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

From rock to living systems: Lanthanides toxicity and biological interactions

Since the discovery of lanthanides, the expanding range of applications and the growing demand for lanthanides in different aspects of life have escalated their dispersion in the environment, raising concerns about their impact on the living world. This review explores the interaction between lanthanides and different groups of living organisms (bacteria, algae, lichens, plants, invertebrates, and low vertebrates), reflecting the current state of scientific knowledge. We have aimed to provide a comprehensive overview of relevant studies, highlight existing gaps, and suggest potential areas for future research to enhance the understanding of this topic.

Ethylene-mediated root endodermal barrier development in impeding Cd radial transport and accumulation in rice (Oryza sativa L.)

Ethylene plays crucial roles in the adaptation to cadmium (Cd) stress. Nevertheless, the impact of endogenous ethylene on radial transport of Cd in different rice cultivars are insufficiently understood. Herein, we investigated how ethylene involved in the formation of endodermal barriers in roots of Nipponbare with low-Cd accumulation and IR32307 with high-Cd accumulation ability and further assessed its influence on Cd radial transport. Our analysis indicated that both Cd stress and external ACC (1-aminocyclopropane-1-carboxylic acid, ethylene biosynthesis precursor) promoted the ethylene production. Intriguingly, the positive response of ethylene signal to Cd was more intensive in roots of Nipponbare than that of IR32307. The increased endogenous ethylene in rice roots promoted development of casparian strips (CSs) and suberin lamellae (SL). Specifically, external addition of ACC decreased the percentage of the DTIP-CS/DTIP-SL to root length by 44.4-79.6%/49.3-11.4% in Nipponbare and 18.7-19.9%/10.7-35.3% in IR32307, individually. The intrinsic molecular mechanism was mainly due to changes in the genes expression levels related to CSs/SL biosynthesis. Simultaneously, the analyses of apoplastic tracer (Propidium Iodide, PI) and cell-to-cell tracer (Fluorescein Diacetate, FDA) confirmed that the ethylene-mediated endodermal barriers were functional, which were in accordance with the increased/reduced Cd transport in roots. Eventually, the results of transcriptome analysis further shed a comprehensive insight that ethylene constructed the endodermal barrier through phenylpropanoid and cutin, suberine and wax biosynthesis to reduce Cd radial transport in rice, which are beneficial for the breeding of rice with low-Cd accumulating capacity in the future.

Salt tolerance in mungbean is associated with controlling Na and Cl transport across roots, regulating Na and Cl accumulation in chloroplasts and maintaining high K in root and leaf mesophyll cells

Salinity tolerance requires coordinated responses encompassing salt exclusion in roots and tissue/cellular compartmentation of salt in leaves. We investigated the possible control points for salt ions transport in roots and tissue tolerance to Na+ and Cl- in leaves of two contrasting mungbean genotypes, salt-tolerant Jade AU and salt-sensitive BARI Mung-6, grown in nonsaline and saline (75 mM NaCl) soil. Cryo-SEM X-ray microanalysis was used to determine concentrations of Na, Cl, K, Ca, Mg, P, and S in various cell types in roots related to the development of apoplastic barriers, and in leaves related to photosynthetic performance. Jade AU exhibited superior salt exclusion by accumulating higher [Na] in the inner cortex, endodermis, and pericycle with reduced [Na] in xylem vessels and accumulating [Cl] in cortical cell vacuoles compared to BARI Mung-6. Jade AU maintained higher [K] in root cells than BARI Mung-6. In leaves, Jade AU maintained lower [Na] and [Cl] in chloroplasts and preferentially accumulated [K] in mesophyll cells than BARI Mung-6, resulting in higher photosynthetic efficiency. Salinity tolerance in Jade AU was associated with shoot Na and Cl exclusion, effective regulation of Na and Cl accumulation in chloroplasts, and maintenance of high K in root and leaf mesophyll cells.

Metabolic adaptations leading to an enhanced lignification in wheat roots under salinity stress

Analysis of salinity tolerance processes in wheat has focused on salt exclusion from shoots while root phenotypes have received limited attention. Here, we consider the varying phenotypic response of four bread wheat varieties that differ in their type and degree of salt tolerance and assess their molecular responses to salinity and changes in root cell wall lignification. These varieties were Westonia introgressed with Nax1 and Nax2 root sodium transporters (HKT1;4-A and HKT1;5-A) that reduce Na+ accumulation in leaves, as well as the 'tissue tolerant' Portuguese landrace Mocho de Espiga Branca that has a mutation in the homologous gene HKT1;5-D and has high Na+ concentration in leaves. These three varieties were compared with the relatively more salt-sensitive cultivar Gladius. Through the use of root histochemical analysis, ion concentrations, as well as differential proteomics and targeted metabolomics, we provide an integrated view of the wheat root response to salinity. We show different metabolic re-arrangements in energy conversion, primary metabolic machinery and phenylpropanoid pathway leading to monolignol production in a genotype and genotype by treatment-dependent manner that alters the extent and localisation of root lignification which correlated with an improved capacity of wheat roots to cope better under salinity stress.

Physiological characterization of the tomato cutin mutant cd1 under salinity and nitrogen stress

MAIN CONCLUSION

We identified tomato leaf cuticle and root suberin monomers that play a role in the response to nitrogen deficiency and salinity stress and discuss their potential agronomic value for breeding. The plant cuticle plays a key role in plant-water relations, and cuticle's agronomic value in plant breeding programs is currently under investigation. In this study, the tomato cutin mutant cd1, with altered fruit cuticle, was physiologically characterized under two nitrogen treatments and three salinity levels. We evaluated leaf wax and cutin load and composition, root suberin, stomatal conductance, photosynthetic rate, partial factor productivity from applied N, flower and fruit number, fruit size and cuticular transpiration, and shoot and root biomass. Both nitrogen and salinity treatments altered leaf cuticle and root suberin composition, regardless of genotype (cd1 or M82). Compared with M82, the cd1 mutant showed lower shoot biomass and reduced partial factor productivity from applied N under all treatments. Under N depletion, cd1 showed altered leaf wax composition, but was comparable to the WT under sufficient N. Under salt treatment, cd1 showed an increase in leaf wax and cutin monomers. Root suberin content of cd1 was lower than M82 under control conditions but comparable under higher salinity levels. The tomato mutant cd1 had a higher fruit cuticular transpiration rate, and lower fruit surface area compared to M82. These results show that the cd1 mutation has complex effects on plant physiology, and growth and development beyond cutin deficiency, and offer new insights on the potential agronomic value of leaf cuticle and root suberin for tomato breeding.

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.

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

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

Primary, seminal and lateral roots of maize show type-specific growth and hydraulic responses to water deficit

The water uptake capacity of a root system is determined by its architecture and hydraulic properties, which together shape the root hydraulic architecture. Here, we investigated root responses to water deficit (WD) in seedlings of a maize (Zea mays) hybrid line (B73H) grown in hydroponic conditions, taking into account the primary root (PR), the seminal roots (SR), and their respective lateral roots. WD was induced by various polyethylene glycol concentrations and resulted in dose-dependent inhibitions of axial and lateral root growth, lateral root formation, and hydraulic conductivity (Lpr), with slightly distinct sensitivities to WD between PR and SR. Inhibition of Lpr by WD showed a half-time of 5 to 6 min and was fully (SR) or partially (PR) reversible within 40 min. In the two root types, WD resulted in reduced aquaporin expression and activity, as monitored by mRNA abundance of 13 plasma membrane intrinsic protein (ZmPIP) isoforms and inhibition of Lpr by sodium azide, respectively. An enhanced suberization/lignification of the epi- and exodermis was observed under WD in axial roots and in lateral roots of the PR but not in those of SR. Inverse modeling revealed a steep increase in axial conductance in root tips of PR and SR grown under WD that may be due to the decreased growth rate of axial roots in these conditions. Overall, our work reveals that these root types show quantitative differences in their anatomical, architectural, and hydraulic responses to WD, in terms of sensitivity, amplitude and reversibility. This distinct functionalization may contribute to integrative acclimation responses of whole root systems to soil WD.

Soil Water Deficit Reduced Root Hydraulic Conductivity of Common Reed (Phragmites australis)

Alterations in root hydraulics in response to varying moisture conditions remain a subject of debate. In our investigation, we subjected common reeds (Phragmites australis) to a 45-day treatment with four distinct soil moisture levels. The findings unveiled that, in response to drought stress, the total root length, surface area, volume, and average diameter exhibited varying degrees of reduction. Anatomically, drought caused a reduction in root diameter (RD), cortex thickness (CT), vessel diameter (VD), and root cross-sectional area (RCA). A decrease in soil moisture significantly reduced both whole- and single-root hydraulic conductivity (Lpwr, Lpsr). The total length, surface area, volume, and average diameter of the reed root system were significantly correlated with Lpwr, while RD, CT, and RCA were significantly correlated with Lpsr. A decrease in soil moisture content significantly influenced root morphological and anatomical characteristics, which, in turn, altered Lpr, and the transcriptome results suggest that this may be associated with the variation in the expression of abscisic acid (ABA) and aquaporins (AQPs) genes. Our initial findings address a gap in our understanding of reed hydraulics, offering fresh theoretical insights into how herbaceous plants respond to external stressors.

Salt Stress-Regulation of Root Water Uptake in a Whole-Plant and Diurnal Context

This review focuses on the regulation of root water uptake in plants which are exposed to salt stress. Root water uptake is not considered in isolation but is viewed in the context of other potential tolerance mechanisms of plants-tolerance mechanisms which relate to water relations and gas exchange. Plants spend between one third and half of their lives in the dark, and salt stress does not stop with sunset, nor does it start with sunrise. Surprisingly, how plants deal with salt stress during the dark has received hardly any attention, yet any growth response to salt stress over days, weeks, months and years is the integrative result of how plants perform during numerous, consecutive day/night cycles. As we will show, dealing with salt stress during the night is a prerequisite to coping with salt stress during the day. We hope to highlight with this review not so much what we know, but what we do not know; and this relates often to some rather basic questions.

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.

Cutinized and suberized barriers in leaves and roots: Similarities and differences

Anatomical, histochemical, chemical, and biosynthetic similarities and differences of cutinized and suberized plant cell walls are presented and reviewed in brief. Based on this, the functional properties of cutinized and suberized plant cell walls acting as transport barriers are compared and discussed in more detail. This is of general importance because fundamental misconceptions about relationships in plant-environment water relations are commonly encountered in the scientific literature. It will be shown here, that cuticles represent highly efficient apoplastic transport barriers significantly reducing the diffusion of water and dissolved compounds. The transport barrier of cuticles is mainly established by the deposition of cuticular waxes. Upon wax extraction, with the cutin polymer remaining, cuticular permeability for water and dissolved non-ionized and lipophilic solutes are increasing by 2-3 orders of magnitude, whereas polar and charged substances (e.g., nutrient ions) are only weakly affected (2- to 3-fold increases in permeability). Suberized apoplastic barriers without the deposition of wax are at least as permeable as the cutin polymer matrix without waxes and hardly offer any resistance to the free movement of water. Only upon the deposition of significant amounts of wax, as it is the case with suberized periderms exposed to the atmosphere, an efficient transport barrier for water can be established by suberized cell walls. Comparing the driving forces (gradients between water potentials inside leaves and roots and the surrounding environment) for water loss acting on leaves and roots, it is shown that leaves must have a genetically pre-defined highly efficient transpiration barrier fairly independent from rapidly changing environmental influences. Roots, in most conditions facing a soil environment with relative humidities very close to 100%, are orders of magnitude more permeable to water than leaf cuticles. Upon desiccation, the permanent wilting point of plants is defined as -1.5 MPa, which still corresponds to nearly 99% relative humidity in soil. Thus, the main reason for plant water stress leading to dehydration is the inability of root tissues to decrease their internal water potential to values more negative than -1.5 MPa and not the lack of a transport barrier for water in roots and leaves. Taken together, the commonly mentioned concepts that a drought-induced increase of cuticular wax or root suberin considerably strengthens the apoplastic leaf or root transport barriers and thus aids in water conservation appears highly questionable.

Induced defense strategies of plants against Ralstonia solanacearum

Plants respond to Ralstonia solanacearum infestation through two layers of immune system (PTI and ETI). This process involves the production of plant-induced resistance. Strategies for inducing resistance in plants include the formation of tyloses, gels, and callose and changes in the content of cell wall components such as cellulose, hemicellulose, pectin, lignin, and suberin in response to pathogen infestation. When R. solanacearum secrete cell wall degrading enzymes, plants also sense the status of cell wall fragments through the cell wall integrity (CWI) system, which activates deep-seated defense responses. In addition, plants also fight against R. solanacearum infestation by regulating the distribution of metabolic networks to increase the production of resistant metabolites and reduce the production of metabolites that are easily exploited by R. solanacearum. We review the strategies used by plants to induce resistance in response to R. solanacearum infestation. In particular, we highlight the importance of plant-induced physical and chemical defenses as well as cell wall defenses in the fight against R. solanacearum.

Plant root suberin: A layer of defence against biotic and abiotic stresses

Plant roots have important functions, such as acquiring nutrients and water from the surrounding soil and transporting them upwards to the shoots. Simultaneously, they must be able to exclude potentially harmful substances and prevent the entry of pathogens into the roots. The endodermis surrounds the vascular tissues and forms hydrophobic diffusion barriers including Casparian strips and suberin lamella. Suberin in cell walls can be induced by a range of environmental factors and contribute to against biotic and abiotic threats. Tremendous progress has been made in biosynthesis of suberin and its function, little is known about the effect of its plasticity and distribution on stress tolerance. In field conditions, biotic and abiotic stress can exist at the same time, and little is known about the change of suberization under that condition. This paper update the progress of research related to suberin biosynthesis and its function, and also discuss the change of suberization in plant roots and its role on biotic and abiotic stresses tolerance.

An ABA-serotonin module regulates root suberization and salinity tolerance

Suberin in roots acts as a physical barrier preventing water/mineral losses. In Arabidopsis, root suberization is regulated by abscisic acid (ABA) and ethylene in response to nutrient stresses. ABA also mediates coordination between microbiota and root endodermis in mineral nutrient homeostasis. However, it is not known whether this regulatory system is common to plants in general, and whether there are other key molecule(s) involved. We show that serotonin acts downstream of ABA in regulating suberization in rice and Arabidopsis and negatively regulates suberization in rice roots in response to salinity. We show that ABA represses transcription of the key gene (OsT5H) in serotonin biosynthesis, thus promoting root suberization in rice. Conversely, overexpression of OsT5H or supplementation with exogenous serotonin represses suberization and reduces tolerance to salt stress. These results identify an ABA-serotonin regulatory module controlling root suberization in rice and Arabidopsis, which is likely to represent a general mechanism as ABA and serotonin are ubiquitous in plants. These findings are of significant importance to breeding novel crop varieties that are resilient to abiotic stresses and developing strategies for production of suberin-rich roots to sequestrate more CO₂ , helping to mitigate the effects of climate change.

Flooding-induced rhizosphere Clostridium assemblage prevents root-to-shoot cadmium translocation in rice by promoting the formation of root apoplastic barriers

Water managements are the most effective agricultural practices for restraining cadmium (Cd) uptake and translocation in rice, which closely correlated with rhizosphere assembly of beneficial microbiome. However, the role of the assemblage of specific microbiota in controlling root-to-shoot Cd translocation in rice remains scarcely clear. The aim of this study was to ascertain how water managements shaped rhizosphere microbiome and mediated root-to-shoot Cd translocation. To disentangle the acting mechanisms of water managements, we performed an experiment monitoring Cd uptake and transport in rice and changes in soil microbial communities in response to continuously flooding and moistening irrigation. Continuously flooding changed rhizosphere microbial communities, leading to the increased abundance of anaerobic bacteria such as Clostridium populations. Weighted gene co-expression network analysis (WGCNA) showed that a dominant OTU163, corresponding to Clostridium sp. CSP1, exhibited a strong negative correlation with root-to-shoot Cd translocation. An integrated analysis of transcriptome and metabolome further indicated that the Clostridium-secreted butyric acid was involved in the regulation of phenylpropanoid pathway in rice roots. The formation of endodermal suberized barriers and lignified xylems was remarkably enhanced in the Clostridium-treated roots, which led to more Cd retained in root cell wall and less Cd in the xylem sap. Collectively, our results indicate that the development of root apoplastic barriers can be orchestrated by beneficial Clostridium strains that are assembled by host plants grown under flooding regime, thereby inhibiting root-to-shoot Cd translocation.

Natural variation in root suberization is associated with local environment in Arabidopsis thaliana

Genetic signature of climate adaptation has been widely recognized across the genome of many organisms; however, the eco-physiological basis for linking genomic polymorphisms with local adaptations remains largely unexplored. Using a panel of 218 world-wide Arabidopsis accessions, we characterized the natural variation in root suberization by quantifying 16 suberin monomers. We explored the associations between suberization traits and 126 climate variables. We conducted genome-wide association analysis and integrated previous genotype-environment association (GEA) to identify the genetic bases underlying suberization variation and their involvements in climate adaptation. Root suberin content displays extensive variation across Arabidopsis populations and significantly correlates with local moisture gradients and soil characteristics. Specifically, enhanced suberization is associated with drier environments, higher soil cation-exchange capacity, and lower soil pH; higher proportional levels of very-long-chain suberin is negatively correlated with moisture availability, lower soil gravel content, and higher soil silt fraction. We identified 94 putative causal loci and experimentally proved that GPAT6 is involved in C16 suberin biosynthesis. Highly significant associations between the putative genes and environmental variables were observed. Roots appear highly responsive to environmental heterogeneity via regulation of suberization, especially the suberin composition. The patterns of suberization-environment correlation and the suberin-related GEA fit the expectations of local adaptation for the polygenic suberization trait.

Phenotyping and modeling of root hydraulic architecture reveal critical determinants of axial water transport

Water uptake by roots is a key adaptation of plants to aerial life. Water uptake depends on root system architecture (RSA) and tissue hydraulic properties that, together, shape the root hydraulic architecture. This work investigates how the interplay between conductivities along radial (e.g. aquaporins) and axial (e.g. xylem vessels) pathways determines the water transport properties of highly branched RSAs as found in adult Arabidopsis (Arabidopsis thaliana) plants. A hydraulic model named HydroRoot was developed, based on multi-scale tree graph representations of RSAs. Root water flow was measured by the pressure chamber technique after successive cuts of a same root system from the tip toward the base. HydroRoot model inversion in corresponding RSAs allowed us to concomitantly determine radial and axial conductivities, providing evidence that the latter is often overestimated by classical evaluation based on the Hagen-Poiseuille law. Organizing principles of Arabidopsis primary and lateral root growth and branching were determined and used to apply the HydroRoot model to an extended set of simulated RSAs. Sensitivity analyses revealed that water transport can be co-limited by radial and axial conductances throughout the whole RSA. The number of roots that can be sectioned (intercepted) at a given distance from the base was defined as an accessible and informative indicator of RSA. The overall set of experimental and theoretical procedures was applied to plants mutated in ESKIMO1 and previously shown to have xylem collapse. This approach will be instrumental to dissect the root water transport phenotype of plants with intricate alterations in root growth or transport functions.

Populus × canescens root suberization in reaction to osmotic and salt stress is limited to the developing younger root tip region

Populus is a valuable and fast-growing tree species commonly cultivated for economic and scientific purposes. But most of the poplar species are sensitive to drought and salt stress. Thus, we compared the physiological effects of osmotic stress (PEG8000) and salt treatment (NaCl) on poplar roots to identify potential strategies for future breeding or genetic engineering approaches. We investigated root anatomy using epifluorescence microscopy, changes in root suberin composition and amount using gas chromatography, transcriptional reprogramming using RNA sequencing, and modifications of root transport physiology using a pressure chamber. Poplar roots reacted to the imposed stress conditions, especially in the developing younger root tip region, with remarkable differences between both types of stress. Overall, the increase in suberin content was surprisingly small, but the expression of key suberin biosynthesis genes was strongly induced. Significant reductions of the radial water transport in roots were only observed for the osmotic and not the hydrostatic hydraulic conductivity. Our data indicate that the genetic enhancement of root suberization processes in poplar might be a promising target to convey increased tolerance, especially against toxic sodium chloride.

Multi-Omics and Integrative Approach towards Understanding Salinity Tolerance in Rice: A Review

Rice (Oryza sativa L.) plants are simultaneously encountered by environmental stressors, most importantly salinity stress. Salinity is the major hurdle that can negatively impact growth and crop yield. Understanding the salt stress and its associated complex trait mechanisms for enhancing salt tolerance in rice plants would ensure future food security. The main aim of this review is to provide insights and impacts of molecular-physiological responses, biochemical alterations, and plant hormonal signal transduction pathways in rice under saline stress. Furthermore, the review highlights the emerging breakthrough in multi-omics and computational biology in identifying the saline stress-responsive candidate genes and transcription factors (TFs). In addition, the review also summarizes the biotechnological tools, genetic engineering, breeding, and agricultural practicing factors that can be implemented to realize the bottlenecks and opportunities to enhance salt tolerance and develop salinity tolerant rice varieties. Future studies pinpointed the augmentation of powerful tools to dissect the salinity stress-related novel players, reveal in-depth mechanisms and ways to incorporate the available literature, and recent advancements to throw more light on salinity responsive transduction pathways in plants. Particularly, this review unravels the whole picture of salinity stress tolerance in rice by expanding knowledge that focuses on molecular aspects.

Radial transport difference mediated by root endodermal barriers contributes to differential cadmium accumulation between japonica and indica subspecies of rice (Oryza sativa L.)

Although Cd concentration of grains is generally lower in japonica than in indica subspecies, the effects of root endodermal barriers on the subspecific differences in Cd accumulation in rice (Oryza sativa L.) are poorly understood. Here, we characterized the differences in endodermal differentiation between japonica and indica subspecies and their effects on Cd radial transport. Casparian strips (CSs) and suberin lamellae (SL) in japonica subspecies were initiated at the 6%- 7% and 21%- 27% position from the root tip, respectively, which were 65% and 26% earlier than in indica subspecies, respectively. The lignin/suberin content in japonica subspecies was 47%/42% greater than that in indica subspecies because of the higher expression of lignin/suberin biosynthesis-related genes (OsCASP1, OsPAL, OsCYP86A1 and OsKCS20). Cd exposure induced endodermal plasticity in both subspecies, but the changes in japonica were greater than in indica subspecies. The earlier formation of CSs/SL in japonica subspecies significantly restricted the flow of radial transport tracer to reach the xylem and decreased Cd influx into roots, that is, endodermal barriers inhibited Cd radial transport via both apoplastic and cell-to-cell pathways, thus decreasing the root-to-shoot transport of Cd in japonica subspecies. Our findings are beneficial for the genetic modification of rice with low-Cd-accumulating ability.

Experimental and conceptual approaches to root water transport

BACKGROUND

Root water transport, which critically contributes to the plant water status and thereby plant productivity, has been the object of extensive experimental and theoretical studies. However, root systems represent an intricate assembly of cells in complex architectures, including many tissues at distinct developmental stages. Our comprehension of where and how molecular actors integrate their function in order to provide the root with its hydraulic properties is therefore still limited.

SCOPE

Based on current literature and prospective discussions, this review addresses how root water transport can be experimentally measured, what is known about the underlying molecular actors, and how elementary water transport processes are scaled up in numerical/mathematical models.

CONCLUSIONS

The theoretical framework and experimental procedures on root water transport that are in use today have been established a few decades ago. However, recent years have seen the appearance of new techniques and models with enhanced resolution, down to a portion of root or to the tissue level. These advances pave the way for a better comprehension of the dynamics of water uptake by roots in the soil.

Hydroponic cultivation conditions allowing the reproducible investigation of poplar root suberization and water transport

BACKGROUND

With increasing joint research cooperation on national and international levels, there is a high need for harmonized and reproducible cultivation conditions and experimental protocols in order to ensure the best comparability and reliability of acquired data. As a result, not only comparisons of findings of different laboratories working with the same species but also of entirely different species would be facilitated. As Populus is becoming an increasingly important genus in modern science and agroforestry, the integration of findings with previously gained knowledge of other crop species is of high significance.

RESULTS

To ease and ensure the comparability of investigations of root suberization and water transport, on a high degree of methodological reproducibility, we set up a hydroponics-based experimental pipeline. This includes plant cultivation, root histochemistry, analytical investigation, and root water transport measurement. A 5-week-long hydroponic cultivation period including an optional final week of stress application resulted in a highly consistent poplar root development. The poplar roots were of conical geometry and exhibited a typical Casparian band development with subsequent continuously increasing suberization of the endodermis. Poplar root suberin was composed of the most frequently described suberin substance classes, but also high amounts of benzoic acid derivatives could be identified. Root transport physiology experiments revealed that poplar roots in this developmental stage have a two- to tenfold higher hydrostatic than osmotic hydraulic conductivity. Lastly, the hydroponic cultivation allowed the application of gradually defined osmotic stress conditions illustrating the precise adjustability of hydroponic experiments as well as the previously reported sensitivity of poplar plants to water deficits.

CONCLUSIONS

By maintaining a high degree of harmonization, we were able to compare our results to previously published data on root suberization and water transport of barley and other crop species. Regarding hydroponic poplar cultivation, we enabled high reliability, reproducibility, and comparability for future experiments. In contrast to abiotic stress conditions applied during axenic tissue culture cultivation, this experimental pipeline offers great advantages including the growth of roots in the dark, easy access to root systems before, during, and after stress conditions, and the more accurate definition of the developmental stages of the roots.

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.

Root plasticity under abiotic stress

Abiotic stresses increasingly threaten existing ecological and agricultural systems across the globe. Plant roots perceive these stresses in the soil and adapt their architecture accordingly. This review provides insights into recent discoveries showing the importance of root system architecture (RSA) and plasticity for the survival and development of plants under heat, cold, drought, salt, and flooding stress. In addition, we review the molecular regulation and hormonal pathways involved in controlling RSA plasticity, main root growth, branching and lateral root growth, root hair development, and formation of adventitious roots. Several stresses affect root anatomy by causing aerenchyma formation, lignin and suberin deposition, and Casparian strip modulation. Roots can also actively grow toward favorable soil conditions and avoid environments detrimental to their development. Recent advances in understanding the cellular mechanisms behind these different root tropisms are discussed. Understanding root plasticity will be instrumental for the development of crops that are resilient in the face of abiotic stress.

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.

Convergent evolution of gene regulatory networks underlying plant adaptations to dry environments

Plants transitioned from an aquatic to a terrestrial lifestyle during their evolution. On land, fluctuations on water availability in the environment became one of the major problems they encountered. The appearance of morpho-physiological adaptations to cope with and tolerate water loss from the cells was undeniably useful to survive on dry land. Some of these adaptations, such as carbon concentrating mechanisms (CCMs), desiccation tolerance (DT) and root impermeabilization, appeared in multiple plant lineages. Despite being crucial for evolution on land, it has been unclear how these adaptations convergently evolved in the various plant lineages. Recent advances on whole genome and transcriptome sequencing are revealing that co-option of genes and gene regulatory networks (GRNs) is a common feature underlying the convergent evolution of these adaptations. In this review, we address how the study of CCMs and DT has provided insight into convergent evolution of GRNs underlying plant adaptation to dry environments, and how these insights could be applied to currently emerging understanding of evolution of root impermeabilization through different barrier cell types. We discuss examples of co-option, conservation and innovation of genes and GRNs at the cell, tissue and organ levels revealed by recent phylogenomic (comparative genomic) and comparative transcriptomic studies.

WRKY9 transcription factor regulates cytochrome P450 genes CYP94B3 and CYP86B1, leading to increased root suberin and salt tolerance in Arabidopsis

Salinity affects crop productivity worldwide and mangroves growing under high salinity exhibit adaptations such as enhanced root apoplastic barrier to survive under such conditions. We have identified two cytochrome P450 family genes, AoCYP94B3 and AoCYP86B1 from the mangrove tree Avicennia officinalis and characterized them using atcyp94b3 and atcyp86b1, which are mutants of their putative Arabidopsis orthologs and the corresponding complemented lines with A. officinalis genes. CYP94B3 and CYP86B1 transcripts were induced upon salt treatment in the roots of both A. officinalis and Arabidopsis. Both AoCYP94B3 and AoCYP86B1 were localized to the endoplasmic reticulum. Heterologous expression of 35S::AoCYP94B3 and 35S::AoCYP86B1 in their respective Arabidopsis mutants (atcyp94b3 and atcyp86b1) increased the salt tolerance of the transgenic seedlings by reducing the amount of Na⁺ accumulation in the shoots. Moreover, the reduced root suberin phenotype of atcyp94b3 was rescued in the 35S::AoCYP94B3;atcyp94b3 transgenic Arabidopsis seedlings. Gas-chromatography and mass spectrometry analyses showed that the amount of suberin monomers (C-16 ω-hydroxy acids, C-16 α, ω-dicarboxylic acids and C-20 eicosanol) were increased in the roots of 35S::AoCYP94B3;atcyp94b3 Arabidopsis seedlings. Using chromatin immunoprecipitation and electrophoretic mobility shift assays, we identified AtWRKY9 as the upstream regulator of AtCYP94B3 and AtCYP86B1 in Arabidopsis. In addition, atwrky9 showed suppressed expression of AtCYP94B3 and AtCYP86B1 transcripts, and reduced suberin in the roots. These results show that AtWRKY9 controls suberin deposition by regulating AtCYP94B3 and AtCYP86B1, leading to salt tolerance. Our data can be used for generating salt-tolerant crop plants in the future.

Nitrogen depletion enhances endodermal suberization without restricting transporter-mediated root NO3- influx

Roots vary their permeability to aid radial transport of solutes towards xylem vessels in response to nutritional cues. Nitrogen (N) depletion was previously shown to induce early suberization of endodermal cell walls and reduce hydraulic conductivity of barley roots suggesting reduced apoplastic transport of ions (Armand et al., 2019). Suberization may also limit transcellular ion movement by blocking access to transporters (Barberon et al., 2016). The aim of this study was to confirm that N depletion induced suberization in the roots of barley and demonstrate that this was a specific effect in response to NO₃^- depletion. Furthermore, in roots with early and enhanced suberization, we assessed their ability for transporter-mediated NO₃^- influx. N depletion induced lateral root elongation and early and enhanced endodermal suberization of the seminal root of each genotype. Both root to shoot NO₃^- translocation and net N uptake was half that of plants supplied with steady-state NO₃^-. Genes with predicted functions in suberin synthesis (HvHORST) and NO₃^- transport (HvNRT2.2) were induced under N-deplete conditions. N-deplete roots had a higher capacity for high-affinity NO₃^- influx in early suberized roots than under optimal NO₃^-. In conclusion, NO₃^- depletion induced early and enhanced suberization in the roots of barley, however, suberization did not restrict transcellular NO₃^- transport.

Root metaxylem and architecture phenotypes integrate to regulate water use under drought stress

At the genus and species level, variation in root anatomy and architecture may interact to affect strategies of drought avoidance. To investigate this idea, root anatomy and architecture of the drought-sensitive common bean (Phaseolus vulgaris) and drought-adapted tepary bean (Phaseolus acutifolius) were analyzed in relation to water use under terminal drought. Intraspecific variation for metaxylem anatomy and axial conductance was found in the roots of both species. Genotypes with high-conductance root metaxylem phenotypes acquired and transpired more water per unit leaf area, shoot mass, and root mass than genotypes with low-conductance metaxylem phenotypes. Interspecific variation in root architecture and root depth was observed where P. acutifolius has a deeper distribution of root length than P. vulgaris. In the deeper-rooted P. acutifolius, genotypes with high root conductance were better able to exploit deep soil water than genotypes with low root axial conductance. Contrastingly, in the shallower-rooted P. vulgaris, genotypes with low root axial conductance had improved water status through conservation of soil moisture for sustained water capture later in the season. These results indicate that metaxylem morphology interacts with root system depth to determine a strategy of drought avoidance and illustrate synergism among architectural and anatomical phenotypes for root function.

Regulation of a Cytochrome P450 Gene CYP94B1 by WRKY33 Transcription Factor Controls Apoplastic Barrier Formation in Roots to Confer Salt Tolerance

Salinity is an environmental stress that causes decline in crop yield. Avicennia officinalis and other mangroves have adaptations such as ultrafiltration at the roots aided by apoplastic cell wall barriers to thrive in saline conditions. We studied a cytochrome P450 gene from A. officinalis, AoCYP94B1, and its putative ortholog in Arabidopsis (Arabidopsis thaliana), AtCYP94B1, which are involved in apoplastic barrier formation. Both genes were induced by 30 min of salt treatment in the roots. Heterologous expression of AoCYP94B1 in the atcyp94b1 Arabidopsis mutant and wild-type rice (Oryza sativa) conferred increased NaCl tolerance to seedlings by enhancing root suberin deposition. Histochemical staining and gas chromatography-tandem mass spectrometry quantification of suberin precursors confirmed the role of CYP94B1 in suberin biosynthesis. Using chromatin immunoprecipitation and yeast one-hybrid and luciferase assays, we identified AtWRKY33 as the upstream regulator of AtCYP94B1 in Arabidopsis. In addition, atwrky33 mutants exhibited reduced suberin and salt-sensitive phenotypes, which were rescued by expressing 35S::AtCYP94B1 in the atwrky33 background. This further confirmed that AtWRKY33-mediated regulation of AtCYP94B1 is part of the salt tolerance mechanism. Our findings may help efforts aimed at generating salt-tolerant crops.

A review of current knowledge about the formation of native peridermal exocarp in fruit

The outer skin layer in any plant is essential in offering a protective barrier against water loss and pathogen attack. Within fleshy fruit, the skin supports internal cell layers and can provide the initial cues in attracting seed-dispersing animals. The skin of a fruit, termed the exocarp, is a key element of consumer preference and a target for many breeding programs. Across fruiting species there is a huge diversity of exocarp types and these range from a simple single living cell layer (epidermis) often covered with a waxy layer, to complex multicellular suberised and dead cell layers (periderm), with various intermediate russet forms in between. Each exocarp can be interspersed with other structures such as hairs or spines. The epidermis has been well characterised and remains pluripotent with the help of the cells immediately under the epidermis. The periderm, in contrast, is the result of secondary meristematic activity, which replaces the epidermal layers, and is not well characterised in fruits. In this review we explore the structure, composition and mechanisms that control the development of a periderm type fruit exocarp. We draw upon literature from non-fleshy fruit species that form periderm tissue, from which a considerable amount of research has been undertaken.

The role of root apoplastic barriers in cadmium translocation and accumulation in cultivars of rice (Oryza sativa L.) with different Cd-accumulating characteristics

The radial translocation of cadmium (Cd) from the root to the shoot is one of the major processes affecting Cd accumulation in rice (Oryza sativa L.) grains, but few studies have focused on Cd apoplastic transport in rice. The aim of this study was to determine how apoplastic barriers affect Cd translocation via the apoplastic pathway, Cd accumulation levels in upper parts (shoot and grains) of rice cultivars, and the possible mechanism involved. Hydroponic and soil pot trials were conducted to study the development and chemical constituents of apoplastic barriers and their permeability to bypass flow, and to determine Cd localization in the roots of rice cultivars with different Cd-accumulating characteristics. The Cd accumulation in upper parts was positively correlated with bypass flow in the root and the apparent Cd concentration in the xylem, indicating that the apoplastic pathway may play an important role in Cd root-shoot translocation in rice. Apoplastic barriers were deposited closer to the root tip and were thicker in low Cd-accumulating cultivars than in high Cd-accumulating cultivars. The amounts and rates of increase in lignin and suberin were significantly higher in ZD14 (a low Cd-accumulating cultivar) than in FYXZ (a high Cd-accumulating cultivar) under Cd stress, indicating that stronger barriers were induced by Cd in ZD14. The stronger and earlier formation of barriers in the low Cd-accumulating cultivar decreased bypass flow more efficiently, so that more Cd was retained in the root during apoplastic translocation. This was confirmed by localization analyses of Cd in root transverse sections. These results suggest that apoplastic barriers reduce Cd root-to-shoot translocation via the apoplastic pathway, leading to lower Cd accumulation in the upper parts of rice plants. Bypass flow may have the potential to be used as a rapid screening indicator for low Cd-accumulating rice cultivars.

TPST is involved in fructose regulation of primary root growth in Arabidopsis thaliana

TPST is involved in fructose signaling to regulate the root development and expression of genes in biological processes including auxin biosynthesis and accumulation in Arabidopsis. Sulfonation of proteins by tyrosine protein sulfotransferases (TPST) has been implicated in many important biological processes in eukaryotic organisms. Arabidopsis possesses a single TPST gene and its role in auxin homeostasis and root development has been reported. Here we show that the Arabidopsis tpst mutants are hypersensitive to fructose. In contrast to sucrose and glucose, fructose represses primary root growth of various ecotypes of Arabidopsis at low concentrations. RNA-seq analysis identified 636 differentially expressed genes (DEGs) in Col-0 seedlings in response to fructose verses glucose. GO and KEGG analyses of the DEGs revealed that fructose down-regulates genes involved in photosynthesis, glucosinolate biosynthesis and IAA biosynthesis, but up-regulates genes involved in the degradation of branched amino acids, sucrose starvation response, and dark response. The fructose responsive DEGs in the tpst mutant largely overlapped with that in Col-0, and most DEGs in tpst displayed larger changes than in Col-0. Interestingly, the fructose up-regulated DEGs includes genes encoding two AtTPST substrate proteins, Phytosulfokine 2 (PSK2) and Root Meristem Growth Factor 7 (RGF7). Synthesized peptides of PSK-α and RGF7 could restore the fructose hypersensitivity of tpst mutant plants. Furthermore, auxin distribution and accumulation at the root tip were affected by fructose and the tpst mutation. Our findings suggest that fructose serves as a signal to regulate the expression of genes involved in various biological processes including auxin biosynthesis and accumulation, and that modulation of auxin accumulation and distribution in roots by fructose might be partly mediated by the TPST substrate genes PSK-α and RGF7.

High water uptake ability was associated with root aerenchyma formation in rice: Evidence from local ammonium supply under osmotic stress conditions

Root water uptake is strongly influenced by the morphology and anatomical structure of roots, which are regulated by nitrogen forms and environmental stimuli. To further illustrate the roles of different nitrogen forms on root water uptake under osmotic stress, a split-root system was supplied with different nitrogen forms and osmotic stress simulated by adding 10% (w/v) polyethylene glycol (PEG, 6000). The local effects of nitrogen form and osmotic stress on root morphology, anatomical structure, root lignin content, and water uptake rate were investigated. Under osmotic stress conditions, ammonium markedly promoted the formation and elongation of the lateral root, whereas a significant decrease in numbers of lateral roots was observed under local nitrate supply. Under nitrate supply in split-root systems, osmotic stress significantly promoted root cell death and more aerenchyma formation, as well as accelerated the lignification of the root. However, osmotic stress had no negative effect on the root anatomical structure under ammonium supply. The root water uptake rate was significantly higher in split-root supplied with ammonium than nitrate under osmotic stress conditions. In conclusion, the high water uptake ability in local ammonium supply was associated with the more lateral roots development and the lower cell death, aerenchyma formation and lignification under osmotic stress.

The barrier function of plant roots: biological bases for selective uptake and avoidance of soil compounds

The root is the main organ through which water and mineral nutrients enter the plant organism. In addition, root fulfils several other functions. Here, we propose that the root also performs the barrier function, which is essential not only for plant survival but for plant acclimation and adaptation to a constantly changing and heterogeneous soil environment. This function is related to selective uptake and avoidance of some soil compounds at the whole plant level. We review the toolkit of morpho-anatomical, structural, and other components that support this view. The components of the root structure involved in selectivity, permeability or barrier at a cellular, tissue, and organ level and their properties are discussed. In consideration of the arguments supporting barrier function of plant roots, evolutionary aspects of this function are also reviewed. Additionally, natural variation in selective root permeability is discussed which suggests that the barrier function is constantly evolving and is subject of natural selection.

MYB41, MYB107, and MYC2 promote ABA-mediated primary fatty alcohol accumulation via activation of AchnFAR in wound suberization in kiwifruit

Wound damage triggers the accumulation of abscisic acid (ABA), which induces the expression of a large number of genes involved in wound suberization in plants. Fatty acyl-CoA reductase (FAR) catalyzes the generation of primary fatty alcohols by the reduction of fatty acids in suberin biosynthesis. However, the regulatory effects of transcription factors (TFs) on AchnFAR in response to ABA are unexplored. In this study, kiwifruit AchnFAR displayed a biological function analogous to that of FAR in transiently overexpressed tobacco (Nicotiana benthamiana) leaves. The positive role of TFs, including AchnMYB41, AchnMYB107, and AchnMYC2, in the regulation of AchnFAR was identified. The three TFs could individually bind to the AchnFAR promoter to activate gene transcription in yeast one-hybrid and dual-luciferase assays. Transient overexpression of TFs in tobacco leaves resulted in the upregulation of aliphatic synthesis genes (including FAR) and the increase in aliphatics, including primary alcohols, α,ω-diacids, ω-hydroxyacids, and fatty acids. Moreover, exogenous ABA treatment elevated TF-mediated AchnFAR expression and the accumulation of primary alcohols. Conversely, fluridone, an inhibitor of ABA biosynthesis, suppressed the expression of AchnFAR and TF genes and reduced the formation of primary alcohols. The results indicate that AchnMYB41, AchnMYB107, and AchnMYC2 activate AchnFAR transcription to promote ABA-mediated primary alcohol formation in wound suberization in kiwifruit.

Overexpression of ANAC046 Promotes Suberin Biosynthesis in Roots of Arabidopsis thaliana

NAC (NAM (no apical meristem), ATAF1/2, and CUC2 (cup-shaped cotyledon)) proteins are one of the largest families of plant-specific transcription factors, and this family is present in a wide range of land plants. Here, we have investigated the role of ANAC046 in the regulation of suberin biosynthesis and deposition in Arabidopsis. Subcellular localization and transcriptional activity assays showed that ANAC046 localizes in the nucleus, where it functions as a transcription activator. Analysis of the P_ANAC046:GUS lines revealed that ANAC046 is mainly expressed in the root endodermis and periderm, and is also induced in leaves by wounding. The transgenic lines overexpressing ANAC046 exhibited defective surfaces on the aerial plant parts compared to the wild-type (WT) as characterized by increased permeability for Toluidine blue stain and greater chlorophyll leaching. Quantitative RT-PCR analysis showed that the expression of suberin biosynthesis genes was significantly higher in the roots and leaves of overexpression lines compared to the WT. The biochemical analysis of leaf cuticular waxes showed that the overexpression lines accumulated 30% more waxes than the WT. Concurrently, overexpression lines also deposited almost twice the amount of suberin content in their roots compared with the WT. Taken together, these results showed that ANAC046 is an important transcription factor that promotes suberin biosynthesis in Arabidopsis thaliana roots.

Regulation of water transport in Arabidopsis by methyl jasmonate

Following a stress event, jasmonate-dependent signaling pathway triggers a shift from growth to defense responses that are accompanied by the cessation of growth in many plants. However, the processes leading to this growth inhibition remain obscure. In this study, we provide evidence for a rapid inhibition of cell hydraulic conductivity (L_p) by methyl jasmonate (MeJA) in the roots of wild-type Arabidopsis within 0.5 h of 20 and 50 μM MeJA treatments. We also demonstrate that MeJA did not affect L_p in fad3-2 and fad7-2 Arabidopsis mutants that are deficient in jasmonate precursor, linolenic acid. The reductions of L_p in wild-type plants were accompanied by the down-regulation of several plasma membrane intrinsic protein (PIP) isoforms, and dephosphorylation. Treatments with HgCl₂ did not further reduce L_p in the wild-type plants, but significantly reduced L_p in the fad3-2 and fad7-2 that had been first treated with MeJA. Continuous prolonged exposure to exogenous 50 μM MeJA inhibited the relative growth rates (RGR) of shoots and net photosynthesis (P_n) in the Arabidopsis wild-type and fad7-2 plants, but had no effect on the RGR of roots. The results demonstrated that a reduction of aquaporin (AQP)-mediated water transport was the initial target of MeJA exposure, and may contribute to the processes of growth inhibition by MeJA.

Regulation of a plant aquaporin by a Casparian strip membrane domain protein-like

The absorption of soil water by roots allows plants to maintain their water status. At the endodermis, water transport can be affected by initial formation of a Casparian strip and further deposition of suberin lamellas and regulated by the function of aquaporins. Four Casparian strip membrane domain protein-like (CASPL; CASPL1B1, CASPL1B2, CASPL1D1, and CASPL1D2) were previously shown to interact with PIP2;1. The present work shows that CASPL1B1, CASPL1B2, and CASPL1D2 are exclusively expressed in suberized endodermal cells, suggesting a cell-specific role in suberization and/or water transport regulation. When compared with wild-type plants, and by contrast to caspl1b1*caspl1b2 double loss of function, caspl1d1*caspl1d2 double mutants showed, in some control or NaCl stress experiments and not upon abscisic acid (ABA) treatment, a weak enlargement of the continuous suberization zone. None of the mutants showed root hydraulic conductivity (Lp_r ) phenotype, whether in control, NaCl, or ABA treatment conditions. The data suggest a slight negative role for CASPL1D1 and CASPL1D2 in suberization under control or salt stress conditions, with no major impact on whole root transport functions. At the molecular level, CASPL1B1 was able to physically interact with PIP2;1 and potentially could influence the regulation of aquaporins by acting on their phosphorylated form.

Surveillance of cell wall diffusion barrier integrity modulates water and solute transport in plants

The endodermis is a key cell layer in plant roots that contributes to the controlled uptake of water and mineral nutrients into plants. In order to provide such functionality the endodermal cell wall has specific chemical modifications consisting of lignin bands (Casparian strips) that encircle each cell, and deposition of a waxy-like substance (suberin) between the wall and the plasma membrane. These two extracellular deposits provide control of diffusion enabling the endodermis to direct the movement of water and solutes into and out of the vascular system in roots. Loss of integrity of the Casparian strip-based apoplastic barrier is sensed by the leakage of a small peptide from the stele into the cortex. Here, we report that such sensing of barrier integrity leads to the rebalancing of water and mineral nutrient uptake, compensating for breakage of Casparian strips. This rebalancing involves both a reduction in root hydraulic conductivity driven by deactivation of aquaporins, and downstream limitation of ion leakage through deposition of suberin. These responses in the root are also coupled to a reduction in water demand in the shoot mediated by ABA-dependent stomatal closure.

Plant Salinity Stress: Many Unanswered Questions Remain

Salinity is a major threat to modern agriculture causing inhibition and impairment of crop growth and development. Here, we not only review recent advances in salinity stress research in plants but also revisit some basic perennial questions that still remain unanswered. In this review, we analyze the physiological, biochemical, and molecular aspects of Na+ and Cl- uptake, sequestration, and transport associated with salinity. We discuss the role and importance of symplastic versus apoplastic pathways for ion uptake and critically evaluate the role of different types of membrane transporters in Na+ and Cl- uptake and intercellular and intracellular ion distribution. Our incomplete knowledge regarding possible mechanisms of salinity sensing by plants is evaluated. Furthermore, a critical evaluation of the mechanisms of ion toxicity leads us to believe that, in contrast to currently held ideas, toxicity only plays a minor role in the cytosol and may be more prevalent in the vacuole. Lastly, the multiple roles of K+ in plant salinity stress are discussed.

OsTPS8 controls yield-related traits and confers salt stress tolerance in rice by enhancing suberin deposition

Class I TREHALOSE-PHOSPHATE-SYNTHASE (TPS) genes affect salinity tolerance and plant development. However, the function of class IITPS genes and their underlying mechanisms of action are unknown. We report the identification and functional analysis of a rice class IITPS gene (OsTPS8). The ostps8 mutant was characterised by GC-MS analysis, an abscisic acid (ABA) sensitivity test and by generating transgenic lines. To identify the underlying mechanism, gene expression analyses, genetic complementation and examination of suberin deposition in the roots were conducted. The ostps8 mutant showed salt sensitivity, ABA sensitivity and altered agronomic traits compared to the wild-type (WT), which could be rescued upon complementation. The dsRNAi line phenocopied the mutant, while the overexpression lines exhibited enhanced salt tolerance. The ostps8 mutant showed significantly reduced soluble sugars, Casparian bands and suberin deposition in the roots compared to the WT and overexpression lines. The mutant also showed downregulation of SAPKs (rice SnRK2s) and ABA-responsive genes. Furthermore, ostps8pUBI::SAPK9 rescued the salt-sensitive phenotype of ostps8. Our results suggest that OsTPS8 may regulate suberin deposition in rice through ABA signalling. Additionally, SAPK9-mediated regulation of altered ABA-responsive genes helps to confer salinity tolerance. Overexpression of OsTPS8 was adequate to confer enhanced salinity tolerance without any yield penalty, suggesting its usefulness in rice genetic improvement.

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

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.

Response to early drought stress and identification of QTLs controlling biomass production under drought in pearl millet

Pearl millet plays a major role in food security in arid and semi-arid areas of Africa and India. However, it lags behind the other cereal crops in terms of genetic improvement. The recent sequencing of its genome opens the way to the use of modern genomic tools for breeding. Our study aimed at identifying genetic components involved in early drought stress tolerance as a first step toward the development of improved pearl millet varieties or hybrids. A panel of 188 inbred lines from West Africa was phenotyped under early drought stress and well-irrigated conditions. We found a strong impact of drought stress on yield components. This impact was variable between inbred lines. We then performed an association analysis with a total of 392,493 SNPs identified using Genotyping-by-Sequencing (GBS). Correcting for genetic relatedness, genome wide association study identified QTLs for biomass production in early drought stress conditions and for stay-green trait. In particular, genes involved in the sirohaem and wax biosynthesis pathways were found to co-locate with two of these QTLs. Our results might contribute to breed pearl millet lines with improved yield under drought stress.

Natural variation at XND1 impacts root hydraulics and trade-off for stress responses in Arabidopsis

Soil water uptake by roots is a key component of plant performance and adaptation to adverse environments. Here, we use a genome-wide association analysis to identify the XYLEM NAC DOMAIN 1 (XND1) transcription factor as a negative regulator of Arabidopsis root hydraulic conductivity (Lp_r). The distinct functionalities of a series of natural XND1 variants and a single nucleotide polymorphism that determines XND1 translation efficiency demonstrate the significance of XND1 natural variation at species-wide level. Phenotyping of xnd1 mutants and natural XND1 variants show that XND1 modulates Lp_r through action on xylem formation and potential indirect effects on aquaporin function and that it diminishes drought stress tolerance. XND1 also mediates the inhibition of xylem formation by the bacterial elicitor flagellin and counteracts plant infection by the root pathogen Ralstonia solanacearum. Thus, genetic variation at XND1, and xylem differentiation contribute to resolving the major trade-off between abiotic and biotic stress resistance in Arabidopsis.

Soil water uptake is a major determinant of plant performance and stress tolerance. Here the authors show that, by affecting xylem formation in the root, natural variation at the Arabidopsis XND1 locus has contrasting effects on root hydraulics and drought tolerance versus pathogen resistance.

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 (Lp_r) 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.

Composite Transport Model and Water and Solute Transport across Plant Roots: An Update

The present review examines recent experimental findings in root transport phenomena in terms of the composite transport model (CTM). It has been a well-accepted conceptual model to explain the complex water and solute flows across the root that has been related to the composite anatomical structure. There are three parallel pathways involved in the transport of water and solutes in roots - apoplast, symplast, and transcellular paths. The role of aquaporins (AQPs), which facilitate water flows through the transcellular path, and root apoplast is examined in terms of the CTM. The contribution of the plasma membrane bound AQPs for the overall water transport in the whole plant level was varying depending on the plant species, age of roots with varying developmental stages of apoplastic barriers, and driving forces (hydrostatic vs. osmotic). Many studies have demonstrated that the apoplastic barriers, such as Casparian bands in the primary anticlinal walls and suberin lamellae in the secondary cell walls, in the endo- and exodermis are not perfect barriers and unable to completely block the transport of water and some solute transport into the stele. Recent research on water and solute transport of roots with and without exodermis triggered the importance of the extension of conventional CTM adding resistances that arrange in series (epidermis, exodermis, mid-cortex, endodermis, and pericycle). The extension of the model may answer current questions about the applicability of CTM for composite water and solute transport of roots that contain complex anatomical structures with heterogeneous cell layers.

The endodermis, a tightly controlled barrier for nutrients

Plant roots acquire nutrients from the soil and transport them upwards to the aerial parts. To reach the central vasculature of the root, water and nutrients radially cross all external cell layers. The endodermis surrounds the vascular tissues and forms diffusion barriers. It thereby compartmentalizes the root and allows control of nutrient transport from the soil to the vasculature, as well as preventing backflow of nutrients from the stele. To achieve this role, endodermal cells undergo two specialized differentiations states consisting of deposition of two impermeable polymers in the cell wall: lignin, forming the Casparian strips, and suberin lamellae. Recent publications showed that endodermal barrier formation is not a hard-wired, irreversible process. Synthesis and degradation of suberin lamellae is highly regulated by plant hormones in response to nutrient stresses. Moreover, Casparian strip continuity seems to be constantly checked by two small peptides produced in the vasculature that diffuse into the apoplastic space in order to test endodermal barrier integrity. This review discusses the recent understanding of endodermal barrier surveillance and plasticity and its role in plant nutrition.

Root cortical senescence decreases root respiration, nutrient content and radial water and nutrient transport in barley

The functional implications of root cortical senescence (RCS) are poorly understood. We tested the hypotheses that RCS in barley (1) reduces the respiration and nutrient content of root tissue; (2) decreases radial water and nutrient transport; and (3) is accompanied by increased suberization to protect the stele. Genetic variation for RCS exists between modern germplasm and landraces. Nitrogen and phosphorus deficiency increased the rate of RCS. Maximal RCS, defined as the disappearance of the entire root cortex, reduced root nitrogen content by 66%, phosphorus content by 63% and respiration by 87% compared with root segments with no RCS. Roots with maximal RCS had 90, 92 and 84% less radial water, nitrate and phosphorus transport, respectively, compared with segments with no RCS. The onset of RCS coincided with 30% greater aliphatic suberin in the endodermis. These results support the hypothesis that RCS reduces root carbon and nutrient costs and may therefore have adaptive significance for soil resource acquisition. By reducing root respiration and nutrient content, RCS could permit greater root growth, soil resource acquisition and resource allocation to other plant processes. RCS merits investigation as a trait for improving the performance of barley, wheat, triticale and rye under edaphic stress.