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

Application of Zinc Oxide Nanoparticles to Mitigate Cadmium Toxicity: Mechanisms and Future Prospects

Cadmium (Cd), as the most prevalent heavy metal contaminant poses serious risks to plants, humans, and the environment. The ubiquity of this toxic metal is continuously increasing due to the rapid discharge of industrial and mining effluents and the excessive use of chemical fertilizers. Nanoparticles (NPs) have emerged as a novel strategy to alleviate Cd toxicity. Zinc oxide nanoparticles (ZnO-NPs) have become the most important NPs used to mitigate the toxicity of abiotic stresses and improve crop productivity. The plants quickly absorb Cd, which subsequently disrupts plant physiological and biochemical processes and increases the production of reactive oxygen species (ROS), which causes the oxidation of cellular structures and significant growth losses. Besides this, Cd toxicity also disrupts leaf osmotic pressure, nutrient uptake, membrane stability, chlorophyll synthesis, and enzyme activities, leading to a serious reduction in growth and biomass productivity. Though plants possess an excellent defense mechanism to counteract Cd toxicity, this is not enough to counter higher concentrations of Cd toxicity. Applying Zn-NPs has proven to have significant potential in mitigating the toxic effects of Cd. ZnO-NPs improve chlorophyll synthesis, photosynthetic efficiency, membrane stability, nutrient uptake, and gene expression, which can help to counter toxic effects of Cd stress. Additionally, ZnO-NPs also help to reduce Cd absorption and accumulation in plants, and the complex relationship between ZnO-NPs, osmolytes, hormones, and secondary metabolites plays an important role in Cd tolerance. Thus, this review concentrates on exploring the diverse mechanisms by which ZnO nanoparticles can alleviate Cd toxicity in plants. In the end, this review has identified various research gaps that need addressing to ensure the promising future of ZnO-NPs in mitigating Cd toxicity. The findings of this review contribute to gaining a deeper understanding of the role of ZnO-NPs in combating Cd toxicity to promote safer and sustainable crop production by remediating Cd-polluted soils. This also allows for the development of eco-friendly approaches to remediate Cd-polluted soils to improve soil fertility and environmental quality.

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.

Overexpression of NtDOGL4 improves cadmium tolerance through abscisic acid signaling pathway in tobacco

The DELAY OF GERMINATION1-LIKE (DOGL) genes play an essential role in diverse biological processes in plants. However, their exact involvement in the response to cadmium (Cd) stress via the ABA pathway remains unclear. Here, we focused on NtDOGL4, a tobacco DOGL gene whose expression is highly induced upon exposure to Cd. Overexpression of NtDOGL4 in tobacco resulted in elevated endogenous ABA levels, reduced Cd accumulation, and increased tolerance to Cd. Moreover, NtDOGL4 overexpression led to decreased accumulation of reactive oxygen species (ROS) and improved ROS scavenging capacity under Cd stress. Further analyses revealed the direct binding of the transcription factor ABSCISIC ACID-INSENSITIVE 5 (ABI5) to the NtDOGL4 promoter, positively regulating its expression in tobacco. Notably, NtDOGL4 overexpression promoted suberin formation and deposition, while suppressing the expression of Cd transporter genes in tobacco roots, as evidenced by histochemical staining, suberin fraction determination, and qRT-PCR assays. Collectively, our results demonstrate that NtDOGL4 overexpression reduces Cd accumulation, thereby improving Cd stress tolerance through the modulation of antioxidant system, transcription of Cd transporters, and suberin deposition. Notably, the NtABI5-NtDOGL4 module functions as a positive regulator in tobacco's Cd tolerance, underscoring its potential as a molecular target for developing low-Cd crops to ensure environmental safety.

Mitigating cadmium accumulation and toxicity in plants: The promising role of nanoparticles

Cadmium (Cd) is a highly toxic heavy metal that adversely affects humans, animals, and plants, even at low concentrations. It is widely distributed and has both natural and anthropogenic sources. Plants readily absorb and distribute Cd in different parts. It may subsequently enter the food chain posing a risk to human health as it is known to be carcinogenic. Cd has a long half-life, resulting in its persistence in plants and animals. Cd toxicity disrupts crucial physiological and biochemical processes in plants, including reactive oxygen species (ROS) homeostasis, enzyme activities, photosynthesis, and nutrient uptake, leading to stunted growth and reduced biomass. Although plants have developed defense mechanisms to mitigate these damages, they are often inadequate to combat high Cd concentrations, resulting in yield losses. Nanoparticles (NPs), typically smaller than 100 nm, possess unique properties such as a large surface area and small size, making them highly reactive compared to their larger counterparts. NPs from diverse sources have shown potential for various agricultural applications, including their use as fertilizers, pesticides, and stress alleviators. Recently, NPs have emerged as a promising strategy to mitigate heavy metal stress, including Cd toxicity. They offer advantages, such as efficient absorption by crop plants, the reduction of Cd uptake, and the enhancement of mineral nutrition, antioxidant defenses, photosynthetic parameters, anatomical structure, and agronomic traits in Cd-stressed plants. The complex interaction of NPs with calcium ions (Ca2+), intracellular ROS, nitric oxide (NO), and phytohormones likely plays a significant role in alleviating Cd stress. This review aims to explore the positive impacts of diverse NPs in reducing Cd accumulation and toxicity while investigating their underlying mechanisms of action. Additionally, it discusses research gaps, recent advancements, and future prospects of utilizing NPs to alleviate Cd-induced stress, ultimately promoting improved plant growth and yield.

Outer apoplastic barriers in roots: prospects for abiotic stress tolerance

Floods and droughts are becoming more frequent as a result of climate change and it is imperative to find ways to enhance the resilience of staple crops to abiotic stresses. This is crucial to sustain food production during unfavourable conditions. Here, we analyse the current knowledge about suberised and lignified outer apoplastic barriers, focusing on the functional roles of the barrier to radial O2 loss formed as a response to soil flooding and we discuss whether this trait also provides resilience to multiple abiotic stresses. The barrier is composed of suberin and lignin depositions in the exodermal and/or sclerenchyma cell walls. In addition to the important role during soil flooding, the barrier can also restrict radial water loss, prevent phytotoxin intrusion, salt intrusion and the main components of the barrier can impede invasion of pathogens in the root. However, more research is needed to fully unravel the induction pathway of the outer apoplastic barriers and to address potential trade-offs such as reduced nutrient or water uptake. Nevertheless, we suggest that the outer apoplastic barriers might act as a jack of all trades providing tolerance to multiple abiotic and/or biotic stressors.

The exodermis: A forgotten but promising apoplastic barrier

The endodermis and exodermis are widely recognized as two important barriers in plant roots that play a role in regulating the movement of water and ions. While the endodermis is present in nearly all plant roots, the exodermis, characterized by Casparian strips and suberin lamellae is absent in certain plant species. The exodermis can be classified into three types: uniform, dimorphic, and inducible exodermis. Apart from its role in water and ion transport, the exodermis acts as a protective barrier against harmful substances present in the external environment. Furthermore, the exodermis is a complex barrier influenced by various environmental factors, and its resistance to water and ions varies depending on the type of exodermis and the maturity of the root. Therefore, investigations concerning the exodermis necessitate a plant-specific approach. However, our current understanding of the exodermal physiological functions and molecular mechanisms governing its development is limited due to the absence of an exodermis in the model plant Arabidopsis. Due to that, unfortunately, the exodermis has been largely overlooked until now. In this review, we aim to summarize the current fundamental knowledge regarding the exodermis in common research used crop species and propose suggestions for future research endeavors.

Root radial apoplastic transport contributes to shoot cadmium accumulation in a high cadmium-accumulating rice line

Radial transport of cadmium (Cd) in roots governs the amount of Cd loaded into xylem vessels, where Cd ions are translocated upward into shoots, while the mechanism of differential Cd radial transport between the high Cd-accumulating rice line Lu527-8 and the normal rice line Lu527-4 remains ambiguous. A higher Cd distribution in cross sections and root apoplast and higher bypass flow of Cd were found in Lu527-8, explaining a greater Cd translocation through the apoplastic pathway. The lower relative area of the epidermis and the constant relative area of the cortex in Lu527-8 opened-up root radial transport for Cd. Deposition of apoplastic barriers (Casparian strips and suberin lamellae) was stimulated by Cd, which effectively prevented Cd from entering the stele through the apoplastic pathway. In Lu527-8, apoplastic barriers were further from the root apex with lower expression of genes responsible for biosynthesis of Casparian strips and suberin lamellae, enhancing radial transport of Cd. Our data revealed that the higher radial apoplastic transport of Cd played an integral role in Cd translocation, contributed to a better understanding of the mechanism involved in high Cd accumulation in Lu527-8 and helped achieve the practical application of phytoextraction.

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.

ABA-dependent suberization and aquaporin activity in rice (Oryza sativa L.) root under different water potentials

Drought is one of the most stressful environments limiting crop growth and yield throughout the world. Therefore, most efforts have been made to document drought-derived genetic and physiological responses and to find better ways to improve drought tolerance. The interaction among them is unclear and/or less investigated. Therefore, the current study is to find a clue of metabolic connectivity among them in rice root experiencing different levels of drought condition. We selected 19 genes directly involved in abscisic acid (ABA) metabolism (6), suberization (6), and aquaporins (AQPs) activity (7) and analyzed the relatively quantitative gene expression using qRT-PCR from rice roots. In addition, we also analyzed proline, chlorophyll, and fatty acids and observed cross-sectional root structure (aerenchyma) and suberin lamella deposition in the endodermis. All drought conditions resulted in an obvious development of aerenchyma and two- to fourfold greater accumulation of proline. The limited water supply (-1.0 and -1.5 MPa) significantly increased gene expression (ABA metabolism, suberization, and AQPs) and developed greater layer of suberin lamella in root endodermis. In addition, the ratio of the unsaturated to the saturated fatty acids was increased, which could be considered as an adjusted cell permeability. Interestingly, these metabolic adaptations were an exception with a severe drought condition (hygroscopic coefficient, -3.1 MPa). Accordingly, we concluded that the drought-tolerant mechanism in rice roots is sophisticatedly regulated until permanent wilting point (-1.5 MPa), and ABA metabolism, suberization, and AQPs activity might be independent and/or concurrent process as a survival strategy against drought.

Diurnal changes in apoplast bypass flow of water and ions in salt-stressed wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.)

The aim of the present study was to quantify the contribution of apoplastic bypass flow to the uptake of water and salt across the root cylinder of wheat and barley during day and night. Plants were grown on hydroponics until they were 14-17 days old and then analysed over a single day (16 h) or night (8 h) period while being exposed to different concentrations of NaCl (50, 100, 150 and 200 mM NaCl). Exposure to salt started just before the experiment (short-term stress) or had started 6d before (longer-term stress). Bypass flow was quantified using the apoplastic tracer dye 8-hydroxy-1,3,6-pyrenesulphonic acid (PTS). The percent contribution of bypass flow to root water uptake increased in response to salt stress and during the night and amounted to up to 4.4%. Bypass flow across the root cylinder of Na+ and Cl- made up 2%-12% of the net delivery of these ions to the shoot; this percentage changed little (wheat) or decreased (barley) during the night. Changes in the contribution of bypass flow to the net uptake of water, Na+ and Cl- in response to salt stress and day/night are the combined result of changes in xylem tension, the contribution of alternative cell-to-cell flow path and the requirement to generate xylem osmotic pressure.

Gibberellin and abscisic acid transporters facilitate endodermal suberin formation in Arabidopsis

The plant hormone gibberellin (GA) regulates multiple developmental processes. It accumulates in the root elongating endodermis, but how it moves into this cell file and the significance of this accumulation are unclear. Here we identify three NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER (NPF) transporters required for GA and abscisic acid (ABA) translocation. We demonstrate that NPF2.14 is a subcellular GA/ABA transporter, presumably the first to be identified in plants, facilitating GA and ABA accumulation in the root endodermis to regulate suberization. Further, NPF2.12 and NPF2.13, closely related proteins, are plasma membrane-localized GA and ABA importers that facilitate shoot-to-root GA12 translocation, regulating endodermal hormone accumulation. This work reveals that GA is required for root suberization and that GA and ABA can act non-antagonistically. We demonstrate how the clade of transporters mediates hormone flow with cell-file-specific vacuolar storage at the phloem unloading zone, and slow release of hormone to induce suberin formation in the maturation zone.

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.

The mechanism of silicon on alleviating cadmium toxicity in plants: A review

Cadmium is one of the most toxic heavy metal elements that seriously threaten food safety and agricultural production worldwide. Because of its high solubility, cadmium can easily enter plants, inhibiting plant growth and reducing crop yield. Therefore, finding a way to alleviate the inhibitory effects of cadmium on plant growth is critical. Silicon, the second most abundant element in the Earth's crust, has been widely reported to promote plant growth and alleviate cadmium toxicity. This review summarizes the recent progress made to elucidate how silicon mitigates cadmium toxicity in plants. We describe the role of silicon in reducing cadmium uptake and transport, improving plant mineral nutrient supply, regulating antioxidant systems and optimizing plant architecture. We also summarize in detail the regulation of plant water balance by silicon, and the role of this phenomenon in enhancing plant resistance to cadmium toxicity. An in-depth analysis of literature has been conducted to identify the current problems related to cadmium toxicity and to propose future research directions.

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.

Root cell types as an interface for biotic interactions

Root responses to environmental stresses show a high level of cell type and developmental stage specificity. Interactions with beneficial and pathogenic organisms - including microbes and parasites - elicit a set of transcriptional responses unique to each root cell type, often dependent on their differentiation state. Localized changes to the cell wall and to the integrity of root cell types can serve as a physical barrier for a range of pests. Conversely, certain microorganisms weaken existing barriers within root cell types. Interactions with microorganisms vary between roots of different developmental origins and cellular architectures. Here we provide an overview of the molecular, architectural, and structural properties of root cell types crucial to both maintaining beneficial interactions and protecting from pathogens.

Novel insights into host specificity of Pyricularia oryzae and Pyricularia grisea in the infection of gramineous plant roots

Pyricularia oryzae and Pyricularia grisea are pathogens that cause blast disease in various monocots. It has been reported that P. oryzae infects the leaves and roots of rice via different mechanisms. However, it is unclear to what extent the tissue types affect the host specificities of P. oryzae and P. grisea. Here, we evaluated the tissue-specific infection strategies of P. oryzae and P. grisea in various gramineous plants. Generally, mycelial plug inoculation caused root browning but the degree of browning did not simply follow the disease index on leaves. Interestingly, the Triticum and Digitaria pathotypes caused strong root growth inhibition in rice, wheat, and barley. Moreover, the Digitaria pathotype inhibited root branching only in rice. Culture filtrate reproduced these inhibitory effects on root, suggesting that some secreted molecules are responsible for the inhibitions. Observation of root sections revealed that most of the infection hyphae penetrated intercellular spaces and further extended into root cells, regardless of pathotype and host plant. The infection hyphae of Digitaria and Triticum pathotypes tended to localize in the outer layer of rice roots, but not in those of wheat and barley roots. The infection hyphae of the Oryza pathotype were distributed in both the intercellular and intracellular spaces of rice root cells. Pathogenesis-related genes and reactive oxygen species accumulation were induced after root inoculation with all combinations. These results suggest that resistance reactions were induced in the roots of gramineous plants against the infection with Pyricularia isolates but failed to prevent fungal invasion.

Evaluation of Protein Extraction Methods for Metaproteomic Analyses of Root-Associated Microbes

Metaproteomics is a powerful tool for the characterization of metabolism, physiology, and functional interactions in microbial communities, including plant-associated microbiota. However, the metaproteomic methods that have been used to study plant-associated microbiota are very laborious and require large amounts of plant tissue, hindering wider application of these methods. We optimized and evaluated different protein extraction methods for metaproteomics of plant-associated microbiota in two different plant species (Arabidopsis and maize). Our main goal was to identify a method that would work with low amounts of input material (40 to 70 mg) and that would maximize the number of identified microbial proteins. We tested eight protocols, each comprising a different combination of physical lysis method, extraction buffer, and cell-enrichment method on roots from plants grown with synthetic microbial communities. We assessed the performance of the extraction protocols by liquid chromatography-tandem mass spectrometry-based metaproteomics and found that the optimal extraction method differed between the two species. For Arabidopsis roots, protein extraction by beating whole roots with small beads provided the greatest number of identified microbial proteins and improved the identification of proteins from gram-positive bacteria. For maize, vortexing root pieces in the presence of large glass beads yielded the greatest number of microbial proteins identified. Based on these data, we recommend the use of these two methods for metaproteomics with Arabidopsis and maize. Furthermore, detailed descriptions of the eight tested protocols will enable future optimization of protein extraction for metaproteomics in other dicot and monocot plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

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.

Dissecting the Relationship between Root Morphological Traits and Yield Attributes in Diverse Rice Cultivars under Subtropical Condition

Understanding the link between root morphological traits and yields is crucial for improving crop management. To evaluate this link, a pot experiment was conducted in the net house of the Department of Agronomy, Bangladesh Agricultural University, Mymensingh, Bangladesh during the boro(dry season irrigated) rice growing season of 2019–20. Thirteen cultivars, named BRRI dhan29, BRRI dhan58, BRRI dhan67, BRRI dhan74, BRRI dhan81, Binadhan-8, Binadhan-10, Hira-2, Tej gold, SL8H, Jagliboro, Rata boro, and Lakhai, were used following a completely randomized design (CRD) with three replications. The cultivars were screened for root number (RN), root length (RL), root volume (RV), root porosity (RP), leaf area index (LAI), total dry matter (TDM), and grain yield (GY). A considerable variation in root traits, LAI, and TDM were found among the studied cultivars, and the highest GY (26.26 g pot−1)was found for Binahan-10. Thirteen cultivars were grouped into three clusters using hierarchical cluster analysis, where clusters 1, 2, and 3 assembled with 3, 5, and 5 cultivars, respectively. Considering all of the studied traits, Cluster 3 (Binadhan-10, Hira-2, BRRI dhan29, BRRI dhan58, and Tejgold) showed promise, followed by Cluster 2 (BRRI dhan81, BRRI dhan67, SL8H, BRRI dhan74, and Binadhan-8). Principal component analysis (PCA) revealed that the RV, RDW, RFW, TDM, and GY are effective traits for rice cultivation.

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

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

Morpho-physiological, biochemical and molecular characterization of coastal rice landraces to identify novel genetic sources of salinity tolerance

Soil salinity is a leading cause for yield losses in rice, affecting nearly 6% of global rice cultivable area. India is host to a rich diversity of coastal rice landraces that are naturally tolerant to salinity and an untapped source to identify novel determinants of salinity tolerance. In the present study, we have assessed the relative salinity tolerance of 43 previously genotyped rice landraces at seedling stage, using thirteen morpho-physiological and biochemical parameters using a hydroponics system. Among 43 rice varieties, 25 were tolerant, 15 were moderately tolerant, 1 was moderately susceptible and 2 sensitive checks were found to be highly susceptible based on standard salinity scoring methods. In addition to previously known saline tolerant genotypes (Pokkali, FL478 and Nona Bokra), the present study has novel genotypes such as Katrangi, Orkyma, Aduisen 1, Orumundakan 1, Hoogla, and Talmugur 2 as potential sources of salinity tolerance through measurement of morpho-physiological and biochemical parameters including Na+, K+ estimations and Na+/K+ ratios. Further, Pallipuram Pokkali may be an important source of the tissue tolerance trait under salinity. Four marker trait associations (RM455-root Na+; RM161-shoot and root Na+/K+ ratios; RM237-salinity tolerance index) accounted for phenotypic variations in the range of 20.97-39.82%. A significant increase in root endodermal and exodermal suberization was observed in selected rice landraces under salinity. For the first time, variation in the number of suberized sclerenchymatous layers as well as passage cells is reported, in addition to expression level changes in suberin biosynthetic genes (CYP86A2, CYP81B1, CYP86A8 and PERL).

Using the Maize Nested Association Mapping (NAM) Population to Partition Arbuscular Mycorrhizal Effects on Drought Stress Tolerance into Hormonal and Hydraulic Components

In this study, a first experiment was conducted with the objective of determining how drought stress alters the radial water flow and physiology in the whole maize nested association mapping (NAM) population and to find out which contrasting maize lines should be tested in a second experiment for their responses to drought in combination with an arbuscular mycorrhizal (AM) fungus. Emphasis was placed on determining the role of plant aquaporins and phytohormones in the responses of these contrasting maize lines to cope with drought stress. Results showed that both plant aquaporins and hormones are altered by the AM symbiosis and are highly involved in the physiological responses of maize plants to drought stress. The regulation by the AM symbiosis of aquaporins involved in water transport across cell membranes alters radial water transport in host plants. Hormones such as IAA, SA, ABA and jasmonates must be involved in this process either by regulating the own plant-AM fungus interaction and the activity of aquaporins, or by inducing posttranscriptional changes in these aquaporins, which in turns alter their water transport capacity. An intricate relationship between root hydraulic conductivity, aquaporins and phytohormones has been observed, revealing a complex network controlling water transport in maize roots.

The Combined Effect of Heat and Osmotic Stress on Suberization of Arabidopsis Roots

The simultaneous occurrence of heat stress and drought is becoming more regular as a consequence of climate change, causing extensive agricultural losses. The application of either heat or osmotic stress increase cell-wall suberization in different tissues, which may play a role in improving plant resilience. In this work, we studied how the suberization process is affected by the combination of drought and heat stress by following the expression of suberin biosynthesis genes, cell-wall suberization and the chemical composition in Arabidopsis roots. The Arabidopsis plants used in this study were at the onset of secondary root development. At this point, one can observe a developmental gradient in the main root, with primary development closer to the root tip and secondary development, confirmed by the suberized phellem, closer to the shoot. Remarkably, we found a differential response depending on the root zone. The combination of drought and heat stress increased cell wall suberization in main root segments undergoing secondary development and in lateral roots (LRs), while the main root zone, at primary development stage, was not particularly affected. We also found differences in the overall chemical composition of the cell walls in both root zones in response to combined stress. The data gathered showed that, under combined drought and heat stress, Arabidopsis roots undergo differential cell wall remodeling depending on developmental stage, with modifications in the biosynthesis and/or assembly of major cell wall components.

Spatiotemporal development of suberized barriers in cork oak taproots

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

Subsurface Flow Phytoremediation Using Barley Plants for Water Recovery from Kerosene-Contaminated Water: Effect of Kerosene Concentration and Removal Kinetics

A phytoremediation experiment was carried out with kerosene as a model for total petroleum hydrocarbons. A constructed wetland of barley was exposed to kerosene pollutants at varying concentrations (1, 2, and 3% v/v) in a subsurface flow (SSF) system. After a period of 42 days of exposure, it was found that the average ability to eliminate kerosene ranged from 56.5% to 61.2%, with the highest removal obtained at a kerosene concentration of 1% v/v. The analysis of kerosene at varying initial concentrations allowed the kinetics of kerosene to be fitted with the Grau model, which was closer than that with the zero order, first order, or second order kinetic models. The experimental study showed that the barley plant designed in a subsurface flow phytoremediation system would have great potential for the reclamation of kerosene-contaminated water.

Reduced apoplastic barriers in tissues of shoot-proximal rhizomes of Oryza coarctata are associated with Na+ sequestration

Oryza coarctata is the only wild rice species with significant salinity tolerance. The present work examines the role of the substantial rhizomatous tissues of O. coarctata in conferring salinity tolerance. Transition to an erect phenotype (shoot emergence) from prostrate growth of rhizome tissues is characterized by marked lignification and suberization of supporting sclerenchymatous tissue, epidermis, and bundle sheath cells in aerial shoot-proximal nodes and internodes in O. coarctata. With salinity, however, aerial shoot-proximal internodal tissues show reductions in lignification and suberization, most probably related to re-direction of carbon flux towards synthesis of the osmporotectant proline. Concurrent with hypolignification and reduced suberization, the aerial rhizomatous biomass of O. coarctata appears to have evolved mechanisms to store Na+ in these specific tissues under salinity. This was confirmed by histochemical staining, quantitative real-time reverse transcription-PCR expression patterns of genes involved in lignification/suberization, Na+ and K+ contents of internodal tissues, as well as non-invasive microelectrode ion flux measurements of NaCl-induced net Na+, K+, and H+ flux profiles of aerial nodes were determined. In O. coarctata, aerial proximal internodes appear to act as 'traffic controllers', sending required amounts of Na+ and K+ into developing leaves for osmotic adjustment and turgor-driven growth, while more deeply positioned internodes assume a Na+ buffering/storage role.

Abscisic acid is required for exodermal suberization to form a barrier to radial oxygen loss in the adventitious roots of rice (Oryza sativa)

To acclimate to waterlogged conditions, wetland plants form a barrier to radial oxygen loss (ROL) that can enhance oxygen transport to the root apex. We hypothesized that one or more hormones are involved in the induction of the barrier and searched for such hormones in rice. We previously identified 98 genes that were tissue-specifically upregulated during ROL barrier formation in rice. The RiceXPro database showed that most of these genes were highly enhanced by exogenous abscisic acid (ABA). We then examined the effect of ABA on ROL barrier formation by using an ABA biosynthesis inhibitor (fluridone, FLU), by applying exogenous ABA and by examining a mutant with a defective ABA biosynthesis gene (osaba1). FLU suppressed barrier formation in a stagnant solution that mimics waterlogged soil. Under aerobic conditions, rice does not naturally form a barrier, but 24 h of ABA treatment induced barrier formation. osaba1 did not form a barrier under stagnant conditions, but the application of ABA rescued the barrier. In parallel with ROL barrier formation, suberin lamellae formed in the exodermis. These findings strongly suggest that ABA is an inducer of suberin lamellae formation in the exodermis, resulting in an ROL barrier formation in rice.

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.

The role of reactive oxygen species and nitric oxide in the formation of root cortical aerenchyma under cadmium contamination

The present study aimed to evaluate root cortical aerenchyma formation in response to Cd-driven hydrogen peroxide (H₂ O₂ ) production and the role of nitric oxide (NO) in the alleviation of Cd oxidative stress in maize roots and its effects on aerenchyma development. Maize plants were subjected to continuous flooding for 30 days, and the following treatments were applied weekly: Cd(NO₃ )₂ at 0, 10, and 50 μM and Na₂ [Fe(CN)₅ NO]·2H₂ O (an NO donor) at 0.5, 0.1, and 0.2 μM. The root biometrics; oxidative stress indicators H₂ O₂ and malondialdehyde (MDA); and activities of catalase (CAT), superoxide dismutase (SOD), and ascorbate peroxidase (APX) were analyzed. The root dry and fresh masses decreased at higher concentrations of NO and Cd. H₂ O₂ also decreased at higher NO concentrations; however, MDA increased only at higher Cd levels. SOD activity decreased at higher concentrations of NO, but CAT activity increased. Aerenchyma development decreased in response to NO. Consequently, NO acts as an antagonist to Cd, decreasing the concentration of H₂ O₂ by reducing SOD activity and increasing CAT activity. Although H₂ O₂ is directly linked to aerenchyma formation, increased H₂ O₂ concentrations are necessary for root cortical aerenchyma development.

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.

Analysis of Extracellular Cell Wall Lipids: Wax, Cutin, and Suberin in Leaves, Roots, Fruits, and Seeds

Extracellular lipids of plants can be analyzed using gas chromatography and mass spectrometry. Soluble waxes are extracted with chloroform and thus separated from the extracellular polymers cutin and suberin. Cutin and suberin have to be depolymerized using boron trifluoride-methanol or methanolic HCl before analysis. The released monomeric hydroxylated fatty acids are then extracted with chloroform or hexane. Prior to gas chromatography, all free polar functional groups (alcohols and carboxylic acids) are derivatized by trimethylsilylation. Internal standards, that is, long chain alkanes, are used for the quantification of wax molecules and cutin or suberin monomers. Lipids are quantified using gas chromatography coupled to flame ionization detection. Qualitative analysis is carried out by gas chromatography coupled to mass spectrometry. Thus, all wax molecules of chain lengths from C₁₆ to C₆₀ and different substance classes (fatty acids, alcohols, esters, aldehydes, alkanes, etc.) or all cutin or suberin monomers of chain lengths from C₁₆ to C₃₂ and different substance classes (hydroxylated fatty acids, diacids, etc.) can be analyzed from one sample.

The regulatory landscape of Arabidopsis thaliana roots at single-cell resolution

The scarcity of accessible sites that are dynamic or cell type-specific in plants may be due in part to tissue heterogeneity in bulk studies. To assess the effects of tissue heterogeneity, we apply single-cell ATAC-seq to Arabidopsis thaliana roots and identify thousands of differentially accessible sites, sufficient to resolve all major cell types of the root. We find that the entirety of a cell's regulatory landscape and its transcriptome independently capture cell type identity. We leverage this shared information on cell identity to integrate accessibility and transcriptome data to characterize developmental progression, endoreduplication and cell division. We further use the combined data to characterize cell type-specific motif enrichments of transcription factor families and link the expression of family members to changing accessibility at specific loci, resolving direct and indirect effects that shape expression. Our approach provides an analytical framework to infer the gene regulatory networks that execute plant development.

The regulatory landscape of Arabidopsis thaliana roots at single-cell resolution

The scarcity of accessible sites that are dynamic or cell type-specific in plants may be due in part to tissue heterogeneity in bulk studies. To assess the effects of tissue heterogeneity, we apply single-cell ATAC-seq to Arabidopsis thaliana roots and identify thousands of differentially accessible sites, sufficient to resolve all major cell types of the root. We find that the entirety of a cell's regulatory landscape and its transcriptome independently capture cell type identity. We leverage this shared information on cell identity to integrate accessibility and transcriptome data to characterize developmental progression, endoreduplication and cell division. We further use the combined data to characterize cell type-specific motif enrichments of transcription factor families and link the expression of family members to changing accessibility at specific loci, resolving direct and indirect effects that shape expression. Our approach provides an analytical framework to infer the gene regulatory networks that execute plant development.

The endodermal passage cell - just another brick in the wall?

The endodermis surrounds and protects the vasculature partly by depositing hydrophobic suberin in the cell walls. Yet, some cells remain unsuberised. These historically termed 'passage cells' are assumed to provide a low-resistance pathway to the xylem. Only recently have we started to gain molecular insights into these cells, which allow us to probe how roots coordinate communication with the environment across barriers with single-cell precision. Increased understanding of root physiology at a high-resolution is intriguing, as it is likely to provide us with new tools to improve overall plant health. With this in mind, we here provide a brief overview of passage cells, their presence across plant species, as well as a molecular update and future directions for passage cell-related research.

Single-cell transcriptome atlas and chromatin accessibility landscape reveal differentiation trajectories in the rice root

Root development relies on the establishment of meristematic tissues that give rise to distinct cell types that differentiate across defined temporal and spatial gradients. Dissection of the developmental trajectories and the transcriptional networks that underlie them could aid understanding of the function of the root apical meristem in both dicots and monocots. Here, we present a single-cell RNA (scRNA) sequencing and chromatin accessibility survey of rice radicles. By temporal profiling of individual root tip cells we reconstruct continuous developmental trajectories of epidermal cells and ground tissues, and elucidate regulatory networks underlying cell fate determination in these cell lineages. We further identify characteristic processes, transcriptome profiles, and marker genes for these cell types and reveal conserved and divergent root developmental pathways between dicots and monocots. Finally, we demonstrate the potential of the platform for functional genetic studies by using spatiotemporal modeling to identify a rice root meristematic mutant from a cell-specific gene cohort.

Functions and structure of roots and their contributions to salinity tolerance in plants

Soil salinity is an increasing threat to the productivity of glycophytic crops worldwide. The root plays vital roles under various stress conditions, including salinity, as well as has diverse functions in non-stress soil environments. In this review, we focus on the essential functions of roots such as in ion homeostasis mediated by several different membrane transporters and signaling molecules under salinity stress and describe recent advances in the impacts of quantitative trait loci (QTLs) or genetic loci (and their causal genes, if applicable) on salinity tolerance. Furthermore, we introduce important literature for the development of barriers against the apoplastic flow of ions, including Na⁺, as well as for understanding the functions and components of the barrier structure under salinity stress.

A barrier to radial oxygen loss helps the root system cope with waterlogging-induced hypoxia

Internal aeration is crucial for root growth under waterlogged conditions. Many wetland plants have a structural barrier that impedes oxygen leakage from the basal part of roots called a radial oxygen loss (ROL) barrier. ROL barriers reduce the loss of oxygen transported via the aerenchyma to the root tips, enabling long-distance oxygen transport for cell respiration at the root tip. Because the root tip does not have an ROL barrier, some of the transferred oxygen is released into the waterlogged soil, where it oxidizes and detoxifies toxic substances (e.g., sulfate and Fe²⁺) around the root tip. ROL barriers are located at the outer part of roots (OPRs). Their main component is thought to be suberin. Suberin deposits may block the entry of potentially toxic compounds in highly reduced soils. The amount of ROL from the roots depends on the strength of the ROL barrier, the length of the roots, and environmental conditions, which causes spatiotemporal changes in the root system's oxidization pattern. We summarize recent achievements in understanding how ROL barrier formation is regulated and discuss opportunities for breeding waterlogging-tolerant crops.

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.

Suberized transport barriers in plant roots: the effect of silicon

Plant roots are the major organs that take up water and dissolved nutrients. It has been widely shown that apoplastic barriers such as Casparian bands and suberin lamellae in the endo- and exodermis of roots have an important effect on regulating radial water and nutrient transport. Furthermore, it has been described that silicon can promote plant growth and survival under different conditions. However, the potential effects of silicon on the formation and structure of apoplastic barriers are controversial. A delayed as well as an enhanced suberization of root apoplastic barriers with silicon has been described in the literature. Here we review the effects of silicon on the formation of suberized apoplastic barriers in roots, and present results of the effect of silicon treatment on the formation of endodermal suberized barriers on barley seminal roots under control conditions and when exposed to osmotic stress. Chemical analysis confirmed that osmotic stress enhanced barley root suberization. While a supplementation with silicon in both, control conditions and osmotic stress, did not enhanced barley root suberization. These results suggest that enhanced stress tolerance of plants after silicon treatment is due to other responses.

Some Mechanisms Modulating the Root Growth of Various Wheat Species under Osmotic-Stress Conditions

The role of the root in water supply and plant viability is especially important if plants are subjected to stress at the juvenile stage. This article describes the study of morphophysiological and cytological responses, as well as elements of the anatomical structure of primary roots of three wheat species, Triticum monococcum L., Triticum dicoccum Shuebl., and Triticum aestivum L., to osmotic stress. It was shown that the degree of plasticity of root morphology in water deficit affected the growth and development of aboveground organs. It was found that in conditions of osmotic stress, the anatomical root modulations were species-specific. In control conditions the increase in absolute values of root diameter was reduced with the increase in the ploidy of wheat species. Species-specific cytological responses to water deficit of apical meristem cells were also shown. The development of plasmolysis, interpreted as a symptom of reduced viability apical meristem cells, was revealed. A significant increase in enzymatic activity of superoxide dismutase under osmotic stress was found to be one of the mechanisms that could facilitate root elongation in adverse conditions. The tetraploid species T. dicoccum Shuebl. were confirmed as a source of traits of drought tolerant primary root system for crosses with wheat cultivars.

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.

The intersection of nitrogen nutrition and water use in plants: new paths toward improved crop productivity

Water and nitrogen availability limit crop productivity globally more than most other environmental factors. Plant availability of macronutrients such as nitrate is, to a large extent, regulated by the amount of water available in the soil, and, during drought episodes, crops can become simultaneously water and nitrogen limited. In this review, we explore the intricate relationship between water and nitrogen transport in plants, from transpiration-driven mass flow in the soil to uptake by roots via membrane transporters and channels and transport to aerial organs. We discuss the roles of root architecture and of suberized hydrophobic root barriers governing apoplastic water and nitrogen movement into the vascular system. We also highlight the need to identify the signalling cascades regulating water and nitrogen transport, as well as the need for targeted physiological analyses of plant traits influencing water and nitrogen uptake. We further advocate for incorporation of new phenotyping technologies, breeding strategies, and agronomic practices to improve crop yield in water- and nitrogen-limited production systems.

QTL Mapping and Candidate Gene Analysis for Alkali Tolerance in Japonica Rice at the bud Stage Based on Linkage Mapping and Genome-Wide Association Study

BACKGROUND

Salinity-alkalinity stress is one of the major factors limiting rice production. Damage caused by alkaline salt stress is more severe than that caused by neutral salt stress. Alkali tolerance at the bud stage in rice directly affects seedling survival and final yield when using the direct seeding cultivation model. However, genetic resources (QTLs and genes) for rice breeders to improve alkali tolerance are limited. In this study, we combined linkage mapping and a genome-wide association study (GWAS) to analyze the genetic structure of this trait in japonica rice at the bud stage.

RESULTS

A population of 184 recombinant inbred lines (RILs) was utilized to map quantitative trait loci (QTLs) for the root length under control condition (RL), alkaline stress (ARL) and relative root length (RRL) at the bud stage. A major QTL related to alkali tolerance at the rice bud stage, qAT11, was detected on chromosome 11. Interestingly, a GWAS identified a lead SNP (Chr_21,999,659) in qAT11 that was significantly associated with alkaline tolerance. After filtering by linkage disequilibrium (LD), haplotype analysis, quantitative real-time PCR, we obtained three candidate genes (LOC_Os11g37300, LOC_Os11g37320 and LOC_Os11g37390). In addition, we performed phenotype verification on the CRISPR/Cas9 mutant of LOC_Os11g37390.

CONCLUSION

Based on these results, LOC_Os11g37300, LOC_Os11g37320 and LOC_Os11g37390 were the candidate genes contributing to alkaline tolerance in japonica rice. This study provides resources for breeding aimed at improving rice responses to alkalinity stress.

Integrative Multi-omics Analyses of Barley Rootzones under Salinity Stress Reveal Two Distinctive Salt Tolerance Mechanisms

The mechanisms underlying rootzone-localized responses to salinity during early stages of barley development remain elusive. In this study, we performed the analyses of multi-root-omes (transcriptomes, metabolomes, and lipidomes) of a domesticated barley cultivar (Clipper) and a landrace (Sahara) that maintain and restrict seedling root growth under salt stress, respectively. Novel generalized linear models were designed to determine differentially expressed genes (DEGs) and abundant metabolites (DAMs) specific to salt treatments, genotypes, or rootzones (meristematic Z1, elongation Z2, and maturation Z3). Based on pathway over-representation of the DEGs and DAMs, phenylpropanoid biosynthesis is the most statistically enriched biological pathway among all salinity responses observed. Together with histological evidence, an intense salt-induced lignin impregnation was found only at stelic cell wall of Clipper Z2, compared with a unique elevation of suberin deposition across Sahara Z2. This suggests two differential salt-induced modulations of apoplastic flow between the genotypes. Based on the global correlation network of the DEGs and DAMs, callose deposition that potentially adjusted symplastic flow in roots was almost independent of salinity in rootzones of Clipper, and was markedly decreased in Sahara. Taken together, we propose two distinctive salt tolerance mechanisms in Clipper (growth-sustaining) and Sahara (salt-shielding), providing important clues for improving crop plasticity to cope with deteriorating global soil salinization.

Drought tolerance of the grapevine, Vitis champinii cv. Ramsey, is associated with higher photosynthesis and greater transcriptomic responsiveness of abscisic acid biosynthesis and signaling

BACKGROUND

Grapevine is an economically important crop for which yield and berry quality is strongly affected by climate change. Large variations in drought tolerance exist across Vitis species. Some of these species are used as rootstock to enhance abiotic and biotic stress tolerance. In this study, we investigated the physiological and transcriptomic responses to water deficit of four different genotypes that differ in drought tolerance: Ramsey (Vitis champinii), Riparia Gloire (Vitis riparia), Cabernet Sauvignon (Vitis vinifera), and SC2 (Vitis vinifera x Vitis girdiana).

RESULTS

Ramsey was particularly more drought tolerant than the other three genotypes. Ramsey maintained a higher stomatal conductance and photosynthesis at equivalent levels of moderate water deficit. We identified specific and common transcriptomic responses shared among the four different Vitis species using RNA sequencing analysis. A weighted gene co-expression analysis identified a water deficit core gene set with the ABA biosynthesis and signaling genes, NCED3, RD29B and ABI1 as potential hub genes. The transcript abundance of many abscisic acid metabolism and signaling genes was strongly increased by water deficit along with genes associated with lipid metabolism, galactinol synthases and MIP family proteins. This response occurred at smaller water deficits in Ramsey and with higher transcript abundance than the other genotypes. A number of aquaporin genes displayed differential and unique responses to water deficit in Ramsey leaves. Genes involved in cysteine biosynthesis and metabolism were constitutively higher in the roots of Ramsey; thus, linking the gene expression of a known factor that influences ABA biosynthesis to this genotype's increased NCED3 transcript abundance.

CONCLUSION

The drought tolerant Ramsey maintained higher photosynthesis at equivalent water deficit than the three other grapevine genotypes. Ramsey was more responsive to water deficit; its transcriptome responded at smaller water deficits, whereas the other genotypes did not respond until more severe water deficits were reached. There was a common core gene network responding to water deficit for all genotypes that included ABA metabolism and signaling. The gene clusters and sub-networks identified in this work represent interesting gene lists to explore and to better understand drought tolerance molecular mechanisms.

Seminal roots of wild and cultivated barley differentially respond to osmotic stress in gene expression, suberization, and hydraulic conductivity

Wild barley, Hordeum vulgare spp. spontaneum, has a wider genetic diversity than its cultivated progeny, Hordeum vulgare spp. vulgare. Osmotic stress leads to a series of different responses in wild barley seminal roots, ranging from no changes in suberization to enhanced endodermal suberization of certain zones and the formation of a suberized exodermis, which was not observed in the modern cultivars studied so far. Further, as a response to osmotic stress, the hydraulic conductivity of roots was not affected in wild barley, but it was 2.5-fold reduced in cultivated barley. In both subspecies, osmotic adjustment by increasing proline concentration and decreasing osmotic potential in roots was observed. RNA-sequencing indicated that the regulation of suberin biosynthesis and water transport via aquaporins were different between wild and cultivated barley. These results indicate that wild barley uses different strategies to cope with osmotic stress compared with cultivated barley. Thus, it seems that wild barley is better adapted to cope with osmotic stress by maintaining a significantly higher hydraulic conductivity of roots during water deficit.

Apoplastic barriers, aquaporin gene expression and root and cell hydraulic conductivity in phosphate-limited sheepgrass plants

Mineral nutrient supply can affect the hydraulic property of roots. The aim of the present work on sheepgrass (Leymus chinensis L.) plants was to test whether any changes in root hydraulic conductivity (Lp; exudation analyses) in response to a growth-limiting supply of phosphate (P) are accompanied by changes in (1) cell Lp via measuring the cell pressure, (2) the aquaporin (AQP) gene expression by performing qPCR and (3) the formation of apoplastic barriers, by analyzing suberin lamella and Casparian bands via cross-sectional analyses in roots. Plants were grown hydroponically on complete nutrient solution, containing 250 µM P, until they were 31-36 days old, and then kept for 2-3 weeks on either complete solution, or transferred on solution containing 2.5 µM (low-P) or no added P (no-P). Phosphate treatments caused significant decreases in root and cell-Lp and AQP gene expression, while the formation of apoplastic barriers increased, particularly in lateral roots. Experiments using the AQP inhibitor mercury (Hg) suggested that a significant portion of radial root water uptake in sheepgrass occurs along a path involving AQPs, and that the Lp of this path is reduced under low- and no-P. It is concluded that a growth-limiting supply of phosphate causes parallel changes in (1) cell Lp and aquaporin gene expression (decrease) and (2) apoplastic barrier formation (increase), and that the two may combine to reduce root Lp. The reduction in root Lp, in turn, facilitates an increased root-to-shoot surface area ratio, which allocates resources to the root, sourcing the limiting nutrient.

Insights into Barley Root Transcriptome under Mild Drought Stress with an Emphasis on Gene Expression Regulatory Mechanisms

Root systems play a pivotal role in coupling with drought stress, which is accompanied with a substantial transcriptome rebuilding in the root tissues. Here, we present the results of global gene expression profiling of roots of two barley genotypes with contrasting abilities to cope with drought that were subjected to a mild level of the stress. We concentrate our analysis on gene expression regulation processes, which allowed the identification of 88 genes from 39 families involved in transcriptional regulation in roots upon mild drought. They include 13 genes encoding transcription factors (TFs) from AP2 family represented by ERFs, DREB, or B3 domain-containing TFs, eight WRKYs, six NACs, five of the HD-domain, MYB or MYB-related, bHLH and bZIP TFs. Also, the representatives of C3H, CPP, GRAS, LOB-domain, TCP, Tiffy, Tubby, and NF-Ys TFs, among others were found to be regulated by the mild drought in barley roots. We found that drought tolerance is accompanied with a lower number of gene expression changes than the amount observed in a susceptible genotype. The better drought acclimation may be related to the activation of transcription factors involved in the maintenance of primary root growth and in the epigenetic control of chromatin and DNA methylation. In addition, our analysis pointed to fives TFs from ERF, LOB, NAC, WRKY and bHLH families that may be important in the mild but not the severe drought response of barley roots.

Phenotypic Plasticity in the Structure of Fine Adventitious Metasequoia glyptostroboides Roots Allows Adaptation to Aquatic and Terrestrial Environments

Metasequoia glyptostroboides (Cupressaceae) is a rare deciduous conifer which grows successfully in both aquatic and terrestrial environments. This tree has a narrow natural distribution in central China but is cultivated worldwide. Using histochemical staining and microscopy (both brightfield and epifluorescent), we investigated whether the phenotypic anatomical and histochemical plasticity in the fine adventitious roots of M. glyptostroboides has promoted the adaptation of this plant to aquatic and terrestrial environments. The fine root development and cortex sloughing of M. glyptostroboides occurs later in aquatic habitats than in terrestrial habitats. Anatomical and histochemical analyses have revealed that the apoplastic barriers in the primary growth of the fine roots consist of the endodermis and exodermis with Casparian bands, suberin lamellae, and secondarily lignified cell walls. There were also lignified phi (Φ) thickenings in the cortex. In both aquatic and terrestrial roots, secondary growth was observed in the vascular cambium, which produced secondary xylem and phloem, as well as in the phellogen, which produced cork. As compared to terrestrial adventitious roots, aquatic adventitious roots had multiple lignified Φ thickenings throughout the cortex, larger air spaces, dilated parenchyma, and dense suberin and lignin depositions in the exodermis. Our results thus indicate that phenotypic plasticity in the anatomical features of the fine adventitious roots, including apoplastic barriers, air spaces, and lignified Φ thickenings, might support the adaptation of M. glyptostroboides to both aquatic and terrestrial environments.