of glycerol esters and phenolic compounds deposited in the cell walls of specific tissues such as endodermis, exodermis, periderm, seed coat and other marginal tissues. Recently, significant progress has been made due to the development of biochemical and genetic techniques. In this review, we not only summarize the aspect of suberin biosynthesis, transport and polymerization, but also elucidate the molecular mechanisms regarding its regulatory network, as well as its adaptive role in abiotic or biotic stress. This will provide important theoretical references for improving crop growth by modifying their adaptive root suberin structure when exposed to environmental changes.

Four MYB transcription factors regulate suberization and nonlocalized lignification at the root endodermis in rice

In response to variable environments, rice (Oryza sativa) roots have developed lignified and suberized diffusion barriers at the endodermis to permit selective nutrient uptake for optimal growth. Here, we demonstrate that endodermal suberization and nonlocalized lignification are redundantly regulated by 4 MYB transcription factors: OsMYB39a, OsMYB41, OsMYB92a, and OsMYB92b. These transcription factors function downstream of the OsMYB36a/b/c, CASPARIAN STRIP INTEGRITY FACTOR (OsCIF)-SCHENGEN3 (OsSGN3), and stress-inducible signaling pathways in rice. Knockout of all 4 MYB genes resulted in the complete absence of endodermal suberin lamellae (SL) and almost no lignin deposition between the Casparian strip and the cortex-facing lignified band at cell corners under all conditions examined. In contrast, endodermis-specific overexpression of any of these MYB genes was sufficient to induce strong endodermal suberization and nonlocalized lignification near the root tip. Furthermore, OsMYB92a-overexpressing lines showed an altered ionomic profile and enhanced salinity tolerance. Transcriptome analysis identified 152 downstream genes regulated by OsMYB39a/41/92a/92b, including the key SL formation gene OsCYP86A1 and other genes involved in endodermal lignification and suberization under normal and stress conditions. Our results provide important insights into the molecular mechanisms underlying suberization and nonlocalized lignification at the root endodermis and their physiological significance in ion homeostasis and acclimation to environmental stress.

Exploring the function of plant root diffusion barriers in sealing and shielding for environmental adaptation

Plant roots serve as the primary interface between the plant and the soil, encountering numerous challenges ranging from water balance to nutrient uptake. One of the central mechanisms enabling plants to thrive in diverse ecosystems is the building of apoplastic diffusion barriers. These barriers control the flow of solutes into and out of the roots, maintaining water and nutrient homeostasis. In this Review, we summarize recent advances in understanding the establishment, function and ecological significance of root apoplastic diffusion barriers. We highlight the plasticity of apoplastic diffusion barriers under various abiotic stresses such as drought, salinity and nutrient deficiency. We also propose new frontiers by discussing the current bottlenecks in the study of plant apoplastic diffusion barriers.

Research Progress and Hotspots Analysis of Apoplastic Barriers in the Roots of Plants Based on Bibliometrics from 2003 to 2023

The apoplastic barriers, composed of Casparian strip (CS) and suberin lamellae (SL), are integral to the regulation of water and plant nutrient uptake in plants, as well as their resilience to abiotic stresses. This study systematically examines the research developments and emerging trends in this field from 2003 to 2023, utilizing bibliometric tools such as Web of Science, CiteSpace, and VOSviewer to analyze a dataset of 642 publications. This paper reviews the cooperation of different countries, institutions, and scholars in apoplastic barriers research based on cooperative network analysis. In the field, China has the highest number of publications, the University of Bolton has the highest number of publications, and Niko Geldner is the author with the maximum number of publications. Notably, 27 publications were identified as highly cited, with their research primarily focusing on (1) genes, proteins, enzymes, and hormones regulating the formation of apoplastic barriers; (2) the influence of adversity stress on apoplastic barriers; (3) the chemical components of apoplastic barriers; (4) the evaluations of research progress on apoplastic barriers. Combined with the keyword co-occurrence network diagram, it is proposed that future research directions in this field should be as follows: (1) physiological functions of apoplastic barriers in plant root; (2) differences in the formation of apoplastic barriers with different root systems; (3) methods to promote apoplastic barriers formation; and (4) application of molecular biology techniques. The present study provides a further understanding of the trends in apoplastic barriers, and the data analyzed can be used as a guide for future research directions.

Multiomics dissection of Brassica napus L. lateral roots and endophytes interactions under phosphorus starvation

Many plants associate with endophytic microbes that improve root phosphorus (P) uptake. Understanding the interactions between roots and endophytes can enable efforts to improve P utilization. Here, we characterize the interactions between lateral roots of endophytes in a core collection of 50 rapeseed (Brassica napus L.) genotypes with differing sensitivities to low P conditions. With the correlation analysis result between bacterial abundance and plant physiological indices of rapeseeds, and inoculation experiments on plates and soil, we identify one Flavobacterium strain (C2) that significantly alleviates the P deficiency phenotype of rapeseeds. The underlying mechanisms are explored by performing the weighted gene coexpression network analysis (WGCNA), and conducting genome-wide association studies (GWAS) using Flavobacterium abundance as a quantitative trait. Under P-limited conditions, C2 regulates fatty acid and lipid metabolic pathways. For example, C2 improves metabolism of linoleic acid, which mediates root suberin biosynthesis, and enhances P uptake efficiency. In addition, C2 suppresses root jasmonic acid biosynthesis, which depends on α-linolenic acid metabolism, improving C2 colonization and activating P uptake. This study demonstrates that adjusting the endophyte composition can modulate P uptake in B. napus plants, providing a basis for developing agricultural microbial agents.

Root endodermal suberization induced by nitrate stress regulate apoplastic pathway rather than nitrate uptake in tobacco (Nicotiana tabacum L.)

Nitrogen levels and distribution in the rhizosphere strongly regulate the root architecture. Nitrate is an essential nutrient and an important signaling molecule for plant growth and development. Hydroponic experiments were conducted to investigate the differences in endodermal suberization in tobacco (Nicotiana tabacum L.) roots at three nitrate levels. Nitrogen accumulation was detected in the roots, shoots, and xylem sap. Nitrate influx on the root surface was also measured using the non-invasive self-referencing microsensor technique (SRMT). RNA-Seq analysis was performed to identify the genes related to endodermal suberization, nitrate transport, and endogenous abscisic acid (ABA) biosynthesis. The results showed that root length, root-shoot ratio, nitrate influx on the root surface, and NiA and NRT2.4 genes were regulated to maintain the nitrogen nutrient supply in tobacco under low nitrate conditions. Low nitrate levels enhanced root endodermal suberization and hence reduced the apoplastic transport pathway, and genes from the KCS, FAR, PAS2, and CYP86 families were upregulated. The results of exogenous fluridone, an ABA biosynthesis inhibitor, indicated that suberization of the tobacco root endodermis had no relevance to radial nitrate transport and accumulation. However, ABA enhances suberization, relating to ABA biosynthesis genes in the CCD family and degradation gene ABA8ox1.

Metabolic and gene-expression analyses reveal developmental dynamics of cutin deposition in pomegranate fruit grown under different environmental conditions

The chemical and transcriptional changes in the cuticle of pomegranate (Punica granatum L.) fruit grown under different environmental conditions were studied. We collected fruit from three orchards located in different regions in Israel, each with a distinct microclimate. Fruit were collected at six phenological stages, and cutin monomers in the fruit cuticle were profiled by gas chromatography-mass spectrometry (GC-MS), along with qPCR transcript-expression analyses of selected cutin-related genes. While fruit phenotypes were comparable along development in all three orchards, principal component analyses of cutin monomer profiles suggested clear separation between cuticle samples of young green fruit to those of maturing fruit. Moreover, total cutin contents in green fruit were lower in the orchard characterized by a hot and dry climate compared to orchards with moderate temperatures. The variances detected in total cutin contents between orchards corresponded well with the expression patterns of BODYGUARD, a key biosynthetic gene operating in the cutin biosynthetic pathway. Based on our extraction protocols, we found that the cutin polyester that builds the pomegranate fruit cuticle accumulates some levels of gallic acid-the precursor of punicalagin, a well-known potent antioxidant metabolite in pomegranate fruit. The gallic acid was also one of the predominant metabolites contributing to the variability between developmental stages and orchards, and its accumulation levels were opposite to the expression patterns of the UGT73AL1 gene which glycosylates gallic acid to synthesize punicalagin. To the best of our knowledge, this is the first detailed composition of the cutin polyester that forms the pomegranate fruit cuticle.

Transcriptomic and Metabolomic Profiling of Root Tissue in Drought-Tolerant and Drought-Susceptible Wheat Genotypes in Response to Water Stress

Wheat is the most widely grown crop in the world; its production is severely disrupted by increasing water deficit. Plant roots play a crucial role in the uptake of water and perception and transduction of water deficit signals. In the past decade, the mechanisms of drought tolerance have been frequently reported; however, the transcriptome and metabolome regulatory network of root responses to water stress has not been fully understood in wheat. In this study, the global transcriptomic and metabolomics profiles were employed to investigate the mechanisms of roots responding to water stresses using the drought-tolerant (DT) and drought-susceptible (DS) wheat genotypes. The results showed that compared with the control group, wheat roots exposed to polyethylene glycol (PEG) had 25941 differentially expressed genes (DEGs) and more upregulated genes were found in DT (8610) than DS (7141). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the DEGs of the drought-tolerant genotype were preferably enriched in the flavonoid biosynthetic process, anthocyanin biosynthesis and suberin biosynthesis. The integrated analysis of the transcriptome and metabolome showed that in DT, the KEGG pathways, including flavonoid biosynthesis and arginine and proline metabolism, were shared by differentially accumulated metabolites (DAMs) and DEGs at 6 h after treatment (HAT) and pathways including alanine, aspartate, glutamate metabolism and carbon metabolism were shared at 48 HAT, while in DS, the KEGG pathways shared by DAMs and DEGs only included arginine and proline metabolism at 6 HAT and the biosynthesis of amino acids at 48 HAT. Our results suggest that the drought-tolerant genotype may relieve the drought stress by producing more ROS scavengers, osmoprotectants, energy and larger roots. Interestingly, hormone signaling plays an important role in promoting the development of larger roots and a higher capability to absorb and transport water in drought-tolerant genotypes.

Polystyrene nanoplastics induce cell type-dependent secondary wall reinforcement in rice (Oryza sativa) roots and reduce root hydraulic conductivity

Nanoplastics (NPs) have been demonstrated the ability to penetrate plant roots and cause stress. However, the extent of NPs penetration into various root tissues and the corresponding plant defense mechanisms remain unclear. This study examined the penetration and accumulation patterns of polystyrene nanoplastics (PS-NPs) in different cell types within rice roots, and explored how the roots quickly modify their cell wall structure in response. The findings showed that fully developed sclerenchyma cells in rice roots effectively prevented the invasion of PS-NPs. Meanwhile, PS-NPs triggered the accumulation of lignin and suberin in specific cells such as the exodermis, sclerenchyma, and xylem vessels. PS-NPs at a concentration of 50 mg L-1 increased cell wall thickness by 18.6 %, 21.1 %, and 22.4 % in epidermis, exodermis, and sclerenchyma cells, respectively, and decreased root hydraulic conductivity by 14.8 %. qPCR analysis revealed that PS-NPs influenced the cell wall synthesis pathway, promoting the deposition of lignin and suberin monomers on the secondary wall through the up-regulation of genes such as OsLAC and OsABCG. These results demonstrate that PS-NPs can induce cell type-specific strengthening of secondary walls and barrier formation in rice roots, suggesting the potential role of plant secondary wall development in mitigating NPs contamination risks in crops.

BIG enhances Arg/N-degron pathway-mediated protein degradation to regulate Arabidopsis hypoxia responses and suberin deposition

BIG/DARK OVEREXPRESSION OF CAB1/TRANSPORT INHIBITOR RESPONSE3 is a 0.5 MDa protein associated with multiple functions in Arabidopsis (Arabidopsis thaliana) signaling and development. However, the biochemical functions of BIG are unknown. We investigated a role for BIG in the Arg/N-degron pathways, in which substrate protein fate is influenced by the N-terminal residue. We crossed a big loss-of-function allele to 2 N-degron pathway E3 ligase mutants, proteolysis6 (prt6) and prt1, and examined the stability of protein substrates. Stability of model substrates was enhanced in prt6-1 big-2 and prt1-1 big-2 relative to the respective single mutants, and the abundance of the PRT6 physiological substrates, HYPOXIA-RESPONSIVE ERF2 (HRE2) and VERNALIZATION2 (VRN2), was similarly increased in prt6 big double mutants. Hypoxia marker expression was enhanced in prt6 big double mutants; this constitutive response required arginyl transferase activity and RAP-type Group VII ethylene response factor (ERFVII) transcription factors. Transcriptomic analysis of roots not only demonstrated increased expression of multiple hypoxia-responsive genes in the double mutant relative to prt6, but also revealed other roles for PRT6 and BIG, including regulation of suberin deposition through both ERFVII-dependent and independent mechanisms, respectively. Our results show that BIG acts together with PRT6 to regulate the hypoxia-response and broader processes in Arabidopsis.

Comparisons of two receptor-MAPK pathways in a single cell-type reveal mechanisms of signalling specificity

Cells harbour numerous receptor pathways to respond to diverse stimuli, yet often share common downstream signalling components. Mitogen-activated protein kinase (MPK) cascades are an example of such common hubs in eukaryotes. How such common hubs faithfully transduce distinct signals within the same cell-type is insufficiently understood, yet of fundamental importance for signal integration and processing in plants. We engineered a unique genetic background allowing direct comparisons of a developmental and an immunity pathway in one cell-type, the Arabidopsis root endodermis. We demonstrate that the two pathways maintain distinct functional and transcriptional outputs despite common MPK activity patterns. Nevertheless, activation of different MPK kinases and MPK classes led to distinct functional readouts, matching observed pathway-specific readouts. On the basis of our comprehensive analysis of core MPK signalling elements, we propose that combinatorial activation within the MPK cascade determines the differential regulation of an endodermal master transcription factor, MYB36, that drives pathway-specific gene activation.

Nitrate transporter protein NPF5.12 and major latex-like protein MLP6 are important defense factors against Verticillium longisporum

Plant defense responses to the soil-borne fungus Verticillium longisporum causing stem stripe disease on oilseed rape (Brassica napus) are poorly understood. In this study, a population of recombinant inbred lines (RILs) using the Arabidopsis accessions Sei-0 and Can-0 was established. Composite interval mapping, transcriptome data, and T-DNA mutant screening identified the NITRATE/PEPTIDE TRANSPORTER FAMILY 5.12 (AtNPF5.12) gene as being associated with disease susceptibility in Can-0. Co-immunoprecipitation revealed interaction between AtNPF5.12 and the MAJOR LATEX PROTEIN family member AtMLP6, and fluorescence microscopy confirmed this interaction in the plasma membrane and endoplasmic reticulum. CRISPR/Cas9 technology was applied to mutate the NPF5.12 and MLP6 genes in B. napus. Elevated fungal growth in the npf5.12 mlp6 double mutant of both oilseed rape and Arabidopsis demonstrated the importance of these genes in defense against V. longisporum. Colonization of this fungus depends also on available nitrates in the host root. Accordingly, the negative effect of nitrate depletion on fungal growth was less pronounced in Atnpf5.12 plants with impaired nitrate transport. In addition, suberin staining revealed involvement of the NPF5.12 and MLP6 genes in suberin barrier formation. Together, these results demonstrate a dependency on multiple plant factors that leads to successful V. longisporum root infection.

Exploration of a European-centered strawberry diversity panel provides markers and candidate genes for the control of fruit quality traits

Fruit quality traits are major breeding targets in cultivated strawberry (Fragaria × ananassa). Taking into account the requirements of both growers and consumers when selecting high-quality cultivars is a real challenge. Here, we used a diversity panel enriched with unique European accessions and the 50 K FanaSNP array to highlight the evolution of strawberry diversity over the past 160 years, investigate the molecular basis of 12 major fruit quality traits by genome-wide association studies (GWAS), and provide genetic markers for breeding. Results show that considerable improvements of key breeding targets including fruit weight, firmness, composition, and appearance occurred simultaneously in European and American cultivars. Despite the high genetic diversity of our panel, we observed a drop in nucleotide diversity in certain chromosomal regions, revealing the impact of selection. GWAS identified 71 associations with 11 quality traits and, while validating known associations (firmness, sugar), highlighted the predominance of new quantitative trait locus (QTL), demonstrating the value of using untapped genetic resources. Three of the six selective sweeps detected are related to glossiness or skin resistance, two little-studied traits important for fruit attractiveness and, potentially, postharvest shelf life. Moreover, major QTL for firmness, glossiness, skin resistance, and susceptibility to bruising are found within a low diversity region of chromosome 3D. Stringent search for candidate genes underlying QTL uncovered strong candidates for fruit color, firmness, sugar and acid composition, glossiness, and skin resistance. Overall, our study provides a potential avenue for extending shelf life without compromising flavor and color as well as the genetic markers needed to achieve this goal.

Rhytidome- and cork-type barks of holm oak, cork oak and their hybrids highlight processes leading to cork formation

BACKGROUND

The periderm is basic for land plants due to its protective role during radial growth, which is achieved by the polymers deposited in the cell walls. In most trees, like holm oak, the first periderm is frequently replaced by subsequent internal periderms yielding a heterogeneous outer bark made of a mixture of periderms and phloem tissues, known as rhytidome. Exceptionally, cork oak forms a persistent or long-lived periderm which results in a homogeneous outer bark of thick phellem cell layers known as cork. Cork oak and holm oak distribution ranges overlap to a great extent, and they often share stands, where they can hybridize and produce offspring showing a rhytidome-type bark.

RESULTS

Here we use the outer bark of cork oak, holm oak, and their natural hybrids to analyse the chemical composition, the anatomy and the transcriptome, and further understand the mechanisms underlying periderm development. We also include a unique natural hybrid individual corresponding to a backcross with cork oak that, interestingly, shows a cork-type bark. The inclusion of hybrid samples showing rhytidome-type and cork-type barks is valuable to approach cork and rhytidome development, allowing an accurate identification of candidate genes and processes. The present study underscores that abiotic stress and cell death are enhanced in rhytidome-type barks whereas lipid metabolism and cell cycle are enriched in cork-type barks. Development-related DEGs showing the highest expression, highlight cell division, cell expansion, and cell differentiation as key processes leading to cork or rhytidome-type barks.

CONCLUSION

Transcriptome results, in agreement with anatomical and chemical analyses, show that rhytidome and cork-type barks are active in periderm development, and suberin and lignin deposition. Development and cell wall-related DEGs suggest that cell division and expansion are upregulated in cork-type barks whereas cell differentiation is enhanced in rhytidome-type barks.

Comprehensive integration of single-cell transcriptomic data illuminates the regulatory network architecture of plant cell fate specification

Plant morphogenesis relies on precise gene expression programs at the proper time and position which is orchestrated by transcription factors (TFs) in intricate regulatory networks in a cell-type specific manner. Here we introduced a comprehensive single-cell transcriptomic atlas of Arabidopsis seedlings. This atlas is the result of meticulous integration of 63 previously published scRNA-seq datasets, addressing batch effects and conserving biological variance. This integration spans a broad spectrum of tissues, including both below- and above-ground parts. Utilizing a rigorous approach for cell type annotation, we identified 47 distinct cell types or states, largely expanding our current view of plant cell compositions. We systematically constructed cell-type specific gene regulatory networks and uncovered key regulators that act in a coordinated manner to control cell-type specific gene expression. Taken together, our study not only offers extensive plant cell atlas exploration that serves as a valuable resource, but also provides molecular insights into gene-regulatory programs that varies from different cell types.

Endodermal suberin restricts root leakage of cesium: a suitable tracer for potassium

An urgent challenge within crop production is to maintain productivity in a world plagued by climate change and its associated plant stresses, such as heat, drought and salinity. A key factor in this endeavor is to understand the dynamics of root suberization, and its role in plant-water relations and nutrient transport. This study focuses on the hypothesis that endodermal suberin, acts as a physical barrier preventing radial potassium (K) movement out of the vascular tissues during translocation. Previous attempts to experimentally support this idea have produced inconsistent results. We developed a Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) method, allowing us to visualize the distribution of mineral elements and track K movement. Cesium (Cs), dosed in optimized concentrations, was found to be an ideal tracer for K, due to its low background and similar chemical/biological properties. In suberin mutants of Arabidopsis thaliana, we observed a positive correlation between suberin levels and K translocation efficiency, indicating that suberin enhances the plant's ability to retain K within the vascular tissues during translocation from root to shoot. In barley (Hordeum vulgare), fully suberized seminal roots maintained higher K concentrations in the stele compared to younger, less suberized root zones. This suggests that suberization increases with root maturity, enhancing the barrier against K leakage. In nodal roots, suberin was scattered towards the phloem in mature root zones. Despite this incomplete suberization, nodal roots still restrict outward K movement, demonstrating that even partial suberin barriers can significantly reduce K loss. Our findings provide evidence that suberin is a barrier to K leakage during root-to-shoot translocation. This understanding is crucial to maintain crop productivity in the face of climate change.

Suberin deficiency and its effect on the transport physiology of young poplar roots

The precise functions of suberized apoplastic barriers in root water and nutrient transport physiology have not fully been elucidated. While lots of research has been performed with mutants of Arabidopsis, little to no data are available for mutants of agricultural crop or tree species. By employing a combined set of physiological, histochemical, analytical, and transport physiological methods as well as RNA-sequencing, this study investigated the implications of remarkable CRISPR/Cas9-induced suberization defects in young roots of the economically important gray poplar. While barely affecting overall plant development, contrary to literature-based expectations significant root suberin reductions of up to 80-95% in four independent mutants were shown to not evidently affect the root hydraulic conductivity during non-stress conditions. In addition, subliminal iron deficiency symptoms and increased translocation of a photosynthesis inhibitor as well as NaCl highlight the involvement of suberin in nutrient transport physiology. The multifaceted nature of the root hydraulic conductivity does not allow drawing simplified conclusions such as that the suberin amount must always be correlated with the water transport properties of roots. However, the decreased masking of plasma membrane surface area could facilitate the uptake but also leakage of beneficial and harmful solutes.

Genome and GWAS analysis identified genes significantly related to phenotypic state of Rhododendron bark

As an important horticultural plant, Rhododendron is often used in urban greening and landscape design. However, factors such as the high rate of genetic recombination, frequent outcrossing in the wild, weak linkage disequilibrium, and the susceptibility of gene expression to environmental factors limit further exploration of functional genes related to important horticultural traits, and make the breeding of new varieties require a longer time. Therefore, we choose bark as the target trait which is not easily affected by environmental factors, but also has ornamental properties. Genome-wide association study (GWAS) of Rhododendron delavayi (30 samples), R. irroratum (30 samples) and their F1 generation R. agastum (200 samples) was conducted on the roughness of bark phenotypes. Finally, we obtained 2416.31 Gbp of clean data and identified 5 328 800 high-quality SNPs. According to the P-value and the degree of linkage disequilibrium of SNPs, we further identified 4 out of 11 candidate genes that affect bark roughness. The results of gene differential expression analysis further indicated that the expression levels of Rhdel02G0243600 and Rhdel08G0220700 in different bark phenotypes were significantly different. Our study identified functional genes that influence important horticultural traits of Rhododendron, and illustrated the powerful utility and great potential of GWAS in understanding and exploiting wild germplasm genetic resources of Rhododendron.

Choreographing root architecture and rhizosphere interactions through synthetic biology

Climate change is driving extreme changes to the environment, posing substantial threats to global food security and bioenergy. Given the direct role of plant roots in mediating plant-environment interactions, engineering the form and function of root systems and their associated microbiota may mitigate these effects. Synthetic genetic circuits have enabled sophisticated control of gene expression in microbial systems for years and a surge of advances has heralded the extension of this approach to multicellular plant species. Targeting these tools to affect root structure, exudation, and microbe activity on root surfaces provide multiple strategies for the advancement of climate-ready crops.

Function identification of Arabidopsis GPAT4 and GPAT8 in the biosynthesis of suberin and cuticular wax

Surface lipids in plants include cutin, cuticular wax and suberin. sn-Glycerol-3-phosphate acyltransferases (GPATs) facilitate the acylation of sn-glycerol-3-phosphate (G3P) utilizing a fatty acyl group from acyl-coenzyme A (acyl-CoA) or acyl-acyl carrier protein (acyl-ACP) as substrates for the biosynthesis of plant extracellular lipids such as suberin and cutin. Here we found that Arabidopsis GPAT4 and GPAT8 are specifically expressed in endodermis cells of roots where suberin was accumulated. GPAT4 mutation significantly decreased the amounts of the C16 and C18 ω-oxidized suberin monomers, whereas the mutation of GPAT8 had little effect on the suberin production, and the functions of both were not redundant. Root suberin phenotype analysis of gpat4-1 and gpat6-1 single or double mutant revealed that GPAT4 and GPAT6 play redundant functions. Interestingly, the gpat4-1 gpat8-1 double mutant displayed a glossy stem phenotype since fewer wax crystals were accumulated. This phenotype was not shown in either parent. Further study showed that the amounts of most wax components were significantly decreased. Taken together, our findings revealed that GPAT4 has an additive effect with GPAT6 in the root suberin biosynthesis, and plays a redundant role in wax production with GPAT8.

Small peptide signaling via OsCIF1/2 mediates Casparian strip formation at the root endodermal and nonendodermal cell layers in rice

The Casparian strip (CS) is a ring-like lignin structure deposited between endodermal cells that forms an apoplastic barrier to control the selective uptake of nutrients in vascular plants. However, the molecular mechanism of CS formation in rice (Oryza sativa), which possesses one CS each in the endodermis and exodermis, is relatively unknown. Here, we functionally characterized CS INTEGRITY FACTOR1 (OsCIF1a, OsCIF1b), OsCIF2, and SCHENGEN3 (OsSGN3a, OsSGN3b) in rice. OsCIF1s and OsCIF2 were mainly expressed in the stele, while OsSGN3s localized around the CS at the endodermis. Knockout of all three OsCIFs or both OsSGN3s resulted in a discontinuous CS and a dramatic reduction in compensatory (less localized) lignification and suberization at the endodermis. By contrast, ectopic overexpression of OsCIF1 or OsCIF2 induced CS formation as well as overlignification and oversuberization at single or double cortical cell layers adjacent to the endodermis. Ectopic co-overexpression of OsCIF1 and SHORTROOT1 (OsSHR1) induced the formation of more CS-like structures at multiple cortical cell layers. Transcriptome analysis identified 112 downstream genes modulated by the OsCIF1/2-OsSGN3 signaling pathway, which is involved in CS formation and activation of the compensatory machinery in native endodermis and nonnative endodermis-like cell layers. Our results provide important insights into the molecular mechanism of CIF-mediated CS formation at the root endodermal and nonendodermal cell layers.

Ferric reduction by a CYBDOM protein counteracts increased iron availability in root meristems induced by phosphorus deficiency

To mobilize sparingly available phosphorus (P) in the rhizosphere, many plant species secrete malate to release P sorbed onto (hydr)oxides of aluminum and iron (Fe). In the presence of Fe, malate can provoke Fe over-accumulation in the root apoplast, triggering a series of events that inhibit root growth. Here, we identified HYPERSENSITIVE TO LOW P1 (HYP1), a CYBDOM protein constituted of a DOMON and a cytochrome b561 domain, as critical to maintain cell elongation and meristem integrity under low P. We demonstrate that HYP1 mediates ascorbate-dependent trans-plasma membrane electron transport and can reduce ferric and cupric substrates in Xenopus laevis oocytes and in planta. HYP1 expression is up-regulated in response to P deficiency in the proximal zone of the root apical meristem. Disruption of HYP1 leads to increased Fe and callose accumulation in the root meristem and causes significant transcriptional changes in roots. We further demonstrate that HYP1 activity overcomes malate-induced Fe accumulation, thereby preventing Fe-dependent root growth arrest in response to low P. Collectively, our results uncover an ascorbate-dependent metalloreductase that is critical to protect root meristems of P-deficient plants from increased Fe availability and provide insights into the physiological function of the yet poorly characterized but ubiquitous CYBDOM proteins.

A suberized exodermis is required for tomato drought tolerance

Plant roots integrate environmental signals with development using exquisite spatiotemporal control. This is apparent in the deposition of suberin, an apoplastic diffusion barrier, which regulates flow of water, solutes and gases, and is environmentally plastic. Suberin is considered a hallmark of endodermal differentiation but is absent in the tomato endodermis. Instead, suberin is present in the exodermis, a cell type that is absent in the model organism Arabidopsis thaliana. Here we demonstrate that the suberin regulatory network has the same parts driving suberin production in the tomato exodermis and the Arabidopsis endodermis. Despite this co-option of network components, the network has undergone rewiring to drive distinct spatial expression and with distinct contributions of specific genes. Functional genetic analyses of the tomato MYB92 transcription factor and ASFT enzyme demonstrate the importance of exodermal suberin for a plant water-deficit response and that the exodermal barrier serves an equivalent function to that of the endodermis and can act in its place.

Identification, evolution, and expression of GDSL-type Esterase/Lipase (GELP) gene family in three cotton species: a bioinformatic analysis

BACKGROUND

GDSL esterase/lipases (GELPs) play important roles in plant growth, development, and response to biotic and abiotic stresses. Presently, an extensive and in-depth analysis of GELP family genes in cotton is still not clear enough, which greatly limits the further understanding of cotton GELP function and regulatory mechanism.

RESULTS

A total of 389 GELP family genes were identified in three cotton species of Gossypium hirsutum (193), G. arboreum (97), and G. raimondii (99). These GELPs could be classified into three groups and eight subgroups, with the GELPs in same group to have similar gene structures and conserved motifs. Evolutionary event analysis showed that the GELP family genes tend to be diversified at the spatial dimension and certain conservative at the time dimension, with a trend of potential continuous expansion in the future. The orthologous or paralogous GELPs among different genomes/subgenomes indicated the inheritance from genome-wide duplication during polyploidization, and the paralogous GELPs were derived from chromosomal segment duplication or tandem replication. GELP genes in the A/D subgenome underwent at least three large-scale replication events in the evolutionary process during the period of 0.6-3.2 MYA, with two large-scale evolutionary events between 0.6-1.8 MYA that were associated with tetraploidization, and the large-scale duplication between 2.6-9.1 MYA that occurred during diploidization. The cotton GELPs indicated diverse expression patterns in tissue development, ovule and fiber growth, and in response to biotic and abiotic stresses, combining the existing cis-elements in the promoter regions, suggesting the GELPs involvements of functions to be diversification and of the mechanisms to be a hormone-mediated manner.

CONCLUSIONS

Our results provide a systematic and comprehensive understanding the function and regulatory mechanism of cotton GELP family, and offer an effective reference for in-depth genetic improvement utilization of cotton GELPs.

Chromosome-level genome assembly of Quercus variabilis provides insights into the molecular mechanism of cork thickness

Quercus variabilis is a deciduous woody species with high ecological and economic value, and is a major source of cork in East Asia. Cork from thick softwood sheets have higher commercial value than those from thin sheets. It is extremely difficult to genetically improve Q. variabilis to produce high quality softwood due to the lack of genomic information. Here, we present a high-quality chromosomal genome assembly for Q. variabilis with length of 791,89 Mb and 54,606 predicted genes. Comparative analysis of protein sequences of Q. variabilis with 11 other species revealed that specific and expanded gene families were significantly enriched in the "fatty acid biosynthesis" pathway in Q. variabilis, which may contribute to the formation of its unique cork. Based on weighted correlation network analysis of time-course (i.e., five important developmental ages) gene expression data in thick-cork versus thin-cork genotypes of Q. variabilis, we identified one co-expression gene module associated with the thick-cork trait. Within this co-expression gene module, 10 hub genes were associated with suberin biosynthesis. Furthermore, we identified a total of 198 suberin biosynthesis-related new candidate genes that were up-regulated in trees with a thick cork layer relative to those with a thin cork layer. Also, we found that some genes related to cell expansion and cell division were highly expressed in trees with a thick cork layer. Collectively, our results revealed that two metabolic pathways (i.e., suberin biosynthesis, fatty acid biosynthesis), along with other genes involved in cell expansion, cell division, and transcriptional regulation, were associated with the thick-cork trait in Q. variabilis, providing insights into the molecular basis of cork development and knowledge for informing genetic improvement of cork thickness in Q. variabilis and closely related species.

The evolutionary innovation of root suberin lamellae contributed to the rise of seed plants

Seed plants overtook ferns to become the dominant plant group during the late Carboniferous, a period in which the climate became colder and dryer1,2. However, the specific innovations driving the success of seed plants are not clear. Here we report that the appearance of suberin lamellae (SL) contributed to the rise of seed plants. We show that the Casparian strip and SL vascular barriers evolved at different times, with the former originating in the most recent common ancestor (MRCA) of vascular plants and the latter in the MRCA of seed plants. Our results further suggest that most of the genes required for suberin formation arose through gene duplication in the MRCA of seed plants. We show that the appearance of the SL in the MRCA of seed plants enhanced drought tolerance through preventing water loss from the stele. We hypothesize that SL provide a decisive selective advantage over ferns in arid environments, resulting in the decline of ferns and the rise of gymnosperms. This study provides insights into the evolutionary success of seed plants and has implications for engineering drought-tolerant crops or fern varieties.

Comparative Analysis of Cd Uptake and Tolerance in Two Mangrove Species (Avicennia marina and Rhizophora stylosa) with Distinct Apoplast Barriers

Mangrove plants demonstrate an impressive ability to tolerate environmental pollutants, but excessive levels of cadmium (Cd) can impede their growth. Few studies have focused on the effects of apoplast barriers on heavy metal tolerance in mangrove plants. To investigate the uptake and tolerance of Cd in mangrove plants, two distinct mangrove species, Avicennia marina and Rhizophora stylosa, are characterized by unique apoplast barriers. The results showed that both mangrove plants exhibited the highest concentration of Cd2+ in roots, followed by stems and leaves. The Cd2+ concentrations in all organs of R. stylosa consistently exhibited lower levels than those of A. marina. In addition, R. stylosa displayed a reduced concentration of apparent PTS and a smaller percentage of bypass flow when compared to A. marina. The root anatomical characteristics indicated that Cd treatment significantly enhanced endodermal suberization in both A. marina and R. stylosa roots, and R. stylosa exhibited a higher degree of suberization. The transcriptomic analysis of R. stylosa and A. marina roots under Cd stress revealed 23 candidate genes involved in suberin biosynthesis and 8 candidate genes associated with suberin regulation. This study has confirmed that suberized apoplastic barriers play a crucial role in preventing Cd from entering mangrove roots.

Tapping into the plasticity of plant architecture for increased stress resilience

Plant architecture develops post-embryonically and emerges from a dialogue between the developmental signals and environmental cues. Length and branching of the vegetative and reproductive tissues were the focus of improvement of plant performance from the early days of plant breeding. Current breeding priorities are changing, as we need to prioritize plant productivity under increasingly challenging environmental conditions. While it has been widely recognized that plant architecture changes in response to the environment, its contribution to plant productivity in the changing climate remains to be fully explored. This review will summarize prior discoveries of genetic control of plant architecture traits and their effect on plant performance under environmental stress. We review new tools in phenotyping that will guide future discoveries of genes contributing to plant architecture, its plasticity, and its contributions to stress resilience. Subsequently, we provide a perspective into how integrating the study of new species, modern phenotyping techniques, and modeling can lead to discovering new genetic targets underlying the plasticity of plant architecture and stress resilience. Altogether, this review provides a new perspective on the plasticity of plant architecture and how it can be harnessed for increased performance under environmental stress.

Time course of changes in the transcriptome during russet induction in apple fruit

BACKGROUND

Russeting is a major problem in many fruit crops. Russeting is caused by environmental factors such as wounding or moisture exposure of the fruit surface. Despite extensive research, the molecular sequence that triggers russet initiation remains unclear. Here, we present high-resolution transcriptomic data by controlled russet induction at very early stages of fruit development. During Phase I, a patch of the fruit surface is exposed to surface moisture. For Phase II, moisture exposure is terminated, and the formerly exposed surface remains dry. We targeted differentially expressed transcripts as soon as 24 h after russet induction.

RESULTS

During moisture exposure (Phase I) of 'Pinova' apple, transcripts associated with the cell cycle, cell wall, and cuticle synthesis (SHN3) decrease, while those related to abiotic stress increase. NAC35 and MYB17 were the earliest induced genes during Phase I. They are therefore linked to the initial processes of cuticle microcracking. After moisture removal (Phase II), the expression of genes related to meristematic activity increased (WOX4 within 24 h, MYB84 within 48 h). Genes related to lignin synthesis (MYB52) and suberin synthesis (MYB93, WRKY56) were upregulated within 3 d after moisture removal. WOX4 and AP2B3 are the earliest differentially expressed genes induced in Phase II. They are therefore linked to early events in periderm formation. The expression profiles were consistent between two different seasons and mirrored differences in russet susceptibility in a comparison of cultivars. Furthermore, expression profiles during Phase II of moisture induction were largely identical to those following wounding.

CONCLUSIONS

The combination of a unique controlled russet induction technique with high-resolution transcriptomic data allowed for the very first time to analyse the formation of cuticular microcracks and periderm in apple fruit immediately after the onset of triggering factors. This data provides valuable insights into the spatial-temporal dynamics of russeting, including the synthesis of cuticles, dedifferentiation of cells, and impregnation of cell walls with suberin and lignin.

Wounding induces suberin deposition, relevant gene expressions and changes of endogenous phytohormones in Chinese yam (Dioscorea opposita) tubers

Wounds on Chinese yam (Dioscorea opposita ) tubers can ocurr during harvest and handling, and rapid suberisation of the wound is required to prevent pathogenic infection and desiccation. However, little is known about the causal relationship among suberin deposition, relevant gene expressions and endogenous phytohormones levels in response to wounding. In this study, the effect of wounding on phytohormones levels and the expression profiles of specific genes involved in wound-induced suberisation were determined. Wounding rapidly increased the expression levels of genes, including PAL , C4H , 4CL , POD , KCSs , FARs , CYP86A1 , CYP86B1 , GPATs , ABCGs and GELPs , which likely involved in the biosynthesis, transport and polymerisation of suberin monomers, ultimately leading to suberin deposition. Wounding induced phenolics biosynthesis and being polymerised into suberin poly(phenolics) (SPP) in advance of suberin poly(aliphatics) (SPA) accumulation. Specifically, rapid expression of genes (e.g. PAL , C4H , 4CL , POD ) associated with the biosynthesis and polymerisation of phenolics, in consistent with SPP accumulation 3days after wounding, followed by the massive accumulation of SPA and relevant gene expressions (e.g. KCSs , FARs , CYP86A1 /B1 , GPATs , ABCGs , GELPs ). Additionally, wound-induced abscisic acid (ABA) and jasmonic acid (JA) consistently correlated with suberin deposition and relevant gene expressions indicating that they might play a central role in regulating wound suberisation in yam tubers.

Effect of NaCl stress on exoproteome profiles of Bacillus amyloliquefaciens EB2003A and Lactobacillus helveticus EL2006H

Salt stress can affect survival, multiplication and ability of plant growth promoting microorganisms to enhance plant growth. Changes in a microbe's proteome profile is one of the mechanisms employed by PGPM to enhance tolerance of salt stress. This study was focused on understanding changes in the exoproteome profile of Bacillus amyloliquefaciens EB2003A and Lactobacillus helveticus EL2006H when exposed to salt stress. The strains were cultured in 100 mL M13 (B. amyloliquefaciens) and 100 mL De man, Rogosa and Sharpe (MRS) (L. helveticus) media, supplemented with 200 and 0 mM NaCl (control), at pH 7.0. The strains were then incubated for 48 h (late exponential growth phase), at 120 rpm and 30 (B. amyloliquefaciens) and 37 (L. helveticus) °C. The microbial cultures were then centrifuged and filtered sterilized, to obtain cell free supernatants whose proteome profiles were studied using LC-MS/MS analysis and quantified using scaffold. Results of the study revealed that treatment with 200 mM NaCl negatively affected the quantity of identified proteins in comparison to the control, for both strains. There was upregulation and downregulation of some proteins, even up to 100%, which resulted in identification of proteins significantly unique between the control or 200 mM NaCl (p ≤ 0.05), for both microbial species. Proteins unique to 200 mM NaCl were mostly those involved in cell wall metabolism, substrate transport, oxidative stress tolerance, gene expression and DNA replication and repair. Some of the identified unique proteins have also been reported to enhance plant growth. In conclusion, based on the results of the work described here, PGPM alter their exoproteome profile when exposed to salt stress, potentially upregulating proteins that enhance their tolerance to this stress.

Pinoresinol rescues developmental phenotypes of Arabidopsis phenylpropanoid mutants overexpressing FERULATE 5-HYDROXYLASE

Most phenylpropanoid pathway flux is directed toward the production of monolignols, but this pathway also generates multiple bioactive metabolites. The monolignols coniferyl and sinapyl alcohol polymerize to form guaiacyl (G) and syringyl (S) units in lignin, components that are characteristic of plant secondary cell walls. Lignin negatively impacts the saccharification potential of lignocellulosic biomass. Although manipulation of its content and composition through genetic engineering has reduced biomass recalcitrance, in some cases, these genetic manipulations lead to impaired growth. The reduced-growth phenotype is often attributed to poor water transport due to xylem collapse in low-lignin mutants, but alternative models suggest that it could be caused by the hyper- or hypoaccumulation of phenylpropanoid intermediates. In Arabidopsis thaliana, overexpression of FERULATE 5-HYDROXYLASE (F5H) shifts the normal G/S lignin ratio to nearly pure S lignin and does not result in substantial changes to plant growth. In contrast, when we overexpressed F5H in the low-lignin mutants cinnamyl dehydrogenase c and d (cadc cadd), cinnamoyl-CoA reductase 1, and reduced epidermal fluorescence 3, plant growth was severely compromised. In addition, cadc cadd plants overexpressing F5H exhibited defects in lateral root development. Exogenous coniferyl alcohol (CA) and its dimeric coupling product, pinoresinol, rescue these phenotypes. These data suggest that mutations in the phenylpropanoid pathway limit the biosynthesis of pinoresinol, and this effect is exacerbated by overexpression of F5H, which further draws down cellular pools of its precursor, CA. Overall, these genetic manipulations appear to restrict the synthesis of pinoresinol or a downstream metabolite that is necessary for plant growth.

Spatiotemporal control of root immune responses during microbial colonization

The entire evolutionary trajectory of plants towards large and complex multi-cellular organisms has been accompanied by incessant interactions with omnipresent unicellular microbes. This led to the evolution of highly complex microbial communities, whose members display the entire spectrum of pathogenic to mutualistic behaviors. Plant roots are dynamic, fractally growing organs and even small Arabidopsis roots harbor millions of individual microbes of diverse taxa. It is evident that microbes at different positions on a root surface could experience fundamentally different environments, which, moreover, rapidly change over time. Differences in spatial scales between microbes and roots compares to humans and the cities they inhabit. Such considerations make it evident that mechanisms of root-microbe interactions can only be understood if analyzed at relevant spatial and temporal scales. This review attempts to provide an overview of the rapid recent progress that has been made in mapping and manipulating plant damage and immune responses at cellular resolution, as well as in visualizing bacterial communities and their transcriptional activities. We further discuss the impact that such approaches will have for a more predictive understanding of root-microbe interactions.

Cell-type-specific transcriptomics reveals that root hairs and endodermal barriers play important roles in beneficial plant-rhizobacterium interactions

Growth- and health-promoting bacteria can boost crop productivity in a sustainable way. Pseudomonas simiae WCS417 is such a bacterium that efficiently colonizes roots, modifies the architecture of the root system to increase its size, and induces systemic resistance to make plants more resistant to pests and pathogens. Our previous work suggested that WCS417-induced phenotypes are controlled by root cell-type-specific mechanisms. However, it remains unclear how WCS417 affects these mechanisms. In this study, we transcriptionally profiled five Arabidopsis thaliana root cell types following WCS417 colonization. We found that the cortex and endodermis have the most differentially expressed genes, even though they are not in direct contact with this epiphytic bacterium. Many of these genes are associated with reduced cell wall biogenesis, and mutant analysis suggests that this downregulation facilitates WCS417-driven root architectural changes. Furthermore, we observed elevated expression of suberin biosynthesis genes and increased deposition of suberin in the endodermis of WCS417-colonized roots. Using an endodermal barrier mutant, we showed the importance of endodermal barrier integrity for optimal plant-beneficial bacterium association. Comparison of the transcriptome profiles in the two epidermal cell types that are in direct contact with WCS417-trichoblasts that form root hairs and atrichoblasts that do not-implies a difference in potential for defense gene activation. While both cell types respond to WCS417, trichoblasts displayed both higher basal and WCS417-dependent activation of defense-related genes compared with atrichoblasts. This suggests that root hairs may activate root immunity, a hypothesis that is supported by differential immune responses in root hair mutants. Taken together, these results highlight the strength of cell-type-specific transcriptional profiling to uncover "masked" biological mechanisms underlying beneficial plant-microbe associations.

Bacterial community and root endodermis: a complementary relationship

There are feedforward and feedback loops along the microbiota-root-shoot axis to maintain plant growth or defense under environmental stresses. Here, we highlight a reciprocal interaction between the endodermis and the plant-bacterial community, which stabilizes the diffusion barriers to maintain nutrient homeostasis under nutritional stress.

Nitrate_dependent suberization regulates cadmium uptake and accumulation in maize

In this study, effect of nitrate-dependent suberization in maize root on cadmium (Cd) uptake and accumulation was investigated. Suberization in maize roots was significantly lower in plants grown with a high nitrate supply compared with low nitrate. This decrease was seen in the total amount of suberin, in which the aliphatic suberin amount was significantly decreased, whereas no difference in aromatic suberin content between different N-treatments. RNA-sequencing showed that suberin biosynthesis genes were upregulated in low nitrate treatment, which correlated well with the increased suberin content. Bioimaging and xylem sap analysis showed that reduced exodermal and endodermal suberization in roots of plants grown under high nitrate promoted radial Cd transport along the crown root. The enhanced suberization in crown roots of plants grown in low nitrate restricted the radial transport of Cd from epidermis to cortex via decreased accessibility to Cd related transporters at the plasmalemma. Also, under low nitrate supply, the Cd transport gene ZmNramp5 was upregulated in the crown root, which may enhance Cd uptake by root tip where exodermis and endodermis were not fully suberized. These results suggest that high nitrate supply enhances Cd uptake and radial transport in maize roots by reducing exodermal and endodermal suberization.

Effect of ipt Gene Induction in Transgenic Tobacco Plants on Hydraulic Conductance, Formation of Apoplastic Barriers and Aquaporin Activity under Heat Shock

Cytokinins are known to keep stomata open, which supports gas exchange and correlates with increased photosynthesis. However, keeping the stomata open can be detrimental if the increased transpiration is not compensated for by water supply to the shoots. In this study, we traced the effect of ipt (isopentenyl transferase) gene induction, which increases the concentration of cytokinins in transgenic tobacco plants, on transpiration and hydraulic conductivity. Since water flow depends on the conductivity of the apoplast, the deposition of lignin and suberin in the apoplast was studied by staining with berberine. The effect of an increased concentration of cytokinins on the flow of water through aquaporins (AQPs) was revealed by inhibition of AQPs with HgCl₂. It was shown that an elevated concentration of cytokinins in ipt-transgenic plants increases hydraulic conductivity by enhancing the activity of aquaporins and reducing the formation of apoplastic barriers. The simultaneous effect of cytokinins on both stomatal and hydraulic conductivity makes it possible to coordinate the evaporation of water from leaves and its flow from roots to leaves, thereby maintaining the water balance and leaf hydration.

Structure-function study of a Ca2+-independent metacaspase involved in lateral root emergence

Metacaspases are part of an evolutionarily broad family of multifunctional cysteine proteases, involved in disease and normal development. As the structure-function relationship of metacaspases remains poorly understood, we solved the X-ray crystal structure of an Arabidopsis thaliana type II metacaspase (AtMCA-IIf) belonging to a particular subgroup not requiring calcium ions for activation. To study metacaspase activity in plants, we developed an in vitro chemical screen to identify small molecule metacaspase inhibitors and found several hits with a minimal thioxodihydropyrimidine-dione structure, of which some are specific AtMCA-IIf inhibitors. We provide mechanistic insight into the basis of inhibition by the TDP-containing compounds through molecular docking onto the AtMCA-IIf crystal structure. Finally, a TDP-containing compound (TDP6) effectively hampered lateral root emergence in vivo, probably through inhibition of metacaspases specifically expressed in the endodermal cells overlying developing lateral root primordia. In the future, the small compound inhibitors and crystal structure of AtMCA-IIf can be used to study metacaspases in other species, such as important human pathogens, including those causing neglected diseases.

Influence of Plant Growth-Promoting Rhizobacteria on the Formation of Apoplastic Barriers and Uptake of Water and Potassium by Wheat Plants

The formation of apoplastic barriers is important for controlling the uptake of water and ions by plants, thereby influencing plant growth. However, the effects of plant growth-promoting bacteria on the formation of apoplastic barriers, and the relationship between these effects and the ability of bacteria to influence the content of hormones in plants, have not been sufficiently studied. The content of cytokinins, auxins and potassium, characteristics of water relations, deposition of lignin and suberin and the formation of Casparian bands in the root endodermis of durum wheat (Triticum durum Desf.) plants were evaluated after the introduction of the cytokinin-producing bacterium Bacillus subtilis IB-22 or the auxin-producing bacterium Pseudomonas mandelii IB-Ki14 into their rhizosphere. The experiments were carried out in laboratory conditions in pots with agrochernozem at an optimal level of illumination and watering. Both strains increased shoot biomass, leaf area and chlorophyll content in leaves. Bacteria enhanced the formation of apoplastic barriers, which were most pronounced when plants were treated with P. mandelii IB-Ki14. At the same time, P. mandelii IB-Ki14 caused no decrease in the hydraulic conductivity, while inoculation with B. subtilis IB-22, increased hydraulic conductivity. Cell wall lignification reduced the potassium content in the roots, but did not affect its content in the shoots of plants inoculated with P. mandelii IB-Ki14. Inoculation with B. subtilis IB-22 did not change the potassium content in the roots, but increased it in the shoots.

Directed growth and fusion of membrane-wall microdomains requires CASP-mediated inhibition and displacement of secretory foci

Casparian strips (CS) are aligned bands of lignin-impregnated cell walls, building an extracellular diffusion barrier in roots. Their structure profoundly differs from tight junctions (TJ), analogous structures in animals. Nonetheless, CS membrane domain (CSD) proteins 1-5 (CASP1-5) are homologues of occludins, TJ components. CASP-marked membranes display cell wall (matrix) adhesion and membrane protein exclusion. A full CASP knock-out now reveals CASPs are not needed for localized lignification, since correctly positioned lignin microdomains still form in the mutant. Ultra-structurally, however, these microdomains are disorganized, showing excessive cell wall growth, lack of exclusion zone and matrix adhesion, and impaired exocyst dynamics. Proximity-labelling identifies a Rab-GTPase subfamily, known exocyst activators, as potential CASP-interactors and demonstrate their localization and function at the CSD. We propose that CASP microdomains displace initial secretory foci by excluding vesicle tethering factors, thereby ensuring rapid fusion of microdomains into a membrane-cell wall band that seals the extracellular space.

Characterization of MdMYB68, a suberin master regulator in russeted apples

INTRODUCTION

Apple russeting is mainly due to the accumulation of suberin in the cell wall in response to defects and damages in the cuticle layer. Over the last decades, massive efforts have been done to better understand the complex interplay between pathways involved in the suberization process in model plants. However, the regulation mechanisms which orchestrate this complex process are still under investigation. Our previous studies highlighted a number of transcription factor candidates from the Myeloblastosis (MYB) transcription factor family which might regulate suberization in russeted or suberized apple fruit skin. Among these, we identified MdMYB68, which was co-expressed with number of well-known key suberin biosynthesis genes.

METHOD

To validate the MdMYB68 function, we conducted an heterologous transient expression in Nicotiana benthamiana combined with whole gene expression profiling analysis (RNA-Seq), quantification of lipids and cell wall monosaccharides, and microscopy.

RESULTS

MdMYB68 overexpression is able to trigger the expression of the whole suberin biosynthesis pathway. The lipid content analysis confirmed that MdMYB68 regulates the deposition of suberin in cell walls. Furthermore, we also investigated the alteration of the non-lipid cell wall components and showed that MdMYB68 triggers a massive modification of hemicelluloses and pectins. These results were finally supported by the microscopy.

DISCUSSION

Once again, we demonstrated that the heterologous transient expression in N. benthamiana coupled with RNA-seq is a powerful and efficient tool to investigate the function of suberin related transcription factors. Here, we suggest MdMYB68 as a new regulator of the aliphatic and aromatic suberin deposition in apple fruit, and further describe, for the first time, rearrangements occurring in the carbohydrate cell wall matrix, preparing this suberin deposition.