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

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

BACKGROUND AND AIMS

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

METHODS

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

KEY RESULTS

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

CONCLUSIONS

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

Overcoming constraints to measuring O2 diffusivity and consumption of intact roots

A method using O2 microsensors enables detailed quantification of respiratory O2 consumption and diffusive resistance to O2 of individual root cell layers.

Climate-resilient crops: Lessons from xerophytes

Developing climate-resilient crops is critical for future food security and sustainable agriculture under current climate scenarios. Of specific importance are drought and soil salinity. Tolerance traits to these stresses are highly complex, and the progress in improving crop tolerance is too slow to cope with the growing demand in food production unless a major paradigm shift in crop breeding occurs. In this work, we combined bioinformatics and physiological approaches to compare some of the key traits that may differentiate between xerophytes (naturally drought-tolerant plants) and mesophytes (to which the majority of the crops belong). We show that both xerophytes and salt-tolerant mesophytes have a much larger number of copies in key gene families conferring some of the key traits related to plant osmotic adjustment, abscisic acid (ABA) sensing and signalling, and stomata development. We show that drought and salt-tolerant species have (i) higher reliance on Na for osmotic adjustment via more diversified and efficient operation of Na+ /H+ tonoplast exchangers (NHXs) and vacuolar H+ - pyrophosphatase (VPPases); (ii) fewer and faster stomata; (iii) intrinsically lower ABA content; (iv) altered structure of pyrabactin resistance/pyrabactin resistance-like (PYR/PYL) ABA receptors; and (v) higher number of gene copies for protein phosphatase 2C (PP2C) and sucrose non-fermenting 1 (SNF1)-related protein kinase 2/open stomata 1 (SnRK2/OST1) ABA signalling components. We also show that the past trends in crop breeding for Na+ exclusion to improve salinity stress tolerance are counterproductive and compromise their drought tolerance. Incorporating these genetic insights into breeding practices could pave the way for more drought-tolerant and salt-resistant crops, securing agricultural yields in an era of climate unpredictability.

Flooding stress and responses to hypoxia in plants

In recent years, research on flooding stress and hypoxic responses in plants has gathered increasing attention due to climate change and the important role of O2 in metabolism and signalling. This Collection of Functional Plant Biology on 'Flooding stress and responses to hypoxia in plants' presents key contributions aimed at progressing our current understanding on how plants respond to low-O2 conditions, flooding stress and a combination of stresses commonly found in flooded areas. The Collection emphasises the characterisation of diverse plant responses across different developmental stages, from seed germination to fully developed plants, and under different water stress conditions ranging from waterlogging to complete submergence, or simply low-O2 conditions resulting from limited O2 diffusivity in bulky tissues. Additionally, this Collection highlights diverse approaches, including eco-physiological characterisation of plant responses, detailed descriptions of root anatomical characteristics and their surrounding microenvironments, evaluation of the seed microbiota under flooding stress, the modification of gene expression, and evaluations of diverse germplasm collections.

Invasion mechanism of Spartina alterniflora by regulating soil sulfur and iron cycling and microbial composition in the Jiuduansha Wetland

Spartina alterniflora, an invasive plant widely distributed in China's coastal regions, has had a significant impact on the stability of wetland ecosystems and elemental biogeochemical cycles. The invasion of S. alterniflora has been found to lead to the accumulation of sulfides in the soil. The cycling of sulfur and iron in the soil is closely interconnected. Coastal estuarine wetlands are influenced by both freshwater in rivers and seawater tides, as well as the frequent variations in redox conditions caused by tidal fluctuations, which makes the cycling of sulfur and iron in the soil invaded by S. alterniflora more intricate. In this study, field surveys and laboratory experiments were conducted to explore the effects of S. alterniflora invasion and hydrological changes on the cycling of sulfur and iron as well as related functional microorganisms in the soil. The invasion of S. alterniflora showed an increase in soil reduced inorganic sulfur (RIS) components in both high and low marshes of Jiuduansha wetland, with higher content observed in summer and autumn. The tidal simulation experiments revealed abundant sulfate in seawater tidal conditions could promote the formation of acid volatile sulfides (AVS) in the soil of low marshes invaded by S. alterniflora and ensuring the continuous increase in AVS content. Diffusive gradients in-thin-films (DGT) technology indicated the existence of high-concentration soluble S2- enrichment zones in the soil of low marshes invaded by S. alterniflora, which may be related to S. alterniflora root exudates. Tidal action increased the relative abundance of sulfur-reducing bacteria (SRB) in the soil of low marshes, and under the influence of seawater tidal action, SRB exhibited higher relative abundance. However, S. alterniflora might inhibit the activity of iron-reducing bacteria (FeRB) in the soil of low marshes. In conclusion, S. alterniflora may enhance the sulfate reduction rate and promote the formation of free sulfides in tidal salt marsh ecosystems by releasing root exudates that stimulate the activity of SRB, while concurrently inhibiting the activity of FeRB and reducing their competition with SRB. This effect is particularly pronounced in low marshes under seawater tidal conditions. Thus, S. alterniflora is capable of rapidly invading tidal salt marshes by utilizing sulfides effectively.

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.

Comparative Analysis of Cd Uptake and Tolerance in Two Mangrove Species (Avicennia marina and Rhizophora stylosa) with Distinct Apoplast Barriers

Mangrove plants demonstrate an impressive ability to tolerate environmental pollutants, but excessive levels of cadmium (Cd) can impede their growth. Few studies have focused on the effects of apoplast barriers on heavy metal tolerance in mangrove plants. To investigate the uptake and tolerance of Cd in mangrove plants, two distinct mangrove species, Avicennia marina and Rhizophora stylosa, are characterized by unique apoplast barriers. The results showed that both mangrove plants exhibited the highest concentration of Cd2+ in roots, followed by stems and leaves. The Cd2+ concentrations in all organs of R. stylosa consistently exhibited lower levels than those of A. marina. In addition, R. stylosa displayed a reduced concentration of apparent PTS and a smaller percentage of bypass flow when compared to A. marina. The root anatomical characteristics indicated that Cd treatment significantly enhanced endodermal suberization in both A. marina and R. stylosa roots, and R. stylosa exhibited a higher degree of suberization. The transcriptomic analysis of R. stylosa and A. marina roots under Cd stress revealed 23 candidate genes involved in suberin biosynthesis and 8 candidate genes associated with suberin regulation. This study has confirmed that suberized apoplastic barriers play a crucial role in preventing Cd from entering mangrove roots.

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.

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.