The GPAT4/6/8 clade functions in Arabidopsis root suberization nonredundantly with the GPAT5/7 clade required for suberin lamellae

Lipid polymers such as cutin and suberin strengthen the diffusion barrier properties of the cell wall in specific cell types and are essential for water relations, mineral nutrition, and stress protection in plants. Land plant-specific glycerol-3-phosphate acyltransferases (GPATs) of different clades are central players in cutin and suberin monomer biosynthesis. Here, we show that the GPAT4/6/8 clade in Arabidopsis thaliana, which is known to mediate cutin formation, is also required for developmentally regulated root suberization, in addition to the established roles of GPAT5/7 in suberization. The GPAT5/7 clade is mainly required for abscisic acid-regulated suberization. In addition, the GPAT5/7 clade is crucial for the formation of the typical lamellated suberin ultrastructure observed by transmission electron microscopy, as distinct amorphous globular polyester structures were deposited in the apoplast of the gpat5 gpat7 double mutant, in contrast to the thinner but still lamellated suberin deposition in the gpat4 gpat6 gpat8 triple mutant. Site-directed mutagenesis revealed that the intrinsic phosphatase activity of GPAT4, GPAT6, and GPAT8, which leads to monoacylglycerol biosynthesis, contributes to suberin formation. GPAT5/7 lack an active phosphatase domain and the amorphous globular polyester structure observed in the gpat5 gpat7 double mutant was partially reverted by treatment with a phosphatase inhibitor or the expression of phosphatase-dead variants of GPAT4/6/8. Thus, GPATs that lack an active phosphatase domain synthetize lysophosphatidic acids that might play a role in the formation of the lamellated structure of suberin. GPATs with active and nonactive phosphatase domains appear to have nonredundant functions and must cooperate to achieve the efficient biosynthesis of correctly structured suberin.

Exogenous abscisic acid induces the formation of a suberized barrier to radial oxygen loss in adventitious roots of barley (Hordeum vulgare)

BACKGROUND AND AIMS

Internal root aeration is essential for root growth in waterlogged conditions. Aerenchyma provides a path for oxygen to diffuse to the roots. In most wetland species, including rice, a barrier to radial oxygen loss (ROL) allows more of the oxygen to diffuse to the root tip, enabling root growth into anoxic soil. Most dryland crops, including barley, do not form a root ROL barrier. We previously found that abscisic acid (ABA) signalling is involved in the induction of ROL barrier formation in rice during waterlogging. Although rice typically does not form a tight ROL barrier in roots in aerated conditions, an ROL barrier with suberized exodermis was induced by application of exogenous ABA. Therefore, we hypothesized that ABA application could also trigger root ROL barrier formation with hypodermal suberization in barley.

METHODS

Formation of an ROL barrier was examined in roots in different exogenous ABA concentrations and at different time points using cylindrical electrodes and Methylene Blue staining. Additionally, we evaluated root porosity and observed suberin and lignin modification. Suberin, lignin and Casparian strips in the cell walls were observed by histochemical staining. We also evaluated the permeability of the apoplast to a tracer.

KEY RESULTS

Application of ABA induced suberization and ROL barrier formation in the adventitious roots of barley. The hypodermis also formed lignin-containing Casparian strips and a barrier to the infiltration of an apoplastic tracer (periodic acid). However, ABA application did not affect root porosity.

CONCLUSIONS

Our results show that in artificial conditions, barley can induce the formation of ROL and apoplastic barriers in the outer part of roots if ABA is applied exogenously. The difference in ROL barrier inducibility between barley (an upland species) and rice (a wetland species) might be attributable to differences in ABA signalling in roots in response to waterlogging conditions.

Function and induction of the root barrier to radial O2 loss. A commentary on 'Exogenous abscisic acid induces the formation of a suberized barrier to radial oxygen loss in adventitious roots of barley (Hordeum vulgare)'

This article comments on:

Katsuhiro Shiono and Haruka Matsuura, Exogenous abscisic acid induces the formation of a suberized barrier to radial oxygen loss in adventitious roots of barley (Hordeum vulgare), Annals of Botany, Volume 133, Issue 7, 6 June 2024, Pages 931–940 https://doi.org/10.1093/aob/mcae010

The soil microbiome modulates the sorghum root metabolome and cellular traits with a concomitant reduction of Striga infection

Sorghum bicolor is among the most important cereals globally and a staple crop for smallholder farmers in sub-Saharan Africa. Approximately 20% of sorghum yield is lost annually in Africa due to infestation with the root parasitic weed Striga hermonthica. Existing Striga management strategies are not singularly effective and integrated approaches are needed. Here, we demonstrate the functional potential of the soil microbiome to suppress Striga infection in sorghum. We associate this suppression with microbiome-mediated induction of root endodermal suberization and aerenchyma formation and with depletion of haustorium-inducing factors, compounds required for the initial stages of Striga infection. We further identify specific bacterial taxa that trigger the observed Striga-suppressive traits. Collectively, our study describes the importance of the soil microbiome in the early stages of root infection by Striga and pinpoints mechanisms of Striga suppression. These findings open avenues to broaden the effectiveness of integrated Striga management practices.

AtMYB41 acts as a dual-function transcription factor that regulates the formation of lipids in an organ- and development-dependent manner

The plant cuticle controls non-stomatal water loss and can serve as a barrier against biotic agents, whereas the heteropolymer suberin and its associated waxes are deposited constitutively at specific cell wall locations. While several transcription factors controlling cuticle formation have been identified, those involved in the transcriptional regulation of suberin biosynthesis remain poorly characterized. The major goal of this study was to further analyse the function of the R2R3-Myeloblastosis (MYB) transcription factor AtMYB41 in formation of the cuticle, suberin, and suberin-associated waxes throughout plant development. For functional analysis, the organ-specific expression pattern of AtMYB41 was analysed and Atmyb41ge alleles were generated using the CRISPR/Cas9 system. These were investigated for root growth and water permeability upon stress. In addition, the fatty acid, wax, cutin, and suberin monomer composition of different organs was evaluated by gas chromatography. The characterization of Atmyb41ge mutants revealed that AtMYB41 negatively regulates the production of cuticular lipids and fatty acid biosynthesis in leaves and seeds, respectively. Remarkably, biochemical analyses indicate that AtMYB41 also positively regulates the formation of cuticular waxes in stems of Arabidopsis thaliana. Overall, these results suggest that the AtMYB41 acts as a negative regulator of cuticle and fatty acid biosynthesis in leaves and seeds, respectively, but also as a positive regulator of wax production in A. thaliana stems.

Rigid Polyurethane Foams' Development and Optimization from Polyols Based on Depolymerized Suberin and Tall Oil Fatty Acids

The utilization of polyols derived from renewable sources presents an opportunity to enhance the sustainability of rigid polyurethane (PUR) foams, thereby contributing to the advancement of a circular bioeconomy. This study explores the development of PUR rigid foams exclusively using polyols sourced from second-generation renewable biomass feedstocks, specifically depolymerized birch bark suberin (suberinic acids) and tall oil fatty acids. The polyols achieved a total renewable material content as high as 74%, with a suberinic acid content of 37%. Response surface modeling was employed to determine the optimal bio-polyol, blowing agents, and catalyst content, hence, optimizing the bio-based foam formulations. In addition, response surface modeling was applied to rigid PUR foam formulations based on commercially available petroleum-based polyols for comparison. The results, including apparent density (~40-44 kg/m3), closed cell content (~95%), compression strength (>0.2 MPa, parallel to the foaming direction), and thermal conductivity (~0.019 W/(m·K)), demonstrated that the suberinic acids-based rigid PUR foam exhibited competitive qualities in comparison to petroleum-based polyols. Remarkably, the bio-based rigid PUR foams comprised up to 29% renewable materials. These findings highlight the potential of suberinic acid-tall oil polyols as effective candidates for developing rigid PUR foams, offering promising solutions for sustainable insulation applications.

Regulation by distinct MYB transcription factors defines the roles of OsCYP86A9 in anther development and root suberin deposition

Cytochrome P450 proteins (CYPs) play critical roles in plant development and adaptation to fluctuating environments. Previous reports have shown that CYP86A proteins are involved in the biosynthesis of suberin and cutin in Arabidopsis. However, the functions of these proteins in rice remain obscure. In this study, a rice mutant with incomplete male sterility was identified. Cytological analyses revealed that this mutant was defective in anther development. Cloning of the mutant gene indicated that the responsible mutation was on OsCYP86A9. OsMYB80 is a core transcription factor in the regulation of rice anther development. The expression of OsCYP86A9 was abolished in the anther of osmyb80 mutant. In vivo and in vitro experiments showed that OsMYB80 binds to the MYB-binding motifs in OsCYP86A9 promoter region and regulates its expression. Furthermore, the oscyp86a9 mutant exhibited an impaired suberin deposition in the root, and was more susceptible to drought stress. Interestingly, genetic and biochemical analyses revealed that OsCYP86A9 expression was regulated in the root by certain MYB transcription factors other than OsMYB80. Moreover, mutations in the MYB genes that regulate OsCYP86A9 expression in the root did not impair the male fertility of the plant. Taken together, these findings revealed the critical roles of OsCYP86A9 in plant development and proposed that OsCYP86A9 functions in anther development and root suberin formation via two distinct tissue-specific regulatory pathways.

Tetracosanoic acids produced by 3-ketoacyl-CoA synthase 17 are required for synthesizing seed coat suberin in Arabidopsis

Very long-chain fatty acids (VLCFAs) are precursors for the synthesis of membrane lipids, cuticular waxes, suberins, and storage oils in plants. 3-Ketoacyl CoA synthase (KCS) catalyzes the condensation of C2 units from malonyl-CoA to acyl-CoA, the first rate-limiting step in VLCFA synthesis. In this study, we revealed that Arabidopsis KCS17 catalyzes the elongation of C22-C24 VLCFAs required for synthesizing seed coat suberin. Histochemical analysis of Arabidopsis plants expressing GUS (β-glucuronidase) under the control of the KCS17 promoter revealed predominant GUS expression in seed coats, petals, stigma, and developing pollen. The expression of KCS17:eYFP (enhanced yellow fluorescent protein) driven by the KCS17 promoter was observed in the outer integument1 of Arabidopsis seed coats. The KCS17:eYFP signal was detected in the endoplasmic reticulum of tobacco epidermal cells. The levels of C22 VLCFAs and their derivatives, primary alcohols, α,ω-alkane diols, ω-hydroxy fatty acids, and α,ω-dicarboxylic acids increased by ~2-fold, but those of C24 VLCFAs, ω-hydroxy fatty acids, and α,ω-dicarboxylic acids were reduced by half in kcs17-1 and kcs17-2 seed coats relative to the wild type (WT). The seed coat of kcs17 displayed decreased autofluorescence under UV and increased permeability to tetrazolium salt compared with the WT. Seed germination and seedling establishment of kcs17 were more delayed by salt and osmotic stress treatments than the WT. KCS17 formed homo- and hetero-interactions with KCR1, PAS2, and ECR, but not with PAS1. Therefore, KCS17-mediated VLCFA synthesis is required for suberin layer formation in Arabidopsis seed coats.

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.

Neither lysigenous nor just oil: Demystifying myrtaceous secretory cavities

PREMISE

Leaf subepidermal secretory cavities are a notable trait in Myrtaceae, but their formation is still controversial because of the lack of consensus on their ontogeny among authors. Knowledge about the compounds present in these cavities has grown over the last few years, demonstrating that terpenoid-rich oils are not their unique content. These two points are the focus of this study on the ontogeny, structure, and contents of secretory cavities in neotropical Myrtaceae.

METHODS

We used histochemical tests and Raman analysis to verify the basic chemical composition of the cavity contents of nine species. We studied the ontogeny of glands in one species, comparing aldehyde-fixed tissues and fresh sections mounted in an inert medium.

RESULTS

We observed schizogenous development and appearance of the secretory cavities and found that sample processing may induce cell breakdown, which can be misinterpreted as lysigeny. The content of these cavities contains putative terpenes, resins, carbonyl groups, and flavonoids.

CONCLUSIONS

Our findings support the hypothesis that the lysigenous appearance of the oil glands is a technical artifact. These tissue distortions must be considered when interpreting the development of this type of secretory structure. Moreover, the basic analyses of chemical constituents show for the first time that the glands of neotropical Myrtaceae are potential reservoirs of some compounds such as flavonoids previously reported as novelties for a few other myrtaceous species. Because some of them are non-lipid compounds, the idea that the glands are just oil repositories is no longer applicable.

Identification and Characterization of ABCG15-A Gene Required for Exocarp Color Differentiation in Pear

Exocarp color is a commercially essential quality for pear which can be divided into two types: green and russet. The occurrence of russet color is associated with deficiencies and defects in the cuticular and epidermal layers, which affect the structure of the cell wall and the deposition of suberin. Until now, the genetic basics triggering this trait have not been well understood, and limited genes have been identified for the trait. To figure out the gene controlling the trait of exocarp color, we perform a comprehensive genome-wide association study, and we describe the candidate genes. One gene encoding the ABCG protein has been verified to be associated with the trait, using an integrative analysis of the metabolomic and transcriptomic data. This review covers a variety of omics resources, which provide a valuable resource for identifying gene-controlled traits of interest. The findings in this study help to elucidate the genetic components responsible for the trait of exocarp color in pear, and the implications of these findings for future pear breeding are evaluated.

Rigid Polyurethane Foams as Thermal Insulation Material from Novel Suberinic Acid-Based Polyols

Developing polyols from biomass sources contributes to a more circular economy by replacing petroleum-based polyols in the vast production of polyurethanes (PUR). One such potential biomass source could be leftover birch bark from which suberinic acids (SA) can be obtained. The purpose of this study was to identify the best synthesis routes for novel SA-based polyols, obtain rigid PUR foams, and evaluate their competitiveness and potential suitability as thermal insulation material. Novel polyols were synthesized from depolymerized SA by esterification with various functionality and molecular weight alcohols in several molar ratios. The moisture content, hydroxyl and acid values, and apparent viscosity were tested. Free-rise rigid PUR foams from the most suitable SA-based polyol and tall oil-based polyol were successfully prepared, reaching ~20 wt.% total renewable material content in the foam. The obtained rigid PUR foams' morphological, mechanical, and thermal properties were investigated and compared to present foam materials, including commercial foams. The apparent density (~33 kg/m³), as well as the closed cell content (~94%), compression strength (0.25 MPa, parallel to the foaming direction), and thermal conductivity (~0.019 W/(m·K)), approved the competitiveness and potential ability of SA-based rigid PUR foam production as thermal insulation material.

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.

The barrier to radial oxygen loss protects roots against hydrogen sulphide intrusion and its toxic effect

The root barrier to radial O₂ loss (ROL) is a key root trait preventing O₂ loss from roots to anoxic soils, thereby enabling root growth into anoxic, flooded soils. We hypothesized that the ROL barrier can also prevent intrusion of hydrogen sulphide (H₂ S), a potent phytotoxin in flooded soils. Using H₂ S- and O₂ -sensitive microsensors, we measured the apparent permeance to H₂ S of rice roots, tested whether restricted H₂ S intrusion reduced its adverse effects on root respiration, and whether H₂ S could induce the formation of a ROL barrier. The ROL barrier reduced apparent permeance to H₂ S by almost 99%, greatly restricting H₂ S intrusion. The ROL barrier acted as a shield towards H₂ S; O₂ consumption in roots with a ROL barrier remained unaffected at high H₂ S concentration (500 μM), compared to a 67% decline in roots without a barrier. Importantly, low H₂ S concentrations induced the formation of a ROL barrier. In conclusion, the ROL barrier plays a key role in protecting against H₂ S intrusion, and H₂ S can act as an environmental signalling molecule for the induction of the barrier. This study demonstrates the multiple functions of the suberized/lignified outer part of the rice root beyond that of restricting ROL.

Leaf cell wall properties and stomatal density influence oxygen isotope enrichment of leaf water

Measurements of oxygen isotope enrichment of leaf water above source water (Δ¹⁸ O_LW ) can improve our understanding of the interaction between leaf anatomy and physiology on leaf water transport. Models have been developed to predict Δ¹⁸ O_LW such as the string-of-lakes model, which describes the mixing of leaf water pools, and the Péclet effect model, which incorporates transpiration rate and the mixing length between unenriched xylem and enriched mesophyll water in the mesophyll (L_m ) or veins (L_v ). Here we compare measurements and models of Δ¹⁸ O_LW on two cell wall composition mutants grown under two light intensities and relative humidities to evaluate cell wall properties on leaf water transport. In maize (Zea mays), the compromised ultrastructure of the suberin lamellae in the bundle sheath of the ALIPHATIC SUBERIN FERULOYL TRANSFERASE mutant (Zmasft) reduced barriers to apoplastic water movement, resulting in higher E and, potentially, L_v and, consequently, lower Δ¹⁸ O_LW . The difference in Δ¹⁸ O_LW in cellulose synthase-like F6 (CslF6) mutants and wild-type of rice (Oryza sativa) grown under two light intensities co-varied with stomatal density. These results show that cell wall composition and stomatal density influence Δ¹⁸ O_LW and that stable isotopes can facilitate the development of a physiologically and anatomically explicit water transport model.

The Components' Roles in Thermal Stability and Flammability of Cork Powder

In this study, an analysis of the influence of extractives, suberin and lignocellulosic components on the pyrolysis decomposition and fire reaction mechanisms of a cork oak powder from Quercus suber L. is presented. The summative chemical composition of cork powder was determined. Suberin was the main component at 40% of the total weight, followed by 24% of lignin, 19% of polysaccharides and 14% of extractives. The absorbance peaks of cork and its individual components were further analyzed by means of ATR-FTIR spectrometry. Thermogravimetric analysis (TGA) showed that the removal of extractives from cork slightly increased the thermal stability between 200 °C and 300 °C and led to the formation of a more thermally stable residue at the end of the cork decomposition. Moreover, by removing suberin, a shift of the onset decomposition temperature to a lower temperature was noticed, indicating that suberin plays a major role in enhancing the thermal stability of cork. Furthermore, non-polar extractives showed the highest flammability with a peak of heat release rate (pHRR) of 365 W/g analyzed by means of micro-scale combustion calorimetry (MCC). Above 300 °C, the heat release rate (HRR) of suberin was lower than that of polysaccharides or lignin. However, below that temperature it released more flammable gases with a pHRR of 180 W/g, without significant charring ability, contrary to the mentioned components that showed lower HRR due to their prominent condensed mode of action that slowed down the mass and heat transfer processes during the combustion process.

The Tomato Feruloyl Transferase FHT Promoter Is an Accurate Identifier of Early Development and Stress-Induced Suberization

As a wall polymer, suberin has a multifaceted role in plant development and stress responses. It is deposited between the plasma membrane and the primary cell wall in specialized tissues such as root exodermis, endodermis, phellem, and seed coats. It is formed de novo in response to stresses such as wounding, salt injury, drought, and pathogen attack and is a complex polyester mainly consisting of fatty acids, glycerol, and minor amounts of ferulic acid that are associated to a lignin-like polymer predominantly composed of ferulates. Metabolomic and transcriptomic studies have revealed that cell wall lignification precedes suberin deposition. The ferulic acid esterified to ω-hydroxy fatty acids, synthetized by the feruloyl transferase FHT (or ASFT), presumably plays a role in coupling both polymers, although the precise mechanism is not understood. Here, we use the promoter of tomato suberin feruloyl transferase (FHT/ASFT) fused to GUS (β-glucuronidase) to demonstrate that ferulate deposition agrees with the site of promoter FHT activation by using a combination of histochemical staining and UV microscopy. Hence, FHT promoter activation and alkali UV microscopy can be used to identify the precise localization of early suberizing cells rich in ferulic acid and can additionally be used as an efficient marker of early suberization events during plant development and stress responses. This line can be used in the future as a tool to identify emerging suberization sites via ferulate deposition in tomato plants, which may contribute to germplasm screening in varietal improvement programs.

Kaempferol-3-rhamnoside overaccumulation in flavonoid 3'-hydroxylase tt7 mutants compromises seed coat outer integument differentiation and seed longevity

Seeds slowly accumulate damage during storage, which ultimately results in germination failure. The seed coat protects the embryo from the external environment, and its composition is critical for seed longevity. Flavonols accumulate in the outer integument. The link between flavonol composition and outer integument development has not been explored. Genetic, molecular and ultrastructural assays on loss-of-function mutants of the flavonoid biosynthesis pathway were used to study the effect of altered flavonoid composition on seed coat development and seed longevity. Controlled deterioration assays indicate that loss of function of the flavonoid 3' hydroxylase gene TT7 dramatically affects seed longevity and seed coat development. Outer integument differentiation is compromised from 9 d after pollination in tt7 developing seeds, resulting in a defective suberin layer and incomplete degradation of seed coat starch. These distinctive phenotypes are not shared by other mutants showing abnormal flavonoid composition. Genetic analysis indicates that overaccumulation of kaempferol-3-rhamnoside is mainly responsible for the observed phenotypes. Expression profiling suggests that multiple cellular processes are altered in the tt7 mutant. Overaccumulation of kaempferol-3-rhamnoside in the seed coat compromises normal seed coat development. This observation positions TRANSPARENT TESTA 7 and the UGT78D1 glycosyltransferase, catalysing flavonol 3-O-rhamnosylation, as essential players in the modulation of seed longevity.

Xylan glucuronic acid side chains fix suberin-like aliphatic compounds to wood cell walls

Wood is the most important repository of assimilated carbon in the biosphere, in the form of large polymers (cellulose, hemicelluloses including glucuronoxylan, and lignin) that interactively form a composite, together with soluble extractives including phenolic and aliphatic compounds. Molecular interactions among these compounds are not fully understood. We have targeted the expression of a fungal α-glucuronidase to the wood cell wall of aspen (Populus tremula L. × tremuloides Michx.) and Arabidopsis (Arabidopsis thaliana (L.) Heynh), to decrease contents of the 4-O-methyl glucuronopyranose acid (mGlcA) substituent of xylan, to elucidate mGlcA's functions. The enzyme affected the content of aliphatic insoluble cell wall components having composition similar to suberin, which required mGlcA for binding to cell walls. Such suberin-like compounds have been previously identified in decayed wood, but here, we show their presence in healthy wood of both hardwood and softwood species. By contrast, γ-ester bonds between mGlcA and lignin were insensitive to cell wall-localized α-glucuronidase, supporting the intracellular formation of these bonds. These findings challenge the current view of the wood cell wall composition and reveal a novel function of mGlcA substituent of xylan in fastening of suberin-like compounds to cell wall. They also suggest an intracellular initiation of lignin-carbohydrate complex assembly.

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.

Disruption of the ABA1 encoding zeaxanthin epoxidase caused defective suberin layers in Arabidopsis seed coats

Suberin, a complex polyester deposited in the seed coat outer integument, acts as a hydrophobic barrier to control the movement of water, ions, and gas. However, relatively little is known about the signal transduction involved in suberin layer formation during seed coat development. In this study, the effect of the plant hormone abscisic acid (ABA) on suberin layer formation in seed coats was investigated by characterizing mutations in Arabidopsis related to ABA biosynthesis and signaling. Seed coat permeability to tetrazolium salt was noticeably elevated in aba1-1 and abi1-1 mutants, but not significantly altered in snrk2.2/3/6, abi3-8, abi5-7, and pyr1pyl1pyl2pyl4 quadruple mutants compared with that in the wild-type (WT). ABA1 encodes a zeaxanthin epoxidase that functions in the first step of ABA biosynthesis. aba1-1 and aba1-8 mutant seed coats showed reduced autofluorescence under UV light and increased tetrazolium salt permeability relative to WT levels. ABA1 disruption resulted in decreased total seed coat polyester levels by approximately 3%, with a remarkable reduction in levels of C24:0 ω-hydroxy fatty acids and C24:0 dicarboxylic acids, which are the most abundant aliphatic compounds in seed coat suberin. Consistent with suberin polyester chemical analysis, RT-qPCR analysis showed a significant reduction in transcript levels of KCS17, FAR1, FAR4, FAR5, CYP86A1, CYP86B1, ASFT, GPAT5, LTPG1, LTPG15, ABCG2, ABCG6, ABCG20, ABCG23, MYB9, and MYB107, which are involved in suberin accumulation and regulation in developing aba1-1 and aba1-8 siliques, as compared with WT levels. Together, seed coat suberization is mediated by ABA and partially processed through canonical ABA signaling.

Analytical Method Cluster Development for Comprehensive Characterisation of Suberinic Acids Derived from Birch Outer Bark

Suberin is a complex polyester biopolymer, and it is practically impossible to estimate the real content of suberin in suberised plant tissues. This indicates the importance of the development of instrumental analytical methods for the comprehensive characterisation of suberin derived from plant biomass for the successful integration of suberinic products into biorefinery production chains. In this study, we optimised two GC-MS methods-one with direct sylilation, and the second with additional depolymerisation, using GPC methods with RI detector and polystyrene calibration and with a three-angle light scattering detector and an eighteen-angle light scattering detector. We also performed MALDI-Tof analysis for non-degraded suberin structure determination. We characterised suberinic acid (SA) samples obtained from birch outer bark after alkaline depolymerisation. The samples were particularly rich in diols, fatty acids and their esters, hydroxyacids and their corresponding esters, diacids and their corresponding esters, as well as extracts (mainly betulin and lupeol) and carbohydrates. To remove phenolic-type admixtures, treatment with ferric chloride (FeCl₃) was used. The SA treatment with FeCl₃ allows the possibility to obtain a sample that has a lower content of phenolic-type compounds and a lower molecular weight than an untreated sample. It was possible to identify the main free monomeric units of SA samples by GC-MS system using direct silylation. By performing an additional depolymerisation step before silylation, it was possible to characterise the complete potential monomeric unit composition in the suberin sample. For the molar mass distribution determination, it is important to perform GPC analysis. Even though chromatographic results can be obtained using a three- laser MALS detector, they are not fully correct because of the fluorescence of the SA samples. Therefore an 18-angle MALS detector with filters was more suitable for SA analysis. MALDI-Tof analysis is a great tool for the polymeric compound structural identification, which cannot be done using GC-MS. Using the MALDI data, we discovered that the main monomeric units that makes up the SA macromolecular structure are octadecanedioic acid and 2-(1,3-dihydroxyprop-2-oxy)decanedioic acid. This corresponds with GC-MS results, showing that after depolymerisation hydroxyacids and diacids were the dominant type of compounds found in the sample.

Effects of a Pseudomonas Strain on the Lipid Transfer Proteins, Appoplast Barriers and Activity of Aquaporins Associated with Hydraulic Conductance of Pea Plants

Lipid transfer proteins (LTPs) are known to be involved in suberin deposition in the Casparian bands of pea roots, thereby reinforcing apoplast barriers. Moreover, the Pseudomonas mandelii IB-Ki14 strain accelerated formation of the Casparian bands in wheat plants, although involvement of LTPs in the process was not studied. Here, we investigated the effects of P. mandelii IB-Ki14 on LTPs, formation of the Casparian bands, hydraulic conductance and activity of aquaporins (AQPs) in pea plants. RT PCR showed a 1.6-1.9-fold up-regulation of the PsLTP-coding genes and an increase in the abundance of LTP proteins in the phloem of pea roots induced by the treatment with P. mandelii IB-Ki14. The treatment was accompanied with increased deposition of suberin in the Casparian bands. Hydraulic conductance did not decrease in association with the bacterial treatment despite strengthening of the apoplast barriers. At the same time, the Fenton reagent, serving as an AQPs inhibitor, decreased hydraulic conductance to a greater extent in treated plants relative to the control group, indicating an increase in the AQP activity by the bacteria. We hypothesize that P. mandelii IB-Ki14 stimulates deposition of suberin, in the biosynthesis of which LTPs are involved, and increases aquaporin activity, which in turn prevents a decrease in hydraulic conductance due to formation of the apoplast barriers in pea roots.

Soil organic matter molecular composition with long-term detrital alterations is controlled by site-specific forest properties

Forest ecosystems are important global soil carbon (C) reservoirs, but their capacity to sequester C is susceptible to climate change factors that alter the quantity and quality of C inputs. To better understand forest soil C responses to altered C inputs, we integrated three molecular composition published data sets of soil organic matter (SOM) and soil microbial communities for mineral soils after 20 years of detrital input and removal treatments in two deciduous forests: Bousson Forest (BF), Harvard Forest (HF), and a coniferous forest: H.J. Andrews Forest (HJA). Soil C turnover times were estimated from radiocarbon measurements and compared with the molecular-level data (based on nuclear magnetic resonance and specific analysis of plant- and microbial-derived compounds) to better understand how ecosystem properties control soil C biogeochemistry and dynamics. Doubled aboveground litter additions did not increase soil C for any of the forests studied likely due to long-term soil priming. The degree of SOM decomposition was higher for bacteria-dominated sites with higher nitrogen (N) availability while lower for the N-poor coniferous forest. Litter exclusions significantly decreased soil C, increased SOM decomposition state, and led to the adaptation of the microbial communities to changes in available substrates. Finally, although aboveground litter determined soil C dynamics and its molecular composition in the coniferous forest (HJA), belowground litter appeared to be more influential in broadleaf deciduous forests (BH and HF). This synthesis demonstrates that inherent ecosystem properties regulate how soil C dynamics change with litter manipulations at the molecular-level. Across the forests studied, 20 years of litter additions did not enhance soil C content, whereas litter reductions negatively impacted soil C concentrations. These results indicate that soil C biogeochemistry at these temperate forests is highly sensitive to changes in litter deposition, which are a product of environmental change drivers.

The Plant Fatty Acyl Reductases

Fatty acyl reductase (FAR) is a crucial enzyme that catalyzes the NADPH-dependent reduction of fatty acyl-CoA or acyl-ACP substrates to primary fatty alcohols, which in turn acts as intermediate metabolites or metabolic end products to participate in the formation of plant extracellular lipid protective barriers (e.g., cuticular wax, sporopollenin, suberin, and taproot wax). FARs are widely present across plant evolution processes and play conserved roles during lipid synthesis. In this review, we provide a comprehensive view of FAR family enzymes, including phylogenetic analysis, conserved structural domains, substrate specificity, subcellular localization, tissue-specific expression patterns, their varied functions in lipid biosynthesis, and the regulation mechanism of FAR activity. Finally, we pose several questions to be addressed, such as the roles of FARs in tryphine, the interactions between transcription factors (TFs) and FARs in various environments, and the identification of post-transcriptional, translational, and post-translational regulators.

Transcriptome and metabolome analyses reveal phenotype formation differences between russet and non-russet apples

The apple is an economically important fruit, and fruit russeting is not conducive to its appearance. Although studies have examined fruit russeting, its mechanism remains unclear. Two apple strains of the F₁ hybrid population derived from 'Fuji' and 'Golden Delicious' were used in this study. We found that the skin of russet apples was rough and fissured, while that of non-russet apples was smooth and waxy. Chemical staining, LC- and GC-MS showed that both lignin and suberin were increased in russet apple skin. Meanwhile, genes involved in lignin and suberin synthetic pathways were upregulated in russet apple skin. Additionally, we found many differentially expressed genes (DEGs¹) involved in hormone biosynthesis and signaling and stress responses in the two apple strains. We found that WRKY13 may influence russeting by regulating lignin synthesis. Our study identified several candidate metabolites and genes, which will provide a good foundation for further research.

Cork Development: What Lies Within

The cork layer present in all dicotyledonous plant species with radial growth is the result of the phellogen activity, a secondary meristem that produces phellem (cork) to the outside and phelloderm inwards. These three different tissues form the periderm, an efficient protective tissue working as a barrier against external factors such as environmental aggressions and pathogen attacks. The protective function offered by cork cells is mainly due to the abundance of suberin in their cell walls. Chemically, suberin is a complex aliphatic network of long chain fatty acids and alcohols with glycerol together with aromatic units. In most woody species growing in temperate climates, the first periderm is replaced by a new functional periderm upon a few years after being formed. One exception to this bark development can be found in cork oak (Quercus suber) which display a single periderm that grows continuously. Quercus suber stands by its thick cork layer development with continuous seasonal growth. Cork raw material has been exploited by man for centuries, especially in Portugal and Spain. Nowadays, its applications have widened vastly, from the most known product, stoppers, to purses or insulating materials used in so many industries, such as construction and car production. Research on how cork develops, and the effect environmental factors on cork oak trees is extremely important to maintain production of good-quality cork, and, by maintaining cork oak stands wealthy, we are preserving a very important ecosystem both by its biodiversity and its vital social and economic role in areas already showing a population declination.

MdMYB52 regulates lignin biosynthesis upon the suberization process in apple

Our previous studies, comparing russeted vs. waxy apple skin, highlighted a MYeloBlastosys (Myb) transcription factor (MdMYB52), which displayed a correlation with genes associated to the suberization process. The present article aims to assess its role and function in the suberization process. Phylogenetic analyses and research against Arabidopsis thaliana MYBs database were first performed and the tissue specific expression of MdMYB52 was investigated using RT-qPCR. The function of MdMYB52 was further investigated using Agrobacterium-mediated transient overexpression in Nicotiana benthamiana leaves. An RNA-Seq analysis was performed to highlight differentially regulated genes in response MdMYB52. Transcriptomic data were supported by analytical chemistry and microscopy. A massive decreased expression of photosynthetic and primary metabolism pathways was observed with a concomitant increased expression of genes associated with phenylpropanoid and lignin biosynthesis, cell wall modification and senescence. Interestingly key genes involved in the synthesis of suberin phenolic components were observed. The analytical chemistry displayed a strong increase in the lignin content in the cell walls during MdMYB52 expression. More specifically, an enrichment in G-Unit lignin residues was observed, supporting transcriptomic data as well as previous work describing the suberin phenolic domain as a G-unit enriched lignin-like polymer. The time-course qPCR analysis revealed that the observed stress response, might be explain by this lignin biosynthesis and by a possible programmed senescence triggered by MdMYB52. The present work supports a crucial regulatory role for MdMYB52 in the biosynthesis of the suberin phenolic domain and possibly in the fate of suberized cells in russeted apple skins.

Endodermal apoplastic barriers are linked to osmotic tolerance in meso-xerophytic grass Elymus sibiricus

Drought is the most serious adversity faced by agriculture and animal husbandry industries. One strategy that plants use to adapt to water deficits is modifying the root growth and architecture. Root endodermis has cell walls reinforced with apoplastic barriers formed by the Casparian strip (CS) and suberin lamellae (SL) deposits, regulates radial nutrient transport and protects the vascular cylinder from abiotic threats. Elymus sibiricus is an economically important meso-xerophytic forage grass, characterized by high nutritional quality and strong environmental adaptability. The purpose of this study was to evaluate the drought tolerance of E. sibiricus genotypes and investigate the root structural adaptation mechanism of drought-tolerant genotypes' responding to drought. Specifically, a drought tolerant (DT) and drought sensitive (DS) genotype were screened out from 52 E. sibiricus genotypes. DT showed less apoplastic bypass flow of water and solutes than DS under control conditions, as determined with a hydraulic conductivity measurement system and an apoplastic fluorescent tracer, specifically PTS trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS). In addition, DT accumulated less Na, Mg, Mn, and Zn and more Ni, Cu, and Al than DS, regardless of osmotic stress. Further study showed more suberin deposition in DT than in DS, which could be induced by osmotic stress in both. Accordingly, the CS and SL were deposited closer to the root tip in DT than in DS. However, osmotic stress induced their deposition closer to the root tips in DS, while likely increasing the thickness of the CS and SL in DT. The stronger and earlier formation of endodermal barriers may determine the radial transport pathways of water and solutes, and contribute to balance growth and drought response in E. sibiricus. These results could help us better understand how altered endodermal apoplastic barriers in roots regulate water and mineral nutrient transport in plants that have adapted to drought environments. Moreover, the current findings will aid in improving future breeding programs to develop drought-tolerant grass or crop cultivars.

Sodium silicate promotes wound healing by inducing the deposition of suberin polyphenolic and lignin in potato tubers

Wound healing is a postharvest characteristic of potato tubers through accumulating suberin and lignin, which could reduce decay and water loss during storage. This study aimed to explore the impact and mechanisms of sodium silicate on wound healing of potatoes. After being wounded, "Atlantic" potato tubers were treated with water or 50 mM sodium silicate. The results showed that sodium silicate treatment accelerated the formation of wound healing structures and significantly reduced the weight loss and disease index of tubers. Furthermore, sodium silicate induced the genes expression and enzyme activity of phenylalanine ammonia lyase (PAL), 4-coumarate: coenzyme A ligase (4CL), and cinnamyl alcohol dehydrogenase (CAD) involved in the phenylpropane metabolism, enhancing the synthesis of the main precursors of suberin polyphenolic (SPP) and lignin, such as coniferyl alcohol, sinapyl alcohol, and cinnamyl alcohol. Meanwhile, the gene expression of StPOD and StNOX was activated, and the production of O^2- and H₂O₂ was promoted, which could be used for injury signal transmission and oxidative crosslinking of SPP monomers and lignin precursors. Besides, antimicrobial compounds, total phenolics, and flavonoids were also induced. We suggest that sodium silicate could promote wound healing by inducing the deposition of SPP, lignin, and antimicrobial compounds in potato tubers.

The making of suberin

Outer protective barriers of animals use a variety of bio-polymers, based on either proteins (e.g. collagens), or modified sugars (e.g. chitin). Plants, however, have come up with a particular solution, based on the polymerisation of lipid-like precursors, giving rise to cutin and suberin. Suberin is a structural lipophilic polyester of fatty acids, glycerol and some aromatics found in cell walls of phellem, endodermis, exodermis, wound tissues, abscission zones, bundle sheath and other tissues. It deposits as a hydrophobic layer between the (ligno)cellulosic primary cell wall and plasma membrane. Suberin is highly protective against biotic and abiotic stresses, shows great developmental plasticity and its chemically recalcitrant nature might assist the sequestration of atmospheric carbon by plants. The aim of this review is to integrate the rapidly accelerating genetic and cell biological discoveries of recent years with the important chemical and structural contributions obtained from very diverse organisms and tissue layers. We critically discuss the order and localisation of the enzymatic machinery synthesising the presumed substrates for export and apoplastic polymerisation. We attempt to explain observed suberin linkages by diverse enzyme activities and discuss the spatiotemporal relationship of suberin with lignin and ferulates, necessary to produce a functional suberised cell wall.

Spatiotemporal development of suberized barriers in cork oak taproots

The longevity and high activity of the cork cambium (or phellogen) from Quercus suber L. (cork oak) are the cornerstones for the sustainable exploitation of a unique raw material. Cork oak is a symbolic model to study cork development and cell wall suberization, yet most genetic and molecular studies on these topics have targeted other model plants. In this study, we explored the potential of taproots as a model system to study phellem development and suberization in cork oak, thereby avoiding the time constraints imposed when studying whole plants. In roots, suberin deposition is found in mature endodermis cells during primary development and in phellem cells during secondary development. By investigating the spatiotemporal characteristics of both endodermis and phellem suberization in young seedling taproots, we demonstrated that secondary growth and phellogen activity are initiated very early in cork oak taproots (approx. 8 days after sowing). We further compared the transcriptomic profile of root segments undergoing primary (PD) and secondary development (SD) and identified multiple candidate genes with predicted roles in cell wall modifications, mainly lignification and suberization, in addition to several regulatory genes, particularly transcription factor- and hormone-related genes. Our results indicate that the molecular regulation of suberization and secondary development in cork oak roots is relatively conserved with other species. The provided morphological characterization creates new opportunities to allow a faster assessment of phellogen activity (as compared with studies using stem tissues) and to tackle fundamental questions regarding its regulation.

Induced ligno-suberin vascular coating and tyramine-derived hydroxycinnamic acid amides restrict Ralstonia solanacearum colonization in resistant tomato

Tomato varieties resistant to the bacterial wilt pathogen Ralstonia solanacearum have the ability to restrict bacterial movement in the plant. Inducible vascular cell wall reinforcements seem to play a key role in confining R. solanacearum into the xylem vasculature of resistant tomato. However, the type of compounds involved in such vascular physico-chemical barriers remain understudied, while being a key component of resistance. Here we use a combination of histological and live-imaging techniques, together with spectroscopy and gene expression analysis to understand the nature of R. solanacearum-induced formation of vascular coatings in resistant tomato. We describe that resistant tomato specifically responds to infection by assembling a vascular structural barrier formed by a ligno-suberin coating and tyramine-derived hydroxycinnamic acid amides. Further, we show that overexpressing genes of the ligno-suberin pathway in a commercial susceptible variety of tomato restricts R. solanacearum movement inside the plant and slows disease progression, enhancing resistance to the pathogen. We propose that the induced barrier in resistant plants does not only restrict the movement of the pathogen, but may also prevent cell wall degradation by the pathogen and confer anti-microbial properties, effectively contributing to resistance.

Translational profile of developing phellem cells in Arabidopsis thaliana roots

The phellem is a specialized boundary tissue providing the first line of defense against abiotic and biotic stresses in organs undergoing secondary growth. Phellem cells undergo several differentiation steps, which include cell wall suberization, cell expansion, and programmed cell death. Yet, the molecular players acting particularly in phellem cell differentiation remain poorly described, particularly in the widely used model plant Arabidopsis thaliana. Using specific marker lines we followed the onset and progression of phellem differentiation in A. thaliana roots and further targeted the translatome of newly developed phellem cells using translating ribosome affinity purification followed by mRNA sequencing (TRAP-SEQ). We showed that phellem suberization is initiated early after phellogen (cork cambium) division. The specific translational landscape was organized in three main domains related to energy production, synthesis and transport of cell wall components, and response to stimulus. Novel players in phellem differentiation related to suberin monomer transport and assembly as well as novel transcription regulators were identified. This strategy provided an unprecedented resolution of the translatome of developing phellem cells, giving a detailed and specific view on the molecular mechanisms acting on cell differentiation in periderm tissues of the model plant Arabidopsis.

Synthesis of C20-38 Fatty Acids in Plant Tissues

Very-long-chain fatty acids (VLCFA) are involved in a number of important plant physiological functions. Disorders in the expression of genes involved in the synthesis of VLCFA lead to a number of phenotypic consequences, ranging from growth retardation to the death of embryos. The elongation of VLCFA in the endoplasmic reticulum (ER) is carried out by multiple elongase complexes with different substrate specificities and adapted to the synthesis of a number of products required for a number of metabolic pathways. The information about the enzymes involved in the synthesis of VLCFA with more than 26 atoms of Carbon is rather poor. Recently, genes encoding enzymes involved in the synthesis of both regular-length fatty acids and VLCFA have been discovered and investigated. Polyunsaturated VLCFA in plants are formed mainly by 20:1 elongation into new monounsaturated acids, which are then imported into chloroplasts, where they are further desaturated. The formation of saturated VLCFA and their further transformation into a number of aliphatic compounds included in cuticular waxes and suberin require the coordinated activity of a large number of different enzymes.

Suberin Biosynthesis, Assembly, and Regulation

Suberin is a specialized cell wall modifying polymer comprising both phenolic-derived and fatty acid-derived monomers, which is deposited in below-ground dermal tissues (epidermis, endodermis, periderm) and above-ground periderm (i.e., bark). Suberized cells are largely impermeable to water and provide a critical protective layer preventing water loss and pathogen infection. The deposition of suberin is part of the skin maturation process of important tuber crops such as potato and can affect storage longevity. Historically, the term "suberin" has been used to describe a polyester of largely aliphatic monomers (fatty acids, ω-hydroxy fatty acids, α,ω-dioic acids, 1-alkanols), hydroxycinnamic acids, and glycerol. However, exhaustive alkaline hydrolysis, which removes esterified aliphatics and phenolics from suberized tissue, reveals a core poly(phenolic) macromolecule, the depolymerization of which yields phenolics not found in the aliphatic polyester. Time course analysis of suberin deposition, at both the transcriptional and metabolite levels, supports a temporal regulation of suberin deposition, with phenolics being polymerized into a poly(phenolic) domain in advance of the bulk of the poly(aliphatics) that characterize suberized cells. In the present review, we summarize the literature describing suberin monomer biosynthesis and speculate on aspects of suberin assembly. In addition, we highlight recent advances in our understanding of how suberization may be regulated, including at the phytohormone, transcription factor, and protein scaffold levels.

Identification of Novel Candidate Genes Involved in Apple Cuticle Integrity and Russeting-Associated Triterpene Synthesis Using Metabolomic, Proteomic, and Transcriptomic Data

Apple russeting develops on the fruit surface when skin integrity has been lost. It induces a modification of fruit wax composition, including its triterpene profile. In the present work, we studied two closely related apple varieties, 'Reinette grise du Canada' and 'Reinette blanche du Canada', which display russeted and non-russeted skin phenotypes, respectively, during fruit development. To better understand the molecular events associated with russeting and the differential triterpene composition, metabolomics data were generated using liquid chromatography coupled to high-resolution mass spectrometry (LC-HRMS) and combined with proteomic and transcriptomic data. Our results indicated lower expression of genes linked to cuticle biosynthesis (cutin and wax) in russet apple throughout fruit development, along with an alteration of the specialized metabolism pathways, including triterpene and phenylpropanoid. We identified a lipid transfer protein (LTP3) as a novel player in cuticle formation, possibly involved in the transport of both cutin and wax components in apple skin. Metabolomic data highlighted for the first time a large diversity of triterpene-hydroxycinnamates in russeted tissues, accumulation of which was highly correlated with suberin-related genes, including some enzymes belonging to the BAHD (HXXXD-motif) acyltransferase family. Overall, this study increases our understanding about the crosstalk between triterpene and suberin pathways.

Trafficking Processes and Secretion Pathways Underlying the Formation of Plant Cuticles

Cuticles are specialized cell wall structures that form at the surface of terrestrial plant organs. They are largely comprised lipidic compounds and are deposited in the apoplast, external to the polysaccharide-rich primary wall, creating a barrier to diffusion of water and solutes, as well as to environmental factors. The predominant cuticle component is cutin, a polyester that is assembled as a complex matrix, within and on the surface of which aliphatic and aromatic wax molecules accumulate, further modifying its properties. To reach the point of cuticle assembly the different acyl lipid-containing components are first exported from the cell across the plasma membrane and then traffic across the polysaccharide wall. The export of cutin precursors and waxes from the cell is known to involve plasma membrane-localized ATP-binding cassette (ABC) transporters; however, other secretion mechanisms may also contribute. Indeed, extracellular vesiculo-tubular structures have recently been reported in Arabidopsis thaliana (Arabidopsis) to be associated with the deposition of suberin, a polyester that is structurally closely related to cutin. Intriguingly, similar membranous structures have been observed in leaves and petals of Arabidopsis, although in lower numbers, but no close association with cutin formation has been identified. The possibility of multiple export mechanisms for cuticular components acting in parallel will be discussed, together with proposals for how cuticle precursors may traverse the polysaccharide cell wall before their assimilation into the cuticle macromolecular architecture.

Physiological roles of Casparian strips and suberin in the transport of water and solutes

The formation of Casparian strips (CS) and the deposition of suberin at the endodermis of plant roots are thought to limit the apoplastic transport of water and ions. We investigated the specific role of each of these apoplastic barriers in the control of hydro-mineral transport by roots and the consequences on shoot growth. A collection of Arabidopsis thaliana mutants defective in suberin deposition and/or CS development was characterized under standard conditions using a hydroponic system and the Phenopsis platform. Mutants altered in suberin deposition had enhanced root hydraulic conductivity, indicating a restrictive role for this compound in water transport. In contrast, defective CS directly increased solute leakage and indirectly reduced root hydraulic conductivity. Defective CS also led to a reduction in rosette growth, which was partly dependent on the hydro-mineral status of the plant. Ectopic suberin was shown to partially compensate for defective CS phenotypes. Altogether, our work shows that the functionality of the root apoplastic diffusion barriers greatly influences the plant physiology, and that their integrity is tightly surveyed.

Dissecting the molecular mechanism of russeting in sand pear (Pyrus pyrifolia Nakai) by metabolomics, transcriptomics, and proteomics

Brown coloration and a rough appearance as russet and semi-russet (partial russet) are features unique to the popular Asian sand pear (Pyrus pyrifolia Nakai). The degree of russeting is different between different genotypes. Russeting is sensitive to water fluctuations, where excessive rainwater can trigger/stimulate its development. However, the molecular mechanism of russeting is currently unclear. Here, we employed multi-omics, i.e., metabolomics, transcriptomics, and proteomics, and analyzed the effect of different sand pear genotypes and artificial rainfall on russeting of pear fruits. This led to the identification of 79, 64, and 29 differentially produced/expressed metabolites, transcripts, and proteins that are involved in the biosynthesis of suberin, phenylpropane, cutin, and waxes. Further analysis of these differentially expressed genes and their encoded proteins revealed that four of them exhibited high expression at both transcript and protein levels. Transient expression of one such gene, PbHHT1 (accession number 103966555), which encodes ω-hydroxypalmitate-O-feruloyl transferase, in young green non-russet fruits triggered premature suberization in the russeting pear genotypes. This coincided with increased production of 16-feruloyloxypalmitic acid, a conjugated compound between phenols and esters during the polymerization for suberin formation. Collectively, our data from the combined three omics demonstrate that russeting in sand pear is a complex process involving the biosynthesis and transport of suberin and many other secondary metabolites.

Role of Basal ABA in Plant Growth and Development

Abscisic acid (ABA) regulates various aspects of plant physiology, including promoting seed dormancy and adaptive responses to abiotic and biotic stresses. In addition, ABA plays an im-portant role in growth and development under non-stressed conditions. This review summarizes phenotypes of ABA biosynthesis and signaling mutants to clarify the roles of basal ABA in growth and development. The promotive and inhibitive actions of ABA in growth are characterized by stunted and enhanced growth of ABA-deficient and insensitive mutants, respectively. Growth regulation by ABA is both promotive and inhibitive, depending on the context, such as concentrations, tissues, and environmental conditions. Basal ABA regulates local growth including hyponastic growth, skotomorphogenesis and lateral root growth. At the cellular level, basal ABA is essential for proper chloroplast biogenesis, central metabolism, and expression of cell-cycle genes. Basal ABA also regulates epidermis development in the shoot, by inhibiting stomatal development, and deposition of hydrophobic polymers like a cuticular wax layer covering the leaf surface. In the root, basal ABA is involved in xylem differentiation and suberization of the endodermis. Hormone crosstalk plays key roles in growth and developmental processes regulated by ABA. Phenotypes of ABA-deficient and insensitive mutants indicate prominent functions of basal ABA in plant growth and development.

Phylogenetic analysis and in-depth characterization of functionally and structurally diverse CE5 cutinases

Cutinases are esterases that release fatty acids from the apoplastic layer in plants. As they accept bulky and hydrophobic substrates, cutinases could be used in many applications, ranging from valorization of bark-rich side streams to plastic recycling. Advancement of these applications, however, requires deeper knowledge of cutinases' biodiversity and structure-function relationships. Here, we mined over 3000 members from carbohydrate esterase family 5 for putative cutinases and condensed it to 151 genes from known or putative lignocellulose-targeting organisms. The 151 genes were subjected to a phylogenetic analysis, which showed that cutinases with available crystal structures were phylogenetically closely related. We then selected nine phylogenic diverse cutinases for recombinant production and characterized their kinetic activity against para-nitrophenol substrates esterified with consecutively longer alkyl chains (pNP-C2 to C16). Each investigated cutinase had a unique activity fingerprint against the tested pNP substrates. The five enzymes with the highest activity on pNP-C12 and C16, indicative of activity on bulky hydrophobic compounds, were selected for in-depth kinetic and structure-function analysis. All five enzymes showed a decrease in kcat values with increasing substrate chain length, whereas KM values and binding energies (calculated from in silico docking analysis) improved. Two cutinases from Fusarium solani and Cryptococcus sp. exhibited outstandingly low KM values, resulting in high catalytic efficiencies toward pNP-C16. Docking analysis suggested that different clades of the phylogenetic tree may harbor enzymes with different modes of substrate interaction, involving a solvent-exposed catalytic triad, a lipase-like lid, or a clamshell-like active site possibly formed by flexible loops.

Root Suberin Plays Important Roles in Reducing Water Loss and Sodium Uptake in Arabidopsis thaliana

Suberin is a cell-wall-associated hetero-polymer deposited in specific plant tissues. The precise role of its composition and lamellae structure in protecting plants against abiotic stresses is unclear. In Arabidopsis thaliana, we tested the biochemical and physiological responses to water deficiency and NaCl treatment in mutants that are differentially affected in suberin composition and lamellae structure. Chronic drought stress increased suberin and suberin-associated waxes in wild-type plants. Suberin-deficient mutants were not more susceptible than the wild-type to the chronic drought stress imposed in this study. Nonetheless, the cyp86a1-1 cyp86b1-1 mutant, which had a severely altered suberin composition and lamellae structure, exhibited increased water loss through the root periderm. Cyp86a1-1 cyp86b1-1 also recorded lower relative water content in leaves. The abcg2-1 abcg6-1 abcg20-1 mutant, which has altered suberin composition and lamellae, was very sensitive to NaCl treatment. Furthermore, cyp86a1-1 cyp86b1-1 recorded a significant drop in the leaf K/Na ratio, indicating salt sensitivity. The far1-2 far4-1 far5-1 mutant, which did not show structural defects in the suberin lamellae, had similar responses to drought and NaCl treatments as the wild-type. Our results provide evidence that the suberin amount and lamellae structure are key features in the barrier function of suberin in reducing water loss and reducing sodium uptake through roots for better performance under drought and salt stresses.

Effects of Salinity and Abscisic Acid on Lipid Transfer Protein Accumulation, Suberin Deposition and Hydraulic Conductance in Pea Roots

Lipid transfer proteins (LTPs) participate in many important physiological processes in plants, including adaptation to stressors, e.g., salinity. Here we address the mechanism of this protective action of LTPs by studying the interaction between LTPs and abscisic acid (ABA, a "stress" hormone) and their mutual participation in suberin deposition in root endodermis of salt-stressed pea plants. Using immunohistochemistry we show for the first time NaCl induced accumulation of LTPs and ABA in the cell walls of phloem paralleled by suberin deposition in the endoderm region of pea roots. Unlike LTPs which were found localized around phloem cells, ABA was also present within phloem cells. In addition, ABA treatment resulted in both LTP and ABA accumulation in phloem cells and promoted root suberization. These results suggested the importance of NaCl-induced accumulation of ABA in increasing the abundance of LTPs and of suberin. Using molecular modeling and fluorescence spectroscopy we confirmed the ability of different plant LTPs, including pea Ps-LTP1, to bind ABA. We therefore hypothesize an involvement of plant LTPs in ABA transport (unloading from phloem) as part of the salinity adaptation mechanism.

Suberin plasticity to developmental and exogenous cues is regulated by a set of MYB transcription factors

Suberin is a hydrophobic biopolymer that can be deposited at the periphery of cells, forming protective barriers against biotic and abiotic stress. In roots, suberin forms lamellae at the periphery of endodermal cells where it plays crucial roles in the control of water and mineral transport. Suberin formation is highly regulated by developmental and environmental cues. However, the mechanisms controlling its spatiotemporal regulation are poorly understood. Here, we show that endodermal suberin is regulated independently by developmental and exogenous signals to fine-tune suberin deposition in roots. We found a set of four MYB transcription factors (MYB41, MYB53, MYB92, and MYB93), each of which is individually regulated by these two signals and is sufficient to promote endodermal suberin. Mutation of these four transcription factors simultaneously through genome editing leads to a dramatic reduction in suberin formation in response to both developmental and environmental signals. Most suberin mutants analyzed at physiological levels are also affected in another endodermal barrier made of lignin (Casparian strips) through a compensatory mechanism. Through the functional analysis of these four MYBs, we generated plants allowing unbiased investigation of endodermal suberin function, without accounting for confounding effects due to Casparian strip defects, and were able to unravel specific roles of suberin in nutrient homeostasis.

Suberin plasticity to developmental and exogenous cues is regulated by a set of MYB transcription factors

Suberin is a hydrophobic biopolymer that can be deposited at the periphery of cells, forming protective barriers against biotic and abiotic stress. In roots, suberin forms lamellae at the periphery of endodermal cells where it plays crucial roles in the control of water and mineral transport. Suberin formation is highly regulated by developmental and environmental cues. However, the mechanisms controlling its spatiotemporal regulation are poorly understood. Here, we show that endodermal suberin is regulated independently by developmental and exogenous signals to fine-tune suberin deposition in roots. We found a set of four MYB transcription factors (MYB41, MYB53, MYB92, and MYB93), each of which is individually regulated by these two signals and is sufficient to promote endodermal suberin. Mutation of these four transcription factors simultaneously through genome editing leads to a dramatic reduction in suberin formation in response to both developmental and environmental signals. Most suberin mutants analyzed at physiological levels are also affected in another endodermal barrier made of lignin (Casparian strips) through a compensatory mechanism. Through the functional analysis of these four MYBs, we generated plants allowing unbiased investigation of endodermal suberin function, without accounting for confounding effects due to Casparian strip defects, and were able to unravel specific roles of suberin in nutrient homeostasis.

Structural Elucidation of Suberin from the Bark of Cultivated Willow (Salix sp.)

Although extractives have been symbolized as major bioactive pharmacological compounds from Salix (Salicaceae) bark, we speculated that these pharmaceutical effects cannot be solely attributed to phenolic components and their derivatives, but the long-chain suberin acids also contribute to their therapeutic effects. Hence, isolation and deconstruction of suberin were conducted, for the first time, to enrich our knowledge about the macromolecular components at the cell wall of willow bark. Saponification was adopted to obtain suberin extracts at a yield of approximately 5 wt % based on the bark of the studied hybrids. Gas chromatography-mass spectrometry allowed qualification and quantification of 23 compounds from the released suberin monomers, from which fatty acids represented majority of the isolated suberin, namely, fatty acid methyl esters (C17-C19); mono-carboxylic acid (C7-C16); alpha, omega-dicarboxylic acid (C7-C16); and omega-hydroxy long-chain fatty acids (C16-C22). Additionally, the lipophilic extractive was dominated by piceol, heptacosane, β-sitosterol, and fatty acids (C16-C28) from the studied hybrids. These findings could boost our integrative approach toward full valorization of willow bark.