Amount Of Suberin Research Articles

Effects of water stress on apoplastic barrier formation in soil grown roots differ from hydroponically grown roots: Histochemical, biochemical and molecular evidence.

In root research, hydroponic plant cultivation is commonly used and soil experiments are rare. We investigated the response of 12-day-old barley roots, cultivated in soil-filled rhizotrons, to different soil water potentials (SWP) comparing a modern cultivar (cv. Scarlett) with a wild accession ICB181243 from Pakistan. Water potentials were quantified in soils with different relative water contents. Root anatomy was studied using histochemistry and microscopy. Suberin and lignin amounts were quantified by analytical chemistry. Transcriptomic changes were observed by RNA-sequencing. Compared with control with decreasing SWP, total root length decreased, the onset of endodermal suberization occurred much closer towards the root tips, amounts of suberin and lignin increased, and corresponding biosynthesis genes were upregulated in response to decreasing SWP. We conclude that decreasing water potentials enhanced root suberization and lignification, like osmotic stress experiments in hydroponic cultivation. However, in soil endodermal cell suberization was initiated very close towards the root tip, and root length as well as suberin amounts were about twofold higher compared with hydroponic cultivation.

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.

PpyMYB144 transcriptionally regulates pear fruit skin russeting by activating the cytochrome P450 gene PpyCYP86B1

PpyMYB144 directly activates the promoter of PpyCYP86B1, promotes the synthesis of α, ω-diacids, and involves in pear fruit skin russeting. Russeting is an economically important surface disorder in pear (Pyrus pyrifolia) fruit. Previous research has demonstrated that suberin is the pivotal chemical component contributing to pear fruit skin russeting, and fruit bagging treatment effectively reduces the amount of suberin of fruits, and thereby reduces the russeting phenotype. However, the mechanisms of pear fruit skin russeting remain largely unclear, particularly the transcriptional regulation. Here, we dissected suberin concentration and composition of pear fruits along fruit development and confirmed that α, ω-diacids are the predominant constituents in russeted pear fruit skins. Two cytochrome P450 monooxygenase (CYP) family genes (PpyCYP86A1 and PpyCYP86B1) and nine MYB genes were isolated from pear fruit. Expressions of PpyCYP86A1, PpyCYP86B1, and five MYB genes (PpyMYB34, PpyMYB138, PpyMYB138-like, PpyMYB139, and PpyMYB144) were up-regulated during fruit russeting and showed significant correlations with the changes of α, ω-diacids. In addition, dual-luciferase assays indicated that PpyMYB144 could trans-activate the promoter of PpyCYP86B1, and the activation was abolished by motif mutagenesis of AC element on the PpyCYP86B1 promoter. Further, Agrobacterium-mediated transient expression of PpyCYP86B1 and PpyMYB144 in pear fruits induced the deposition of aliphatic suberin. Thus, PpyMYB144 is a novel direct activator of PpyCYP86B1 and contributes to pear fruit skin russeting.

Low temperature reduces potato wound formation by inhibiting phenylpropanoid metabolism and fatty acid biosynthesis.

Potato tubers have the healing capacity to prevent surface water transpiration and pathogen invasion after mechanical damage. Previous research has shown the inability to form healing periderm in potatoes under low temperatures, but the potential mechanism is still unclear. To explore the effects and mechanisms of low-temperature potato healing, wounded potatoes were stored at low temperature (4°C) and room temperature (22°C), respectively. In this study, compared with 22°C healing, low temperature reduced the content of hydrogen peroxide, and the down-regulation of StAMY23 inhibited the conversion of starch to sugar, alleviated the degradation of starch, and reduced the content of soluble sugars and sucrose. Meanwhile, inhibition of phenylalanine metabolism by suppression of StPAL1 and St4CL expression reduced lignin accumulation. Low temperature also down-regulated the expression of StKCS6, StFAOH, StGPAT5, and StPrx, causing the lower deposition amount of suberin in wounds of potato tubers. The above results suggested that low temperature led to less wound tissue deposition at the wound surfaces via suppressing phenylpropanoid metabolism and fatty acid biosynthesis in potato tubers.

Comparing anatomy, chemical composition, and water permeability of suberized organs in five plant species: wax makes the difference

Main conclusionThe efficiency of suberized plant/environment interfaces as transpiration barriers is not established by the suberin polymer but by the wax molecules sorbed to the suberin polymer.Suberized cell walls formed as barriers at the plant/soil or plant/atmosphere interface in various plant organs (soil-grown roots, aerial roots, tubers, and bark) were enzymatically isolated from five different plant species (Clivia miniata, Monstera deliciosa, Solanum tuberosum, Manihot esculenta, and Malus domestica). Anatomy, chemical composition and efficiency as transpiration barriers (water loss in m s−1) of the different suberized cell wall samples were quantified. Results clearly indicated that there was no correlation between barrier properties of the suberized interfaces and the number of suberized cell layers, the amount of soluble wax and the amounts of suberin. Suberized interfaces of C. miniata roots, M. esculenta roots, and M. domestica bark periderms formed poor or hardly any transpiration barrier. Permeances varying between 1.1 and 5.1 × 10−8 m s−1 were very close to the permeance of water (7.4 × 10−8 m s−1) evaporating from a water/atmosphere interface. Suberized interfaces of aerial roots of M. deliciosa and tubers of S. tuberosum formed reasonable transpiration barriers with permeances varying between 7.4 × 10−10 and 4.2 × 10−9 m s−1, which were similar to the upper range of permeances measured with isolated cuticles (about 10−9 m s−1). Upon wax extraction, permeances of M. deliciosa and S. tuberosum increased nearly tenfold, which proves the importance of wax establishing a transpiration barrier. Finally, highly opposite results obtained with M. esculenta and S. tuberosum periderms are discussed in relation to their agronomical importance for postharvest losses and tuber storage.

Pinus radiata bark sequentially processed using scCO2 and an ionic liquid catalyst yields plentiful resin acids and alkanoic acids enriched suberin

The bark of Pinus radiata is an under-utilized forest residue that is renewable, abundant and has the potential to become a source of sustainable high-value chemicals. However, the use of this bark within a biorefinery for advanced applications is hindered by its intractable characteristics: high integrity, complex composition, and high heterogeneity. Most of the bark is burnt to provide energy and heat. The bark contains a high portion of phenolic extractives, constituting a potential source of valuable compounds. It also contains the heteropolymer suberin, a source of unique building blocks for developing innovative materials with potential broad bactericidal properties. Removal of phenolic extractives and suberin from bark simplifies down-streaming pulping processing of bark’s lignocellulosic part. Herein, we describe an effective green strategy to sequentially extract the lipophilic bark constituents and suberin, exploring scCO2 (40, 50 or 60 °C / 200, 350 or 500 bar) and a biocompatible ionic liquid catalyst. The obtained scCO2 extracts had similar diversity of lipophilic compounds and predominantly contained resin acids. Further extraction of the scCO2 extracted bark yielded suberin amounts of 2.25% wt. The bark’s suberin structure shows archetypal chemical features yet has an idiosyncratic high abundance of alkanoic acids, which is not common in most sources. The findings of this opening bark biorefinery study deserve further development and complementary techno-economic analyses to secure new value chains for the bark's major lipophilic compounds consisting of resin acids and bark suberin.

Valorization of Cork Using Subcritical Water.

Granulated cork was submitted to subcritical water extraction/hydrolysis in a semi-continuous reactor at temperatures in the range of 120–200 °C and with a constant pressure of 100 bar. The influence of temperature on the composition of the cork extracts obtained was assessed—namely, their content of carbohydrates and phenolics. The extraction yield increased with the temperature, and this was associated with the decrease in the dielectric constant of water and the increase in its ionic product. Extracts composed of up to 36% phenolics were obtained at temperatures of up to 120 °C, with an antioxidant activity only two times lower than that of pure gallic acid, but in low amounts. Assays at higher temperatures generated extracts richer in carbohydrates and with a phenolics content of ca. 20 wt.% in comparatively far higher amounts. Neither the amount of suberin nor its structure were affected by the subcritical water treatment.

Insights on the Structural and Metabolic Resistance of Potato (Solanum tuberosum) Cultivars to Tuber Black Dot (Colletotrichum coccodes).

Black dot is a blemish disease of potato tubers caused by the phytopathogenic fungus Colletotrichum coccodes. Qualitative resistance (monogenic) that leads to the hypersensitive response has not been reported against black dot, but commercial potato cultivars show different susceptibility levels to the disease, indicating that quantitative resistance (polygenic) mechanisms against this pathogen exist. Cytological studies are essential to decipher pathogen colonization of the plant tissue, and untargeted metabolomics has been shown effective in highlighting resistance-related metabolites in quantitative resistance. In this study, we used five commercial potato cultivars with different susceptibility levels to black dot, and studied the structural and biochemical aspects that correlate with resistance to black dot using cytological and untargeted metabolomics methods. The cytological approach using semithin sections of potato tuber periderm revealed that C. coccodes colonizes the tuber periderm, but does not penetrate in cortical cells. Furthermore, skin thickness did not correlate with disease susceptibility, indicating that other factors influence quantitative resistance to black dot. Furthermore, suberin amounts did not correlate with black dot severity, and suberin composition was similar between the five potato cultivars studied. On the other hand, the untargeted metabolomics approach allowed highlighting biomarkers of infection, as well as constitutive and induced resistance-related metabolites. Hydroxycinnamic acids, hydroxycinnamic acid amides and steroidal saponins were found to be biomarkers of resistance under control conditions, while hydroxycoumarins were found to be specifically induced in the resistant cultivars. Notably, some of these biomarkers showed antifungal activity in vitro against C. coccodes. Altogether, our results show that quantitative resistance of potatoes to black dot involves structural and biochemical mechanisms, including the production of specialized metabolites with antifungal properties.

Osmotic stress enhances suberization of apoplastic barriers in barley seminal roots: analysis of chemical, transcriptomic and physiological responses.

Summary Barley (Hordeum vulgare) is more drought tolerant than other cereals, thus making it an excellent model for the study of the chemical, transcriptomic and physiological effects of water deficit. Roots are the first organ to sense soil water deficit. Therefore, we studied the response of barley seminal roots to different water potentials induced by polyethylene glycol (PEG) 8000.We investigated changes in anatomical parameters by histochemistry and microscopy, quantitative and qualitative changes in suberin composition by analytical chemistry, transcript changes by RNA‐sequencing (RNA‐Seq), and the radial water and solute movement of roots using a root pressure probe.In response to osmotic stress, genes in the suberin biosynthesis pathway were upregulated that correlated with increased suberin amounts in the endodermis and an overall reduction in hydraulic conductivity (Lpr). In parallel, transcriptomic data indicated no or only weak effects of osmotic stress on aquaporin expression.These results indicate that osmotic stress enhances cell wall suberization and markedly reduces Lpr of the apoplastic pathway, whereas Lpr of the cell‐to‐cell pathway is not altered. Thus, the sealed apoplast markedly reduces the uncontrolled backflow of water from the root to the medium, whilst keeping constant water flow through the highly regulated cell‐to‐cell path.

Suberized transport barriers in Arabidopsis, barley and rice roots: From the model plant to crop species

Water is the most important prerequisite for life and plays a major role during uptake and transport of nutrients. Roots are the plant organs that take up the major part of water, from the surrounding soil. Water uptake is related to the root system architecture, root growth, age and species dependent complex developmental changes in the anatomical structures. The latter is mainly attributed to the deposition of suberized barriers in certain layers of cell walls, such as endo- and exodermis. With respect to water permeability, changes in the suberization of roots are most relevant. Water transport or hydraulic conductivity of roots (Lpr) can be described by the composite transport model and is known to be very variable between plant species and growth conditions and root developmental states. In this review, we summarize how anatomical structures and apoplastic barriers of roots can diversely affect water transport, comparing the model plant Arabidopsis with crop plants, such as barley and rice. Results comparing the suberin amounts and water transport properties indicate that the common assumption that suberin amount negatively correlates with water and solute transport through roots may not always be true. The composition, microstructure and localization of suberin may also have a great impact on the formation of efficient barriers to water and solutes.

The composite water and solute transport of barley (Hordeum vulgare) roots: effect of suberized barriers.

Background and Aims Roots have complex anatomical structures, and certain localized cell layers develop suberized apoplastic barriers. The size and tightness of these barriers depend on the growth conditions and on the age of the root. Such complex anatomical structures result in a composite water and solute transport in roots. Methods Development of apoplastic barriers along barley seminal roots was detected using various staining methods, and the suberin amounts in the apical and basal zones were analysed using gas chromatography–mass spectometry (GC-MS). The hydraulic conductivity of roots (Lpr) and of cortical cells (Lpc) was measured using root and cell pressure probes. Key Results When grown in hydroponics, barley roots did not form an exodermis, even at their basal zones. However, they developed an endodermis. Endodermal Casparian bands first appeared as ‘dots’ as early as at 20 mm from the apex, whereas a patchy suberin lamellae appeared at 60 mm. The endodermal suberin accounted for the total suberin of the roots. The absolute amount in the basal zone was significantly higher than in the apical zone, which was inversely proportional to the Lpr. Comparison of Lpr and Lpc suggested that cell to cell pathways dominate for water transport in roots. However, the calculation of Lpr from Lpc showed that at least 26 % of water transport occurs through the apoplast. Roots had different solute permeabilities (Psr) and reflection coefficients (σsr) for the solutes used. The σsr was below unity for the solutes, which have virtually zero permeability for semi-permeable membranes. Conclusions Suberized endodermis significantly reduces Lpr of seminal roots. The water and solute transport across barley roots is composite in nature and they do not behave like ideal osmometers. The composite transport model should be extended by adding components arranged in series (cortex, endodermis) in addition to the currently included components arranged in parallel (apoplastic, cell to cell pathways).

Environmental and physiological effects on grouping of drought-tolerant and susceptible rice varieties related to rice (Oryza sativa) root hydraulics under drought.

Background and Aims Root hydraulic limitations (i.e. intra-plant restrictions to water movement) may be related to crop performance under drought, and groupings in the hydraulic function of drought-tolerant and drought-susceptible rice (Oryza sativa) varieties have been previously reported. This study aimed to better understand the environmental and physiological relationships with rice root hydraulics under drought. Methods Xylem sap bleeding rates in the field (gsap g-1shoot) were measured on seasonal and diurnal time frames, during which time environmental conditions were monitored and physiological measurements were conducted. Complementary experiments on the effects of vapour pressure deficit (VPD) on root hydraulic conductivity and on transpiration rates of de-rooted tillers were conducted in growth chambers. Key Results The diurnal effects on bleeding rate were more closely related to irradiance than VPD, and VPD effects on root hydraulic conductivity measured on 21-day-old plants were due to effects on plant growth including root surface area, maximum root depth and root:shoot ratio. Leaf osmotic potential was related to the grouping of drought-tolerant and drought-susceptible varieties in rice root hydraulics, and these groupings were independent of differences in phenology. Low single-tiller bleeding rates were observed under high evapo-transpirational demand, higher bleeding rates were observed at more negative leaf osmotic potentials in drought-susceptible varieties, and drought-tolerant and susceptible varieties differed in the VPD-induced increase in transpiration rates of de-rooted tillers. Low root suberin amounts in some of the drought-susceptible varieties may have resulted in higher ion transport, as evidenced by higher sap K+ concentration and higher bleeding rates in those varieties. Conclusions These results provide evidence of the environmental effects on shoots that can influence root hydraulics. The consistent groupings of drought-tolerant and susceptible varieties suggest that traits affecting plant osmotic status may regulate root hydraulic response to drought in rice.

Fractioning and chemical characterization of barks of Betula pendula and Eucalyptus globulus

The composition of birch (Betula pendula Roth.) and eucalypt (Eucalyptus globulus Labill.) barks was studied after grinding and fractioning into different particles sizes.There was a significant difference in the fractionation of both barks in relation to the yield of fines (5.9% and 28.3% of particles under 0.450 for birch and eucalypt, respectively) and of coarser particles over 2mm (70.7% and 41.4%).The chemical composition of birch and eucalypt barks, as a mass weighed average of all granulometric fractions was, respectively: ash 2.9% and 12.1%; total extractives 17.6% and 6.5% (hydrophilic extractives were dominant), lignin 27.9% and 28.8% and holocellulose 49.8% and 62.6%. Birch bark contained a considerable amount of suberin (5.9%) whereas eucalypt bark contained a very small amount (<1%). The carbohydrate composition differed between birch and eucalypt barks, i.e., respectively, glucose 47.0% and 68.4%, and xylose 33.8% and 23.2% of total neutral monosaccharides.Ash elemental composition was different in both species. Birch bark contained in relation to eucalypt bark, in the 0.250–0.450mm fraction, more N (0.69% vs. 0.26%) and P (0.075% vs. 0.001%), and less Ca (0.39% vs. 0.62%), K (0.24% vs. 0.31%) and Mg (0.07% vs. 0.15%). High concentration of Zn was found in birch bark (217mg/kg vs. 11mg/kg in eucalypt bark).After grinding and granulometric separation, extractives were present preferentially in the finest fraction with an enrichment in dichloromethane and ethanol solubles especially in the case of birch bark. Eucalypt bark had a high content of cellulose and hemicelluloses especially in the coarser fraction. The fibrous character of this fraction shows its potential as a fiber source.

Stagnant deoxygenated growth enhances root suberization and lignifications, but differentially affects water and NaCl permeabilities in rice (Oryza sativa L.) roots

It has been shown that rice roots grown in a stagnant medium develop a tight barrier to radial oxygen loss (ROL), whereas aerated roots do not. This study investigated whether the induction of a barrier to ROL affects water and solute permeabilities. Growth in stagnant medium markedly reduced the root growth rate relative to aerated conditions. Histochemical studies revealed an early deposition of Casparian bands (CBs) and suberin lamellae (SL) in both the endodermis (EN) and exodermis, and accelerated lignification of stagnant roots. The absolute amounts of suberin, lignin and esterified aromatics (coumaric and ferulic acid) in these barriers were significantly higher in stagnant roots. However, correlative permeability studies revealed that early deposition of barriers in stagnant roots failed to reduce hydraulic conductivity (Lp(r) ) below those of aerated roots. In contrast to Lp(r) , the NaCl permeability (P(sr) ) of stagnant roots was markedly lower than that of aerated roots, as indicated by an increased reflection coefficient (σ(sr) ). In stagnant roots, P(sr) decreased by 60%, while σ(sr) increased by 55%. The stagnant medium differentially affected the Lp(r) and P(sr) of roots, which can be explained in terms of the physical properties of the molecules used and the size of the pores in the apoplast.

Water and solute permeabilities of Arabidopsis roots in relation to the amount and composition of aliphatic suberin

Although it is implied that suberized apoplastic barriers of roots negatively correlate with water and solute permeabilities, direct transport measurements across roots with altered amounts and compositions of aliphatic suberin are scarce. In the present study, hydroponically grown Arabidopsis wild types (Col8 and Col0) and different suberin mutants with altered amounts and/or compositions (horst, esb1-1, and esb1-2) were used to test this hypothesis. Detailed histochemical studies revealed late development of Casparian bands and suberin lamellae in the horst mutant compared with wild types and esb mutants. Suberin analysis with gas chromatography and mass spectrometry (GC-MS) showed that the horst mutant had ∼33% lower amounts of aliphatic monomers than Col8 and Col0. In contrast, enhanced suberin mutants (esb1-1 and esb1-2) had twice the amount of suberin as the wild types. Correlative permeability measurements, which were carried out for the first time with a root pressure probe for Arabidopsis, revealed that the hydraulic conductivity (Lpr) and NaCl permeability (Psr) of the whole root system of the horst mutant were markedly greater than in the respective wild types. This was reflected by the total amounts of aliphatic suberin determined in the roots. However, increased levels of aliphatic suberin in esb mutants failed to reduce either water or NaCl permeabilities below those of the wild types. It was concluded that the simple view and the conventional assumption that the amount of root suberin negatively correlates with permeability may not always be true. The aliphatic monomer arrangement in the suberin biopolymer and its microstructure also play a role in apoplastic barrier formation.

Comparison of Solid‐State Carbon‐13 Nuclear Magnetic Resonance and Organic Matter Biomarkers for Assessing Soil Organic Matter Degradation

Soil organic matter (SOM) is a heterogeneous mixture of compounds that are derived from a number of sources. Therefore, trying to assess the stage of SOM alteration is challenging. Recently, our research has proposed a number of degradation parameters based on the measurement of SOM biomarkers. These biomarkers, which can be attributed to a specific organism source and the stage of alteration (oxidation) of a compound can provide information regarding the stage and rate of SOM degradation. The previously developed biomarker ratios for assessing SOM degradation have not been tested with degradation parameters derived from other molecular methods. This study investigates the use of solid‐state 13C nuclear magnetic resonance (NMR) and organic matter biomarker methods for the assessment of SOM degradation in four soils from Western Canada. Total solvent extraction was used to isolate free lipids, base hydrolysis was used to extract bound lipids, and copper (II) oxide (CuO) oxidation was used to liberate lignin monomers and dimers from the soil. The presence of plant sterols and their oxidative degradation products facilitated the analysis of SOM alteration. Cutin and suberin degradation parameters were applied to study the relative amounts of suberin versus cutin inputs into soil as well as the stage of cutin and suberin degradation. The extent of oxidation for lignin‐derived compounds was also tabulated. The trends observed by solid‐state 13C NMR (alkyl/O‐alkyl ratios) were consistent with those found with the biomarker techniques, however, it should be noted that the biomarker methods provide detailed information on specific compounds whereas solid‐state 13C NMR only provides information on the “bulk” structure of SOM. Because only a small portion of SOM components are amenable to analysis by biomarker methods, it is advantageous to use these methods in tandem with solid‐state 13C NMR spectroscopy and other molecular techniques when investigating SOM sources and biogeochemistry.

The chemical composition of suberin in apoplastic barriers affects radial hydraulic conductivity differently in the roots of rice (Oryza sativa L. cv. IR64) and corn (Zea mays L. cv. Helix)

Apoplastic transport barriers in the roots of rice (Oryza sativa L. cv. IR64) and corn (Zea mays L. cv. Helix) were isolated enzymatically. Following chemical degradation (monomerization, derivatization), the amounts of aliphatic and aromatic suberin monomers were analysed quantitatively by gas chromatography and mass spectrometry. In corn, suberin was determined for isolated endodermal (ECW) and rhizo-hypodermal (RHCW) cell walls. In rice, the strong lignification of the central cylinder (CC), did not allow the isolation of endodermal cell walls. Similarly, exodermal walls could not be separated from the rhizodermal and sclerenchyma cell layers. Suberin analyses of ECW and RHCW of rice, thus, refer to either the entire CC or to the entire outer part of the root (OPR), the latter lacking the inner cortical cell layer. In both species, aromatic suberin was mainly composed of coumaric and ferulic acids. Aliphatic suberin monomers released from rice and corn belonged to five substance classes: primary fatty acids, primary alcohols, diacids, omega-hydroxy fatty acids, and 2-hydroxy fatty acids, with omega-hydroxy fatty acids being the most prominent substance class. Qualitative composition of aliphatic suberin of rice was different from that of corn; (i) it was much less diverse, and (ii) besides monomers with chain lengths of C(16), a second maximum of C(28) was evident. In corn, C(24) monomers represented the most prominent class of chain lengths. When suberin quantities were related to surface areas of the respective tissues of interest (hypodermis and/or exodermis and endodermis), exodermal cell walls of rice contained, on average, six-times more aliphatic suberin than those of corn. In endodermal cell walls, amounts were 34 times greater in rice than in corn. Significantly higher amounts of suberin detected in the apoplastic barriers of rice corresponded with a substantially lower root hydraulic conductivity (Lp(r)) compared with corn, when water flow was driven by hydrostatic pressure gradients across the apoplast. As the OPR of rice is highly porous and permeable to water, it is argued that this holds true only for the endodermis. The results imply that some caution is required when discussing the role of suberin in terms of an efficient transport barrier for water. The simple view that only the quantity of suberin present is important, may not hold. A more detailed consideration of both the chemical nature of suberins and of the microstructure of deposits is required, i.e. how suberins impregnate wall pores.

Effects of NO 3 deficiency and NaCl stress on suberin deposition in rhizo- and hypodermal (RHCW) and endodermal cell walls (ECW) of castor bean (Ricinus communis L.) roots

Castor bean (Ricinus communis L.) plants were hydroponically cultivated to achieve NO3 deficiency (N starvation), salt stress (addition of 100 mM NaCl), or normal conditions. Endodermal (ECW) and rhizodermal and hypodermal cell walls (RHCW) were isolated enzymatically from roots, and suberin monomers were released by transesterification after solvent extraction. Aromatic and aliphatic suberin monomers were identified and quantified by gas chromatography and mass spectrometry. Between 90 and 95% of the released suberin monomers were linear, long-chain, aliphatic compounds (alcohols, acids, diacids, ω-hydroxy acids and 2-hydroxy acids) with an average chain length of 19 C-atoms. The remainder was an aromatic suberin fraction mainly composed of coumaric and ferulic acid. Suberin amounts were significantly increased in ECW and RHCW in the presence of NaCl. In contrast, N starvation led to significantly reduced levels of suberization in ECW and RHCW. It is concluded that R. communis plants reinforce their apoplastic transport barriers in roots in adaptation to NaCl stress in order to minimize NaCl uptake. Under conditions of N starvation the opposite occurs and plants reduce the suberization of their apoplastic transport barriers to facilitate nutrient uptake form the soil.

Wax and suberin development of native and wound periderm of potato (Solanum tuberosum L.) and its relation to peridermal transpiration.

Native and wound periderm was isolated enzymatically from potato (Solanum tuberosum L. cv. Desirée) tubers at different time intervals between 0 days up to 4 weeks after harvesting. Wound periderm formation was induced by carefully removing native periderm from freshly harvested tubers before storage. The chemical composition of lipids (waxes) obtained by chloroform extraction, as well as the monomeric composition of native and wound suberin polymer after transesterification by boron trifluoride/methanol, was analyzed using gas chromatography and mass spectrometry. Both types of periderm contained up to 20% extractable lipids. Besides linear long-chain aliphatic wax compounds, alkyl ferulates were detected as significant constituents. In wound periderm they amounted to more than 60% of the total extracts. Within 1 month of storage, suberin amounts in the polymer increased 2-fold in native periderm (180 microg cm(-2)), whereas in wound periderm about 75.0 microg cm(-2) suberin polymer was newly synthesized. Native potato tuber periderm developed a very efficient transport barrier for water with permeances decreasing from 6.4 x 10(-10) m s(-1) to 5.5 x 10(-11) m s(-1) within 1 month of storage. However, the water permeability of wound periderm was on average 100 times higher with permeances decreasing from 4.7 x 10(-8) m s(-1) after 3 days to only 5.4 x 10(-9) m s(-1) after 1 month of storage, although suberin and wax amounts in wound periderm amounted to about 60% of native periderm. From this result it must be concluded that the occurrence of suberin with wax depositions in cell walls does not necessarily allow us to conclude that these cell walls must be nearly perfect barriers to water transport. In addition to the occurrence of the lipophilic biopolymer suberin and associated waxes, the still unknown molecular arrangement and precisely localized deposition of suberin within the cell wall must contribute to the efficiency of suberin as a barrier to water transport.

Chemical composition of apoplastic transport barriers in relation to radial hydraulic conductivity of corn roots (Zea mays L.).

The hydraulic conductivity of roots (Lp(r)) of 6- to 8-d-old maize seedlings has been related to the chemical composition of apoplastic transport barriers in the endodermis and hypodermis (exodermis), and to the hydraulic conductivity of root cortical cells. Roots were cultivated in two different ways. When grown in aeroponic culture, they developed an exodermis (Casparian band in the hypodermal layer), which was missing in roots from hydroponics. The development of Casparian bands and suberin lamellae was observed by staining with berberin-aniline-blue and Sudan-III. The compositions of suberin and lignin were analyzed quantitatively and qualitatively after depolymerization (BF(3)/methanol-transesterification, thioacidolysis) using gas chromatography/mass spectrometry. Root Lp(r) was measured using the root pressure probe, and the hydraulic conductivity of cortical cells (Lp) using the cell pressure probe. Roots from the two cultivation methods differed significantly in (i) the Lp(r) evaluated from hydrostatic relaxations (factor of 1.5), and (ii) the amounts of lignin and aliphatic suberin in the hypodermal layer of the apical root zone. Aliphatic suberin is thought to be the major reason for the hydrophobic properties of apoplastic barriers and for their relatively low permeability to water. No differences were found in the amounts of suberin in the hypodermal layers of basal root zones and in the endodermal layer. In order to verify that changes in root Lp(r) were not caused by changes in hydraulic conductivity at the membrane level, cell Lp was measured as well. No differences were found in the Lp values of cells from roots cultivated by the two different methods. It was concluded that changes in the hydraulic conductivity of the apoplastic rather than of the cell-to-cell path were causing the observed changes in root Lp(r).