Ethylene Differentially Modulates Hypoxia Responses and Tolerance across Solanum Species

ACS2 and ACS6, especially ACS2 is involved in MPK6 evoked production of ethylene under Cd stress, which exacerbated Cd toxicity in Arabidopsis thaliana

As one of the heavy metal pollutants with strong biological toxicity, cadmium (Cd) is easily absorbed by plant roots, which seriously restricts the growth of plants, causes the quality of agricultural products to decline and threatens human health. Many complex signal transduction pathways are involved in the process of plant response to Cd stress. Among them, plant hormone ethylene is an important signal molecule for plant response to various environmental stresses, and its regulatory mechanism and signal transduction pathway in Cd stress response need to be further clarified. Here, we discovered that Cd stress induced a significant increment in ethylene production in Arabidopsis roots, and the amount of ethylene produced was positively correlated with the inhibition of Arabidopsis root growth and Cd accumulation. Simultaneously, Cd stress stimulated the detoxification mechanism within cells and promoted the expression of METAL TOLERANCE PROTEIN 3 (MTP3), IRON-REGULATED TRANSPORTER2 (IRT2), IRON REGULATED GENE 2 (IREG2) genes implicated in Cd vacuolar compartmentation. However, whether this is associated with ethylene signal transduction remains to be further explored. Further studies have revealed that the Cd induced ethylene burst is attributed to the up-regulation of the expression of 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHASE (ACS) genes that mediated by MITONGEN-ACTIVATED PROTEIN KINASE 6 (MAPK6) in Arabidopsis roots, and among them, ACS2 and ACS6, especially ACS2, are involved in MAPK6-induced ethylene production under Cd stress. The results of this study provide new ideas for understanding the signal transduction pathway of plant response to Cd stress.

Possible role of autophagy in microbial volatile pollutant-induced starch degradation and expression of hypoxia responsive genes

Autophagy is thought to be critically involved in the regulation of nutrient metabolism and gene expression. Nevertheless, little is known about its role in regulating starch metabolism and hypoxia responsive genes in plants exposed to microbial volatile pollutants. In the present study, we found that exposure of Arabidopsis to Enterobacter aerogene (E. aerogene) volatile pollutants induced autophagy, as indicated by autophagosome formation. The exposure also caused upregulation of autophagy-associated genes, such as ATGs, NBR1, ATI1, and ATG8e-regulating transcription factors. Additionally, exposure to E. aerogenes volatile pollutants induced starch degradation in the roots of Arabidopsis seedlings. Finally, we found that ATG7-deficiency negatively affected the expression of hypoxia-responsive genes (i.e HRE1, HRA1, and ADH1) and starch degradation induced by E. aerogenes volatile pollutants. Overall, our study reveals that microbial volatile pollutants can induce starch degradation and autophagy, which participates in the regulation of some hypoxia-responsive genes and starch metabolism. These findings help to define the role of autophagy in plant nutrient metabolism and regulation of gene expression under microbial volatile pollutant exposure. The insights gained may contribute to agricultural management when living organisms face challenges from microbial volatile pollutants.

Reciprocal modulation of responses to nitrate starvation and hypoxia in roots and leaves of Arabidopsis thaliana

The flooding of agricultural land leads to hypoxia and nitrate leaching. While understanding the plant's response to these conditions is essential for crop improvement, the effect of extended nitrate limitation on subsequent hypoxia has not been studied in an organ-specific manner. We cultivated Arabidopsis thaliana without nitrate for 1 week before inducing hypoxia by bubbling the hydroponic solution with nitrogen gas for 16 h. In the roots, the transcripts of two transcription factor genes (HRA1, HRE2) and three genes involved in fermentation (SUS4, PDC1, ADH1) were ~10- to 100-fold upregulated by simultaneous hypoxia and nitrate starvation compared to the control condition (replete nitrate and oxygen). In contrast, this hypoxic upregulation was ~5 to 10 times stronger when nitrate was available. The phytoglobin genes PGB1 and PGB2, involved in nitric oxide (NO) scavenging, were massively downregulated by nitrate starvation (~1000-fold and 105-fold, respectively), but only under ambient oxygen levels; this was reflected in a 2.5-fold increase in NO concentration. In the leaves, HRA1, SUS4, and RAP2.3 were upregulated ~20-fold by hypoxia under nitrate starvation, whereas this upregulation was virtually absent in the presence of nitrate. Our results highlight that the plant's responses to nitrate starvation and hypoxia can influence each other.

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.

Low-oxygen-induced root bending is altered by phytoglobin1 through mediation of ethylene response factors (ERFs) and auxin signaling

MAIN CONCLUSION

phytoglobin1 positively regulates root bending in hypoxic Arabidopsis roots through regulation of ethylene response factors and auxin transport. Hypoxia-induced root bending is known to be mediated by the redundant activity of the group VII ethylene response factors (ERFVII) RAP2.12 and HRE2, causing changes in polar auxin transport (PAT). Here, we show that phytoglobin1 (Pgb1), implicated in hypoxic adaptation through scavenging of nitric oxide (NO), can alter root direction under low oxygen. Hypoxia-induced bending is exaggerated in roots over-expressing Pgb1 and attenuated in those where the gene is suppressed. These effects were attributed to Pgb1 repressing both RAP2.12 and HRE2. Expression, immunological and genetic data place Pgb1 upstream of RAP2.12 and HRE2 in the regulation of root bending in oxygen-limiting environments. The attenuation of slanting in Pgb1-suppressing roots was associated with depletion of auxin activity at the root tip because of depression in PAT, while exaggeration of root bending in Pgb1-over-expressing roots with the retention of auxin activity. Changes in PIN2 distribution patterns, suggestive of redirection of auxin movement during hypoxia, might contribute to the differential root bending responses of the transgenic lines. In the end, Pgb1, by regulating NO levels, controls the expression of 2 ERFVIIs which, in a cascade, modulate PAT and, therefore, root bending.

Looking for Resistance to Soft Rot Disease of Potatoes Facing Environmental Hypoxia

Plants are exposed to various stressors, including pathogens, requiring specific environmental conditions to provoke/induce plant disease. This phenomenon is called the "disease triangle" and is directly connected with a particular plant-pathogen interaction. Only a virulent pathogen interacting with a susceptible plant cultivar will lead to disease under specific environmental conditions. This may seem difficult to accomplish, but soft rot Pectobacteriaceae (SRPs) is a group virulent of pathogenic bacteria with a broad host range. Additionally, waterlogging (and, resulting from it, hypoxia), which is becoming a frequent problem in farming, is a favoring condition for this group of pathogens. Waterlogging by itself is an important source of abiotic stress for plants due to lowered gas exchange. Therefore, plants have evolved an ethylene-based system for hypoxia sensing. Plant response is coordinated by hormonal changes which induce metabolic and physiological adjustment to the environmental conditions. Wetland species such as rice (Oryza sativa L.), and bittersweet nightshade (Solanum dulcamara L.) have developed adaptations enabling them to withstand prolonged periods of decreased oxygen availability. On the other hand, potato (Solanum tuberosum L.), although able to sense and response to hypoxia, is sensitive to this environmental stress. This situation is exploited by SRPs which in response to hypoxia induce the production of virulence factors with the use of cyclic diguanylate (c-di-GMP). Potato tubers in turn reduce their defenses to preserve energy to prevent the negative effects of reactive oxygen species and acidification, making them prone to soft rot disease. To reduce the losses caused by the soft rot disease we need sensitive and reliable methods for the detection of the pathogens, to isolate infected plant material. However, due to the high prevalence of SRPs in the environment, we also need to create new potato varieties more resistant to the disease. To reach that goal, we can look to wild potatoes and other Solanum species for mechanisms of resistance to waterlogging. Potato resistance can also be aided by beneficial microorganisms which can induce the plant's natural defenses to bacterial infections but also waterlogging. However, most of the known plant-beneficial microorganisms suffer from hypoxia and can be outcompeted by plant pathogens. Therefore, it is important to look for microorganisms that can withstand hypoxia or alleviate its effects on the plant, e.g., by improving soil structure. Therefore, this review aims to present crucial elements of potato response to hypoxia and SRP infection and future outlooks for the prevention of soft rot disease considering the influence of 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.

Effect of ethylene pretreatment on tomato plant responses to salt, drought, and waterlogging stress

Salinity, drought, and waterlogging are common environmental stresses that negatively impact plant growth, development, and productivity. One of the responses to abiotic stresses is the production of the phytohormone ethylene, which induces different coping mechanisms that help plants resist or tolerate stress. In this study, we investigated if an ethylene pretreatment can aid plants in activating stress-coping responses prior to the onset of salt, drought, and waterlogging stress. Therefore, we measured real-time transpiration and CO2 assimilation rates and the impact on biomass during and after 3 days of abiotic stress. Our results showed that an ethylene pretreatment of 1 ppm for 4 h did not significantly influence the negative effects of waterlogging stress, while plants were more sensitive to salt stress as reflected by enhanced water losses due to a higher transpiration rate. However, when exposed to drought stress, an ethylene pretreatment resulted in reduced transpiration rates, reducing water loss during drought stress. Overall, our findings indicate that pretreating tomato plants with ethylene can potentially regulate their responses during the forthcoming stress period, but optimization of the ethylene pre-treatment duration, timing, and dose is needed. Furthermore, it remains tested if the effect is related to the stress duration and severity and whether an ethylene pretreatment has a net positive or negative effect on plant vigor during stress recovery. Further investigations are needed to elucidate the mode of action of how ethylene priming impacts subsequent stress responses.

Perspectives in Plant Abiotic Stress Signaling

State-of-the-art collections of strategies, approaches, and methods are immediately useful for ongoing characterizations or for novel discoveries in the scientific field of plant abiotic stress signaling. It must however be kept in mind that, in the future, these strategies, approaches, and methods will be facing a number of increasingly complex issues. The development of the necessary confrontation of laboratory-based knowledge on abiotic stress signaling mechanisms with real-life in natura situations of plant-stress interactions involves at least five levels of complexity: (i) plant biodiversity, (ii) the spatio-temporal heterogeneity of stress-related parameters, (iii) the unknowns of future stress-related constraints, (iv) the influence of biotic interactions, (v) the crosstalk between various signaling pathways and their final integration into physiological responses. These complexities are major bottlenecks for assessing the evolutionary, ecological, and agronomical relevance of abiotic stress signaling studies. All of the presently-described strategies, approaches, and methods will have to be gradually complemented with the development of real-time and in natura tools, with systematic application of mathematical modeling to complex interactions and with further research on the impact of stress memory mechanisms on long-term responses.

Interplay of Methodology and Conceptualization in Plant Abiotic Stress Signaling

Characterizing the mechanisms of plant sensitivity and reactivity to physicochemical cues related to abiotic stresses is of utmost importance for understanding plant-environment interactions, adaptations of the sessile lifestyle, and the evolutionary dynamics of plant species and populations. Moreover, plant communities are confronted with an environmental context of global change, involving climate changes, planetary pollutions of soils, waters and atmosphere, and additional anthropogenic changes. The mechanisms through which plants perceive abiotic stress stimuli and transduce stress perception into physiological responses constitute the primary line of interaction between the plant and the environment, and therefore between the plant and global changes. Understanding how plants perceive complex combinations of abiotic stress signals and transduce the resulting information into coordinated responses of abiotic stress tolerance is therefore essential for devising genetic, agricultural, and agroecological strategies that can ensure climate change resilience, global food security, and environmental protection. Discovery and characterization of sensing and signaling mechanisms of plant cells are usually carried out within the general framework of eukaryotic sensing and signal transduction. However, further progress depends on a close relationship between the conceptualization of sensing and signaling processes with adequate methodologies and techniques that encompass biochemical and biophysical approaches, cell biology, molecular biology, and genetics. The integration of subcellular and cellular analyses as well as the integration of in vitro and in vivo analyses are particularly important to evaluate the efficiency of sensing and signaling mechanisms in planta. Major progress has been made in the last 10-20 years with the caveat that cell-specific processes and in vivo processes still remain difficult to analyze and with the additional caveat that the range of plant models under study remains rather limited relatively to plant biodiversity and to the diversity of stress situations.

Ethylene augments root hypoxia tolerance via growth cessation and reactive oxygen species amelioration

Flooded plants experience impaired gas diffusion underwater, leading to oxygen deprivation (hypoxia). The volatile plant hormone ethylene is rapidly trapped in submerged plant cells and is instrumental for enhanced hypoxia acclimation. However, the precise mechanisms underpinning ethylene-enhanced hypoxia survival remain unclear. We studied the effect of ethylene pretreatment on hypoxia survival of Arabidopsis (Arabidopsis thaliana) primary root tips. Both hypoxia itself and re-oxygenation following hypoxia are highly damaging to root tip cells, and ethylene pretreatments reduced this damage. Ethylene pretreatment alone altered the abundance of transcripts and proteins involved in hypoxia responses, root growth, translation, and reactive oxygen species (ROS) homeostasis. Through imaging and manipulating ROS abundance in planta, we demonstrated that ethylene limited excessive ROS formation during hypoxia and subsequent re-oxygenation and improved oxidative stress survival in a PHYTOGLOBIN1-dependent manner. In addition, we showed that root growth cessation via ethylene and auxin occurred rapidly and that this quiescence behavior contributed to enhanced hypoxia tolerance. Collectively, our results show that the early flooding signal ethylene modulates a variety of processes that all contribute to hypoxia survival.

Waterlogging Stress Induces Antioxidant Defense Responses, Aerenchyma Formation and Alters Metabolisms of Banana Plants

Flooding caused or exacerbated by climate change has threatened plant growth and food production worldwide. The lack of knowledge on how crops respond and adapt to flooding stress imposes a major barrier to enhancing their productivity. Hence, understanding the flooding-responsive mechanisms of crops is indispensable for developing new flooding-tolerant varieties. Here, we examined the banana (Musa acuminata cv. Berangan) responses to soil waterlogging for 1, 3, 5, 7, 14, and 24 days. After waterlogging stress, banana root samples were analyzed for their molecular and biochemical changes. We found that waterlogging treatment induced the formation of adventitious roots and aerenchyma with conspicuous gas spaces. In addition, the antioxidant activities, hydrogen peroxide, and malondialdehyde contents of the waterlogged bananas increased in response to waterlogging stress. To assess the initial response of bananas toward waterlogging stress, we analyzed the transcriptome changes of banana roots. A total of 3508 unigenes were differentially expressed under 1-day waterlogging conditions. These unigenes comprise abiotic stress-related transcription factors, such as ethylene response factors, basic helix-loop-helix, myeloblastosis, plant signal transduction, and carbohydrate metabolisms. The findings of the study provide insight into the complex molecular events of bananas in response to waterlogging stress, which could later help develop waterlogging resilient crops for the future climate.

Ultra-Low Oxygen and Preconditioning Storage Regulate Ethylene Synthesis to Prevent Corky Disorders in 'Fuji' Apple

Corky disorders in apples represent a significant problem for long-term storage where controlled atmosphere (CA) is mainly used. Ultra-low oxygen (ULO) is an alternative to CA, which consists of low partial pressure of O₂ to maintain a low metabolism in the apple fruit, achieving an effective decrease in the ethylene production and physiological disorders. The aim of this research was to study the effectiveness of a short hypoxia period on the development of cork physiological disorders during the storage of apple. 'Fuji' apples were prestored under ULO (0.5 kPa O₂) for two periods of time (15 and 30 days) and at two temperatures (0 or 5^°C). Corky physiological disorders increased at 5^°C prestorage temperature; however, ULO treatments for 15 or 30 days at 0 or 5^°C achieved a significant reduction in corky disorders near to 1%, compared with control treatments. In addition, a considerable reduction in ethylene production for up to 30 days was observed in ULO-treated fruit at 0 and 5^°C. ULO for 30 days at 0 and 5^°C increased the internal production of ethanol and acetaldehyde, causing a lower sensory quality due to the presence of fermentative flavors in fruit stored at 5^°C. ULO of 15 days of conditioning decreased the relative expression of ethylene biosynthesis genes MdACS1 and MdACO1, resulting in lower ethylene production.

A digital sensor to measure real-time leaf movements and detect abiotic stress in plants

Plant and plant organ movements are the result of a complex integration of endogenous growth and developmental responses, partially controlled by the circadian clock, and external environmental cues. Monitoring of plant motion is typically done by image-based phenotyping techniques with the aid of computer vision algorithms. Here we present a method to measure leaf movements using a digital inertial measurement unit (IMU) sensor. The lightweight sensor is easily attachable to a leaf or plant organ and records angular traits in real-time for two dimensions (pitch and roll) with high resolution (measured sensor oscillations of 0.36 ± 0.53° for pitch and 0.50 ± 0.65° for roll). We were able to record simple movements such as petiole bending, as well as complex lamina motions, in several crops, ranging from tomato to banana. We also assessed growth responses in terms of lettuce rosette expansion and maize seedling stem movements. The IMU sensors are capable of detecting small changes of nutations (i.e. bending movements) in leaves of different ages and in different plant species. In addition, the sensor system can also monitor stress-induced leaf movements. We observed that unfavorable environmental conditions evoke certain leaf movements, such as drastic epinastic responses, as well as subtle fading of the amplitude of nutations. In summary, the presented digital sensor system enables continuous detection of a variety of leaf motions with high precision, and is a low-cost tool in the field of plant phenotyping, with potential applications in early stress detection.

Hydrogen Sulfide Enhances Plant Tolerance to Waterlogging Stress

Hydrogen sulfide (H₂S) is considered the third gas signal molecule in recent years. A large number of studies have shown that H₂S not only played an important role in animals but also participated in the regulation of plant growth and development and responses to various environmental stresses. Waterlogging, as a kind of abiotic stress, poses a serious threat to land-based waterlogging-sensitive plants, and which H₂S plays an indispensable role in response to. In this review, we summarized that H₂S improves resistance to waterlogging stress by affecting lateral root development, photosynthetic efficiency, and cell fates. Here, we reviewed the roles of H₂S in plant resistance to waterlogging stress, focusing on the mechanism of its promotion to gained hypoxia tolerance. Finally, we raised relevant issues that needed to be addressed.

Ethylene Supplementation Combined with Split Application of Nitrogen and Sulfur Protects Salt-Inhibited Photosynthesis through Optimization of Proline Metabolism and Antioxidant System in Mustard (Brassica juncea L.)

In the present study, the potential of ethylene as ethephon (an ethylene source) was investigated individually and in combination with split doses of nitrogen (N) and sulfur (S) soil treatments for removal of the damaging effects of salt stress (100 mM NaCl) in mustard (Brassica juncea L.). Plants were grown with 50 mg N plus 50 mg S kg^-1 soil at sowing time and an equivalent dose at 20 days after sowing [N50 + S50]_{0d and 20d}. Ethephon at 200 μL L^‒1 was applied to combined split doses of N and S with or without NaCl. Plants subjected to NaCl showed a decrease in growth and photosynthetic characteristics as well as N and S assimilation, whereas proline metabolism and antioxidants increased. The application of ethephon to plants grown with split N and S doses significantly enhanced photosynthetic efficiency by increasing the assimilation of N and S, improving the concentration of proline and induction of the antioxidant system with or without NaCl. The regulation of ethylene and/or split forms of N and S application may be potential tools for not just overcoming salt stress effects in this species and in related Brassicaceae but also enhancing their photosynthesis and growth potential through increased nutrient assimilation.

Identification of ABA-Mediated Genetic and Metabolic Responses to Soil Flooding in Tomato (Solanum lycopersicum L. Mill)

Soil flooding is a compound abiotic stress that alters soil properties and limits atmospheric gas diffusion (O₂ and CO₂) to the roots. The involvement of abscisic acid (ABA) in the regulation of soil flooding-specific genetic and metabolic responses has been scarcely studied despite its key importance as regulator in other abiotic stress conditions. To attain this objective, wild type and ABA-deficient tomatoes were subjected to short-term (24 h) soil waterlogging. After this period, gas exchange parameters were reduced in the wild type but not in ABA-deficient plants that always had higher E and g _s . Transcript and metabolite alterations were more intense in waterlogged tissues, with genotype-specific variations. Waterlogging reduced the ABA levels in the roots while inducing PYR/PYL/RCAR ABA receptors and ABA-dependent transcription factor transcripts, of which induction was less pronounced in the ABA-deficient genotype. Ethylene/O₂-dependent genetic responses (ERFVIIs, plant anoxia survival responses, and genes involved in the N-degron pathway) were induced in hypoxic tissues independently of the genotype. Interestingly, genes encoding a nitrate reductase and a phytoglobin involved in NO biosynthesis and scavenging and ERFVII stability were induced in waterlogged tissues, but to a lower extent in ABA-deficient tomato. At the metabolic level, flooding-induced accumulation of Ala was enhanced in ABA-deficient lines following a differential accumulation of Glu and Asp in both hypoxic and aerated tissues, supporting their involvement as sources of oxalacetate to feed the tricarboxylic acid cycle in waterlogged tissues and constituting a potential advantage upon long periods of soil waterlogging. The promoter analysis of upregulated genes indicated that the production of oxalacetate from Asp via Asp oxidase, energy processes such as acetyl-CoA, ATP, and starch biosynthesis, and the lignification process were likely subjected to ABA regulation. Taken together, these data indicate that ABA depletion in waterlogged tissues acts as a positive signal, inducing several specific genetic and metabolic responses to soil flooding.

Tolerance to excess moisture in soybean is enhanced by over-expression of the Glycine max Phytoglobin (GmPgb1)

Excess moisture in the form of waterlogging or full submergence can cause severe conditions of hypoxia or anoxia compromising several physiological and biochemical processes. A decline in photosynthetic rate due to accumulation of ROS and damage of leaf tissue are the main consequences of excess moisture. These effects compromise crop yield and quality, especially in sensitive species, such as soybean (Glycine max.). Phytoglobins (Pgbs) are expressed during hypoxia and through their ability to scavenge nitric oxide participate in several stress-related responses. Soybean plants over-expressing or suppressing the Pgb1 gene GmPgb1 were generated and their ability to cope with waterlogging and full submergence conditions was assessed. Plants over-expressing GmPgb1 exhibited a higher retention of photosynthetic rate during waterlogging and survival rate during submergence relative to wild type plants. The same plants also had lower levels of ROS due to a reduction in expression of Respiratory Burst Oxidase Homologs (RBOH), components of the NADPH oxidase enzyme, and enhanced antioxidant system characterized by higher expression of catalases (CAT) and superoxide dismutase (SOD), as well as elevated expression and activity of ascorbate peroxidase (APX). Plants over-expressing GmPgb1 also exhibited an expression pattern of aquaporins typical of excess moisture resilience. This was in contrast to plants downregulating GmPgb1 which were characterized by the lowest photosynthetic rates, higher ROS signal, and reduced expression and activities of many antioxidant enzymes. Results from these studies suggest that GmPgb1 exercises a protective role during conditions of excess moisture with similar mechanisms operating during waterlogging and submergence.

Plant Responses to Hypoxia: Signaling and Adaptation

Molecular oxygen deficiency leads to altered cellular metabolism and can dramatically reduce crop productivity. Nearly all crops are negatively affected by lack of oxygen (hypoxia) due to adverse environmental conditions such as excessive rain and soil waterlogging. Extensive efforts to fully understand how plants sense oxygen deficiency and their ability to respond using different strategies are crucial to increase hypoxia tolerance. It was estimated that 57% of crop losses are due to floods [1]. Progress in our understanding has been significant in the last years. This topic deserved more attention from the academic community; therefore, we have compiled a Special Issue including four reviews and thirteen research articles reflecting the advancements made thus far.[...].