Root length is proxy for high-throughput screening of waterlogging tolerance in Urochloa spp. grasses

Clear effects on root system architecture of winter wheat cultivars (Triticum aestivum L.) from cultivation environment and practices

Roots play a pivotal role in the adaption of a plant to its environment, with different root traits adapting the plant to different stresses. The environment affects the Root System Architecture (RSA), but the genetic factors determine to what extent, and whether stress brought about by extreme environmental conditions is detrimental to a specific crop. This study aimed to identify differences in winter wheat RSA caused by cultivation region and practice, in the form of preceding crop (precrop), and to identify if modern cultivars used in Sweden differ in their reaction to these environments. This was undertaken using high-throughput phenotyping to assess the RSA. Clear differences in the RSA were observed between the Swedish cultivation regions, precrop treatments, and interaction of these conditions with each other and the genetics. Julius showed a large difference between cultivars, with 9.3-17.1% fewer and 12-20% narrower seminal roots. Standardized yield decreased when grown after wheat, 23% less compared to oilseed rape (OSR), and when grown in the Southern region, 14% less than the Central region. Additionally, correlations were shown between the root number, angle, and grain yield, with different root types being correlated depending on the precrop. Cultivars on the Swedish market show differences that can be adapted to the region-precrop combinations. The differences in precrop effect on RSA between regions show global implications and a need for further assessment. Correlations between RSA and yield, based on root-type × precrop, indicate different needs of the RSA depending on the management practices and show the potential for improving crop yield through targeting genotypic and environmental conditions in a holistic manner. Understanding this RSA variance, and the mechanisms of conditional response, will allow targeted cultivar breeding for specific environments, increasing plant health and food security.

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.

Changes in morphological traits associated with waterlogging, salinity and saline waterlogging in Festuca arundinacea

Rising incidences of waterlogging and salinity, particularly in extensive livestock farming areas, pose increasing challenges to plant growth. This study investigated the morphological growth responses and tolerance of 39 Festuca arundinacea accessions to these stresses, with tolerance quantified by the relative growth rate under stress versus control conditions. Notably, more productive accessions under normal conditions also showed greater stress tolerance. Waterlogging was generally well-tolerated (89-113% of control relative growth rate), without significantly altering growth morphological components as increases in specific leaf area were offset by reductions in leaf weight ratio, maintaining stable leaf area ratios. Conversely, salinity and combined saline waterlogging significantly reduced relative growth rate (56-94% of control), with a substantial variation among accessions. A decrease in specific leaf area, suggestive of thicker leaves, correlated with higher tolerance to salinity and saline waterlogging (r =0.63). In summary, F. arundinacea displays diverse tolerance to these stresses, warranting further study into the adaptive mechanisms. Specific leaf area emerges as a potential selection marker for breeding programs targeting saline and waterlogging tolerance.

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.

Physiology of Plant Responses to Water Stress and Related Genes: A Review

Drought and waterlogging seriously affect the growth of plants and are considered severe constraints on agricultural and forestry productivity; their frequency and degree have increased over time due to global climate change. The morphology, photosynthetic activity, antioxidant enzyme system and hormone levels of plants could change in response to water stress. The mechanisms of these changes are introduced in this review, along with research on key transcription factors and genes. Both drought and waterlogging stress similarly impact leaf morphology (such as wilting and crimping) and inhibit photosynthesis. The former affects the absorption and transportation mechanisms of plants, and the lack of water and nutrients inhibits the formation of chlorophyll, which leads to reduced photosynthetic capacity. Constitutive overexpression of 9-cis-epoxydioxygenase (NCED) and acetaldehyde dehydrogenase (ALDH), key enzymes in abscisic acid (ABA) biosynthesis, increases drought resistance. The latter forces leaf stomata to close in response to chemical signals, which are produced by the roots and transferred aboveground, affecting the absorption capacity of CO2, and reducing photosynthetic substrates. The root system produces adventitious roots and forms aerenchymal to adapt the stresses. Ethylene (ETH) is the main response hormone of plants to waterlogging stress, and is a member of the ERFVII subfamily, which includes response factors involved in hypoxia-induced gene expression, and responds to energy expenditure through anaerobic respiration. There are two potential adaptation mechanisms of plants (“static” or “escape”) through ETH-mediated gibberellin (GA) dynamic equilibrium to waterlogging stress in the present studies. Plant signal transduction pathways, after receiving stress stimulus signals as well as the regulatory mechanism of the subsequent synthesis of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) enzymes to produce ethanol under a hypoxic environment caused by waterlogging, should be considered. This review provides a theoretical basis for plants to improve water stress tolerance and water-resistant breeding.

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.