A Potassium-Dependent Oxygen Sensing Pathway Regulates Plant Root Hydraulics

Melatonin promotes the recovery of apple plants after waterlogging by shaping the structure and function of the rhizosphere microbiome

The dynamics of the physiological adaptability of plants and the rhizosphere soil environment after waterlogging remain unclear. Here we investigated the mechanisms regulating plant condition and shaping of the rhizosphere microbiome in a pot experiment. In the experiment, we added melatonin to waterlogged plants, which promoted waterlogging relief. The treatment significantly enhanced photosynthesis and the antioxidant capacity of apple plants, and significantly promoted nitrogen (N) utilization efficiency by upregulating genes related to N transport and metabolism. Multiperiod soil microbiome analysis showed the dynamic effects of melatonin on the diversity of the microbial community during waterlogging recovery. Random forest and linear regression analyses were used to screen for potential beneficial bacteria (e.g., Azoarcus, Pseudomonas and Nocardioides) specifically regulated by melatonin and revealed a positive correlation with soil nutrient levels and plant growth. Furthermore, metagenomic analyses revealed the regulatory effects of melatonin on genes involved in N cycling in soil. Melatonin positively contributed to the accumulation of plant dry weight by upregulating the expression of nifD and nifK (N fixation). In summary, melatonin positively regulates physiological functions in plants and the structure and function of the microbial community; it promoted the recovery of apple plants after waterlogging stress.

Plant physiology: RAF kinases claim a conserved role in rapid auxin responses

A recent study spotlights B-RAF kinases as central mediators of rapid auxin responses across diverse plant species. Coupled with other current studies, this discovery illuminates the essential role of B-RAF kinases in orchestrating growth, stress responses, and various other biological processes in plants.

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.

RAF-like protein kinases mediate a deeply conserved, rapid auxin response

The plant-signaling molecule auxin triggers fast and slow cellular responses across land plants and algae. The nuclear auxin pathway mediates gene expression and controls growth and development in land plants, but this pathway is absent from algal sister groups. Several components of rapid responses have been identified in Arabidopsis, but it is unknown if these are part of a conserved mechanism. We recently identified a fast, proteome-wide phosphorylation response to auxin. Here, we show that this response occurs across 5 land plant and algal species and converges on a core group of shared targets. We found conserved rapid physiological responses to auxin in the same species and identified rapidly accelerated fibrosarcoma (RAF)-like protein kinases as central mediators of auxin-triggered phosphorylation across species. Genetic analysis connects this kinase to both auxin-triggered protein phosphorylation and rapid cellular response, thus identifying an ancient mechanism for fast auxin responses in the green lineage.

Protein kinase SnRK2.4 is a key regulator of aquaporins and root hydraulics in Arabidopsis

Soil water uptake by roots is a key component of plant water homeostasis contributing to plant growth and survival under ever-changing environmental conditions. The water transport capacity of roots (root hydraulic conductivity; Lpr ) is mostly contributed by finely regulated Plasma membrane Intrinsic Protein (PIP) aquaporins. In this study, we used natural variation of Arabidopsis for the identification of quantitative trait loci (QTLs) contributing to Lpr . Using recombinant lines from a biparental cross (Cvi-0 x Col-0), we show that the gene encoding class 2 Sucrose-Non-Fermenting Protein kinase 2.4 (SnRK2.4) in Col-0 contributes to >30% of Lpr by enhancing aquaporin-dependent water transport. At variance with the inactive and possibly unstable Cvi-0 SnRK2.4 form, the Col-0 form interacts with and phosphorylates the prototypal PIP2;1 aquaporin at Ser121 and stimulates its water transport activity upon coexpression in Xenopus oocytes and yeast cells. Activation of PIP2;1 by Col-0 SnRK2.4 in yeast also requires its protein kinase activity and can be counteracted by clade A Protein Phosphatases 2C. SnRK2.4 shows all hallmarks to be part of core abscisic acid (ABA) signaling modules. Yet, long-term (>3 h) inhibition of Lpr by ABA possibly involves a SnRK2.4-independent inhibition of PIP2;1. SnRK2.4 also promotes stomatal aperture and ABA-induced inhibition of primary root growth. The study identifies a key component of Lpr and sheds new light on the functional overlap and specificity of SnRK2.4 with respect to other ABA-dependent or independent SnRK2s.

Plasma membrane aquaporins regulate root hydraulic conductivity in the model plant Setaria viridis

The high rate of productivity observed in panicoid crops is in part due to their extensive root system. Recently, green foxtail (Setaria viridis) has emerged as a genetic model system for panicoid grasses. Natural accessions of S. viridis originating from different parts of the world, with differential leaf physiological behavior, have been identified. This work focused on understanding the physiological and molecular mechanisms controlling root hydraulic conductivity and root-to-shoot gas exchange signaling in S. viridis. We identified 2 accessions, SHA and ZHA, with contrasting behavior at the leaf, root, and whole-plant levels. Our results indicated a role for root aquaporin (AQP) plasma membrane (PM) intrinsic proteins in the differential behavior of SHA and ZHA. Moreover, a different root hydraulic response to low levels of abscisic acid between SHA and ZHA was observed, which was associated with root AQPs. Using cell imaging, biochemical, and reverse genetic approaches, we identified PM intrinsic protein 1;6 (PIP1;6) as a possible PIP1 candidate that regulates radial root hydraulics and root-to-shoot signaling of gas exchange in S. viridis. In heterologous systems, PIP1;6 localized in the endoplasmic reticulum, and upon interaction with PIP2s, relocalization to the PM was observed. PIP1;6 was predominantly expressed at the root endodermis. Generation of knockout PIP1;6 plants (KO-PIP1;6) in S. viridis showed altered root hydraulic conductivity, altered gas exchange, and alteration of root transcriptional patterns. Our results indicate that PIPs are essential in regulating whole-plant water homeostasis in S. viridis. We conclude that root hydraulic conductivity and gas exchange are positively associated and are regulated by AQPs.

Deciphering the role of mechanosensitive channels in plant root biology: perception, signaling, and adaptive responses

MAIN CONCLUSION

Mechanosensitive channels are integral membrane proteins that rapidly translate extrinsic or intrinsic mechanical tensions into biological responses. They can serve as potential candidates for developing smart-resilient crops with efficient root systems. Mechanosensitive (MS) calcium channels are molecular switches for mechanoperception and signal transduction in all living organisms. Although tremendous progress has been made in understanding mechanoperception and signal transduction in bacteria and animals, this remains largely unknown in plants. However, identification and validation of MS channels such as Mid1-complementing activity channels (MCAs), mechanosensitive-like channels (MSLs), and Piezo channels (PIEZO) has been the most significant discovery in plant mechanobiology, providing novel insights into plant mechanoperception. This review summarizes recent advances in root mechanobiology, focusing on MS channels and their related signaling players, such as calcium ions (Ca2+), reactive oxygen species (ROS), and phytohormones. Despite significant advances in understanding the role of Ca2+ signaling in root biology, little is known about the involvement of MS channel-driven Ca2+ and ROS signaling. Additionally, the hotspots connecting the upstream and downstream signaling of MS channels remain unclear. In light of this, we discuss the present knowledge of MS channels in root biology and their role in root developmental and adaptive traits. We also provide a model highlighting upstream (cell wall sensors) and downstream signaling players, viz., Ca2+, ROS, and hormones, connected with MS channels. Furthermore, we highlighted the importance of emerging signaling molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), and neurotransmitters (NTs), and their association with root mechanoperception. Finally, we conclude with future directions and knowledge gaps that warrant further research to decipher the complexity of root mechanosensing.

Calcium-dependent activation of CPK12 facilitates its cytoplasm-to-nucleus translocation to potentiate plant hypoxia sensing by phosphorylating ERF-VII transcription factors

Calcium-dependent protein kinases (CDPKs/CPKs) are key regulators of plant stress signaling that translate calcium signals into cellular responses by phosphorylating diverse substrate proteins. However, the molecular mechanism by which plant cells relay calcium signals in response to hypoxia remains elusive. Here, we show that one member of the CDPK family in Arabidopsis thaliana, CPK12, is rapidly activated during hypoxia through calcium-dependent phosphorylation of its Ser-186 residue. Phosphorylated CPK12 shuttles from the cytoplasm to the nucleus, where it interacts with and phosphorylates the group VII ethylene-responsive transcription factors (ERF-VII) that are core regulators of plant hypoxia sensing, to enhance their stabilities. Consistently, CPK12 knockdown lines show attenuated tolerance of hypoxia, whereas transgenic plants overexpressing CPK12 display improved hypoxia tolerance. Nonethelss, loss of function of five ERF-VII proteins in an erf-vii pentuple mutant could partially suppress the enhanced hypoxia-tolerance phenotype of CPK12-overexpressing lines. Moreover, we also discovered that phosphatidic acid and 14-3-3κ protein serve as positive and negative modulators of the CPK12 cytoplasm-to-nucleus translocation, respectively. Taken together, these findings uncover a CPK12-ERF-VII regulatory module that is key to transducing calcium signals from the cytoplasm into the nucleus to potentiate hypoxia sensing in plants.

Complexity of Abiotic Stress Stimuli: Mimicking Hypoxic Conditions Experimentally on the Basis of Naturally Occurring Environments

Plants require oxygen to respire and produce energy. Plant cells are exposed to low oxygen levels (hypoxia) in different contexts and have evolved conserved molecular responses to hypoxia. Both environmental and developmental factors can influence intracellular oxygen concentrations. In nature, plants can experience hypoxic conditions when the soil becomes saturated with water following heavy precipitation (i.e., waterlogging). Hypoxia can also arise in specific tissues that have poor gas exchange with atmospheric oxygen. In this case, hypoxic niches that are physiologically and developmentally relevant may form. To dissect the molecular mechanisms underlying the regulation of hypoxia response in plants, a wide range of hypoxia-inducing methods have been used in the laboratory setting. Yet, the different characteristics, pros and cons of each of these hypoxia treatments are seldom compared between methods, and with natural forms of hypoxia. In this chapter, we present both environmental and developmental forms of hypoxia that plants encounter in the wild, as well as the different experimental hypoxia treatments used to mimic them in the laboratory setting, with the aim of informing on what experimental approaches might be most appropriate to the questions addressed, including stress signaling and regulation.

Phenotyping and modeling of root hydraulic architecture reveal critical determinants of axial water transport

Water uptake by roots is a key adaptation of plants to aerial life. Water uptake depends on root system architecture (RSA) and tissue hydraulic properties that, together, shape the root hydraulic architecture. This work investigates how the interplay between conductivities along radial (e.g. aquaporins) and axial (e.g. xylem vessels) pathways determines the water transport properties of highly branched RSAs as found in adult Arabidopsis (Arabidopsis thaliana) plants. A hydraulic model named HydroRoot was developed, based on multi-scale tree graph representations of RSAs. Root water flow was measured by the pressure chamber technique after successive cuts of a same root system from the tip toward the base. HydroRoot model inversion in corresponding RSAs allowed us to concomitantly determine radial and axial conductivities, providing evidence that the latter is often overestimated by classical evaluation based on the Hagen-Poiseuille law. Organizing principles of Arabidopsis primary and lateral root growth and branching were determined and used to apply the HydroRoot model to an extended set of simulated RSAs. Sensitivity analyses revealed that water transport can be co-limited by radial and axial conductances throughout the whole RSA. The number of roots that can be sectioned (intercepted) at a given distance from the base was defined as an accessible and informative indicator of RSA. The overall set of experimental and theoretical procedures was applied to plants mutated in ESKIMO1 and previously shown to have xylem collapse. This approach will be instrumental to dissect the root water transport phenotype of plants with intricate alterations in root growth or transport functions.

The Role of Aquaporins in Plant Growth under Conditions of Oxygen Deficiency

Plants frequently experience hypoxia due to flooding caused by intensive rainfall or irrigation, when they are partially or completely submerged under a layer of water. In the latter case, some resistant plants implement a hypoxia avoidance strategy by accelerating shoot elongation, which allows lifting their leaves above the water surface. This strategy is achieved due to increased water uptake by shoot cells through water channels (aquaporins, AQPs). It remains a puzzle how an increased flow of water through aquaporins into the cells of submerged shoots can be achieved, while it is well known that hypoxia inhibits the activity of aquaporins. In this review, we summarize the literature data on the mechanisms that are likely to compensate for the decline in aquaporin activity under hypoxic conditions, providing increased water entry into cells and accelerated shoot elongation. These mechanisms include changes in the expression of genes encoding aquaporins, as well as processes that occur at the post-transcriptional level. We also discuss the involvement of hormones, whose concentration changes in submerged plants, in the control of aquaporin activity.

Roots’ Drought Adaptive Traits in Crop Improvement

Drought is one of the biggest concerns in agriculture due to the projected reduction of global freshwater supply with a concurrent increase in global food demand. Roots can significantly contribute to improving drought adaptation and productivity. Plants increase water uptake by adjusting root architecture and cooperating with symbiotic soil microbes. Thus, emphasis has been given to root architectural responses and root–microbe relationships in drought-resilient crop development. However, root responses to drought adaptation are continuous and complex processes and involve additional root traits and interactions among themselves. This review comprehensively compiles and discusses several of these root traits such as structural, physiological, molecular, hydraulic, anatomical, and plasticity, which are important to consider together, with architectural changes, when developing drought resilient crop varieties. In addition, it describes the significance of root contribution in improving soil structure and water holding capacity and its implication on long-term resilience to drought. In addition, various drought adaptive root ideotypes of monocot and dicot crops are compared and proposed for given agroclimatic conditions. Overall, this review provides a broader perspective of understanding root structural, physiological, and molecular regulators, and describes the considerations for simultaneously integrating multiple traits for drought tolerance and crop improvement, under specific growing environments.

The Role of Phytohormones in Plant Response to Flooding

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

Creativity comes from interactions: modules of protein interactions in plants

Protein interactions are the foundation of cell biology. For robust signal transduction to occur, proteins interact selectively and modulate their behavior to direct specific biological outcomes. Frequently, modular protein interaction domains are central to these processes. Some of these domains bind proteins bearing post-translational modifications, such as phosphorylation, whereas other domains recognize and bind to specific amino acid motifs. Other modules act as diverse protein interaction scaffolds or can be multifunctional, forming head-to-head homodimers and binding specific peptide sequences or membrane phospholipids. Additionally, the so-called head-to-tail oligomerization domains (SAM, DIX, and PB1) can form extended polymers to regulate diverse aspects of biology. Although the mechanism and structures of these domains are diverse, they are united by their modularity. Together, these domains are versatile and facilitate the evolution of complex protein interaction networks. In this review, we will highlight the role of select modular protein interaction domains in various aspects of plant biology.

What can mechanistic models tell us about guard cells, photosynthesis, and water use efficiency?

Stomatal pores facilitate gaseous exchange between the inner air spaces of the leaf and the atmosphere. The pores open to enable CO₂ entry for photosynthesis and close to reduce transpirational water loss. How stomata respond to the environment has long attracted interest in modeling as a tool to understand the consequences for the plant and for the ecosystem. Models that focus on stomatal conductance for gas exchange make intuitive sense, but such models need also to connect with the mechanics of the guard cells that regulate pore aperture if we are to understand the 'decisions made' by stomata, their impacts on the plant and on the global environment.

Potassium in plants: Growth regulation, signaling, and environmental stress tolerance

Potassium (K) is an essential element for the growth and development of plants; however, its scarcity or excessive level leads to distortion of numerous functions in plants. It takes part in the control of various significant functions in plant advancement. Because of the importance index, K is regarded second after nitrogen for whole plant growth. Approximately, higher than 60 enzymes are reliant on K for activation within the plant system, in which K plays a vital function as a regulator. Potassium provides assistance in plants against abiotic stress conditions in the environment. With this background, the present paper reviews the physiological functions of K in plants like stomatal regulation, photosynthesis and water uptake. The article also focuses upon the uptake and transport mechanisms of K along with its role in detoxification of reactive oxygen species and in conferring tolerance to plants against abiotic stresses. It also highlights the research progress made in the direction of K mediated signaling cascades.

Try or Die: Dynamics of Plant Respiration and How to Survive Low Oxygen Conditions

Fluctuations in oxygen (O₂) availability occur as a result of flooding, which is periodically encountered by terrestrial plants. Plant respiration and mitochondrial energy generation rely on O₂ availability. Therefore, decreased O₂ concentrations severely affect mitochondrial function. Low O₂ concentrations (hypoxia) induce cellular stress due to decreased ATP production, depletion of energy reserves and accumulation of metabolic intermediates. In addition, the transition from low to high O₂ in combination with light changes-as experienced during re-oxygenation-leads to the excess formation of reactive oxygen species (ROS). In this review, we will update our current knowledge about the mechanisms enabling plants to adapt to low-O₂ environments, and how to survive re-oxygenation. New insights into the role of mitochondrial retrograde signaling, chromatin modification, as well as moonlighting proteins and mitochondrial alternative electron transport pathways (and their contribution to low O₂ tolerance and survival of re-oxygenation), are presented.

Experimental and conceptual approaches to root water transport

BACKGROUND

Root water transport, which critically contributes to the plant water status and thereby plant productivity, has been the object of extensive experimental and theoretical studies. However, root systems represent an intricate assembly of cells in complex architectures, including many tissues at distinct developmental stages. Our comprehension of where and how molecular actors integrate their function in order to provide the root with its hydraulic properties is therefore still limited.

SCOPE

Based on current literature and prospective discussions, this review addresses how root water transport can be experimentally measured, what is known about the underlying molecular actors, and how elementary water transport processes are scaled up in numerical/mathematical models.

CONCLUSIONS

The theoretical framework and experimental procedures on root water transport that are in use today have been established a few decades ago. However, recent years have seen the appearance of new techniques and models with enhanced resolution, down to a portion of root or to the tissue level. These advances pave the way for a better comprehension of the dynamics of water uptake by roots in the soil.

Hormonal and environmental signaling pathways target membrane water transport

Plant water transport and its molecular components including aquaporins are responsive, across diverse time scales, to an extremely wide array of environmental and hormonal signals. These include water deficit and abscisic acid (ABA) but also more recently identified stimuli such as peptide hormones or bacterial elicitors. The present review makes an inventory of corresponding signalling pathways. It identifies some main principles, such as the central signalling role of ROS, with a dual function of aquaporins in water and hydrogen peroxide transport, the importance of aquaporin phosphorylation that is targeted by multiple classes of protein kinases, and the emerging role of lipid signalling. More studies including systems biology approaches are now needed to comprehend how plant water transport can be adjusted in response to combined stresses.

Identification of Raf-Like Kinases B Subfamily Genes in Gossypium Species Revealed GhRAF42 Enhanced Salt Tolerance in Cotton

Salinity is a critical abiotic factor that significantly reduces agricultural production. Cotton is an important fiber crop and a pioneer on saline soil, hence genetic architecture that underpins salt tolerance should be thoroughly investigated. The Raf-like kinase B-subfamily (RAF) genes were discovered to regulate the salt stress response in cotton plants. However, understanding the RAFs in cotton, such as Enhanced Disease Resistance 1 and Constitutive Triple Response 1 kinase, remains a mystery. This study obtained 29, 28, 56, and 54 RAF genes from G. arboreum, G. raimondii, G. hirsutum, and G. barbadense, respectively. The RAF gene family described allopolyploidy and hybridization events in allotetraploid cotton evolutionary connections. Ka/Ks analysis advocates that cotton evolution was subjected to an intense purifying selection of the RAF gene family. Interestingly, integrated analysis of synteny and gene collinearity suggested dispersed and segmental duplication events involved in the extension of RAFs in cotton. Transcriptome studies, functional validation, and virus-induced gene silencing on salt treatments revealed that GhRAF42 is engaged in salt tolerance in upland cotton. This research might lead to a better understanding of the role of RAFs in plants and the identification of suitable candidate salt-tolerant genes for cotton breeding.

6-Benzyladenine increasing subsequent waterlogging-induced waterlogging tolerance of summer maize by increasing hormone signal transduction

Summer maize is frequently subjected to waterlogging damage because of increased and variable rainfall during the growing season. The application of 6-benzyladenine (6-BA) can effectively mitigate the waterlogging effects on plant growth and increase the grain yield of waterlogged summer maize. However, the mechanisms underlying this process and the involvement of 6-BA in relevant signal transduction pathways remain unclear. In this study, we explored the effects of 6-BA on waterlogged summer maize using a phosphoproteomic technique to better understand the mechanism by which summer maize growth improves following waterlogging. Application of 6-BA inhibited the waterlogging-induced increase in abscisic acid (ABA) content and increased the phosphorylation levels of proteins involved in ABA signaling; accordingly, stomatal responsiveness to exogenous ABA increased. In addition, the application of 6-BA had a long-term effect on signal transduction pathways and contributed to rapid responses to subsequent stresses. Plants primed with 6-BA accumulated more ethylene and jasmonic acid in response to subsequent waterlogging; accordingly, leaf SPAD, antioxidase activity, and root traits improved by 6-BA priming. These results suggest that the effects of 6-BA on hormone signal transduction pathways are anamnestic, which enables plants to show faster or stronger defense responses to stress.

Potassium: A track to develop salinity tolerant plants

Salinity is one of the major constraints to plant growth and development across the globe that leads to the huge crop productivity loss. Salinity stress causes impairment in plant's metabolic and cellular processes including disruption in ionic homeostasis due to excess of sodium (Na⁺) ion influx and potassium (K⁺) efflux. This condition subsequently results in a significant reduction of the cytosolic K⁺ levels, eventually inhibiting plant growth attributes. K⁺ plays a crucial role in alleviating salinity stress by recasting key processes of plants. In addition, K⁺ acquisition and retention also serve as the perquisite trait to establish salt tolerant mechanism. In addition, an intricate network of genes and their regulatory elements are involved in coordinating salinity stress responses. Furthermore, plant growth regulators (PGRs) and other signalling molecules influence K⁺-mediated salinity tolerance in plants. Recently, nanoparticles (NPs) have also been found several implications in plants with respect to their roles in mediating K⁺ homoeostasis during salinity stress in plants. The present review describes salinity-induced adversities in plants and role of K⁺ in mitigating salinity-induced damages. The review also highlights the efficacy of PGRs and other signalling molecules in regulating K⁺ mediated salinity tolerance along with nano-technological perspective for improving K⁺ mediated salinity tolerance in plants.

How roots and shoots communicate through stressful times

When plants face an environmental stress such as water deficit, soil salinity, high temperature, or shade, good communication between above- and belowground organs is necessary to coordinate growth and development. Various signals including hormones, peptides, proteins, hydraulic signals, and metabolites are transported mostly through the vasculature to distant tissues. How shoots and roots synchronize their response to stress using mobile signals is an emerging field of research. We summarize recent advances on mobile signals regulating shoot stomatal movement and root development in response to highly localized environmental cues. In addition, we highlight how the vascular system is not only a conduit but is also flexible in its development in response to abiotic stress.

The hypoxia-reoxygenation stress in plants

Plants are very plastic in adapting growth and development to changing adverse environmental conditions. This feature will be essential for plants to survive climate changes characterized by extreme temperatures and rainfall. Although plants require molecular oxygen (O2) to live, they can overcome transient low-O2 conditions (hypoxia) until return to standard 21% O2 atmospheric conditions (normoxia). After heavy rainfall, submerged plants in flooded lands undergo transient hypoxia until water recedes and normoxia is recovered. The accumulated information on the physiological and molecular events occurring during the hypoxia phase contrasts with the limited knowledge on the reoxygenation process after hypoxia, which has often been overlooked in many studies in plants. Phenotypic alterations during recovery are due to potentiated oxidative stress generated by simultaneous reoxygenation and reillumination leading to cell damage. Besides processes such as N-degron proteolytic pathway-mediated O2 sensing, or mitochondria-driven metabolic alterations, other molecular events controlling gene expression have been recently proposed as key regulators of hypoxia and reoxygenation. RNA regulatory functions, chromatin remodeling, protein synthesis, and post-translational modifications must all be studied in depth in the coming years to improve our knowledge on hypoxia-reoxygenation transition in plants, a topic with relevance in agricultural biotechnology in the context of global climate change.

Overlapping and stress-specific transcriptomic and hormonal responses to flooding and drought in soybean

Flooding and drought are serious constraints that reduce crop productivity worldwide. Previous studies identified genes conferring tolerance to both water extremes in various plants. However, overlapping responses to flooding and drought at the genome-scale remain obscure. Here, we defined overlapping and stress-specific transcriptomic and hormonal responses to submergence, drought and recovery from these stresses in soybean (Glycine max). We performed comparative RNA-sequencing and hormone profiling, identifying genes, hormones and biological processes that are differentially regulated in an overlapping or stress-specific manner. Overlapping responses included positive regulation of trehalose and sucrose metabolism and negative regulation of cellulose, tubulin, photosystem II and I, and chlorophyll biosynthesis, facilitating the economization of energy reserves under both submergence and drought. Additional energy-consuming pathways were restricted in a stress-specific manner. Downregulation of distinct pathways for energy saving under each stress suggests energy-consuming processes that are relatively unnecessary for each stress adaptation are turned down. Our newly developed transcriptomic-response analysis revealed that abscisic acid and ethylene responses were activated in common under both stresses, whereas stimulated auxin response was submergence-specific. The energy-saving strategy is the key overlapping mechanism that underpins adaptation to both submergence and drought in soybean. Abscisic acid and ethylene are candidate hormones that coordinate transcriptomic energy-saving processes under both stresses. Auxin may be a signaling component that distinguishes submergence-specific regulation of the stress response.

Ethylene enhances root water transport and aquaporin expression in trembling aspen (Populus tremuloides) exposed to root hypoxia

BACKGROUND

Root hypoxia has detrimental effects on physiological processes and growth in most plants. The effects of hypoxia can be partly alleviated by ethylene. However, the tolerance mechanisms contributing to the ethylene-mediated hypoxia tolerance in plants remain poorly understood.

RESULTS

In this study, we examined the effects of root hypoxia and exogenous ethylene treatments on leaf gas exchange, root hydraulic conductance, and the expression levels of several aquaporins of the plasma membrane intrinsic protein group (PIP) in trembling aspen (Populus tremuloides) seedlings. Ethylene enhanced net photosynthetic rates, transpiration rates, and root hydraulic conductance in hypoxic plants. Of the two subgroups of PIPs (PIP1 and PIP2), the protein abundance of PIP2s and the transcript abundance of PIP2;4 and PIP2;5 were higher in ethylene-treated trembling aspen roots compared with non-treated roots under hypoxia. The increases in the expression levels of these aquaporins could potentially facilitate root water transport. The enhanced root water transport by ethylene was likely responsible for the increase in leaf gas exchange of the hypoxic plants.

CONCLUSIONS

Exogenous ethylene enhanced root water transport and the expression levels of PIP2;4 and PIP2;5 in hypoxic roots of trembling aspen. The results suggest that ethylene facilitates the aquaporin-mediated water transport in plants exposed to root hypoxia.

Potassium Control of Plant Functions: Ecological and Agricultural Implications

Potassium, mostly as a cation (K+), together with calcium (Ca2+) are the most abundant inorganic chemicals in plant cellular media, but they are rarely discussed. K+ is not a component of molecular or macromolecular plant structures, thus it is more difficult to link it to concrete metabolic pathways than nitrogen or phosphorus. Over the last two decades, many studies have reported on the role of K+ in several physiological functions, including controlling cellular growth and wood formation, xylem-phloem water content and movement, nutrient and metabolite transport, and stress responses. In this paper, we present an overview of contemporary findings associating K+ with various plant functions, emphasizing plant-mediated responses to environmental abiotic and biotic shifts and stresses by controlling transmembrane potentials and water, nutrient, and metabolite transport. These essential roles of K+ account for its high concentrations in the most active plant organs, such as leaves, and are consistent with the increasing number of ecological and agricultural studies that report K+ as a key element in the function and structure of terrestrial ecosystems, crop production, and global food security. We synthesized these roles from an integrated perspective, considering the metabolic and physiological functions of individual plants and their complex roles in terrestrial ecosystem functions and food security within the current context of ongoing global change. Thus, we provide a bridge between studies of K+ at the plant and ecological levels to ultimately claim that K+ should be considered at least at a level similar to N and P in terrestrial ecological studies.

Abscisic Acid as an Emerging Modulator of the Responses of Plants to Low Oxygen Conditions

Different environmental and developmental cues involve low oxygen conditions, particularly those associated to abiotic stress conditions. It is widely accepted that plant responses to low oxygen conditions are mainly regulated by ethylene (ET). However, interaction with other hormonal signaling pathways as gibberellins (GAs), auxin (IAA), or nitric oxide (NO) has been well-documented. In this network of interactions, abscisic acid (ABA) has always been present and regarded to as a negative regulator of the development of morphological adaptations to soil flooding: hyponastic growth, adventitious root emergence, or formation of secondary aerenchyma in different plant species. However, recent evidence points toward a positive role of this plant hormone on the modulation of plant responses to hypoxia and, more importantly, on the ability to recover during the post-hypoxic period. In this work, the involvement of ABA as an emerging regulator of plant responses to low oxygen conditions alone or in interaction with other hormones is reviewed and discussed.

Role of Raf-like kinases in SnRK2 activation and osmotic stress response in plants

Environmental drought and high salinity impose osmotic stress, which inhibits plant growth and yield. Thus, understanding how plants respond to osmotic stress is critical to improve crop productivity. Plants have multiple signalling pathways in response to osmotic stress in which the phytohormone abscisic acid (ABA) plays important roles. However, since little is known concerning key early components, the global osmotic stress-signalling network remains to be elucidated. Here, we review recent advances in the identification of osmotic-stress activated Raf-like protein kinases as regulators of ABA-dependent and -independent signalling pathways and discuss the plant stress-responsive kinase network from an evolutionary perspective.

A better understanding of how plants respond to osmotic stress could potentially help improve crop yields. Here Fàbregas et al. review the recent characterization of Raf-like kinases that act in both in ABA-dependent and -independent responses to osmotic stress.

Root architecture and hydraulics converge for acclimation to changing water availability

Because of intense transpiration and growth, the needs of plants for water can be immense. Yet water in the soil is most often heterogeneous if not scarce due to more and more frequent and intense drought episodes. The converse context, flooding, is often associated with marked oxygen deficiency and can also challenge the plant water status. Under our feet, roots achieve an incredible challenge to meet the water demand of the plant's aerial parts under such dramatically different environmental conditions. For this, they continuously explore the soil, building a highly complex, branched architecture. On shorter time scales, roots keep adjusting their water transport capacity (their so-called hydraulics) locally or globally. While the mechanisms that directly underlie root growth and development as well as tissue hydraulics are being uncovered, the signalling mechanisms that govern their local and systemic adjustments as a function of water availability remain largely unknown. A comprehensive understanding of root architecture and hydraulics as a whole (in other terms, root hydraulic architecture) is needed to apprehend the strategies used by plants to optimize water uptake and possibly improve crops regarding this crucial trait.

The calcineurin β-like interacting protein kinase CIPK25 regulates potassium homeostasis under low oxygen in Arabidopsis

Hypoxic conditions often arise from waterlogging and flooding, affecting several aspects of plant metabolism, including the uptake of nutrients. We identified a member of the CALCINEURIN β-LIKE INTERACTING PROTEIN KINASE (CIPK) family in Arabidopsis, CIPK25, which is induced in the root endodermis under low-oxygen conditions. A cipk25 mutant exhibited higher sensitivity to anoxia in conditions of potassium limitation, suggesting that this kinase is involved in the regulation of potassium uptake. Interestingly, we found that CIPK25 interacts with AKT1, the major inward rectifying potassium channel in Arabidopsis. Under anoxic conditions, cipk25 mutant seedlings were unable to maintain potassium concentrations at wild-type levels, suggesting that CIPK25 likely plays a role in modulating potassium homeostasis under low-oxygen conditions. In addition, cipk25 and akt1 mutants share similar developmental defects under waterlogging, further supporting an interplay between CIPK25 and AKT1.

Measuring the Hydraulic Conductivity of Grass Root Systems

Root-system hydraulic conductivity (RSHC) is an important physiological characteristic that describes the inherent ability of roots to conduct water across a water-potential gradient between the root and the stem xylem. RSHC is commonly used as an indicator of plant functioning and adaptability to a given environment. A simple, fast, and easy-to-use protocol is described for the quantification of RSHC at the seedling stage in two important monocot species grown in hydroponic solution: Setaria viridis, a C4 model plant, and wheat, a C3 crop plant. This protocol can also be easily modified for use with almost any grass species and environmental treatments, such as salinity or hormone treatments. © 2020 by John Wiley & Sons, Inc. Basic Protocol: Setaria hydrostatic root-system hydraulic conductivity Alternate Protocol: Measuring the root conductivity of young plants with soft stems.

Transcription Factors in Plant Stress Responses: Challenges and Potential for Sugarcane Improvement

Increasing vulnerability of crops to a wide range of abiotic and biotic stresses can have a marked influence on the growth and yield of major crops, especially sugarcane (Saccharum spp.). In response to various stresses, plants have evolved a variety of complex defense systems of signal perception and transduction networks. Transcription factors (TFs) that are activated by different pathways of signal transduction and can directly or indirectly combine with cis-acting elements to modulate the transcription efficiency of target genes, which play key regulators for crop genetic improvement. Over the past decade, significant progresses have been made in deciphering the role of plant TFs as key regulators of environmental responses in particular important cereal crops; however, a limited amount of studies have focused on sugarcane. This review summarizes the potential functions of major TF families, such as WRKY, NAC, MYB and AP2/ERF, in regulating gene expression in the response of plants to abiotic and biotic stresses, which provides important clues for the engineering of stress-tolerant cultivars in sugarcane.

Plant Raf-like kinases regulate the mRNA population upstream of ABA-unresponsive SnRK2 kinases under drought stress

SNF1-related protein kinases 2 (SnRK2s) are key regulators governing the plant adaptive responses to osmotic stresses, such as drought and high salinity. Subclass III SnRK2s function as central regulators of abscisic acid (ABA) signalling and orchestrate ABA-regulated adaptive responses to osmotic stresses. Seed plants have acquired other types of osmotic stress-activated but ABA-unresponsive subclass I SnRK2s that regulate mRNA decay and promote plant growth under osmotic stresses. In contrast to subclass III SnRK2s, the regulatory mechanisms underlying the rapid activation of subclass I SnRK2s in response to osmotic stress remain elusive. Here, we report that three B4 Raf-like MAP kinase kinase kinases (MAPKKKs) phosphorylate and activate subclass I SnRK2s under osmotic stress. Transcriptome analyses reveal that genes downstream of these MAPKKKs largely overlap with subclass I SnRK2-regulated genes under osmotic stress, which indicates that these MAPKKKs are upstream factors of subclass I SnRK2 and are directly activated by osmotic stress.

Regulation of Photosynthetic Carbohydrate Metabolism by a Raf-Like Kinase in the Liverwort Marchantia polymorpha

To optimize growth and development, plants monitor photosynthetic activities and appropriately regulate various cellular processes. However, signaling mechanisms that coordinate plant growth with photosynthesis remain poorly understood. To identify factors that are involved in signaling related to photosynthetic stimuli, we performed a phosphoproteomic analysis with Marchantia polymorpha, an extant bryophyte species in the basal lineage of land plants. Among proteins whose phosphorylation status changed differentially between dark-treated plants and those after light irradiation but failed to do so in the presence of a photosynthesis inhibitor, we identified a B4-group Raf-like kinase, named PHOTOSYNTHESIS-RELATED RAF (MpPRAF). Biochemical analyses confirmed photosynthesis-activity-dependent changes in the phosphorylation status of MpPRAF. Mutations in the MpPRAF gene resulted in growth retardation. Measurement of carbohydrates demonstrated both hyper-accumulation of starch and reduction of sucrose in Mppraf mutants. Neither inhibition of starch synthesis nor exogenous supply of sucrose alleviated the growth defect, suggesting serious impairment of Mppraf mutants in both the synthesis of sucrose and the repression of its catabolism. As a result of the compromised photosynthate metabolism, photosynthetic electron transport was downregulated in Mppraf mutants. A mutated MpPRAF with a common amino acid substitution for inactivating kinase activity was unable to rescue the Mppraf mutant defects. Our results provide evidence that MpPRAF is a photosynthesis signaling kinase that regulates sucrose metabolism.

A RAF-SnRK2 kinase cascade mediates early osmotic stress signaling in higher plants

Osmoregulation is important for plant growth, development and response to environmental changes. SNF1-related protein kinase 2s (SnRK2s) are quickly activated by osmotic stress and are central components in osmotic stress and abscisic acid (ABA) signaling pathways; however, the upstream components required for SnRK2 activation and early osmotic stress signaling are still unknown. Here, we report a critical role for B2, B3 and B4 subfamilies of Raf-like kinases (RAFs) in early osmotic stress as well as ABA signaling in Arabidopsis thaliana. B2, B3 and B4 RAFs are quickly activated by osmotic stress and are required for phosphorylation and activation of SnRK2s. Analyses of high-order mutants of RAFs reveal critical roles of the RAFs in osmotic stress tolerance and ABA responses as well as in growth and development. Our findings uncover a kinase cascade mediating osmoregulation in higher plants.

Rapid activation of SnRK2 kinases is central to plant responses to osmotic stress and abscisic acid. Here the authors show that a group of Raf-like kinases are very quickly activated by osmotic stress, and then phosphorylate and activate SnRK2s.

Cryptic variation in RNA-directed DNA-methylation controls lateral root development when auxin signalling is perturbed

Maintaining the right balance between plasticity and robustness in biological systems is important to allow adaptation while maintaining essential functions. Developmental plasticity of plant root systems has been the subject of intensive research, but the mechanisms underpinning robustness remain unclear. Here, we show that potassium deficiency inhibits lateral root organogenesis by delaying early stages in the formation of lateral root primordia. However, the severity of the symptoms arising from this perturbation varies within a natural population of Arabidopsis and is associated with the genetic variation in CLSY1, a key component of the RNA-directed DNA-methylation machinery. Mechanistically, CLSY1 mediates the transcriptional repression of a negative regulator of root branching, IAA27, and promotes lateral root development when the auxin-dependent proteolysis pathway fails. Our study identifies DNA-methylation-mediated transcriptional repression as a backup system for post-translational protein degradation which ensures robust development and performance of plants in a challenging environment.

Developmental plasticity of plant root systems has been intensively studied, but the mechanisms underpinning robustness remain unclear. Here, the authors show that DNA-methylation-mediated transcriptional repression serves as a backup system to control lateral root development when auxin signalling is perturbed.

Regulation of a plant aquaporin by a Casparian strip membrane domain protein-like

The absorption of soil water by roots allows plants to maintain their water status. At the endodermis, water transport can be affected by initial formation of a Casparian strip and further deposition of suberin lamellas and regulated by the function of aquaporins. Four Casparian strip membrane domain protein-like (CASPL; CASPL1B1, CASPL1B2, CASPL1D1, and CASPL1D2) were previously shown to interact with PIP2;1. The present work shows that CASPL1B1, CASPL1B2, and CASPL1D2 are exclusively expressed in suberized endodermal cells, suggesting a cell-specific role in suberization and/or water transport regulation. When compared with wild-type plants, and by contrast to caspl1b1*caspl1b2 double loss of function, caspl1d1*caspl1d2 double mutants showed, in some control or NaCl stress experiments and not upon abscisic acid (ABA) treatment, a weak enlargement of the continuous suberization zone. None of the mutants showed root hydraulic conductivity (Lp_r ) phenotype, whether in control, NaCl, or ABA treatment conditions. The data suggest a slight negative role for CASPL1D1 and CASPL1D2 in suberization under control or salt stress conditions, with no major impact on whole root transport functions. At the molecular level, CASPL1B1 was able to physically interact with PIP2;1 and potentially could influence the regulation of aquaporins by acting on their phosphorylated form.

After The Deluge: Plant Revival Post-Flooding

Increasing flooding events have detrimentally impacted food security amid a growing global population. Complete submergence of plants represents the most severe flooding stress and studies have identified underwater responses to low oxygen and light availability. However, knowledge on plant responses during the post-submergence phase is limited. It is important to consider how plants can resume vegetative growth after enduring submergence and post-submergence stress. This review highlights current knowledge on physiological and molecular adaptations following desubmergence. Interplays of reactive oxygen species (ROS), energy depletion, photoinhibition, desiccation stress, and hormonal signaling have been characterized as components of the post-submergence stress response. Active elucidation of key genes and traits enhancing post-submergence adaptations is highly relevant for the improvement of submergence tolerance and ultimately crop yield.

Photosynthetic and Growth Responses of Arundo donax L. Plantlets Under Different Oxygen Deficiency Stresses and Reoxygenation

Promotion of nonfood species production to marginal, degraded lands abandoned by mainstream agriculture is affected by extremes of water availability (droughts and floods), which have increased in frequency and intensity and account for severe yield reduction. Arundo donax L., known as giant cane or giant reed, spontaneously grows in different kinds of environments with limitation to low temperature and is thus widespread in temperate and hot areas around the world. Moreover, this perennial rhizomatous grass has been recognized as a leading candidate crop in the Mediterranean for lignocellulosic feedstock due to its high C3 photosynthetic capacity, positive energy balance and low agroecological management demand. In this study, the photosynthetic performance and growth response of A. donax to waterlogging and submergence stress following a time course as well as their respective re-oxygenation were analyzed under reproducible and controlled environment conditions. Results of growth response showed that biomass production was strongly conditioned by the availability of oxygen. In fact, only waterlogged plants showed similar growth capacity to those under control conditions, while plants under submergence resulted in a dramatic reduction of this trait. The simultaneous measurements of both gas exchanges and chlorophyll fluorescence highlighted an alteration of both stomatal and non-stomatal photosynthetic behaviors during a short/medium period of oxygen deprivation and re-oxygenation. Photosynthetic CO2 uptake was strictly related to a combination of stomatal and mesophyll diffusional constrains, depending on the severity of the treatment and exposure time. Conditions of waterlogging and hypoxia revealed a slight growth plasticity of the species in response to prolonged stress conditions, followed by a fast recovery upon reoxygenation. Moreover, the rapid restoration of physiological functions after O2 deprivation testifies to the environmental plasticity of this species, although prolonged O2 shortage proved detrimental to A. donax by hampering growth and photosynthetic CO2 uptake.

Submergence and Waterlogging Stress in Plants: A Review Highlighting Research Opportunities and Understudied Aspects

Soil flooding creates composite and complex stress in plants known as either submergence or waterlogging stress depending on the depth of the water table. In nature, these stresses are important factors dictating the species composition of the ecosystem. On agricultural land, they cause economic damage associated with long-term social consequences. The understanding of the plant molecular responses to these two stresses has benefited from research studying individual components of the stress, in particular low-oxygen stress. To a lesser extent, other associated stresses and plant responses have been incorporated into the molecular framework, such as ion and ROS signaling, pathogen susceptibility, and organ-specific expression and development. In this review, we aim to highlight known or suspected components of submergence/waterlogging stress that have not yet been thoroughly studied at the molecular level in this context, such as miRNA and retrotransposon expression, the influence of light/dark cycles, protein isoforms, root architecture, sugar sensing and signaling, post-stress molecular events, heavy-metal and salinity stress, and mRNA dynamics (splicing, sequestering, and ribosome loading). Finally, we explore biotechnological strategies that have applied this molecular knowledge to develop cultivars resistant to flooding or to offer alternative uses of flooding-prone soils, like bioethanol and biomass production.

Natural variation at XND1 impacts root hydraulics and trade-off for stress responses in Arabidopsis

Soil water uptake by roots is a key component of plant performance and adaptation to adverse environments. Here, we use a genome-wide association analysis to identify the XYLEM NAC DOMAIN 1 (XND1) transcription factor as a negative regulator of Arabidopsis root hydraulic conductivity (Lp_r). The distinct functionalities of a series of natural XND1 variants and a single nucleotide polymorphism that determines XND1 translation efficiency demonstrate the significance of XND1 natural variation at species-wide level. Phenotyping of xnd1 mutants and natural XND1 variants show that XND1 modulates Lp_r through action on xylem formation and potential indirect effects on aquaporin function and that it diminishes drought stress tolerance. XND1 also mediates the inhibition of xylem formation by the bacterial elicitor flagellin and counteracts plant infection by the root pathogen Ralstonia solanacearum. Thus, genetic variation at XND1, and xylem differentiation contribute to resolving the major trade-off between abiotic and biotic stress resistance in Arabidopsis.

Soil water uptake is a major determinant of plant performance and stress tolerance. Here the authors show that, by affecting xylem formation in the root, natural variation at the Arabidopsis XND1 locus has contrasting effects on root hydraulics and drought tolerance versus pathogen resistance.

The effect of exogenous calcium on cucumber fruit quality, photosynthesis, chlorophyll fluorescence, and fast chlorophyll fluorescence during the fruiting period under hypoxic stress

BACKGROUND

Plants often suffer from hypoxic stress during waterlogging and hydroponic culturing. This study investigated the response of cucumber (Cucumis sativus L.) plant growth parameters, leaf photosynthesis, chlorophyll fluorescence, fast chlorophyll a fluorescence transient (OJIP), and fruit quality parameters to hypoxic stress alleviated by exogenous calcium. During the fruiting period, cucumber plants were exposed to hypoxia and hypoxia + Ca²⁺ treatment (4 mM Ca²⁺) for 9 d.

RESULT

Exogenous calcium application enhanced the biomass and fruit quality of hypoxic stressed cucumber and also increased the net photosynthesis rate, stomatal conductance, intercellular CO₂ concentration, maximum quantum efficiency of photosystem II photochemistry, actual photochemical efficiency of PSII, photochemical quenching coefficient, and non-photochemical quenching coefficient. Additionally, measurement of chlorophyll a fluorescence transients showed the positive K- and L-bands were more pronounced in leaves treated with hypoxia compared with those with hypoxia + Ca²⁺, indicating that hypoxic treatment induced uncoupling of the oxygen-evolving complex and inhibited electron transport beyond plastoquinone pool (Q_a, Q_b) including possible constraints on the reduction of end electron acceptors of photosystem I. Exogenous calcium can reduce these stress-induced damages in cucumber.

CONCLUSION

This research focused the effect of exogenous calcium on cucumber photosynthesis during the fruiting period under hypoxic stress. Hypoxic stress might impair the photosynthetic electron-transport chain from the donor side of PSII up to the reduction of end acceptors of PSI, and exogenous calcium enhanced electron transport capacity and reduced hypoxic damage of cucumber leaves.

Preformed and induced mechanisms underlies the differential responses of Prunus rootstock to hypoxia

Analysis of the transcriptomic changes produced in response to hypoxia in root tissues from two rootstock Prunus genotypes differing in their sensitivity to waterlogging: resistant Myrobalan 'P.2175' (P. cerasifera Erhr.), and sensitive 'Felinem' hybrid [P. amygdalus Batsch × P. persica (L.) Batsch] revealed alterations in both metabolism and regulatory processes. Early hypoxia response in both genotypes is characterized by a molecular program aimed to adapt the cell metabolism to the new conditions. Upon hypoxia conditions, tolerant Myrobalan represses first secondary metabolism gene expression as a strategy to prevent the waste of resources/energy, and by the up-regulation of protein degradation genes probably leading to structural adaptations to long-term response to hypoxia. In response to the same conditions, sensitive 'Felinem' up-regulates a core of signal transduction and transcription factor genes. A combination of PLS-DA and qRT-PCR approaches revealed a set of transcription factors and signalling molecules as differentially regulated in the sensitive and tolerant genotypes including the peach orthologs for oxygen sensors. Apart from providing insights into the molecular processes underlying the differential response to waterlogging of two Prunus rootstocks, our approach reveals a set of candidate genes to be used expression biomarkers for biotech or breeding approaches to waterlogging tolerance.

Plant water transport and aquaporins in oxygen-deprived environments

Oxygen deprivation commonly affects plants exposed to flooding and soil compaction. The resulting root hypoxia has an immediate effect on plant water relations and upsets water balance. Hypoxia inhibits root water transport and triggers stomatal closure. The processes contributing to the inhibition of root hydraulic conductivity and conductance (hydraulic conductivity of the whole root system) are complex and involve changes in root morphology and the functions of aquaporins. Aquaporins (AQPs) comprise a group of membrane intrinsic proteins that are responsible for the transport of water, as well as some small neutral solutes and ions. They respond to a wide range of environmental stresses including O₂ deprivation, but the underlying functional mechanisms are still elusive. The aquaporin-mediated water transport is affected by the acidification of the cytoplasm and depletion of ATP that is required for aquaporin phosphorylation and membrane functions. Cytoplasmic pH, phosphorylation, and intracellular Ca²⁺ concentration directly control AQP gating, all of which are related to O₂ deprivation. This review addresses the structural determinants that are essential for pore conformational changes in AQPs, to highlight the underlying mechanisms triggered by O₂ deprivation stress. Gene expression of AQPs is modified in hypoxic plants, which may constitute an important, yet little explored, mechanism of hypoxia tolerance. In addition to water transport, AQPs may contribute to hypoxia tolerance by transporting O₂, H₂O₂, and lactic acid. Responses of plants to O₂ deprivation, and especially those that contribute to maintenance of water transport, are highly complex and entail the signals originating in roots and shoots that lead to and follow the stomatal closure. These complex responses may involve ethylene, abscisic acid, and possibly other hormonal factors and signaling molecules in ways that remain to be elucidated.

Arabidopsis thaliana Raf22 protein kinase maintains growth capacity during postgerminative growth arrest under stress

When seeds are exposed to drought and salinity during germination, newly germinated embryos stop growth and enter a quiescent state called postgerminative growth (PGG) arrest. PGG arrest protects embryos from the stress, but it is not known how PGG is restored from the arrest when stress is eased. In this study, we show that under stress- or abscisic acid-induced PGG arrest conditions, Arabidopsis thaliana Raf-type protein kinase 22 (AtRaf22) overexpression accelerated photoautotrophic seedling establishment, whereas atraf22 knockout mutations enhanced the arrest. Furthermore, when the stress intensity was reduced subsequently, AtRaf22 overexpression plants resumed growth and accomplished photoautotrophic transition much faster than the knockout or wild-type plants. These results suggest that AtRaf22 activity is important for maintaining growth capacity during stress-induced PGG arrest, which is most likely critical for competitive growth when the stress subsides and plants resume growth. Such a factor has not been reported before.

A stress recovery signaling network for enhanced flooding tolerance in Arabidopsis thaliana

Abiotic stresses in plants are often transient, and the recovery phase following stress removal is critical. Flooding, a major abiotic stress that negatively impacts plant biodiversity and agriculture, is a sequential stress where tolerance is strongly dependent on viability underwater and during the postflooding period. Here we show that in Arabidopsis thaliana accessions (Bay-0 and Lp2-6), different rates of submergence recovery correlate with submergence tolerance and fecundity. A genome-wide assessment of ribosome-associated transcripts in Bay-0 and Lp2-6 revealed a signaling network regulating recovery processes. Differential recovery between the accessions was related to the activity of three genes: RESPIRATORY BURST OXIDASE HOMOLOG D, SENESCENCE-ASSOCIATED GENE113, and ORESARA1, which function in a regulatory network involving a reactive oxygen species (ROS) burst upon desubmergence and the hormones abscisic acid and ethylene. This regulatory module controls ROS homeostasis, stomatal aperture, and chlorophyll degradation during submergence recovery. This work uncovers a signaling network that regulates recovery processes following flooding to hasten the return to prestress homeostasis.

Influences of Air, Oxygen, Nitrogen, and Carbon Dioxide Nanobubbles on Seed Germination and Plant Growth

Nanobubbles (NBs) hold promise in green and sustainable engineering applications in diverse fields (e.g., water/wastewater treatment, food processing, medical applications, and agriculture). This study investigated the effects of four types of NBs on seed germination and plant growth. Air, oxygen, nitrogen, and carbon dioxide NBs were generated and dispersed in tap water. Different plants, including lettuce, carrot, fava bean, and tomato, were used in germination and growth tests. The seeds in water-containing NBs exhibited 6-25% higher germination rates. Especially, nitrogen NBs exhibited considerable effects in the seed germination, whereas air and carbon dioxide NBs did not significantly promote germination. The growth of stem length and diameter, leave number, and leave width were promoted by NBs (except air). Furthermore, the promotion effect was primarily ascribed to the generation of exogenous reactive oxygen species by NBs and higher efficiency of nutrient fixation or utilization.

Hormone dependent survival mechanisms of plants during post-waterlogging stress

Waterlogging stress has two phases like waterlogging phase and post-waterlogging phase where both are injurious to plants. Susceptible plants normally die at post-waterlogging phase due to damaged root system, sudden rexoygenation, dehydration and photoinhibition of the desubmerged tissues. Formation of reactive oxygen species (ROS) is the main result of reoxygenation stress that can cause oxidative damage of the functional tissues responsible for normal physiological activities. There are almost all types of hormones responsible to recover plants from these destructive phenomenons. Among these hormones ethylene and abscisic acid (ABA) are the main regulators to overcome the reoxygenation and drought like stresses in plants at post-waterlogging condition. The balanced crosstalk among the hormones is highly important for the survival of plants at these stresses. So this paper is completely a precise summary of hormonal homeostasis of post-waterlogged plants through physiological, biochemical and signaling pathways.