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

: Drought and waterlogging seriously affect the growth of plants and are considered severe constraints on agricultural and forestry productivity; their frequency and degree have increased over time due to global climate change. The morphology, photosynthetic activity, antioxidant enzyme system and hormone levels of plants could change in response to water stress. The mechanisms of these changes are introduced in this review, along with research on key transcription factors and genes. Both drought and waterlogging stress similarly impact leaf morphology (such as wilting and crimping) and inhibit photosynthesis. The former affects the absorption and transportation mechanisms of plants, and the lack of water and nutrients inhibits the formation of chlorophyll, which leads to reduced photosynthetic capacity. Constitutive overexpression of 9-cis-epoxydioxygenase (NCED) and acetaldehyde dehydrogenase (ALDH), key enzymes in abscisic acid (ABA) biosynthesis, increases drought resistance. The latter forces leaf stomata to close in response to chemical signals, which are produced by the roots and transferred aboveground, affecting the absorption capacity of CO 2 , and reducing photosynthetic substrates. The root system produces adventitious roots and forms aerenchymal to adapt the stresses. Ethylene (ETH) is the main response hormone of plants to waterlogging stress, and is a member of the ERFVII subfamily, which includes response factors involved in hypoxia-induced gene expression, and responds to energy expenditure through anaerobic respiration. There are two potential adaptation mechanisms of plants (“static” or “escape”) through ETH-mediated gibberellin (GA) dynamic equilibrium to waterlogging stress in the present studies. Plant signal transduction pathways, after receiving stress stimulus signals as well as the regulatory mechanism of the subsequent synthesis of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) enzymes to produce ethanol under a hypoxic environment caused by waterlogging, should be considered. This review provides a theoretical basis for plants to improve water stress tolerance and water-resistant breeding.


Introduction
In recent years, drought and waterlogging stress have seriously affected the growth of plants due to extreme climate change; these stresses are an important limiting factor for global agricultural and forestry productivity [1].Over the past decade, the total area of the world's drylands has increased dramatically, with a clear upward trend in the scope, extent and frequency of drought, resulting in a total global loss of crop production of approximately $30 billion [2,3].Waterlogging is the second most important climate disaster after drought.Since the 1990s, the scope of waterlogging disasters has been expanding year by year, and the frequency has also been increasing [4,5].Due to the frequency and severity of drought and waterlogging, the global vegetation loss caused by these stresses is equivalent.The response and adaptation mechanisms of plants have been the focus of physiological and ecological research related to water stress (including drought stress and waterlogging stress), and are also very important for breeding water-tolerant varieties.
When plants are damaged by water stress, they will respond to adverse environments with changes to different morphological structures and physiological metabolisms, such as leaf and root morphology, photosynthesis, antioxidant enzyme systems and hormone levels [6,7].A large number of stress response genes are activated through complex signal transduction networks and synthesize many functional proteins to improve the ability of plants to resist water stress [8,9].To date, it is believed that drought stress mainly affects the absorption and transport of nutrients from roots to leaves [10][11][12], while waterlogging stress is an anaerobic respiratory metabolism caused by the environment around the roots [13][14][15].Based on the research results, this review discusses and compares the changes to plant morphology, structure, physiology and molecular mechanisms under drought and waterlogging stress.These are important factors to understand plant regulatory mechanisms in response to drought and waterlogging stress, and to increase plant productivity in adverse environments.

Morphological Structure Responses to Water Stress in Plants
The response of plants to water stress is mainly reflected in leaves and roots, and their external morphological characteristics and internal anatomical structure can best reflect the adaptability to adverse environments [16][17][18][19] (Table 1).Leaves are the most variable organs in long-term adaptation to the environment.They react similarly under drought and waterlogging stress, showing signs of etiolation, atrophy, curling, senescence and even abscission [20,21].In some cases, stress resulted in stunted leaf growth and reduced leaf number and area [22][23][24] (Figure 1).
physiological and ecological research related to water stress (including drought stress and waterlogging stress), and are also very important for breeding water-tolerant varieties.
When plants are damaged by water stress, they will respond to adverse environments with changes to different morphological structures and physiological metabolisms, such as leaf and root morphology, photosynthesis, antioxidant enzyme systems and hormone levels [6,7].A large number of stress response genes are activated through complex signal transduction networks and synthesize many functional proteins to improve the ability of plants to resist water stress [8,9].To date, it is believed that drought stress mainly affects the absorption and transport of nutrients from roots to leaves [10][11][12], while waterlogging stress is an anaerobic respiratory metabolism caused by the environment around the roots [13][14][15].Based on the research results, this review discusses and compares the changes to plant morphology, structure, physiology and molecular mechanisms under drought and waterlogging stress.These are important factors to understand plant regulatory mechanisms in response to drought and waterlogging stress, and to increase plant productivity in adverse environments.

Morphological Structure Responses to Water Stress in Plants
The response of plants to water stress is mainly reflected in leaves and roots, and their external morphological characteristics and internal anatomical structure can best reflect the adaptability to adverse environments [16][17][18][19] (Table 1).Leaves are the most variable organs in long-term adaptation to the environment.They react similarly under drought and waterlogging stress, showing signs of etiolation, atrophy, curling, senescence and even abscission [20,21].In some cases, stress resulted in stunted leaf growth and reduced leaf number and area [22][23][24] (Figure 1).

Morphological Structure Responses to Drought Stress
Drought can limit plant growth by inhibiting the cell division of leaf meristematic tissue and cell expansion in elongation areas, as well as inducing complex changes in leaf thickness, palisade tissue and spongy tissue during adaptation [25][26][27].Rueda et al. [28] Figure 1.Changes to the morphological and anatomical structure of plant leaves and roots due to water stress.P n : net photosynthetic rate; G s : stomatal conductance; T r : transpiration rate; ROS: reactive oxygen species; SOD: superoxide dismutase; CAT: catalase; APX: ascorbic peroxidase; GPX: peroxidase; GSSG: L-glutathione oxidized; MDHA: dehydroascorbic acid reductase; MDHAR: monodehydroascorbic acid reductase; DHAR: dehydroascorbate reductase glutathione; GR: glutathione reductase; GSH: glutathione peroxidase; AA: ascorbic acid.

Morphological Structure Responses to Drought Stress
Drought can limit plant growth by inhibiting the cell division of leaf meristematic tissue and cell expansion in elongation areas, as well as inducing complex changes in leaf thickness, palisade tissue and spongy tissue during adaptation [25][26][27].Rueda et al. [28] found that the conifers (water-holding capacity of plants) could be improved by increasing the thickness of leaves and decreasing the thickness of palisade tissue and spongy tissue in drought environments.However, Zheng et al. [29] found that Lycium barbarum increased the thickness of palisade tissue and reduced the thickness of spongy tissue, inhibiting transpiration and preventing tissue from excessive dehydration.The above results presented that the internal structure of the leaf changes resulted in transpiration reduction, as well as photosynthetic rate.
The root is an important organ for plants to fix and absorb substances from the soil.Drought stress reduces the stele area, vessel diameter and secondary root cortex cells and increases the number of vessels in the stele to facilitate water flow [30][31][32].To improve water retention and drought resistance, plants not only extend the root system by increasing the number of functional roots, but also increase the water-absorbing capacity of the root sheath [33,34].Furthermore, plants improve resistance by changing the root structure (such as root hair and root density) to influence root spatial distribution, soil fixation and nutrient absorption [35][36][37].Therefore, plants could improve water absorption capacity by changing root length and internal structure under drought stress conditions.

Morphological Structure Responses to Waterlogging Stress
The main response symptoms of leaves to waterlogging stress are curling, yellowing, wilting, falling off, rotting, etc. Leaves have two kinds of adaptation to waterlogging stress: one is to increase the thickness, while the other is to reduce the thickness.For the former, the water loss is reduced and the water holding capacity of plants is improved by increasing palisade tissue and spongy tissue, as well as the decrease in leaf and stomata size [38][39][40].The latter takes place because leaves cannot complete morphogenesis normally due to lack of water and nutrition [41].Thereby, some plants thin their leaves or form special leaves to promote the infiltration ability of CO 2 and inorganic nutrients into the leaves [42,43], and improve gas exchange to restore and maintain respiration under waterlogging stress [44,45].Therefore, the internal anatomy variation of the leaf is to adjust the stomata and improve transpiration under waterlogging stress, but the reason is uncertain and further study is needed.
Aerenchyma forming in the adventitious roots are the most obvious adaptation features under waterlogging stress.Meanwhile, the epithelial cell wall keratinizes gradually under a waterlogged environment to promote oxygen capture by underwater tissue, and enhance waterlogging tolerance [46,47].Yamauchi et al. [48] found that there are a lot of root hairs in the adventitious roots, the surface area is large, and the cuticle of the adventitious root is thin, but the aerenchyma is well developed, which can improve the oxygen content of waterlogging-tolerant plants.Moreover, lignified and embolized vascular bundle cortical cells contribute to long-distance oxygen diffusion to the root tips, and block the entry of soil toxins into plants effectively.For instance, Ranathunge et al. [49] found that rice promoted the early formation and increased lignin deposition in both the internal and external epidermis of roots, and prevented ion penetration more effectively under waterlogged conditions.Abiko et al. [50] found that waterlogging-tolerant teosinte formed adventitious roots and produced larger aerenchyma, a stronger lignified vascular bundle cell barrier, and the transport of oxygen from stem base to root tip was better than normal maize under a waterlogging environment.Therefore, the ways of producing adventitious roots are diverse in different types of plants under waterlogging stress, and strong waterlogging-tolerant plants are more likely to have the ability to form adventitious roots.It has been indicated that roots could improve adaptability by creating air cavities in the aerenchyma to expand storage space, and block the entry of soil toxins into plants.

Photosynthetic Characteristics of Plant Responses to Drought Stress
To maintain photosynthesis, plants form a series of defense mechanisms to protect their photosynthetic organs from damage in the process of adapting to water stress [70,71].For most plants, light water stress can control stomata and transpiration, directly regulate leaf water potential, and self-repair after a return to a normal water supply; some plants even increase photosynthesis [72,73].For example, light drought stress usually leads to a stomatal conductance and transpiration increase, while moderate and severe drought stress results in a net photosynthetic rate (P n ), stomatal conductance (G s ) and transpiration rate (T r ) decrease.However, the intercellular carbon dioxide concentration (C i ) shows a different trend.C i increases or decreases with the deepening of stress, while the stomatal limit (L s ) first increases and then decreases.These results indicate that the decrease in P n under drought stress is mainly caused by nonstomatal factors [74,75].Most nonstomatal factors, including chlorophyll content, photosynthetic enzyme activity and active oxygen metabolism, are induced by moderate and severe drought stress.Drought not only inhibits the formation of chlorophyll directly [76,77], but also causes difficulty in absorbing mineral elements from the soil, causing leaf nutrient deficiency (for example, leaf etiolation) [78,79] (Figure 1).The regulation of photosynthetic enzymes is a very complicated process.Light drought stress may slightly affect the photosynthetic carboxylation efficiency, but it can inhibit the activity of RuBPCase, which may result in a decrease in the photosynthetic carboxylation efficiency under severe drought stress [80].

Photosynthetic Characteristics of Plant Responses to Waterlogging Stress
Under waterlogging stress, both stomatal and nonstomatal factors inhibit photosynthesis.For stomatal factors, the chemical signals from roots are transferred to the ground, forcing the stomata of leaves to close, and reducing the photosynthetic rate by decreasing the absorption capacity of the photosynthetic substrate CO 2 [81][82][83]; Another aspect of stomatal conductance increasing is the supply of CO 2 , which increases the amount of assimilates to maintain growth under waterlogging.For non-stomatal factors, there is the anaerobic respiration of the plant under hypoxic surroundings.Lactic acid and ethanol are produced, which break the balance of active oxygen metabolism, degrade chlorophyll and damage the photosynthetic apparatus, producing excess excitation energy and causing photoinhibition [84,85].For severe waterlogging-tolerant plants, the stomata closed quickly due to the stress reaction of plants at the initial stage.For poor waterlogging-tolerant plants, leaf carbohydrates may accumulate rapidly within a few days, because root anaerobic respiration restrains sugar transfer from the stem to the root by reducing sugar consumption in the root, and the accumulation of photoassimilated products in leaves can form a negative feedback inhibition to the photosynthetic rate.

Antioxidant System of Plant Responses to Water Stress
Under normal physiological activities, plants produce reactive oxygen species (ROS), such as superoxide anion radicals (O 2 − ), singlet oxygen (O 2 ), hydroxyl radicals (•OH) and hydrogen peroxide (H 2 O 2 ), as signal transmitters to regulate gene and protein expression in plant cells, and the production and elimination of ROS are always in a state of dynamic equilibrium [86].When the plant is stressed, the balance will be broken, the physiological and biochemical functions of the plant cell membrane will be disturbed, and the production of reactive oxygen species will increase [87].Plants have similar responses to drought and waterlogging, and both stresses activate the antioxidant defense system of plants to avoid cell damage.The components of the antioxidant defense system are enzymatic and nonenzymatic antioxidants.The enzymatic antioxidants include superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase glutathione (DHAR) and monodehydroascorbic acid reductase (MDHAR).The nonenzymatic antioxidants are glutathione (GSH), ascorbic acid (AA) (both water soluble), carotenoids and tocopherols (lipid soluble).Both components counteract the harm caused by reactive oxygen species [88][89][90][91].
The response of antioxidant enzymes in plants to water stress is mainly related to tolerance and the level of stress.The activity of SOD in leaves and roots of the same species increases with an increasing level of water stress.Furthermore, the disproportionation conversion of O  [92].This mechanism has been demonstrated in mosses [93], trifoliate orange seedlings [94], and tobacco [95].There are different antioxidant enzyme activities in different tolerant varieties under the same water stress.The adaptive mechanism of plants is a very complicated process, and there are no fixed rules to follow.For example, the SOD activity of Poa pratensis and Festuca arundinacea increased briefly and then decreased, while the CAT activity of F. arundinacea decreased with increasing drought stress [96].The SOD activity of the drought-sensitive cultivar Trifolium repens was inhibited under stress, but there was no significant change in the drought-tolerant cultivar Debut, which may be related to its higher ability to mitigate oxidative damage [97].These results showed that plants could increase the activity of antioxidant enzymes to cope with adverse environments, but the dynamic changes across individuals and stress degrees.

Phytohormones and Related Genes in Plant Responses to Drought Stress
Phytohormones play a vital role in plant growth and metabolism, as well as the transport and distribution of nutrients, as their synthesis and signal transduction pathways are interrelated.The physiological function is changed to a specific antistress mechanism through regulating hormone metabolism and signal transduction [98][99][100].Drought stimulates abscisic acid (ABA) production in different plant organs, especially in the root, which can reach leaf guard cells and send signals through xylem transport and transpiration.ABA combines cytokinin (CTK) and jasmonic acid (JA) to regulate stomatal movement.They reduce the leaf transpiration rate and guard cell turgor pressure, which causes stomatal closure to adapt to external environments stress [101][102][103][104], and ABA accumulation also activates downstream signal components and enhances root antioxidant capacity to improve stress resistance [105].These results indicated that ABA could play an important role in plant cells receiving drought signals.Therefore, it is of great significance to understand the involvement of ABA in regulating cell metabolism, energy supply, growth, and the expression of functional genes at the transcriptional level under drought stress.
To avoid drought, plants have evolved complex mechanisms to adapt (such as strictly controlling stomatal opening and closing), and endogenous ABA plays an important role in this process [106].There are many ways to synthesize ABA under a drought environment.One is the involvement of key regulatory factors (such as 9-cis-epoxydioxygenase (NCED) and acetaldehyde dehydrogenase (ALDH)) in the last step of the ABA biosynthesis pathway, as the accumulation of ABA activates downstream signals and specifically binds to genes, which play an important role in drought environments [107] (Figure 2).We grouped them into drought adjustment (Table 2).Increased expression of the TaNCED1 gene isolated from Triticum aestivum, significantly improved drought tolerance in tobacco transgenic plants [108].Moreover, different levels of OsALDH expression were detected in rice seedlings under drought stress.Transgenic rice overexpressing OsALDH showed elevated stress tolerances and a down-regulation of OsALDH in the RNA interference (RNAi).Repression transgenic lines manifest a declined stress tolerance [109].
The second method plays an important role in the upstream enhancement of the expression of downstream genes to increase NCED enzyme activity, and promote ABA biosynthesis.The ABA-mediated signal transduction pathway leads to stomatal closure involved in ABA synthesis, including NGA1, ATAF1, HAT1 and ATX1 [110,111].NGA1 (a B3 transcription factor) binds directly to the NCED3 promoter and activates its expression in vitro and in vivo under drought stress [112].The regulatory target gene of ATAF1 (a NAC protein) is NCED3, which binds specifically to the transcription factor NAC, regulates the ABA biosynthesis gene directly, and activates its expression.Drought-stimulated plants can enhance the expression of downstream genes by binding specific transcription factors (such as B3, NAC and MIKC) to cis-regulatory elements.Transcription factors such as MYB and WRKY bind specifically to cis-regulatory elements and induce the expression of drought-responsive genes to maintain osmotic balance [113][114][115].Moreover, some genes can suppress ABA synthesis and signaling, such as HAT1 (an HD-ZIP transcription factor) binding to their promoters and the ABA/drought-responsive genes RD29A and RD22 directly, by down-regulating the expression of ABA3 and NCED3 [116].ATX1 not only upregulates NCED3 transcription but also affects ABA production in response to drought stress directly [117].
The third method is changes in leaf stomatal density, leaf water loss rate and reactive oxygen species levels.AGL16 (a MIKC transcription factor) plays an important role in the upstream of the AAO3 gene (abscisic aldehyde oxidase 3, the gene encodes an aldehyde oxidase).AGL16 binds to the CArG motif in the AAO3 promoter, regulates transcription, and changes ABA levels and leaf stomatal density [118].GbMYB5 and GhWRKY17 play an active role by regulating the expression of drought-related genes and the production of reactive oxygen species under drought stress [119,120].
In addition, ABA-independent signaling includes both the NAC and DREB2 pathways [121][122][123].The former, SINAC4, plays a role as a transcription factor in the positive regulation of stress tolerance.Zhu et al. [9] found that the chlorophyll content and leaf water content of transgenic tomato with SINAC4-RNAi were lower than those of wildtype plants, and the leaf water loss rate was higher under drought stress.Drought also directly induces the binding of HcDREB2 to the DRE cis-regulatory element and activates downstream gene expression to significantly improve the drought resistance of plants [124] (Figure 2).These results showed that genes can regulate signal transduction and induce the drought resistance gene expression under drought stress, and the functional genes can transcribe and synthesize proteins that play a direct role in stress tolerance.The activity of transcription factors was enhanced, and the interaction between transcription factors and cis-regulatory elements could further induce the expression of functional genes under drought stress.

Phytohormones and Related Genes in Plant Responses to Waterlogging Stress
The root is the most sensitive and responsive organ, and its primary responsibility is to adapt to waterlogging by controlling growth [125,126].Similar to drought stress, waterlogging stress induces ABA synthesis in the root system and adjusts stomatal movement to adapt to the external environment [127].The difference is that ethylene (ETH) is one of the more sensitive hormones to waterlogging, and it is increased in an anoxic environment [128,129].It has been reported that the regulatory mechanism of waterlogging in plants involves not only the production of ABA in the root system but also the regulation of stomatal opening and closing.First, plants respond to a lack of energy by increasing anaerobic respiration.Hypoxia stress caused by waterlogging leads to the inhibition of aerobic respiration to increase the ATP supply, and plants create energy through ethanol fermentation (mainly through pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH)) [130,131].Second, plants adapt to waterlogging through a "static" strategy [132,133].ETH can regulate gibberellin (GA) synthesis, inhibit internode elongation and reduce energy consumption [134][135][136].Third, plants adapt to long-term waterlogging through an "escape" strategy [137].ETH maintains the stability of GA and ABA in plants to increase the contact between plants and the air, and promotes stem elongation to the water surface for photosynthesis and rapid aerobic absorption to maintain growth [138,139] (Figure 3).
Ethylene response factor (ERFVII) subfamily members are response factors involved in hypoxia-induced gene expression [140,141].Plant hypoxia-responsive genes are involved in fermentation and glycometabolism pathways and affect gene expression related

Phytohormones and Related Genes in Plant Responses to Waterlogging Stress
The root is the most sensitive and responsive organ, and its primary responsibility is to adapt to waterlogging by controlling growth [125,126].Similar to drought stress, waterlogging stress induces ABA synthesis in the root system and adjusts stomatal movement to adapt to the external environment [127].The difference is that ethylene (ETH) is one of the more sensitive hormones to waterlogging, and it is increased in an anoxic environment [128,129].It has been reported that the regulatory mechanism of waterlogging in plants involves not only the production of ABA in the root system but also the regulation of stomatal opening and closing.First, plants respond to a lack of energy by increasing anaerobic respiration.Hypoxia stress caused by waterlogging leads to the inhibition of aerobic respiration to increase the ATP supply, and plants create energy through ethanol fermentation (mainly through pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH)) [130,131].Second, plants adapt to waterlogging through a "static" strategy [132,133].ETH can regulate gibberellin (GA) synthesis, inhibit internode elongation and reduce energy consumption [134][135][136].Third, plants adapt to long-term waterlogging through an "escape" strategy [137].ETH maintains the stability of GA and ABA in plants to increase the contact between plants and the air, and promotes stem elongation to the water surface for photosynthesis and rapid aerobic absorption to maintain growth [138,139] (Figure 3).
Ethylene response factor (ERFVII) subfamily members are response factors involved in hypoxia-induced gene expression [140,141].Plant hypoxia-responsive genes are involved in fermentation and glycometabolism pathways and affect gene expression related to ethylene biosynthesis [142].When breathing is restricted, lactate dehydrogenase converts the pyruvate produced during glycolysis into lactic acid.The PDC and ADH genes can convert pyruvic acid into lactic acid and change it into ethanol; that is, PDC converts pyruvic acid into acetaldehyde, and ADH converts acetaldehyde into ethanol.Additionally, NAD + and a finite amount of ATP are produced [143,144].At present, it has been shown that ADH and PDC activity are regulated by SUB1, HRE1 and HRE2 under waterlogging.We grouped them into waterlogging adjustment (Table 2), as waterlogging could increase the transcription level of Sub1A and Sub1C and affect PDC and ADH activity to inhibit the chlorophyll degradation and carbohydrate consumption of waterlogged plants [145].HRE1 overexpression increased the induction of anaerobic genes in a hypoxic environment.Compared with normal oxygen conditions, the overexpression of HRE1 and ATERF73/HRE1 has a positive regulatory role in the absence of oxygen, in which plants not only increase PDC enzyme activity, ADH enzyme activity, and ethanol content, but also induce elongated adventitious roots to adapt to waterlogging [146,147].Moreover, amino-oxyacetic acid, an inhibitor of ethylene biosynthesis, can partially inhibit the anoxic induction of ADH, but this partial inhibition could be reversed by adding 1-aminocyclopropane-1-carboxylic acid, which is a direct precursor of ethylene [148,149].CgACO (1-aminocyclopropane-1-carboxylate oxidase) expression in roots of the waterlogging-tolerant species (Chrysanthemum zawadskii) were higher than the sensitive species (Chrysanthemum nankingense) after 12 h waterlogging treatment.This indicated that higher CgACO expression possibly contributed to higher accumulation of ethylene in the waterlogging-tolerant species [150].At present, research on this pathway mainly focuses on the enhancement of PDC and ADH enzyme activity after the overexpression of ERFVII subfamily members.The signal transduction mechanism of increased PDC and ADH activity in the synthesis of ethanol in an anoxic environment caused by waterlogging needs further study [151][152][153][154].
The waterlogging environment showed two opposite growth responses: "static" and "escape".Both were mainly regulated by SK and Sub1 transcription factors induced by ETH [155,156].Sub1A inhibits ETH production and the expression of the related downstream genes of ETH to promote the synthesis of brassinosteroids (BRs), and activates Ga2oxidase7 expression to inhibit the synthesis of gibberellin (GA) while increasing the expression of the suppressor of the GA signaling pathway SLR1 [157,158].This process is a "static" strategy to adapt to short-term waterlogging by inhibiting internode elongation and reducing energy consumption until the stress is relieved [159].Rice SK1, SK2 and Sub1 upregulate ABA-inactivating enzyme genes OsCYP707A5 or OsABA8ox1 and GA anabolism genes (OsGA20ox and OsGA3ox) under deep water, which induces a decline in ABA in rice internodes and increases the accumulation of GA in the subaqueous internodes, eventually upregulating growth-related genes to rapidly elongate stems to the water surface.This process is an "escape" strategy for the long-term submergence of plants [139,160].The ERFVII transcripts downstream genes in a cascade amplification mode, which converts extracellular signals into intracellular, and then induces a series of adaptive mechanisms, such as accelerated glycolysis, elongated stem, formation of aerenchyma and increased oxygen transport rate, etc., to adapt to the waterlogging environment (Figure 3).

A View to the Future
In recent years, more research has been devoted to the study of the harmful effects of extreme climate on plants, and some important progress has been made into the adaptability of different plants to drought and waterlogging.However, great differences were observed in the response mechanisms of different plants under water stress.To date, although scholars have proposed many mechanisms of plant tolerance, none of them have been universally accepted due to their complexity.Currently, gene cloning and genetic transformation are mainly focused on model plants and some crops, but these methods are still in their infancy in some species.On the one hand, the regulatory mechanism of plants under drought and waterlogging stress should be further compared to explore the gene expression regulation and functional identification of resistance genes.On the other hand, the response mechanism of roots and leaves to water stress and the generation and transformation of important regulatory factors should be further studied.In particular, the signal transduction pathway, after receiving a stimulus but before hormone production, should be focused on.In addition, the gene regulation mechanism of inducing PDC and ADH enzymes to create ethanol under an anoxic environment caused by waterlogging in order to improve the plant stress-resistance signaling network also needs further study.

Figure 2 .
Figure 2. Regulatory mechanisms of abscisic acid (ABA) and related genes in response to drought stress in plants.

Figure 2 .
Figure 2. Regulatory mechanisms of abscisic acid (ABA) and related genes in response to drought stress in plants.

Figure 3 .
Figure 3. Regulatory mechanisms of phytohormones and related genes in response to waterlogging stress in plants.

Figure 3 .
Figure 3. Regulatory mechanisms of phytohormones and related genes in response to waterlogging stress in plants.

Table 1 .
Characteristics of plant roots and leaves under water stress.
2 − to H 2 O 2 increases and the content of O 2 − decreases.POD and CAT decompose H 2 O 2 to H 2 O, inhibit the accumulation of H 2 O 2 effectively, protect plants from oxidative damage, and reduce the toxic effect on plants caused by water stress

Table 2 .
Genes involved in drought and waterlogging adjustment.

Table 2 .
Genes involved in drought and waterlogging adjustment.