Plant roots sense soil compaction through restricted ethylene diffusion

Optimizing Root Phenotypes for Compacted Soils: Enhancing Root-Soil-Microbe Interactions

Soil compaction impedes root growth, reduces crop yields, and threatens global food security and sustainable agriculture. Addressing this challenge requires a comprehensive understanding of root-soil interactions in compacted environments. This review examines key root traits-architectural, anatomical, biochemical, and biomechanical-that enhance plant resilience in compacted soils. We discuss how these traits influence root penetration and the formation of more favorable soil pore structures, which are crucial for alleviating compaction stress. Additionally, we explore the molecular mechanisms underlying root adaptation, identifying key genetic and biochemical factors that contribute to stress-tolerant root phenotypes. The review emphasizes the role of root-microbe interactions in boosting root adaptability under compaction. By integrating these insights, we propose a framework for breeding crops with resilient root systems that thrive in high soil strength, supporting sustainable agricultural practices essential for food security amidst environmental challenges.

Responses of root hairs to soil compaction: A review

In recent years, many studies have investigated the effects of soil compaction on plant root growth. However, root hairs, which are important parts of plants that anchor the soil and absorb nutrients and water, under compacted conditions have received limited attention. We reviewed the responses of root hair structure (behaviors), the rhizosheath, water and nutrient uptake by root hairs, plant hormones and crop species associated with root hairs to soil compaction and proposed potential solutions to mitigate the impacts of soil compaction on root hairs. Soil compaction generally reduces root hair length and density; however, a few studies have reported opposite results for reasons that are unclear. Root hairs exhibit limited water and nutrient uptake capacity, whereas high levels of ethylene have been observed in response to soil compaction. The scales of the effects described above are closely related to genotype. Bio-tillage, the application of ethylene inhibitors, the use of microorganisms and the breeding of soil compaction-tolerant crops may be effective methods to promote root hair growth under soil compaction. High-resolution computed tomography (CT) techniques are needed in the future to study root hair interactions with non-homogeneous soils over large scales.

Hormonal regulation of primary root development

Plant post-embryonic development is characterized by the unceasing generation of new tissues and organs that allow plants to adapt their growth and architecture to environmental conditions. Primary root indeterminate growth is sustained by the activity of the root apical meristem (RAM), an organized structure containing stem cells that continuously divide to generate new tissues. RAM, thanks to its simple and symmetric structure, is an excellent system for studying how hormone distributions and interactions regulate growth. Plant hormones are pivotal actors in establishing and maintaining RAM patterning, and their developmental outputs are dependent on their integrated activities. In this review, we discuss recent findings on hormonal crosstalk that shapes the root meristem, focusing our attention on both the root radial and longitudinal axes.

Ca2+ waves and ethylene/JA crosstalk orchestrate wound responses in Arabidopsis roots

Wounding triggers complex and multi-faceted responses in plants. Among these, calcium (Ca2+) waves serve as an immediate and localized response to strong stimuli, such as nematode infection or laser ablation. Here, we investigate the propagation patterns of Ca2+ waves induced by laser ablation and observe that glutamate-receptor-like channels (GLR3.3/GLR3.6), the stretch-activated anion channel MSL10, and the mechanosensitive Ca2+-permeable channels MCA1/MCA2 influence this process. These channels contribute to ethylene-associated signaling pathways, potentially through the WRKY33-ACS6 regulatory network. Furthermore, our findings show that ACC/ethylene signaling modulates Ca2+ wave propagation following laser ablation. Ethylene perception and synthesis at the site of damage regulate the local jasmonate response, which displays tissue-specific patterns upon laser ablation. Overall, our data provide new insights into the molecular and cellular processes underlying plant responses to localized damage, highlighting the roles of specific ion channels and hormone signaling pathways in shaping these responses in Arabidopsis roots.

Identification of a Specific Role of Dihydrozeatin in the Regulation of the Cell Differentiation Activity in Arabidopsis Roots

The plant hormones cytokinins are a class of heterogeneous active compounds that control multiple aspects of development and physiology. Among cytokinins, trans-zeatin (tZ), the most abundant cytokinin, has been extensively studied in relation to its effects on development, and it plays a key role in promoting cell differentiation. In analogy with tZ, here we demonstrate that dihydrozeatin (DHZ) controls (root) development by promoting cell differentiation. By means of pharmacological and genetic analysis, we demonstrate that DHZ is specifically and uniquely perceived by the histidine kinase (HK) receptor AHK3, and that this interaction is sufficient to promote cell differentiation in the root meristem via activation of the transcription factors ARABIDOPSIS RESPONSE REGULATOR 1, 12, and 11. We also show that DHZ and tZ activity might be conserved among plants. Our results support the idea that different types of cytokinins act via specific receptors to exert their roles and suggest new approaches to study their activity in differentiation.

Roles of hormones in regulating root growth-water interactions

Water stress presents a critical challenge affecting plant growth and agricultural productivity, with drought alone causing substantial yield losses. Roots serve as the primary site for water uptake, enabling plants to detect water stress by sensing changes in soil moisture levels. This initial perception prompts roots to initiate a spectrum of adaptive responses at morphological, anatomical, and biochemical levels. In addition to coping with severe water stress conditions such as drought, roots also respond to microscale variations in water availability within the rhizosphere as they navigate through soil, exhibiting responses such as hydrotropism, xerobranching, and hydropatterning. These adaptive responses are orchestrated by dynamic and sophisticated sensing and signalling mechanisms mediated by plant hormones at the cellular level. This review explores recent advances in our understanding of root responses to water stress, emphasizing the hormonal mechanisms underpinning these adaptations. Furthermore, it outlines future perspectives aimed at enhancing crop resilience to water stress through improved understanding and manipulation of root-water interactions.

Soil compaction sensing mechanisms and root responses

Soil compaction is an agricultural challenge with profound influence on the physical, chemical, and biological properties of the soil. It causes drastic changes by increasing mechanical impedance, reducing water infiltration, gaseous exchange, and biological activities. Soil compaction hinders root growth, limiting nutrient and water foraging abilities of plants. Recent research reveals that plant roots sense soil compaction due to higher ethylene accumulation in and around root tips. Ethylene orchestrates auxin and abscisic acid as downstream signals to regulate root adaptive responses to soil compaction. In this review, we describe the changes inflicted by soil compaction ranging from cell to organ scale and explore the latest research regarding plant root compaction sensing and response.

Root Growth and Development in "Real Life": Advances and Challenges in Studying Root-Environment Interactions

Plant roots play myriad roles that include foraging for resources in complex soil environments. Within this highly dynamic soil environment roots must sense, interact with, and acclimate to factors such as water availability, microbiota, and heterogeneous distribution of nutrients. To aid their acclimation, roots alter their growth and development to optimize their architecture and actively regulate the physical, chemical, and biological properties of their rhizosphere. Understanding the complex interactions between roots and rhizosphere is critical for designing future crops with improved root traits better adapted to diverse and challenging soil conditions. However, studying roots and their interactions with soil under real-world conditions presents significant challenges. Addressing these challenges demands developing realistic laboratory-based model systems and innovative field-based root imaging techniques. Our review surveys the current knowledge and recent advances in understanding root-environment interactions while proposing future solutions to study roots under more "real-life" soil conditions.

Single-cell transcriptomics reveal how root tissues adapt to soil stress

Land plants thrive in soils showing vastly different properties and environmental stresses1. Root systems can adapt to contrasting soil conditions and stresses, yet how their responses are programmed at the individual cell scale remains unclear. Using single-cell RNA sequencing and spatial transcriptomic approaches, we showed major expression changes in outer root cell types when comparing the single-cell transcriptomes of rice roots grown in gel versus soil conditions. These tissue-specific transcriptional responses are related to nutrient homeostasis, cell wall integrity and defence in response to heterogeneous soil versus homogeneous gel growth conditions. We also demonstrate how the model soil stress, termed compaction, triggers expression changes in cell wall remodelling and barrier formation in outer and inner root tissues, regulated by abscisic acid released from phloem cells. Our study reveals how root tissues communicate and adapt to contrasting soil conditions at single-cell resolution.

Mechanosensing antagonizes ethylene signaling to promote root gravitropism in rice

Root gravitropism relies on gravity perception by the root cap and requires tightly regulated phytohormone signaling. Here, we isolate a rice mutant that displays root coiling in hydroponics but normal gravitropic growth in soil. We identify COILING ROOT IN WATER 1 (CRW1) encoding an ETHYLENE-INSENSITIVE3 (EIN3)-BINDING F-BOX PROTEIN (OsEBF1) as the causative gene for the mutant phenotype. We show that the OsCRW1-EIN3 LIKE 1 and 2 (OsEIL1/2)-ETHYLENE RESPONSE FACTOR 82 (OsERF82) module controls the production of reactive oxygen species in the root tip, subsequently impacting root cap stability, polar localization of PIN-FORMED 2 (OsPIN2), symmetric distribution of auxin, and ultimately gravitropic growth of roots. The OsEIL1/2-OsERF82 ethylene signaling module is effectively impeded by applying gentle mechanical resistance to root tips, including growing in water-saturated paddy soil. We further show that mechanosensing-induced calcium signaling is required and sufficient for antagonizing the ethylene signaling pathway. This study has revealed previously unanticipated interplay among ethylene, auxin, and mechanosensing in the control of plant gravitropism.

Root RADAR: how 'rhizocrine' signals allow roots to detect and respond to their soil environment and stresses

Agricultural intensification coupled with changing climate are causing soils to become increasingly vulnerable to stresses such as drought, soil erosion, and compaction. The mechanisms by which roots detect and respond to soil stresses remain poorly understood. Recent breakthroughs show that roots release volatile and soluble hormone signals into the surrounding soil, then monitor their levels to sense soil stresses. Our review discusses how hormones can act 'outside the plant' as 'rhizocrine' signals that function to improve plant resilience to different soil stresses. We also propose a novel signalling paradigm which we term 'root RADAR' where 'rhizocrine' levels change in soil in response to environmental stresses, feeding back to roots and triggering adaptive responses.

Foot traffic on turf primarily shaped the endophytic bacteriome of the soil-rhizosphere-root continuum

Foot traffic on turf can cause grass wear-stress and soil compaction, adversely impacting turf health. The root microbiome, consisting of diverse microbes, plays a crucial role in enhancing plant resilience to abiotic stressors. However, the effects of foot traffic on these microbes and the mechanisms they employ to help plant survival remain largely unknown. Here, we investigated how foot traffic affected microbial communities of the root endosphere, rhizosphere, and bulk soil in Bermudagrass (Cynodon spp.) and Zoysiagrass (Zoysia spp.) turfs. Foot traffic was simulated to mimic six professional football games per week using a modified Baldree traffic simulator. High-throughput amplicon sequencing targeting 16S rRNA for bacteria and ITS for fungi was employed to analyze microbial communities. Foot traffic slightly and significantly reduced soil moisture and inorganic nitrogen, likely due to soil compaction and associated impairment on microbial activity. Microbial alpha diversity varied across microhabitats, with no discernible effect of foot traffic. However, microbial community composition was impacted by foot traffic, being more pronounced on bacteria of the root endosphere and on fungi of the bulk soil. In light of the genetic potential predicted by PICRUSt2, foot traffic enriched a few pathways of the endophytic bacteriome, including nitrifier denitrification (PWY7084) and mannosylglycerate biosynthesis (PWY5656). This indicated that root endophytes could help turfgrass to tolerate foot traffic via controls on the concentration of nitric oxide, the signaling molecule for root growth, and mannosylglycerate, the compatible solute for protecting enzymes against osmotic stress. Foot traffic also enhanced degradation pathways of carbohydrates and 4-coumarate, the constituent of turfgrass cell walls (PWY-3801, PWY-2221, PWY-7046), indicating the faster turnover of root tissues. Along the root-rhizosphere-bulk soil continuum, the bacteriome varied substantially in composition and also exhibited contrasting genetic potentials from stress alleviation to nutrient supply in coping with grass growth. But foot traffic had little effect on the genetic potential of bacteriome in rhizosphere and bulk soil. Our findings indicated that the endophytic bacteriome was more sensitive to foot traffic than the bacteriome in the rhizosphere and bulk soil and could potentially help turf survival via influences on plant signal molecules and compatible solutes.

Mechanical stimulation in plants: molecular insights, morphological adaptations, and agricultural applications in monocots

Mechanical stimulation, including wind exposure, is a common environmental factor for plants and can significantly impact plant phenotype, development, and growth. Most responses to external mechanical stimulation are defined by the term thigmomorphogenesis. While these morphogenetic changes in growth and development may not be immediately apparent, their end-results can be substantial. Although mostly studied in dicotyledonous plants, recently monocot grasses, particularly cereal crops, have received more attention. This review summarizes current knowledge on mechanical stimulation in plants, particularly focusing on the molecular, physiological, and phenological responses in cereals, and explores practical applications to sustainably improve the resilience of agricultural crops.

The phytohormone brassinosteroid (BR) promotes early seedling development via auxin signaling pathway in rapeseed

The phytohormone brassinosteroid (BR) regulate various developmental and physiological processes in plants. However, the function of BR during early seedling development stage in rapeseed is largely unknown. To understand the effects of exogenous BR during early seedling development, the ZS11 and BR-INSENSITIVE (bin2) mutants were treated with BR before seed sowing and seed germination stage under 16/8 hours light/dark cycle. The phenotype results indicated that BR promotes only seedling establishment but not seed germination stage in ZS11, while no function in bin2 mutants. Since BRs play a crucial role in regulation of developmental transition between growth in the dark (skotomorphogenesis) and growth in the light (photomorphogenesis), the ZS11 and bin2 mutants were treated with BR under continuous light and dark. The BR treatment also showed the same functions as 16/8 hours light/dark cycle. To understand the function of BR on expression levels, the differentially expressed genes (DEGs) and differentially expressed metabolites (DEMs) between mock- and BR-treated seedlings were explored. A total of 234 significantly DEGs were identified between the mock- and BR-treated groups by transcriptomic analyses. These DEGs were markedly enriched in BR biosynthesis, pentose and glucuronate interconversions and plant hormone signal transduction pathways. Meanwhile, a total of 145 DEMs were identified through metabolomics analyses, with a significant enrichment in lipid substances. Interestingly, some genes and metabolites associated with auxin pathway were identified, which exhibited up-regulation in both DEGs and DEMs after BR treatment. Subsequently, functional enrichment analyses revealed that the majority of DEGs and DEMs were primarily enriched in ascorbate and aldehyde metabolism, arginine and proline metabolism, tryptophan metabolism (the main route for auxin synthesis) and cyanogenic amino acid metabolism. Furthermore, it was found that glutamate was up-regulated in nitrogen metabolism, glyoxylate and dicarboxylate metabolism, and arginine and proline metabolism pathways. These indicated that the glutamate signaling pathway was a key regulatory pathway for exogenous BR to induce seedling establishment. These evidence implied that exogenous BR treatment lead to up-regulation of auxin-related genes expression, then promoted seedling establishment in rapeseed.

Moisture-responsive root-branching pathways identified in diverse maize breeding germplasm

Plants grow complex root systems to extract unevenly distributed resources from soils. Spatial differences in soil moisture are perceived by root tips, leading to the patterning of new root branches toward available water in a process called hydropatterning. Little is known about hydropatterning behavior and its genetic basis in crop plants. Here, we developed an assay to measure hydropatterning in maize and revealed substantial differences between tropical/subtropical and temperate maize breeding germplasm that likely resulted from divergent selection. Genetic analysis of hydropatterning confirmed the regulatory role of auxin and revealed that the gaseous hormone ethylene locally inhibits root branching from air-exposed tissues. Our results demonstrate how distinct signaling pathways translate spatial patterns of water availability to developmental programs that determine root architecture.

A lipid synthase maintains metabolic flux for jasmonate synthesis to regulate root growth and phosphate homeostasis

Plants require phosphate (Pi) for proper growth and development but often face scarcity of this vital nutrient in the soil. Pi starvation triggers membrane lipid remodeling to utilize the membrane phospholipid-bound Pi in plants. In this process, phospholipids are replaced by non-Pi-containing galactolipids (monogalactosyldiacylglycerol, MGDG; digalactosyldiacylglycerol, DGDG) and sulfolipids. The galactolipids ratio (MGDG:DGDG) is suggested to influence jasmonic acid (JA) biosynthesis. However, how the MGDG:DGDG ratio, JA levels, and root growth are coordinated under Pi deficiency in rice (Oryza sativa) remains unknown. Here, we characterized DGDG synthase 1 (OsDGD1) for its role in regulating root development by maintaining metabolic flux for JA biosynthesis. We showed that OsDGD1 is responsive under low Pi and is under the direct control of Phosphate Starvation Response 2, the master regulator of low Pi adaptations. Further, OsDGD1 knockout (KO) lines showed marked phenotypic differences compared to the wild type, including a significant reduction in root length and biomass, leading to reduced Pi uptake. Further, lipidome analyses revealed reduced DGDG levels in the KO line, leading to reduced membrane remodeling, thus affecting P utilization efficiency. We also observed an increase in the MGDG:DGDG ratio in KO lines, which enhanced the endogenous JA levels and signaling. This imbalance of JA in KO plants led to changes in auxin levels, causing drastic root growth inhibition. These findings indicate the critical role of OsDGD1 in maintaining optimum levels of JA during Pi deficiency for conducive root growth. Besides acting as signaling molecules and structural components, our study widens the role of lipids as metabolic flux controllers for phytohormone biosynthesis.

Plants breathing under pressure: mechanistic insights into soil compaction-induced physiological, molecular and biochemical responses in plants

MAIN CONCLUSION

This review highlights the molecular, biochemical and physiological responses of plants under soil compaction and presents suitable strategies for optimizing soil compaction for sustainable and intelligent plant production. Soil compaction (SC) increases the mechanical impedance of agricultural crops, which restricts plant growth, root elongation, and productivity. Therefore, exploring the impacts of SC-induced alterations in plants and developing optimization strategies are crucial for sustainable agricultural production and ensuring global food security. However, the regulation of molecular, biochemical and physiological responses to SC in plants has not yet been well explored. Here, we conducted a thorough analysis of the relevant literature regarding the primary factors behind SC in agricultural soils, mechanistic insights into SC-mediated molecular and physiological alterations in plants, the impact of SC on plant productivity, and SC-minimization strategies for eco-friendly and intelligent agricultural production. The existing information suggests that plant roots sense SC-induced changes in soil properties, including decreased soil water content, hypoxia, nutrient deficiency and mechanical stimuli, through altering the expression of membrane-located ion channel- or stimulus receptor-related genes, such as MSLs, MCA1, and AHK. After signal transduction, the synthesis and transport of several plant hormones, mainly ABA, ethylene and auxin, change and restrict root deepening but promote root thickening. In addition, the changes in plant hormones in combination with decreased water availability and decreased root hydraulic conductance induced by SC affect aboveground physiological responses, such as decreasing leaf hydraulic conductance, promoting stomatal closure and inhibiting plant photosynthesis. Comprehensive physiological insights into SC in plants and SC optimization strategies could be useful to soil biologists and plant eco-physiologists seeking to improve soil management and sustainable agricultural plant production to promote global food security.

Ethylene signals through an ethylene receptor to modulate biofilm formation and root colonization in a beneficial plant-associated bacterium

Ethylene is a plant hormone involved in many aspects of plant growth and development as well as responses to stress. The role of ethylene in plant-microbe interactions has been explored from the perspective of plants. However, only a small number of studies have examined the role of ethylene in microbes. We demonstrated that Azospirillum brasilense contains a functional ethylene receptor that we call Azospirillum Ethylene Response1 (AzoEtr1) after the nomenclature used in plants. AzoEtr1 directly binds ethylene with high affinity. Treating cells with ethylene or disrupting the receptor reduces biofilm formation and colonization of plant root surfaces. Additionally, RNA sequencing and untargeted metabolomics showed that ethylene causes wide-spread metabolic changes that affect carbon and nitrogen metabolism. One result is the accumulation of poly-hydroxybutyrate. Our data suggests a model in which ethylene from host plants alters the density of colonization by A. brasilense and re-wires its metabolism, suggesting that the bacterium implements an adaptation program upon sensing ethylene. These data provide potential new targets to regulate beneficial plant-microbe interactions.

Tillage type and sentinel insect species affect the relative prevalence of the entomopathogenic fungus, Metarhizium robertsii, in soil

Because the use of synthetic agrochemicals is generally not allowed in organic crop production systems, growers rely on natural substances and processes, such as microbial control, to suppress insect pests. Reduced tillage practices are associated with beneficial soil organisms, such as entomopathogenic fungi, that can contribute to the natural control of insect pests. The impacts of management, such as tillage, in a cropping system can affect soil biota in the current season and can also persist over time as legacy effects. We investigated the in-season and legacy effects of soil management in four, three-year organic feed grain and forage production systems that varied in number and intensity of soil disturbances on the relative prevalence of the entomopathogenic fungus, Metarhizium robertsii. Employing sentinel bait assays with Tenebrio molitor and Galleria mellonella, we found that relative prevalence, measured as infection rate of sentinel insects, was lowest in systems utilizing a shallow high-speed disk (G. mellonella: 14%; T. molitor: 23%) in the current and previous seasons compared to systems that included inversion and non-inversion tillage (G. mellonella: 22%; T. molitor: 34%) or no-till planting (G. mellonella: 21%; T. molitor: 30%,). There was no difference in prevalence in systems that included the use of a high-speed disk compared to a perennial hay crop (G. mellonella: 16%; T. molitor: 28%). There were no negative legacy effects of inversion tillage on the prevalence of M. robertsii in subsequent crops. Sentinel assays with G. mellonella (19%) produced overall lower estimates of relative prevalence of M. robertsii than T. molitor (29%) but the association of relative prevalence with environmental variables was greater in assays with G. mellonella. We suggest that the use of occasional inversion tillage is not damaging to populations of M. robertsii in soil and that surveys using assays with multiple sentinel insect species will improve our ability to understand the effects of agricultural practices on entomopathogenic fungi.

The recent genetic modification techniques for improve soil conservation, nutrient uptake and utilization

Advances in genetic modification (GM) techniques have generated huge interest in improving nutrient utilization, maximizing nutrient uptake, and conserving soil in the pursuit of sustainable agriculture. Unfortunately, little is still known about the recent advancements in the application of GM tactics to enhance each of these areas. This review explores the latest GM strategies intended to support soil conservation, maximize nutrient uptake, and improve nutrient utilization in farming, highlighting the critical roles that soil health and nutrient management play in sustainable farming. GM strategies such as improving the efficiency of nutrient uptake through enhanced root systems and increased nutrient transport mechanisms are well discussed. This study suggests that addressing potential obstacles, such as ethical and regulatory concerns, is a necessity for long-term sustainability applications of GM technologies to raise agricultural yields.

A mechanistic integration of hypoxia signaling with energy, redox, and hormonal cues

Oxygen deficiency (hypoxia) occurs naturally in many developing plant tissues but can become a major threat during acute flooding stress. Consequently, plants as aerobic organisms must rapidly acclimate to hypoxia and the associated energy crisis to ensure cellular and ultimately organismal survival. In plants, oxygen sensing is tightly linked with oxygen-controlled protein stability of group VII ETHYLENE-RESPONSE FACTORs (ERFVII), which, when stabilized under hypoxia, act as key transcriptional regulators of hypoxia-responsive genes (HRGs). Multiple signaling pathways feed into hypoxia signaling to fine-tune cellular decision-making under stress. First, ATP shortage upon hypoxia directly affects the energy status and adjusts anaerobic metabolism. Secondly, altered redox homeostasis leads to reactive oxygen and nitrogen species (ROS and RNS) accumulation, evoking signaling and oxidative stress acclimation. Finally, the phytohormone ethylene promotes hypoxia signaling to improve acute stress acclimation, while hypoxia signaling in turn can alter ethylene, auxin, abscisic acid, salicylic acid, and jasmonate signaling to guide development and stress responses. In this Update, we summarize the current knowledge on how energy, redox, and hormone signaling pathways are induced under hypoxia and subsequently integrated at the molecular level to ensure stress-tailored cellular responses. We show that some HRGs are responsive to changes in redox, energy, and ethylene independently of the oxygen status, and we propose an updated HRG list that is more representative for hypoxia marker gene expression. We discuss the synergistic effects of hypoxia, energy, redox, and hormone signaling and their phenotypic consequences in the context of both environmental and developmental hypoxia.

ZmSPL12 Enhances Root Penetration and Elongation in Maize Under Compacted Soil Conditions by Responding to Ethylene Signaling

Soil compaction poses a significant challenge in modern agriculture, as it constrains root development and hinders crop growth. The increasing evidence indicated that various phytohormones collaborate in distinct root zones to regulate root growth in compacted soils. However, the study of root development in maize under such conditions has been relatively limited. Here, we identified that the ZmSPL12 gene, belonging to the SPL transcription factor family, plays a crucial and positive role in regulating root development in the compacted soil. Specifically, the overexpression of ZmSPL12 resulted in significantly less inhibition of root growth than the wild-type plants when subjected to soil compaction. Histological analysis revealed that the capacity for root growth in compacted soil is closely associated with the development of the root cap. Further exploration demonstrated that ZmSPL12 modulates root growth through regulating ethylene signaling. Our findings underscored that ZmSPL12 expression level is induced by soil compaction and then enhances root penetration by regulating root cap and development, thereby enabling roots to thrive better in the compacted soil environment.

Imaging the spatial distribution of structurally diverse plant hormones

Plant hormones are essential and structurally diverse molecules that regulate various aspects of plant growth, development, and stress responses. However, the precise analysis of plant hormones in complex biological samples poses a challenge due to their low concentrations, dynamic levels, and intricate spatial distribution. Moreover, the complexity and interconnectedness of hormone signaling networks make it difficult to simultaneously trace multiple hormone spatial distributions. In this review, we provide an overview of currently recognized small-molecule plant hormones, signal peptide hormones, and plant growth regulators, along with the analytical methods employed for their analysis. We delve into the latest advancements in mass spectrometry imaging and in situ fluorescence techniques, which enable the examination of the spatial distribution of plant hormones. The advantages and disadvantages of these imaging techniques are further discussed. Finally, we propose potential avenues in imaging techniques to further enhance our understanding of plant hormone biology.

Cytokinins regulate spatially specific ethylene production to control root growth in Arabidopsis

Two principal growth regulators, cytokinins and ethylene, are known to interact in the regulation of plant growth. However, information about the underlying molecular mechanism and positional specificity of cytokinin/ethylene crosstalk in the control of root growth is scarce. We have identified the spatial specificity of cytokinin-regulated root elongation and root apical meristem (RAM) size, both of which we demonstrate to be dependent on ethylene biosynthesis. Upregulation of the cytokinin biosynthetic gene ISOPENTENYLTRANSFERASE (IPT) in proximal and peripheral tissues leads to both root and RAM shortening. By contrast, IPT activation in distal and inner tissues reduces RAM size while leaving the root length comparable to that of mock-treated controls. We show that cytokinins regulate two steps specific to ethylene biosynthesis: production of the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) by ACC SYNTHASEs (ACSs) and its conversion to ethylene by ACC OXIDASEs (ACOs). We describe cytokinin- and ethylene-specific regulation controlling the activity of ACSs and ACOs that are spatially discrete along both proximo/distal and radial root axes. Using direct ethylene measurements, we identify ACO2, ACO3, and ACO4 as being responsible for ethylene biosynthesis and ethylene-regulated root and RAM shortening in cytokinin-treated Arabidopsis. Direct interaction between ARABIDOPSIS RESPONSE REGULATOR 2 (ARR2), a member of the multistep phosphorelay cascade, and the C-terminal portion of ETHYLENE INSENSITIVE 2 (EIN2-C), a key regulator of canonical ethylene signaling, is involved in the cytokinin-induced, ethylene-mediated control of ACO4. We propose tight cooperation between cytokinin and ethylene signaling in the spatially specific regulation of ethylene biosynthesis as a key aspect of the hormonal control of root growth.

Diverse functional interactions between ABA and ethylene in plant development and responses to stress

Abscisic acid (ABA) and ethylene are two essential hormones that play crucial roles throughout the entire plant life cycle and in their tolerance to abiotic or biotic stress. In recent decades, increasing research has revealed that, in addition to their individual roles, these two hormones are more likely to function through their interactions, forming a complex regulatory network. More importantly, their functions change and their interactions vary from synergistic to antagonistic depending on the specific plant organ and development stage, which is less focused, compared and systematically summarized. In this review, we first introduce the general synthesis and action signaling pathways of these two plant hormones individually and their interactions in relation to seed dormancy and germination, primary root growth, shoot development, fruit ripening, leaf senescence and abscission, and stomatal movement regulation under both normal and stress conditions. A better understanding of the complex interactions between ABA and ethylene will enhance our knowledge of how plant hormones regulate development and respond to stress and may facilitate the development of crops with higher yields and greater tolerance to stressful environments through tissue-specific genetic modifications in the future.

Belowground cascading biotic interactions trigger crop diversity benefits

Crop diversification practices offer numerous synergistic benefits. So far, research has traditionally been confined to exploring isolated, unidirectional single-process interactions among plants, soil, and microorganisms. Here, we present a novel and systematic perspective, unveiling the intricate web of plant-soil-microbiome interactions that trigger cascading effects. Applying the principles of cascading interactions can be an alternative way to overcome soil obstacles such as soil compaction and soil pathogen pressure. Finally, we introduce a research framework comprising the design of diversified cropping systems by including commercial varieties and crops with resource-efficient traits, the exploration of cascading effects, and the innovation of field management. We propose that this provides theoretical and methodological insights that can reveal new mechanisms by which crop diversity increases productivity.

Understanding plant-soil interactions underpins enhanced sustainability of crop production

The Green Revolution transformed agriculture with high-yielding, stress-resistant varieties. However, the urgent need for more sustainable agricultural development presents new challenges: increasing crop yield, improving nutritional quality, and enhancing resource-use efficiency. Soil plays a vital role in crop-production systems and ecosystem services, providing water, nutrients, and physical anchorage for crop growth. Despite advancements in plant and soil sciences, our understanding of belowground plant-soil interactions, which impact both crop performance and soil health, remains limited. Here, we argue that a lack of understanding of these plant-soil interactions hinders sustainable crop production. We propose that targeted engineering of crops and soils can provide a fresh approach to achieve higher yields, more efficient sustainable crop production, and improved soil health.

Artificial macropores improve maize performance at the seedling stage under poor aeration

Maize is susceptible to hypoxia stress in soils with poor aeration, but the macropores have the potential to improve soil aeration. We studied the impact of artificial macropores on maize performance under poor aeration. Three levels of air-filled porosity (5%, 10% and 15%) were established, and soil columns with (28 vertical artificial macropores with 0.5 mm diameter) or without macropores were created for each level of air-filled porosity with a bulk density of 1.3 g cm-3. Root-macropore interactions were visualized using CT scanning (41 μm in resolution). Our results showed that root length density significantly increased by 114%, as air-filled porosity increased from 5% to 15%. However, when artificial macropores were present, an increase in air-filled porosity had no significant effect on root length density. The treatment of 5% air-filled porosity with macropores significantly increased root length density and root biomass by 108% and 65%, respectively, relative to the treatment of 5% air-filled porosity without macropores, whereas there was no significant difference in root growth between the treatments of 15% air-filled porosity with and without macropores. Compared to the treatment of 5% air-filled porosity with macropores, there was a significant reduction of 49% in the number of macropores colonized by roots under the treatment of 15% air-filled porosity with macropores. Our results demonstrate that macropores provide preferential paths for the colonization of maize roots, thereby promoting root growth under poor aeration. Creating macropores with bio-tillage can serve as a crucial strategy for enhancing crop performance in poorly aerated soils.

Unveiling the secrets of abiotic stress tolerance in plants through molecular and hormonal insights

Phytohormones are signaling substances that control essential elements of growth, development, and reactions to environmental stress. Drought, salt, heat, cold, and floods are a few examples of abiotic factors that have a significant impact on plant development and survival. Complex sensing, signaling, and stress response systems are needed for adaptation and tolerance to such pressures. Abscisic acid (ABA) is a key phytohormone that regulates stress responses. It interacts with the jasmonic acid (JA) and salicylic acid (SA) signaling pathways to direct resources toward reducing the impacts of abiotic stressors rather than fighting against pathogens. Under exposure to nanoparticles, the plant growth hormones also function as molecules that regulate stress and are known to be involved in a variety of signaling cascades. Reactive oxygen species (ROS) are detected in excess while under stress, and nanoparticles can control their formation. Understanding the way these many signaling pathways interact in plants will tremendously help breeders create food crops that can survive in deteriorating environmental circumstances brought on by climate change and that can sustain or even improve crop production. Recent studies have demonstrated that phytohormones, such as the traditional auxins, cytokinins, ethylene, and gibberellins, as well as more recent members like brassinosteroids, jasmonates, and strigolactones, may prove to be significant metabolic engineering targets for creating crop plants that are resistant to abiotic stress. In this review, we address recent developments in current understanding regarding the way various plant hormones regulate plant responses to abiotic stress and highlight instances of hormonal communication between plants during abiotic stress signaling. We also discuss new insights into plant gene and growth regulation mechanisms during stress, phytohormone engineering, nanotechnological crosstalk of phytohormones, and Plant Growth-Promoting Rhizobacteria's Regulatory Powers (PGPR) via the involvement of phytohormones.

Exploring the puzzle of reactive oxygen species acting on root hair cells

Reactive oxygen species (ROS) are essential signaling molecules that enable cells to respond rapidly to a range of stimuli. The ability of plants to recognize various stressors, incorporate a variety of environmental inputs, and initiate stress-response networks depends on ROS. Plants develop resilience and defensive systems as a result of these processes. Root hairs are central components of root biology since they increase the surface area of the root, anchor it in the soil, increase its ability to absorb water and nutrients, and foster interactions between microorganisms. In this review, we specifically focused on root hair cells and we highlighted the identification of ROS receptors, important new regulatory hubs that connect ROS production, transport, and signaling in the context of two hormonal pathways (auxin and ethylene) and under low temperature environmental input related to nutrients. As ROS play a crucial role in regulating cell elongation rates, root hairs are rapidly gaining traction as a very valuable single plant cell model for investigating ROS homeostasis and signaling. These promising findings might soon facilitate the development of plants and roots that are more resilient to environmental stressors.

Root plasticity versus elasticity - when are responses acclimative?

Spatiotemporal soil heterogeneity and the resulting edaphic stress cycles can be decisive for crop growth. However, our understanding of the acclimative value of root responses to heterogeneous soil conditions remains limited. We outline a framework to evaluate the acclimative value of root responses that distinguishes between stress responses that are persistent and reversible upon stress release, termed 'plasticity' and 'elasticity', respectively. Using energy balances, we provide theoretical evidence that the advantage of plasticity over elasticity increases with the number of edaphic stress cycles and if responses lead to comparatively high energy gains. Our framework provides a conceptual basis for assessing the acclimative value of root responses to soil heterogeneity and can catalyse research on crop adaptations to heterogeneous belowground environments.

Roots of synthetic ecology: microbes that foster plant resilience in the changing climate

Microbes orchestrate nearly all major biogeochemical processes. The ability to program their influence on plant growth and development is attractive for sustainable agriculture. However, the complexity of microbial ecosystems and our limited understanding of the mechanisms by which plants and microbes interact with each other and the environment make it challenging to use microbiomes to influence plant growth. Novel technologies at the intersection of microbial ecology, systems biology, and bioengineering provide new tools to probe the role of plant microbiomes across environments. Here, we summarize recent studies on plant and microbe responses to abiotic stresses, showcasing key molecules and micro-organisms that are important for plant health. We highlight opportunities to use synthetic microbial communities to understand the complexity of plant-microbial interactions and discuss future avenues of programming ecology to improve plant and ecosystem health.

OsEIN2-OsEIL1/2 pathway negatively regulates chilling tolerance by attenuating OsICE1 function in rice

Low temperature severely affects rice development and yield. Ethylene signal is essential for plant development and stress response. Here, we reported that the OsEIN2-OsEIL1/2 pathway reduced OsICE1-dependent chilling tolerance in rice. The overexpressing plants of OsEIN2, OsEIL1 and OsEIL2 exhibited severe stress symptoms with excessive reactive oxygen species (ROS) accumulation under chilling, while the mutants (osein2 and oseil1) and OsEIL2-RNA interference plants (OsEIL2-Ri) showed the enhanced chilling tolerance. We validated that OsEIL1 and OsEIL2 could form a heterxodimer and synergistically repressed OsICE1 expression by binding to its promoter. The expression of OsICE1 target genes, ROS scavenging- and photosynthesis-related genes were downregulated by OsEIN2 and OsEIL1/2, which were activated by OsICE1, suggesting that OsEIN2-OsEIL1/2 pathway might mediate ROS accumulation and photosynthetic capacity under chilling by attenuating OsICE1 function. Moreover, the association analysis of the seedling chilling tolerance with the haplotype showed that the lower expression of OsEIL1 and OsEIL2 caused by natural variation might confer chilling tolerance on rice seedlings. Finally, we generated OsEIL2-edited rice with an enhanced chilling tolerance. Taken together, our findings reveal a possible mechanism integrating OsEIN2-OsEIL1/2 pathway with OsICE1-dependent cascade in regulating chilling tolerance, providing a practical strategy for breeding chilling-tolerant rice.

An orchestrated ethylene-gibberellin signaling cascade contributes to mesocotyl elongation and emergence of rice direct seeding

A mechanized direct seeding of rice with less labor and water usage, has been widely adopted. However, this approach requires varieties that exhibit uniform seedling emergence. Mesocotyl elongation (ME) offers the main drive of fast emergence of rice seedlings from soils; nevertheless, its genetic basis remains unknown. Here, we identify a major rice quantitative trait locus Mesocotyl Elongation1 (qME1), an allele of the Green Revolution gene Semi-Dwarf1 (SD1), encoding GA20-oxidase for gibberellin (GA) biosynthesis. ME1 expression is strongly induced by soil depth and ethylene. When rice grains are direct-seeded in soils, the ethylene core signaling factor OsEIL1 directly promotes ME1 transcription, accelerating bioactive GA biosynthesis. The GAs further degrade the DELLA protein SLENDER RICE 1 (SLR1), alleviating its inhibition of rice PHYTOCHROME-INTERACTING FACTOR-LIKE13 (OsPIL13) to activate the downstream expansion gene OsEXPA4 and ultimately promote rice seedling ME and emergence. The ancient traits of long mesocotyl and strong emergence ability in wild rice and landrace were gradually lost in company with the Green Revolution dwarf breeding process, and an elite ME1-R allele (D349H) is found in some modern Geng varieties (long mesocotyl lengths) in northern China, which can be used in the direct seeding and dwarf breeding of Geng varieties. Furthermore, the ectopic and high expression of ME1 driven by mesocotyl-specific promoters resulted in rice plants that could be direct-seeded without obvious plant architecture or yield penalties. Collectively, we reveal the molecular mechanism of rice ME, and provide useful information for breeding new Green Revolution varieties with long mesocotyl suitable for direct-seeding practice.

The OsEIL1-OsWOX11 transcription factor module controls rice crown root development in response to soil compaction

Optimizing the root architecture of crops is an effective strategy for improving crop yields. Soil compaction is a serious global problem that limits crop productivity by restricting root growth, but the underlying molecular mechanisms are largely unclear. Here, we show that ethylene stimulates rice (Oryza sativa) crown root development in response to soil compaction. First, we demonstrate that compacted soil promotes ethylene production and the accumulation of ETHYLENE INSENSITIVE 3-LIKE 1 (OsEIL1) in rice roots, stimulating crown root primordia initiation and development, thereby increasing crown root number in lower stem nodes. Through transcriptome profiling and molecular analyses, we reveal that OsEIL1 directly activates the expression of WUSCHEL-RELATED HOMEOBOX 11 (OsWOX11), an activator of crown root emergence and growth, and that OsWOX11 mutations delay crown root development, thus impairing the plant's response to ethylene and soil compaction. Genetic analysis demonstrates that OsWOX11 functions downstream of OsEIL1. In summary, our results demonstrate that the OsEIL1-OsWOX11 module regulates ethylene action during crown root development in response to soil compaction, providing a strategy for the genetic modification of crop root architecture and grain agronomic traits.

Engineering Rapeseed Germination and Root Growth with Mechanical Strength of Polysaccharide Hydrogel

Plant roots are highly sensitive to physical stress in the soil, with appropriate mechanical impedance promoting root elongation and lateral root growth. However, few studies have quantitatively explored the relationship between the mechanical impedance of the growth medium and the phenotypes of plant roots. In this study, we used a tensile machine equipped with a self-made steel needle mimicking the root tip to measure the force needed to penetrate the hydrogel medium (agar, low acyl gellan gum, and κ-carrageenan), providing insights into the force required for the rapeseed root tip to enter the medium following germination. These findings indicate that root penetration length is inversely associated with the mechanical strength of the growth medium, with variations observed in the root system adaptability across different substrates. Specifically, when the gel puncture resistance of the culture medium without adding MS reached approximately 18.4 mN, root penetration and growth were significantly hindered. With the addition of 1/2 MS medium, the polysaccharide concentration is 1.0 wt %, which is more suitable for cultivating rapeseed. This research not only offers a method for quantifying root phenotypes and medium mechanical impedance but also presents an approach for plant growth regulation and crop breeding.

Root hairs facilitate rice root penetration into compacted layers

Compacted soil layers adversely affect rooting depth and access to deeper nutrient and water resources, thereby impacting climate resilience of crop production and global food security. Root hair plays well-known roles in facilitating water and nutrient acquisition. Here, we report that root hair also contributes to root penetration into compacted layers. We demonstrate that longer root hair, induced by elevated auxin response during a root compaction response, improves the ability of rice roots to penetrate harder layers. This compaction-induced auxin response in the root hair zone is dependent on the root apex-expressed auxin synthesis gene OsYUCCA8 (OsYUC8), which is induced by compaction stress. This auxin source for root hair elongation relies on the auxin influx carrier AUXIN RESISTANT 1 (OsAUX1), mobilizing this signal from the root apex to the root hair zone. Mutants disrupting OsYUC8 and OsAUX1 genes exhibit shorter root hairs and weaker penetration ability into harder layers compared with wild type (WT). Root-hair-specific mutants phenocopy these auxin-signaling mutants, as they also exhibit an attenuated root penetration ability. We conclude that compaction stress upregulates OsYUC8-mediated auxin biosynthesis in the root apex, which is subsequently mobilized to the root hair zone by OsAUX1, where auxin promotes root hair elongation, improving anchorage of root tips to their surrounding soil environment and aiding their penetration ability into harder layers.

Not so hidden anymore: Advances and challenges in understanding root growth under water deficits

Limited water availability is a major environmental factor constraining plant development and crop yields. One of the prominent adaptations of plants to water deficits is the maintenance of root growth that enables sustained access to soil water. Despite early recognition of the adaptive significance of root growth maintenance under water deficits, progress in understanding has been hampered by the inherent complexity of root systems and their interactions with the soil environment. We highlight selected milestones in the understanding of root growth responses to water deficits, with emphasis on founding studies that have shaped current knowledge and set the stage for further investigation. We revisit the concept of integrated biophysical and metabolic regulation of plant growth and use this framework to review central growth-regulatory processes occurring within root growth zones under water stress at subcellular to organ scales. Key topics include the primary processes of modifications of cell wall-yielding properties and osmotic adjustment, as well as regulatory roles of abscisic acid and its interactions with other hormones. We include consideration of long-recognized responses for which detailed mechanistic understanding has been elusive until recently, for example hydrotropism, and identify gaps in knowledge, ongoing challenges, and opportunities for future research.

Role of sugars in the apical hook development of Arabidopsis etiolated seedlings

KEY MESSAGE

The sugar supply in the medium affects the apical hook development of Arabidopsis etiolated seedlings. In addition, we provided the mechanism insights of this process. Dicotyledonous plants form an apical hook structure to shield their young cotyledons from mechanical damage as they emerge from the rough soil. Our findings indicate that sugar molecules, such as sucrose and glucose, are crucial for apical hook development. The presence of sucrose and glucose allows the apical hooks to be maintained for a longer period compared to those grown in sugar-free conditions, and this effect is dose-dependent. Key roles in apical hook development are played by several sugar metabolism pathways, including oxidative phosphorylation and glycolysis. RNA-seq data revealed an up-regulation of genes involved in starch and sucrose metabolism in plants grown in sugar-free conditions, while genes associated with phenylpropanoid metabolism were down-regulated. This study underscores the significant role of sugar metabolism in the apical hook development of etiolated Arabidopsis seedlings.

Ethylene insensitive 2 (EIN2) destiny shaper: The post-translational modification

PTMs (Post-Translational Modifications) of proteins facilitate rapid modulation of protein function in response to various environmental stimuli. The EIN2 (Ethylene Insensitive 2) protein is a core regulatory of the ethylene signaling pathway. Recent findings have demonstrated that PTMs, including protein phosphorylation, ubiquitination, and glycosylation, govern EIN2 trafficking, subcellular localization, stability, and physiological roles. The cognition of multiple PTMs in EIN2 underscores the stringent regulation of protein. Consequently, a thorough review of the regulatory role of PTMs in EIN2 functions will improve our profound comprehension of the regulation mechanism and various physiological processes of EIN2-mediated signaling pathways. This review discusses the evolution, functions, structure and characteristics of EIN2 protein in plants. Additionally, this review sheds light on the progress of protein ubiquitination, phosphorylation, O-Glycosylation in the regulation of EIN2 functions, and the unresolved questions and future perspectives.

Alfalfa Responses to Intensive Soil Compaction: Effects on Plant and Root Growth, Phytohormones and Internal Gene Expression

The perennial legume alfalfa (Medicago sativa L.) is of high value in providing cheap and high-nutritive forages. Due to a lack of tillage during the production period, the soil in which alfalfa grows prunes to become compacted through highly mechanized agriculture. Compaction deteriorates the soil's structure and fertility, leading to compromised alfalfa development and productivity. However, the way alfalfa responses to different levels of soil compaction and the underlying molecular mechanism are still unclear. In this study, we systematically evaluated the effects of gradient compacted soil on the growth of different cultivars of alfalfa, especially the root system architecture, phytohormones and internal gene expression profile alterations. The results showed that alfalfa growth was facilitated by moderate soil compaction, but drastically inhibited when compaction was intensified. The inhibition effect was universal across different cultivars, but with different severity. Transcriptomic and physiological studies revealed that the expression of a set of genes regulating the biosynthesis of lignin and flavonoids was significantly repressed in compaction treated alfalfa roots, and this might have resulted in a modified secondary cell wall and xylem vessel formation. Phytohormones, like ABA, are supposed to play pivotal roles in the regulation of the overall responses. These findings provide directions for the improvement of field soil management in alfalfa production and the molecular breeding of alfalfa germplasm with better soil compaction resilience.

Breaking new ground: Decoding the root's molecular circuits to penetrate compacted soil

Penetration capacity is a central root phenotype for enhancing rooting into compacted soils and improving abiotic stress tolerance in crop plants, but the molecular mechanisms are unclear. In this issue of Developmental Cell, Xu et al. demonstrate the vital role of the FER-PIF3-PIEZO circuitry in controlling primary root penetrability into compacted substrates.

Chemically Mediated Plant-Plant Interactions: Allelopathy and Allelobiosis

Plant-plant interactions are a central driver for plant coexistence and community assembly. Chemically mediated plant-plant interactions are represented by allelopathy and allelobiosis. Both allelopathy and allelobiosis are achieved through specialized metabolites (allelochemicals or signaling chemicals) produced and released from neighboring plants. Allelopathy exerts mostly negative effects on the establishment and growth of neighboring plants by allelochemicals, while allelobiosis provides plant neighbor detection and identity recognition mediated by signaling chemicals. Therefore, plants can chemically affect the performance of neighboring plants through the allelopathy and allelobiosis that frequently occur in plant-plant intra-specific and inter-specific interactions. Allelopathy and allelobiosis are two probably inseparable processes that occur together in plant-plant chemical interactions. Here, we comprehensively review allelopathy and allelobiosis in plant-plant interactions, including allelopathy and allelochemicals and their application for sustainable agriculture and forestry, allelobiosis and plant identity recognition, chemically mediated root-soil interactions and plant-soil feedback, and biosynthesis and the molecular mechanisms of allelochemicals and signaling chemicals. Altogether, these efforts provide the recent advancements in the wide field of allelopathy and allelobiosis, and new insights into the chemically mediated plant-plant interactions.

Whispers in the dark: Signals regulating underground plant-plant interactions

Plants are able to actively detect and respond to the presence in neighboring plants, in order to optimize their physiology to promote survival and reproduction despite the presence of competing organisms. A key but still poorly understood mechanism for neighbor detection is through the perception of root exudates. In this review, we explore recent findings on the role of root exudates in plant-plant interactions, focusing both on general interactions and also the highly specialized example of root parasite-host plant interactions.

Ethylene Modulates Rice Root Plasticity under Abiotic Stresses

Plants live in constantly changing environments that are often unfavorable or stressful. Root development strongly affects plant growth and productivity, and the developmental plasticity of roots helps plants to survive under abiotic stress conditions. This review summarizes the progress being made in understanding the regulation of the phtyohormone ethylene in rice root development in response to abiotic stresses, highlighting the complexity associated with the integration of ethylene synthesis and signaling in root development under adverse environments. Understanding the molecular mechanisms of ethylene in regulating root architecture and response to environmental signals can contribute to the genetic improvement of crop root systems, enhancing their adaptation to stressful environmental conditions.

How plant roots respond to waterlogging

Plant submergence is a major abiotic stress that impairs plant performance. Under water, reduced gas diffusion exposes submerged plant cells to an environment that is enriched in gaseous ethylene and is limited in oxygen (O2) availability (hypoxia). The capacity for plant roots to avoid and/or sustain critical hypoxia damage is essential for plants to survive waterlogging. Plants use spatiotemporal ethylene and O2 dynamics as instrumental flooding signals to modulate potential adaptive root growth and hypoxia stress acclimation responses. However, how non-adapted plant species modulate root growth behaviour during actual waterlogged conditions to overcome flooding stress has hardly been investigated. Here we discuss how changes in the root growth rate, lateral root formation, density, and growth angle of non-flood adapted plant species (mainly Arabidopsis) could contribute to avoiding and enduring critical hypoxic conditions. In addition, we discuss current molecular understanding of how ethylene and hypoxia signalling control these adaptive root growth responses. We propose that future research would benefit from less artificial experimental designs to better understand how plant roots respond to and survive waterlogging. This acquired knowledge would be instrumental to guide targeted breeding of flood-tolerant crops with more resilient root systems.

Uncovering root compaction response mechanisms: new insights and opportunities

Compaction disrupts soil structure, reducing root growth, nutrient and water uptake, gas exchange, and microbial growth. Root growth inhibition by soil compaction was originally thought to reflect the impact of mechanical impedance and reduced water availability. However, using a novel gas diffusion-based mechanism employing the hormone ethylene, recent research has revealed that plant roots sense soil compaction. Non-compacted soil features highly interconnected pore spaces that facilitate diffusion of gases such as ethylene which are released by root tips. In contrast, soil compaction stress disrupts the pore network, causing ethylene to accumulate around root tips and trigger growth arrest. Genetically disrupting ethylene signalling causes roots to become much less sensitive to compaction stress. Such new understanding about the molecular sensing mechanism and emerging root anatomical traits provides novel opportunities to develop crops resistant to soil compaction by targeting key genes and their signalling pathways. This expert view discusses these recent advances and the molecular mechanisms associated with root-soil compaction responses.