Rice acclimation to soil flooding: Low concentrations of organic acids can trigger a barrier to radial oxygen loss in roots

Phytic acid is an available phosphorus source for maize plants in juvenile phase belonging to two populations with different breeding backgrounds

BACKGROUND

Applying animal effluent/digestate to forage crops can improve the sustainability of intensive livestock husbandry. Organic phosphorus (P) forms, in particular phytic acid (InsP6) present in animal effluent/digestate, would benefit from an effective uptake and assimilation by crops thus representing an alternative to mineral P fertilization and controlling P losses in water system. A maize (Zea mays L.) traditional Italian population (VA572), bred before the widespread diffusion of crop chemical fertilization, and a modern commercial hybrid (P1547) were used in this study to investigate their ability of growing in soilless medium using phytic acid (Po) vs phosphate P (Pi) as P sources in a 28-day experiment. The hypothesis was that the different agronomical context in which the two varieties were selected could have brought to different abilities in exploiting P sources for plant growth.

RESULTS

Quantitative and qualitative growth parameters, root enzymatic phosphatase activities and root transcriptome by RNA-seq analysis were analyzed in this study. Both maize populations were able to grow using phytic acid as the sole P source but organic P utilization was less efficient than Pi; a significant root-released phytase activity, induced by the presence of InsP6, was detected in Po treatment. The RNA-seq analysis showed different expression patterns induced by organic P treatment (Po) in the two populations. The upregulation in Po treatment of a Purple Acid Phosphatase (PAP) gene and of genes involved in inositol transport indicate that both phosphate hydrolysis from InsP6 by root-secreted PAPs and a direct uptake of myo-inositol at various degrees of phosphorylation could be involved in maize phytic acid exploitation. Root system development and the relationship of P sources with other macro and micro nutrient uptake (N, K, metal ions) were also implied in the response to Po treatment.

CONCLUSIONS

This study indicates that phytic acid is a bioavailable P source for maize seedling growth. A wider mobilization of genes/pathways was induced by Po treatment in VA572 with respect to P1547 hybrid. The physiological responses to Po treatment were similar in both populations but the patterns of genes involved often differed being specific to each one.

Willow oak (Quercus phellos) seedling roots continue respiration and growth during fall and winter in a soil temperature-dependent manner

Many greentree reservoirs (GTRs) and other bottomland hardwood forests have experienced a shift in tree species composition away from desired red oaks (Quercus section Lobatae), like willow oak (Quercus phellos L.), due to flood stress mortality. Trees experience flood stress primarily through their root system, so it is surmised that GTR flooding may be occurring before root systems have reduced their activity entering the winter. Because soils buffer seasonal temperature changes, we hypothesized that root activity would respond to the belowground environment rather than the aboveground environment. To investigate whether cold soil temperatures reduce root growth and respiration in willow oak during winter, soil temperatures for container seedlings were either held at 15 °C or transitioned to 10 or 5 °C in the late fall. Root elongation was measured in seedlings grown in rhizotron pots by analyzing repeated images of roots during the fall-winter transition period. Root respiration, measured at soil temperature levels, was used as an indicator of root energetic expenses. Also, root respiration was measured at 15 and 5 °C to determine Q10 values to test for acclimation to low soil temperature. Root elongation continued in winter, even after stem elongation stopped in soil temperatures ≥5 °C, a condition usually met throughout most of the native range of willow oak. Both root elongation and respiration rates decreased in cooler soil temperatures. However, Q10 values were unaffected by soil temperature treatment. These findings do not support root dormancy or cold acclimation of root respiratory activity but indicate that temperature directly and reversibly affected root respiration rate. Root elongation may have been dependent on photoassimilates produced by green leaves that were retained through much of winter. Overall, our results suggest that willow oak roots may continue a high rate of growth throughout winter, unlike most temperate species measured to date, and that soil temperature has a major influence over their growth and respiration rates.

Exploring the Role of Non-Structural Carbohydrates (NSCs) Under Abiotic Stresses on Woody Plants: A Comprehensive Review

Global climate change has increased the severity and frequency of abiotic stresses, posing significant challenges to the survival and growth of woody plants. Non-structural carbohydrates (NSCs), including starch and sugars, play a vital role in enabling plants to withstand these stresses, helping to stabilize cellular functions by buffering plant energy demands and facilitating recovery on the alleviation of stress. Despite the recognized multiple functions of NSCs, the contrasting effects of multiple abiotic stresses on NSCs dynamics in woody plants remain poorly understood. This review aims to explore the current knowledge of the contrasting effects of abiotic stress conditions including drought, salinity, heat, water logging, and cold on NSCs dynamics. The roles of NSCs in regulating stress-resilience responses in woody plants are also discussed, along with the challenges in NSC measurement, and options for future research directions are explored. This review is based on comprehensive literature research across different search engines like Scopus, Web of Science, and Google Scholar (2000-2024) using targeted keywords. This study compiles the current research on NSCs functions and provides insights into the adaptive strategies of woody plants in response to changing climate conditions, providing groundwork for future research to improve stress tolerance in woody plants.

Leaf Gas Film 1 promotes glycerol ester accumulation and formation of a tight root barrier to radial O2 loss in rice

Rice (Oryza sativa L.) and many other wetland plants form an apoplastic barrier in the outer parts of the roots to restrict radial O2 loss to the rhizosphere during soil flooding. This barrier facilitates longitudinal internal O2 diffusion via gas-filled tissues from shoot to root apices, enabling root growth in anoxic soils. We tested the hypothesis that Leaf Gas Film 1 (LGF1), which influences leaf hydrophobicity in rice, plays a crucial role in tight outer apoplastic barrier formation in rice roots. We examined the roots of a rice mutant (dripping wet leaf 7, drp7) lacking functional LGF1, its wild type, and an LGF1 overexpression line for their capacity to develop outer apoplastic barriers that restrict radial O2 loss. We quantified the chemical composition of the outer part of the root and measured radial O2 diffusion from intact roots. The drp7 mutant exhibited a weak barrier to radial O2 loss compared to the wild type. However, introducing functional LGF1 into the mutant fully restored tight barrier function. The formation of a tight barrier to radial O2 loss was associated with increased glycerol ester levels in exodermal cells, rather than differences in total root suberization or lignification. These results demonstrate that, in addition to its role in leaf hydrophobicity regulation, LGF1 plays an important role in controlling the function of the outer apoplastic barriers in roots. Our study suggests that increased deposition of glycerol esters in the suberized root exodermis establishes a tight barrier to radial O2 loss in rice roots.

Positive contribution of shoot apex to the growth and flooding tolerance of Hemarthria altissima upon complete submergence

Flooding events tend to destroy the original flood-intolerant vegetation in riparian zones, but the flood-tolerant species can confront the stress, and contribute to the riparian ecosystem. Grass species, Hemarthria altissima, are usually dominant in the riparian zones. This species is considered as good forage which is usually grazed by livestock or mowed by local people. Therefore, the apical tissues of the plants are often removed, and the plants have to grow without stem apexes, during their life cycle. In this study, we aimed to examine the differences in growth performance of intact versus apex-cut individuals of H. altissima upon complete submergence. Two groups of H. altissima plants (with and without shoot apexes) were treated with dark non-submergence and dark complete submergence conditions for 200 days. During the experiment, we measured plant growth, biomass changes in plant organs, and the consumption of non-structural carbohydrates (NSC) by different tissues. During submergence, shoot elongation stopped, and around six lateral buds were developed averagely by each plant without apexes. This growth performance finally caused 60% decline of NSC in underground parts. The relatively intensive consumption of carbohydrates in submerged apex-removed plants induced the 21% stem length decreased under water, which indicated the decreasing submergence tolerance of plants with shoot apex removed. Therefore, we suggest that when using H. altissima for restoring degraded riparian ecosystems, the shoot apexes should be protected from grazing by livestock or harvesting by local people in order to maintain the submergence tolerance of H. altissima.

Low nitrate under waterlogging triggers exodermal suberization to form a barrier to radial oxygen loss in rice roots

To acclimate to hypoxic waterlogged conditions, the roots of wetland plants form a radial oxygen loss (ROL) barrier that can promote oxygen diffusion to the root tips. We hypothesized that the low-nitrate concentrations that occur after molecular oxygen is consumed in waterlogged soils are an environmental trigger for ROL barrier formation in rice (Oryza sativa). We previously identified 128 tissue-specific up/downregulated genes during rice ROL barrier formation. The RiceXPro database showed that many of these genes were differentially regulated in response to nitrogen deficiency. Therefore, we assessed changes in the concentrations of ionic species of nitrogen under stagnant conditions, i.e. in a nutrient solution that mimics waterlogged soil conditions, and examined the effects of an increase or decrease of nitrate in the nutrient solution on ROL barrier formation and exodermal suberization. Preventing nitrate deficiency in the stagnant nutrient solution suppressed the formation of an ROL barrier. Conversely, a decrease in nitrate strongly induced ROL barrier formation, even under aerated conditions. In parallel with ROL barrier formation, suberin lamellae formed at the exodermis. Nitrate deficiency also promoted aerenchyma formation and the enlargement of root diameters. These findings suggest that the severe decline of nitrates under waterlogged conditions is an environmental cue for exodermal suberization to form an ROL barrier in rice roots.

Uncovering the effect of waterlogging stress on plant microbiome and disease development: current knowledge and future perspectives

Waterlogging is a constant threat to crop productivity and ecological biodiversity. Plants face multiple challenges during waterlogging stress like metabolic reprogramming, hypoxia, nutritional depletion, reduction in gaseous exchange, pH modifications, microbiome alterations and disease promotion all of which threaten plants survival. Due to global warming and climatic change, the occurrence, frequency and severity of flooding has dramatically increased posing a severe threat to food security. Thus, developing innovative crop management technologies is critical for ensuring food security under changing climatic conditions. At present, the top priority among scientists is to find nature-based solutions to tackle abiotic or biotic stressors in sustainable agriculture in order to reduce climate change hazards to the environment. In this regard, utilizing plant beneficial microbiome is one of the viable nature based remedial tool for mitigating abiotic stressors like waterlogging. Beneficial microbiota provides plants multifaceted benefits which improves their growth and stress resilience. Plants recruit unique microbial communities to shield themselves against the deleterious effects of biotic and abiotic stress. In comparison to other stressors, there has been limited studies on how waterlogging stress affects plant microbiome structure and their functional traits. Therefore, it is important to understand and explore how waterlogging alters plant microbiome structure and its implications on plant survival. Here, we discussed the effect of waterlogging stress in plants and its microbiome. We also highlighted how waterlogging stress promotes pathogen occurrence and disease development in plants. Finally, we highlight the knowledge gaps and areas for future research directions on unwiring how waterlogging affects plant microbiome and its functional traits. This will pave the way for identifying resilient microbiota that can be engineered to promote their positive interactions with plants during waterlogging stress.

Regulation of photosynthetic function and reactive oxygen species metabolism in sugar beet (Beta vulgaris L.) cultivars under waterlogging stress and associated tolerance mechanisms

Sugar beet (Beta vulgaris L.) is an economically important sugar crop worldwide that is susceptible to sudden waterlogging stress during seedling cultivation, which poses a major threat to sugar beet development and production. Our understanding of the physiological basis of waterlogging tolerance in sugar beet is limited. To investigate the photosynthetic adaptation strategies of sugar beet to waterlogging stress conditions, the tolerant cultivar KUHN1260 (KU) and sensitive cultivar SV1433 (SV) were grown under waterlogging stress, and their photosynthetic function and reactive oxygen species (ROS) metabolism were assessed. Our results showed that waterlogging stress significantly reduced the photosynthetic pigment content, rubisco activity, and expression level of the photosynthetic enzyme genes SvRuBP, SvGAPDH, and SvPRK, gas exchange parameters, and chlorophyll fluorescence parameters, induced damage to the ultrastructure of the chloroplast of the two sugar beet cultivars, inhibited the photosynthetic carbon assimilation capacity of sugar beet leaves, damaged the structural stability of photosystem II (PSII), and disturbed the equilibrium between electrons at the acceptor and donor sides of PSII, which was the result of stomatal and non-stomatal limiting factors. Moreover, the level of ROS, H2O2, and O2▪-, antioxidant enzyme activity, and gene expression levels in the leaves of the two sugar beet cultivars increased over time under waterlogging stress; ROS accumulation was lower and antioxidant enzyme activities and gene expression levels were higher in the waterlogging-tolerant cultivar (KU) than the waterlogging-sensitive cultivar (SV). In sum, these responses in the more tolerant cultivars are associated with their resistance to waterlogging stress. Our findings will aid the breeding of waterlogging-tolerant sugar beet cultivars.

The role of priming and memory in rice environmental stress adaptation: Current knowledge and perspectives

Plant responses to abiotic stresses are dynamic, following the unpredictable changes of physical environmental parameters such as temperature, water and nutrients. Physiological and phenotypical responses to stress are intercalated by periods of recovery. An earlier stress can be remembered as 'stress memory' to mount a response within a generation or transgenerationally. The 'stress priming' phenomenon allows plants to respond quickly and more robustly to stressors to increase survival, and therefore has significant implications for agriculture. Although evidence for stress memory in various plant species is accumulating, understanding of the mechanisms implicated, especially for crops of agricultural interest, is in its infancy. Rice is a major food crop which is susceptible to abiotic stresses causing constraints on its cultivation and yield globally. Advancing the understanding of the stress response network will thus have a significant impact on rice sustainable production and global food security in the face of climate change. Therefore, this review highlights the effects of priming on rice abiotic stress tolerance and focuses on specific aspects of stress memory, its perpetuation and its regulation at epigenetic, transcriptional, metabolic as well as physiological levels. The open questions and future directions in this exciting research field are also laid out.

The hidden roots of wetland methane emissions

Wetlands are the largest natural source of methane (CH4 ) globally. Climate and land use change are expected to alter CH4 emissions but current and future wetland CH4 budgets remain uncertain. One important predictor of wetland CH4 flux, plants, play an important role in providing substrates for CH4 -producing microbes, increasing CH4 consumption by oxygenating the rhizosphere, and transporting CH4 from soils to the atmosphere. Yet, there remain various mechanistic knowledge gaps regarding the extent to which plant root systems and their traits influence wetland CH4 emissions. Here, we present a novel conceptual framework of the relationships between a range of root traits and CH4 processes in wetlands. Based on a literature review, we propose four main CH4 -relevant categories of root function: gas transport, carbon substrate provision, physicochemical influences and root system architecture. Within these categories, we discuss how individual root traits influence CH4 production, consumption, and transport (PCT). Our findings reveal knowledge gaps concerning trait functions in physicochemical influences, and the role of mycorrhizae and temporal root dynamics in PCT. We also identify priority research needs such as integrating trait measurements from different root function categories, measuring root-CH4 linkages along environmental gradients, and following standardized root ecology protocols and vocabularies. Thus, our conceptual framework identifies relevant belowground plant traits that will help improve wetland CH4 predictions and reduce uncertainties in current and future wetland CH4 budgets.

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.

Flooding-adaptive root and shoot traits in rice

Wetland plants, including rice (Oryza spp.), have developed multiple functional adaptive traits to survive soil flooding, partial submergence or even complete submergence. In waterlogged soils and under water, diffusion of O2 and CO2 is extremely slow with severe impacts on photosynthesis and respiration. As a response to shallow floods or rising floodwater, several rice varieties, including deepwater rice, elongate their stems to keep their leaves above the water surface so that photosynthesis can occur unhindered during partial submergence. In stark contrast, some other varieties hardly elongate even if they become completely submerged. Instead, their metabolism is reduced to an absolute minimum so that carbohydrates are conserved enabling fast regrowth once the floodwater recedes. This review focuses on the fascinating functional adaptive traits conferring tolerance to soil flooding, partial or complete submergence. We provide a general analysis of these traits focusing on molecular, anatomical and morphological, physiological and ecological levels. Some of these key traits have already been introgressed into modern high-yielding genotypes improving flood tolerance of several cultivars used by millions of farmers in Asia. However, with the ongoing changes in climate, we propose that even more emphasis should be placed on improving flood tolerance of rice by breeding for rice that can tolerate longer periods of complete submergence or stagnant flooding. Such tolerance could be achieved via additional tissues; i.e. aquatic adventitious roots relevant during partial submergence, and leaves with higher underwater photosynthesis caused by a longer gas film retention time.

Restricted O2 consumption in pea roots induced by hexanoic acid is linked to depletion of Krebs cycle substrates

Plant roots are exposed to hypoxia in waterlogged soils, and they are further challenged by specific phytotoxins produced by microorganisms in such conditions. One such toxin is hexanoic acid (HxA), which, at toxic levels, causes a strong decline in root O2 consumption. However, the mechanism underlying this process is still unknown. We treated pea (Pisum sativum L.) roots with 20 mM HxA at pH 5.0 and 6.0 for a short time (1 h) and measured leakage of key electrolytes such as metal cations, malate, citrate and nonstructural carbohydrates (NSC). After treatment, mitochondria were isolated to assess their functionality evaluated as electrical potential and O2 consumption rate. HxA treatment resulted in root tissue extrusion of K+ , malate, citrate and NSC, but only the leakage of the organic acids and NSC increased at pH 5.0, concomitantly with the inhibition of O2 consumption. The activity of mitochondria isolated from treated roots was almost unaffected, showing just a slight decrease in oxygen consumption after treatment at pH 5.0. Similar results were obtained by treating the pea roots with another organic acid with a short carbon chain, that is, butyric acid. Based on these results, we propose a model in which HxA, in its undissociated form prevalent at acidic pH, stimulates the efflux of citrate, malate and NSC, which would, in turn, cause starvation of mitochondrial respiratory substrates of the Krebs cycle and a consequent decline in O2 consumption. Cation extrusion would be a compensatory mechanism in order to restore plasma membrane potential.

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

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

Mitigation of Greenhouse Gas Emissions from Rice via Manipulation of Key Root Traits

Rice production worldwide represents a major anthropogenic source of greenhouse gas emissions. Nitrogen fertilization and irrigation practices have been fundamental to achieve optimal rice yields, but these agricultural practices together with by-products from plants and microorganisms, facilitate the production, accumulation and venting of vast amounts of CO₂, CH₄ and N₂O. We propose that the development of elite rice varieties should target root traits enabling an effective internal O₂ diffusion, via enlarged aerenchyma channels. Moreover, gas tight barriers impeding radial O₂ loss in basal parts of the roots will increase O₂ diffusion to the root apex where molecular O₂ diffuses into the rhizosphere. These developments result in plants with roots penetrating deeper into the flooded anoxic soils, producing higher volumes of oxic conditions in the interface between roots and rhizosphere. Molecular O₂ in these zones promotes CH₄ oxidation into CO₂ by methanotrophs and nitrification (conversion of NH₄⁺ into NO₃^-), reducing greenhouse gas production and at the same time improving plant nutrition. Moreover, roots with tight barriers to radial O₂ loss will have restricted diffusional entry of CH₄ produced in the anoxic parts of the rhizosphere and therefore plant-mediated diffusion will be reduced. In this review, we describe how the exploitation of these key root traits in rice can potentially reduce greenhouse gas emissions from paddy fields.

A quantitative trait locus conferring flood tolerance to deepwater rice regulates the formation of two distinct types of aquatic adventitious roots

A key trait conferring flood tolerance is the ability to grow adventitious roots as a response to submergence. The genetic traits of deepwater rice determining the development and characteristics of aquatic adventitious roots (AAR) had not been evaluated. We used near-isogenic lines introgressed to test the hypothesis that the impressive shoot elongation ability of deepwater rice linked to quantitative trait loci 1 and 12 also promote the development of AAR. The deepwater rice genotype NIL-12 possessed expanded regions at the stem nodes where numerous AAR developed as a response to submergence. Two types (AR1 and AR2) of roots with distinct timing of emergence and large differences in morphological and anatomical traits formed within 3 (AR1) to 7 (AR2) d of submergence. The mechanical impedance provided by the leaf sheath caused AR2 to emerge later promoting thicker roots, higher elongation capacity and higher desiccation tolerance. Upregulation of key genes suggests a joint contribution in activating the meristem in AAR enhancing the development of these in response to submergence. The morphological and anatomical traits suggested that AR2 is better adapted to long-term flooding than AR1. We therefore propose that AR2 in deepwater rice functions as an evolutionary defence strategy to tackle periodic submergence.

Beneficial Root-Associated Microbiome during Drought and Flooding Stress in Plants

Crop productivity is seriously threatened by the rise in the frequency and severity of drought and flood events around the world. Reduced drought and flooding stress in vulnerable species and ecosystems depends on our ability to comprehend how drought and flooding affect plant physiology and plant-associated microbes. Involvement of both abscisic acid ABA-dependent and ABA-independent pathways has been noted during drought. Hypoxic conditions impede hydraulic conductance, nutrient uptake and plant growth and development, as well as root aerobic respiration. The root microbiome, which works with the roots during drought and flood, is made up of plant growth-promoting rhizosphere, endophytes and mycorrhizas. A large number of phytohormones, primarily auxins, cytokinin and ethylene, as well as enzymes like 1-Aminocyclopropane-1-Carboxylate deaminase (ACC deaminase) and metabolites like exopolysaccharides are produced by rhizospheric microbes. These phytohormones, enzymes and metabolites have role in the induction of systemic drought tolerance in plants. Under hypoxia, anaerobic microbes with the potential to harm the plant due to their pathogenic behavior or soil denitrification ability are more likely to be present in the rhizosphere and roots. This review concentrates on the primary mechanisms of plant-microbe interactions under drought and flood stress as well as the importance of flood and drought-tolerant microbes in maintaining and increasing crop plant productivity under stress.

Plant responses to limited aeration: Advances and future challenges

Limited aeration that is caused by tissue geometry, diffusion barriers, high elevation, or a flooding event poses major challenges to plants and is often, but not exclusively, associated with low oxygen. These processes span a broad interest in the research community ranging from whole plant and crop responses, post-harvest physiology, plant morphology and anatomy, fermentative metabolism, plant developmental processes, oxygen sensing by ERF-VIIs, gene expression profiles, the gaseous hormone ethylene, and O2 dynamics at cellular resolution. The International Society for Plant Anaerobiosis (ISPA) gathers researchers from all over the world contributing to understand the causes, responses, and consequences of limited aeration in plants. During the 14th ISPA meeting, major research progress was related to the evolution of O2 sensing mechanisms and the intricate network that balances low O2 signaling. Here, the work moved beyond flooding stress and emphasized novel underexplored roles of low O2 and limited aeration in altitude adaptation, fruit development and storage, and the vegetative development of growth apices. Regarding tolerance towards flooding, the meeting stressed the relevance and regulation of developmental plasticity, aerenchyma, and barrier formation to improve internal aeration. Additional newly explored flood tolerance traits concerned resource balance, senescence, and the exploration of natural genetic variation for novel tolerance loci. In this report, we summarize and synthesize the major progress and future challenges for low O2 and aeration research presented at the conference.

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.

Wheat Crop under Waterlogging: Potential Soil and Plant Effects

Inundation, excessive precipitation, or inadequate field drainage can cause waterlogging of cultivated land. It is anticipated that climate change will increase the frequency, intensity, and unpredictability of flooding events. This stress affects 10-15 million hectares of wheat every year, resulting in 20-50% yield losses. Since this crop greatly sustains a population's food demands, providing ca. 20% of the world's energy and protein diets requirements, it is crucial to understand changes in soil and plant physiology under excess water conditions. Variations in redox potential, pH, nutrient availability, and electrical conductivity of waterlogged soil will be addressed, as well as their impacts in major plant responses, such as root system and plant development. Waterlogging effects at the leaf level will also be addressed, with a particular focus on gas exchanges, photosynthetic pigments, soluble sugars, membrane integrity, lipids, and oxidative stress.

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.

Beneficial and pathogenic plant-microbe interactions during flooding stress

The number and intensity of flood events will likely increase in the future, raising the risk of flooding stress in terrestrial plants. Understanding flood effects on plant physiology and plant-associated microbes is key to alleviate flooding stress in sensitive species and ecosystems. Reduced oxygen supply is the main constrain to the plant and its associated microbiome. Hypoxic conditions hamper root aerobic respiration and, consequently, hydraulic conductance, nutrient uptake, and plant growth and development. Hypoxia favours the presence of anaerobic microbes in the rhizosphere and roots with potential negative effects to the plant due to their pathogenic behaviour or their soil denitrification ability. Moreover, plant physiological and metabolic changes induced by flooding stress may also cause dysbiotic changes in endosphere and rhizosphere microbial composition. The negative effects of flooding stress on the holobiont (i.e., the host plant and its associated microbiome) can be mitigated once the plant displays adaptive responses to increase oxygen uptake. Stress relief could also arise from the positive effect of certain beneficial microbes, such as mycorrhiza or dark septate endophytes. More research is needed to explore the spiralling, feedback flood responses of plant and microbes if we want to promote plant flood tolerance from a holobiont perspective.

The Role of Phytohormones in Plant Response to Flooding

Climatic variations influence the morphological, physiological, biological, and biochemical states of plants. Plant responses to abiotic stress include biochemical adjustments, regulation of proteins, molecular mechanisms, and alteration of post-translational modifications, as well as signal transduction. Among the various abiotic stresses, flooding stress adversely affects the growth of plants, including various economically important crops. Biochemical and biological techniques, including proteomic techniques, provide a thorough understanding of the molecular mechanisms during flooding conditions. In particular, plants can cope with flooding conditions by embracing an orchestrated set of morphological adaptations and physiological adjustments that are regulated by an elaborate hormonal signaling network. With the help of these findings, the main objective is to identify plant responses to flooding and utilize that information for the development of flood-tolerant plants. This review provides an insight into the role of phytohormones in plant response mechanisms to flooding stress, as well as different mitigation strategies that can be successfully administered to improve plant growth during stress exposure. Ultimately, this review will expedite marker-assisted genetic enhancement studies in crops for developing high-yield lines or varieties with flood tolerance.

Deconstructing the root system of grasses through an exploration of development, anatomy and function

Well-adapted root systems allow plants to grow under resource-limiting environmental conditions and are important determinants of yield in agricultural systems. Important staple crops such as rice and maize belong to the family of grasses, which develop a complex root system that consists of an embryonic root system that emerges from the seed, and a postembryonic nodal root system that emerges from basal regions of the shoot after germination. While early seedling establishment is dependent on the embryonic root system, the nodal root system, and its associated branches, gains in importance as the plant matures and will ultimately constitute the bulk of below-ground growth. In this review, we aim to give an overview of the different root types that develop in cereal grass root systems, explore the different physiological roles they play by defining their anatomical features, and outline the genetic networks that control their development. Through this deconstructed view of grass root system function, we provide a parts-list of elements that function together in an integrated root system to promote survival and crop productivity.

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.

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.

Radial Oxygen Loss from Plant Roots—Methods

In flooded soils, an efficient internal aeration system is essential for root growth and plant survival. Roots of many wetland species form barriers to restrict radial O2 loss (ROL) to the rhizosphere. The formation of such barriers greatly enhances longitudinal O2 diffusion from basal parts towards the root tip, and the barrier also impedes the entry of phytotoxic compounds produced in flooded soils into the root. Nevertheless, ROL from roots is an important source of O2 for rhizosphere oxygenation and the oxidation of toxic compounds. In this paper, we review the methodological aspects for the most widely used techniques for the qualitative visualization and quantitative determination of ROL from roots. Detailed methodological approaches, practical set-ups and examples of ROL from roots with or without barriers to ROL are included. This paper provides practical knowledge relevant to several disciplines, including plant–soil interactions, biogeochemistry and eco-physiological aspects of roots and soil biota.

Novel functions of the root barrier to radial oxygen loss - radial diffusion resistance to H2 and water vapour

The root barrier to radial O₂ loss (ROL) is a trait enabling waterlogging tolerance of plants. The ROL barrier restricts O₂ diffusion to the anoxic soil so that O₂ is retained inside root tissues. We hypothesised that the ROL barrier can also restrict radial diffusion of other gases (H₂ and water vapour) in rice roots with a barrier to ROL. We used O₂ and H₂ microsensors to measure ROL and permeability of rice roots, and gravimetric measurements to assess the influence of the ROL barrier on radial water loss (RWL). The ROL barrier greatly restricted radial diffusion of O₂ as well as H₂ . At 60 kPa pO₂ , we found no radial diffusion of O₂ across the barrier, and for H₂ the barrier reduced radial diffusion by 73%. Similarly, RWL was reduced by 93% in roots with a ROL barrier. Our study showed that the root barrier to ROL not only completely blocks radial O₂ diffusion under steep concentration gradients but is also a diffusive barrier to H₂ and to water vapour. The strong correlation between ROL and RWL presents a case in which simple measurements of RWL can be used to predict ROL in screening studies with a focus on waterlogging tolerance.

Heterotrimeric G protein γ subunit DEP1 is involved in hydrogen peroxide signaling and promotes aerenchyma formation in rice roots

Heterotrimeric G-protein α and β-subunits regulate H₂O₂-mediated aerenchyma formation. The rice G-protein γ-subunit, dense and erect panicle 1 (DEP1), is known to interact with the α-subunit and regulate nitrogen utilization and yield. However, it is unclear whether DEP1 regulates cell death for aerenchyma formation in rice roots. Using wild-type WYJ8 and its transgenic line WYJ8(DEP1), we confirmed that DEP1 is involved in H₂O₂-mediated aerenchyma formation. The rates of aerenchyma formation varied in different parts of the roots in both varieties, with the highest rate in the 4-7 cm segments, reaching a plateau in the 7-8 cm segments. Compared with WYJ8, the aerenchyma area and H₂O₂ content in WYJ8(DEP1) were increased by 55.98% and 53.37%, respectively; however, the responses of aerenchyma formation to exogenous H₂O₂ were basically the same in the two varieties. Diphenylene iodonium (DPI) treatment had no effect on H₂O₂ production and elimination processes in WYJ8, but significantly reduced the activity of the key enzyme that catalyzes H₂O₂ biosynthesis in WYJ8(DEP1). Importantly, exogenous H₂O₂ treatment did not offset the effect of the decrease in endogenous H₂O₂ level caused by DPI on aerenchyma formation. These results indicated that DEP1 enhanced H₂O₂ biosynthesis and promoted the cell death of the root cortex, thus contributing to aerenchyma development in WYJ8(DEP1).

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.

Plant Morphological, Physiological and Anatomical Adaption to Flooding Stress and the Underlying Molecular Mechanisms

Globally, flooding is a major threat causing substantial yield decline of cereal crops, and is expected to be even more serious in many parts of the world due to climatic anomaly in the future. Understanding the mechanisms of plants coping with unanticipated flooding will be crucial for developing new flooding-tolerance crop varieties. Here we describe survival strategies of plants adaptation to flooding stress at the morphological, physiological and anatomical scale systemically, such as the formation of adventitious roots (ARs), aerenchyma and radial O₂ loss (ROL) barriers. Then molecular mechanisms underlying the adaptive strategies are summarized, and more than thirty identified functional genes or proteins associated with flooding-tolerance are searched out and expounded. Moreover, we elaborated the regulatory roles of phytohormones in plant against flooding stress, especially ethylene and its relevant transcription factors from the group VII Ethylene Response Factor (ERF-VII) family. ERF-VIIs of main crops and several reported ERF-VIIs involving plant tolerance to flooding stress were collected and analyzed according to sequence similarity, which can provide references for screening flooding-tolerant genes more precisely. Finally, the potential research directions in the future were summarized and discussed. Through this review, we aim to provide references for the studies of plant acclimation to flooding stress and breeding new flooding-resistant crops in the future.

Regulation of root adaptive anatomical and morphological traits during low soil oxygen

Flooding causes oxygen deprivation in soils. Plants adapt to low soil oxygen availability by changes in root morphology, anatomy, and architecture to maintain root system functioning. Essential traits include aerenchyma formation, a barrier to radial oxygen loss, and outgrowth of adventitious roots into the soil or the floodwater. We highlight recent findings of mechanisms of constitutive aerenchyma formation and of changes in root architecture. Moreover, we use modelling of internal aeration to demonstrate the beneficial effect of increasing cortex-to-stele ratio on sustaining root growth in waterlogged soils. We know the genes for some of the beneficial traits, and the next step is to manipulate these genes in breeding in order to enhance the flood tolerance of our crops.

Lateral roots, in addition to adventitious roots, form a barrier to radial oxygen loss in Zea nicaraguensis and a chromosome segment introgression line in maize

Plants typically respond to waterlogging by producing new adventitious roots with aerenchyma and many wetland plants form a root barrier to radial O₂ loss (ROL), but it was not known if this was also the case for lateral roots. We tested the hypothesis that lateral roots arising from adventitious roots can form a ROL barrier, using root-sleeving electrodes and O₂ microsensors to assess ROL of Zea nicaraguensis, the maize (Zea mays ssp. mays) introgression line with a locus for ROL barrier formation (introgression line (IL) #468) from Z. nicaraguensis and a maize inbred line (Mi29). Lateral roots of Z. nicaraguensis and IL #468 both formed a ROL barrier under stagnant, deoxygenated conditions, whereas Mi29 did not. Lateral roots of Z. nicaraguensis had higher tissue O₂ status than for IL #468 and Mi29. The ROL barrier was visible as suberin in the root hypodermis/exodermis. Modelling showed that laterals roots can grow to a maximum length of 74 mm with a ROL barrier, but only to 33 mm without a barrier. Presence of a ROL barrier in lateral roots requires reconsideration of the role of these roots as sites of O₂ loss, which for some species now appears to be less than hitherto thought.

Selenium enhances iron plaque formation by elevating the radial oxygen loss of roots to reduce cadmium accumulation in rice (Oryza sativa L.)

The inhibition of cadmium (Cd) absorption by selenium (Se) in rice may be associated with iron plaque (IP) formation, but the driving mechanisms are still unclear. This study investigated the effects of Se on the growth, oxidative toxicity, radial oxygen loss (ROL), IP formation, and Cd absorption of rice exposed to Cd. The results of this study showed that Cd stress elevated the levels of O₂- and H₂O₂ and depressed superoxide dismutase (SOD) and catalase (CAT) activities. The maximum ROL and IP were reduced by 43.3 % and 74.5 %, respectively. However, Se alleviated Cd toxicity by stimulating SOD and CAT activities by scavenging O₂- and H₂O₂ and enhancing the ROL profiles. Under culture conditions without Fe²⁺, Se had no impact on the total Cd levels in rice (T_Cd). However, with the addition of Fe²⁺, T_Cd was significantly reduced by 23.3 % due to the enhancement of IP formation by Se. These results indicated that Se can reduce Cd accumulation in rice in the presence of Fe²⁺ treatments. However, Se just alleviated Cd toxicity in the absence of Fe²⁺ treatments. The enhancement of ROL was a potential reason for the elevated IP formation induced by Se.

Dynamics of radial oxygen loss in mangroves subjected to waterlogging

Tidal flooding can directly result in oxygen (O₂) shortage, however the functions of root aeration in flooding tolerance and O₂ dynamics within mangroves are still poorly understood. Thus, in this study, the correlations among waterlogging tolerance, root porosity and O₂ movement within the plants were investigated using two mangrove species (Aegiceras corniculatum and Bruguiera gymnorrhiza) and a semi-mangrove Heritiera littoralis. Based on the present data, the species A. corniculatum and B. gymnorrhiza, which possessed higher root porosity, exhibited higher waterlogging tolerance, while H. littoralis is intolerant. Increased root porosity, leaf stoma, and total ROL were observed in the roots of A. corniculatum and B. gymnorrhiza growing in stagnant solution when compared to respective aerated controls. As for ROL spatial pattern along roots, external anaerobic condition could promote ROL from apical root regions but reduce ROL from basal roots, leading to a 'tighter barrier'. In summary, the present study indicated that the plants (e.g., A. corniculatum and B. gymnorrhiza) prioritized to ensure O₂ diffusion towards root tips under waterlogging by increasing aerenchyma formation and reducing O₂ leakage at basal root regions.

Some Accessions of Amazonian Wild Rice (Oryza glumaepatula) Constitutively Form a Barrier to Radial Oxygen Loss along Adventitious Roots under Aerated Conditions

A barrier to radial oxygen loss (ROL), which reduces the loss of oxygen transported via the aerenchyma to the root tips, enables the roots of wetland plants to grow into anoxic/hypoxic waterlogged soil. However, little is known about its genetic regulation. Quantitative trait loci (QTLs) mapping can help to understand the factors that regulate barrier formation. Rice (Oryza sativa) inducibly forms an ROL barrier under stagnant conditions, while a few wetland plants constitutively form one under aerated conditions. Here, we evaluated the formation of a constitutive ROL barrier in a total of four accessions from two wild rice species. Three of the accessions were wetland accessions of O. glumaepatula, and the fourth was a non-wetland species of O. rufipogon. These species have an AA type genome, which allows them to be crossed with cultivated rice. The three O. glumaepatula accessions (W2165, W2149, and W1183) formed an ROL barrier under aerated conditions. The O. rufipogon accession (W1962) did not form a constitutive ROL barrier, but it formed an inducible ROL barrier under stagnant conditions. The three O. glumaepatula accessions should be useful for QTL mapping to understand how a constitutive ROL barrier forms. The constitutive barrier of W2165 was closely associated with suberization and resistance to penetration by an apoplastic tracer (periodic acid) at the exodermis but did not include lignin at the sclerenchyma.

Root O2 consumption, CO2 production and tissue concentration profiles in chickpea, as influenced by environmental hypoxia

Roots in flooded soils experience hypoxia, with the least O₂ in the vascular cylinder. Gradients in CO₂ across roots had not previously been measured. The respiratory quotient (RQ; CO₂ produced : O₂ consumed) is expected to increase as O₂ availability declines. A new CO₂ microsensor and an O₂ microsensor were used to measure profiles across roots of chickpea seedlings in aerated or hypoxic conditions. Simultaneous, nondestructive flux measurements of O₂ consumption, CO₂ production, and thus RQ, were taken for roots with declining O₂ . Radial profiling revealed severe hypoxia and c. 0.8 kPa CO₂ within the root vascular cylinder. The distance penetrated by O₂ into the roots was shorter at lower O₂ . The gradient in CO₂ was in the opposite direction to that of O₂ , across the roots and diffusive boundary layer. RQ increased as external O₂ was lowered. For chickpea roots in solution at air equilibrium, O₂ was very low and CO₂ was elevated within the vascular cylinder; the extent of the severely hypoxic core increased as external O₂ was reduced. The increased RQ in roots in response to declining external O₂ highlighted the shift from respiration to ethanolic fermentation as the severely hypoxic/anoxic core became a progressively greater proportion of the root tissues.