The stimulating effects of ethylene and auxin on petiole elongation and on hyponastic curvature are independent processes in submerged Rumex palustris

Comparable and adaptable strategies to waterlogging stress regulated by adventitious roots between two contrasting species

Cleistocalyx operculatus and Syzygium cumini possess a certain waterlogging tolerance. However, the comparable and adaptable strategies to waterlogging stress between these two species on the basis of waterlogging adventitious root (AR) regulation were still unclear. In this study, the plant performance in response to AR regulation based on AR removal (AR-R) and exogenous hormone application was investigated in terms of plant morphology, physiology, photosynthesis and AR traits. Results showed that C. operculatus possesses stronger waterlogging tolerance than S. cumini based on waterlogging tolerance coefficient, which is mainly due to the higher root biomass, root porosity and length, and activity of ARs, and shorter emergence time of ARs in C. operculatus than in S. cumini. The AR-R treatment increased activity and porosity of primary root, and induced a large amount of up-vertical ARs from the primary root systems in C. operculatus, while similar adaptive morphological changes in roots did not occur in AR-R-treated S. cumini. Exogenous abscisic acid (ABA) application had better effects on alleviating waterlogging damages than exogenous auxin (IAA) in balancing endogenous hormones (ABA and zeatin riboside), promoting AR development (porosity and activity, and the ratio of cortex area to stele area), improving the photosynthesis process and the antioxidant system (soluble protein, free proline and peroxidase). Moreover, under waterlogging conditions, exogenous ABA application induced greater increases in net photosynthesis rate, stomatal conductance, chlorophyll b and carotenoid in S. cumini than in C. operculatus, which suggested that S. cumini responded more positively and efficiently to exogenous ABA application than C. operculatus under waterlogging conditions. Thus, the findings provided new insights into the waterlogging adaptable strategies in waterlogging tolerant woody species on the basis of ARs and could provide scientific guidance for the application of these two species during revegetation activities in wetlands. Cleistocalyx operculatus could alternatively form a majority of up-vertical adventitious roots (ARs) from the primary roots after removing the normal ARs, but Syzygium cumini could not.Cleistocalyx operculatus possessed positive strategies to waterlogging stress, while S. cumini used traditional passive strategies.Exogenous abscisic acid (ABA) application had better effects on alleviating waterlogging damages in both species than exogenous auxin application.Syzygium cumini could more positively and efficiently respond to exogenous ABA application than C. operculatus.Waterlogging tolerance coefficient was significantly controlled by the chlorophyll contents and AR factors in C. operculatus and the AR factors and O2- in S. cumini.The best development of the AR number (ARN) and AR length (ARL) in exogenous ABA-treated C. operculatus may be closely related with positive zeatin riboside accumulation.The development of ARN and ARL was more important to waterlogging tolerance than that of AR porosity under waterlogging conditions.

Understanding a Mechanistic Basis of ABA Involvement in Plant Adaptation to Soil Flooding: The Current Standing

Soil flooding severely impairs agricultural crop production. Plants can cope with flooding conditions by embracing an orchestrated set of morphological adaptations and physiological adjustments that are regulated by the elaborated hormonal signaling network. The most prominent of these hormones is ethylene, which has been firmly established as a critical signal in flooding tolerance. ABA (abscisic acid) is also known as a “stress hormone” that modulates various responses to abiotic stresses; however, its role in flooding tolerance remains much less established. Here, we discuss the progress made in the elucidation of morphological adaptations regulated by ABA and its crosstalk with other phytohormones under flooding conditions in model plants and agriculturally important crops.

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.

Keeping the shoot above water - submergence triggers antithetical growth responses in stems and petioles of watercress (Nasturtium officinale)

The molecular mechanisms controlling underwater elongation are based extensively on studies on internode elongation in the monocot rice (Oryza sativa) and petiole elongation in Rumex rosette species. Here, we characterize underwater growth in the dicot Nasturtium officinale (watercress), a wild species of the Brassicaceae family, in which submergence enhances stem elongation and suppresses petiole growth. We used a genome-wide transcriptome analysis to identify the molecular mechanisms underlying the observed antithetical growth responses. Though submergence caused a substantial reconfiguration of the petiole and stem transcriptome, only little qualitative differences were observed between both tissues. A core submergence response included hormonal regulation and metabolic readjustment for energy conservation, whereas tissue-specific responses were associated with defense, photosynthesis, and cell wall polysaccharides. Transcriptomic and physiological characterization suggested that the established ethylene, abscisic acid (ABA), and GA growth regulatory module for underwater elongation could not fully explain underwater growth in watercress. Petiole growth suppression is likely attributed to a cell cycle arrest. Underwater stem elongation is driven by an early decline in ABA and is not primarily mediated by ethylene or GA. An enhanced stem elongation observed in the night period was not linked to hypoxia and suggests an involvement of circadian regulation.

Dimorphic Leaf Development of the Aquatic Plant Callitriche palustris L. Through Differential Cell Division and Expansion

Heterophylly, or phenotypic plasticity in leaf form, is a remarkable feature of amphibious plants. When the shoots of these plants grow underwater, they often develop surprisingly different leaves from those that emerge in air. Among aquatic plants, it is typical for two or more distinct leaf development processes to be observed in the same individual exposed to different environments. Here, we analyze the developmental processes of heterophylly in the amphibious plant Callitriche palustris L. (Plantaginaceae). First, we reliably cultured this species under laboratory conditions and established a laboratory strain. We also established a framework for molecular-based developmental analyses, such as whole-mount in situ hybridization. We observed several developmental features of aerial and submerged leaves, including changes in form, stomata and vein formation, and transition of the meristematic zone. Then we defined developmental stages for C. palustris leaves. We found that in early stages, aerial and submerged leaf primordia had similar forms, but became discriminable through cell divisions with differential direction, and later became highly distinct via extensive cell elongation in submerged leaf primordia.

Indoleacetic Acid Levels in Wheat and Rice Seedlings under Oxygen Deficiency and Subsequent Reoxygenation

The lack of oxygen and post-anoxic reactions cause significant alterations of plant growth and metabolism. Plant hormones are active participants in these alterations. This study focuses on auxin-a phytohormone with a wide spectrum of effects on plant growth and stress tolerance. The indoleacetic acid (IAA) content in plants was measured by ELISA. The obtained data revealed anoxia-induced accumulation of IAA in wheat and rice seedlings related to their tolerance of oxygen deprivation. The highest IAA accumulation was detected in rice roots. Subsequent reoxygenation was accompanied with a fast auxin reduction to the control level. A major difference was reported for shoots: wheat seedlings contained less than one-third of normoxic level of auxin during post-anoxia, while IAA level in rice seedlings rapidly recovered to normoxic level. It is likely that the mechanisms of auxin dynamics resulted from oxygen-induced shift in auxin degradation and transport. Exogenous IAA treatment enhanced plant survival under anoxia by decreased electrolyte leakage, production of hydrogen peroxide and lipid peroxidation. The positive effect of external IAA application coincided with improvement of tolerance to oxygen deprivation in the 35S:iaaM × 35S:iaaH lines of transgene tobacco due to its IAA overproduction.

The role of nitrite and nitric oxide under low oxygen conditions in plants

Plant tissues, particularly roots, can be subjected to periods of hypoxia due to environmental circumstances. Plants have developed various adaptations in response to hypoxic stress and these have been described extensively. Less well-appreciated is the body of evidence demonstrating that scavenging of nitric oxide (NO) and the reduction of nitrate/nitrite regulate important mechanisms that contribute to tolerance to hypoxia. Although ethylene controls hyponasty and aerenchyma formation, NO production apparently regulates hypoxic ethylene biosynthesis. In the hypoxic mitochondrion, cytochrome c oxidase, which is a major source of NO, also is inhibited by NO, thereby reducing the respiratory rate and enhancing local oxygen concentrations. Nitrite can maintain ATP generation under hypoxia by coupling its reduction to the translocation of protons from the inner side of mitochondria and generating an electrochemical gradient. This reaction can be further coupled to a reaction whereby nonsymbiotic haemoglobin oxidizes NO to nitrate. In addition to these functions, nitrite has been reported to influence mitochondrial structure and supercomplex formation, as well as playing a role in oxygen sensing via the N-end rule pathway. These studies establish that nitrite and NO perform multiple functions during plant hypoxia and suggest that further research into the underlying mechanisms is warranted.

Differentially Expressed microRNAs and Target Genes Associated with Plastic Internode Elongation in Alternanthera philoxeroides in Contrasting Hydrological Habitats

Phenotypic plasticity is crucial for plants to survive in changing environments. Discovering microRNAs, identifying their targets and further inferring microRNA functions in mediating plastic developmental responses to environmental changes have been a critical strategy for understanding the underlying molecular mechanisms of phenotypic plasticity. In this study, the dynamic expression patterns of microRNAs under contrasting hydrological habitats in the amphibious species Alternanthera philoxeroides were identified by time course expression profiling using high-throughput sequencing technology. A total of 128 known and 18 novel microRNAs were found to be differentially expressed under contrasting hydrological habitats. The microRNA:mRNA pairs potentially associated with plastic internode elongation were identified by integrative analysis of microRNA and mRNA expression profiles, and were validated by qRT-PCR and 5' RLM-RACE. The results showed that both the universal microRNAs conserved across different plants and the unique microRNAs novelly identified in A. philoxeroides were involved in the responses to varied water regimes. The results also showed that most of the differentially expressed microRNAs were transiently up-/down-regulated at certain time points during the treatments. The fine-scale temporal changes in microRNA expression highlighted the importance of time-series sampling in identifying stress-responsive microRNAs and analyzing their role in stress response/tolerance.

How plants handle multiple stresses: hormonal interactions underlying responses to abiotic stress and insect herbivory

Adaptive plant responses to specific abiotic stresses or biotic agents are fine-tuned by a network of hormonal signaling cascades, including abscisic acid (ABA), ethylene, jasmonic acid (JA) and salicylic acid. Moreover, hormonal cross-talk modulates plant responses to abiotic stresses and defenses against insect herbivores when they occur simultaneously. How such interactions affect plant responses under multiple stresses, however, is less understood, even though this may frequently occur in natural environments. Here, we review our current knowledge on how hormonal signaling regulates abiotic stress responses and defenses against insects, and discuss the few recent studies that attempted to dissect hormonal interactions occurring under simultaneous abiotic stress and herbivory. Based on this we hypothesize that drought stress enhances insect resistance due to synergistic interactions between JA and ABA signaling. Responses to flooding or waterlogging involve ethylene signaling, which likely reduces plant resistance to chewing herbivores due to its negative cross-talk with JA. However, the outcome of interactions between biotic and abiotic stress signaling is often plant and/or insect species-dependent and cannot simply be predicted based on general knowledge on the involvement of signaling pathways in single stress responses. More experimental data on non-model plant and insect species are needed to reveal general patterns and better understand the molecular mechanisms allowing plants to optimize their responses in complex environments.

Flood adaptive traits and processes: an overview

Unanticipated flooding challenges plant growth and fitness in natural and agricultural ecosystems. Here we describe mechanisms of developmental plasticity and metabolic modulation that underpin adaptive traits and acclimation responses to waterlogging of root systems and submergence of aerial tissues. This includes insights into processes that enhance ventilation of submerged organs. At the intersection between metabolism and growth, submergence survival strategies have evolved involving an ethylene-driven and gibberellin-enhanced module that regulates growth of submerged organs. Opposing regulation of this pathway is facilitated by a subgroup of ethylene-response transcription factors (ERFs), which include members that require low O₂ or low nitric oxide (NO) conditions for their stabilization. These transcription factors control genes encoding enzymes required for anaerobic metabolism as well as proteins that fine-tune their function in transcription and turnover. Other mechanisms that control metabolism and growth at seed, seedling and mature stages under flooding conditions are reviewed, as well as findings demonstrating that true endurance of submergence includes an ability to restore growth following the deluge. Finally, we highlight molecular insights obtained from natural variation of domesticated and wild species that occupy different hydrological niches, emphasizing the value of understanding natural flooding survival strategies in efforts to stabilize crop yields in flood-prone environments.

Growth promotion and inhibition of the Amazonian wild rice species Oryza grandiglumis to survive flooding

In Asian cultivated rice (Oryza sativa), distinct mechanisms to survive flooding are activated in two groups of varieties. Submergence-tolerant rice varieties possessing the SUBMERGENCE1A (SUB1A) gene display reduced growth during flash floods at the seedling stage and resume growth after the flood recedes, whereas deepwater rice varieties possessing the SNORKEL1 (SK1) and SNORKEL2 (SK2) genes display enhanced growth based on internodal elongation during prolonged submergence at the mature stage. In this study, we investigated the occurrence of these growth responses to submergence in the wild rice species Oryza grandiglumis, which is native to the Amazon floodplains. When subjected to gradual submergence, adult plants of O. grandiglumis accessions showed enhanced internodal elongation with rising water level and their growth response closely resembled that of deepwater varieties of O. sativa with high floating capacity. On the other hand, when subjected to complete submergence, seedlings of O. grandiglumis accessions displayed reduced shoot growth and resumed normal growth after desubmergence, similar to the response of submergence-tolerant varieties of O. sativa. Neither SUB1A nor the SK genes were detected in the O. grandiglumis accessions. These results indicate that the O. grandiglumis accessions are capable of adapting successfully to flooding by activating two contrasting mechanisms as the situation demands and that each mechanism of adaptation to flooding is not mediated by SUB1A or the SK genes.

The role of gibberellin signalling in plant responses to abiotic stress

Plant hormones are small molecules that regulate plant growth and development, as well as responses to changing environmental conditions. By modifying the production, distribution or signal transduction of these hormones, plants are able to regulate and coordinate both growth and/or stress tolerance to promote survival or escape from environmental stress. A central role for the gibberellin (GA) class of growth hormones in the response to abiotic stress is becoming increasingly evident. Reduction of GA levels and signalling has been shown to contribute to plant growth restriction on exposure to several stresses, including cold, salt and osmotic stress. Conversely, increased GA biosynthesis and signalling promote growth in plant escape responses to shading and submergence. In several cases, GA signalling has also been linked to stress tolerance. The transcriptional regulation of GA metabolism appears to be a major point of regulation of the GA pathway, while emerging evidence for interaction of the GA-signalling molecule DELLA with components of the signalling pathway for the stress hormone jasmonic acid suggests additional mechanisms by which GA signalling may integrate multiple hormone signalling pathways in the response to stress. Here, we review the evidence for the role of GA in these processes, and the regulation of the GA signalling pathway on exposure to abiotic stress. The potential mechanisms by which GA signalling modulates stress tolerance are also discussed.

Petiole hyponasty: an ethylene-driven, adaptive response to changes in the environment

BACKGROUND

Many plant species can actively reorient their organs in response to dynamic environmental conditions. Organ movement can be an integral part of plant development or can occur in response to unfavourable external circumstances. These active reactions take place with or without a directional stimulus and can be driven either by changes in turgor pressure or by asymmetric growth. Petiole hyponasty is upward movement driven by a higher rate of cell expansion on the lower (abaxial) compared with the upper (adaxial) side. Hyponasty is common among rosette species facing environmental stresses such as flooding, proximity of neighbours or elevated ambient temperature. The complex regulatory mechanism of hyponasty involves activation of pathways at molecular and developmental levels, with ethylene playing a crucial role.

SCOPE

We present current knowledge on the mechanisms that promote hyponasty in the context of other organ movements, including tropic and nastic reactions together with circumnutation. We describe major environmental cues resulting in hyponasty and briefly discuss their perception and signal transduction. Since ethylene is a central agent triggering hyponasty, we focus on ethylene in controlling different stages during plant development and summarize current knowledge on the relationship between ethylene and cell growth.

Photosynthetic acclimation is important for post-submergence recovery of photosynthesis and growth in two riparian species

BACKGROUND AND AIMS

Concomitant increases in O(2) and irradiance upon de-submergence can cause photoinhibition and photo-oxidative damage to the photosynthetic apparatus of plants. As energy and carbohydrate supply from photosynthesis is needed for growth, it was hypothesized that post-submergence growth recovery may require efficient photosynthetic acclimation to increased O(2) and irradiance to minimize photo-oxidative damage. The hypothesis was tested in two flood-tolerant species: a C(3) herb, Alternanthera philoxeroides; and a C(4) grass, Hemarthria altissima. The impact of low O(2) and low light, typical conditions in turbid floodwater, on post-submergence recovery was assessed by different flooding treatments combined with shading.

METHODS

Experiments were conducted during 30 d of flooding (waterlogging or submergence) with or without shading and subsequent recovery of 20 d under growth conditions. Changes in dry mass, number of branches/tillers, and length of the longest internodes and main stems were recorded to characterize growth responses. Photosynthetic parameters (photosystem II efficiency and non-photochemical quenching) were determined in mature leaves based on chlorophyll a fluorescence measurements.

KEY RESULTS

In both species growth and photosynthesis recovered after the end of the submergence treatment, with recovery of photosynthesis (starting shortly after de-submergence) preceding recovery of growth (pronounced on days 40-50). The effective quantum yield of photosystem II and non-photochemical quenching were diminished during submergence but rapidly increased upon de-submergence. Similar changes were found in all shaded plants, with or without flooding. Submerged plants did not suffer from photoinhibition throughout the recovery period although their growth recovery was retarded.

CONCLUSIONS

After sudden de-submergence the C(3) plant A. philoxeroides and the C(4) plant H. altissima were both able to maintain the functionality of the photosynthetic apparatus through rapid acclimation to changing O(2) and light conditions. The ability for photosynthetic acclimation may be essential for adaptation to wetland habitats in which water levels fluctuate.

Abscisic acid antagonizes ethylene-induced hyponastic growth in Arabidopsis

Ethylene induces enhanced differential growth in petioles of Arabidopsis (Arabidopsis thaliana), resulting in an upward movement of the leaf blades (hyponastic growth). The amplitude of this effect differs between accessions, with Columbia-0 (Col-0) showing a large response, while in Landsberg erecta (Ler), hyponastic growth is minimal. Abscisic acid (ABA) was found to act as an inhibitory factor of this response in both accessions, but the relationship between ethylene and ABA differed between the two; the ability of ABA to inhibit ethylene-induced hyponasty was significantly more pronounced in Col-0. Mutations in ABI1 or ABI3 induced a strong ethylene-regulated hyponastic growth in the less responsive accession Ler, while the response was abolished in the ABA-hypersensitive era1 in Col-0. Modifications in ABA levels altered petiole angles in the absence of applied ethylene, indicating that ABA influences petiole angles also independently from ethylene. A model is proposed whereby the negative effect of ABA on hyponastic growth is overcome by ethylene in Col-0 but not in Ler. However, when ABA signaling is artificially released in Ler, this regulatory mechanism is bypassed, resulting in a strong hyponastic response in this accession.

The Janus face of ethylene: growth inhibition and stimulation

The gaseous plant hormone ethylene modulates many internal processes and growth responses to environmental stimuli. Ethylene has long been recognized as a growth inhibitor, but evidence is accumulating that ethylene can also promote growth. Therefore, the concept of ethylene as a general growth inhibitor needs reconsideration: a close examination of recent literature can help to understand the two contrasting faces of growth control by ethylene. Here, we propose a hypothesis that integrates growth inhibition and growth stimulation into one biphasic ethylene response model. Focusing on photosynthesis and cell expansion, we highlight several mechanisms through which ethylene affects plant growth, thereby interacting with various other signal transduction routes.

New perspectives in flooding research: the use of shade avoidance and Arabidopsis thaliana

BACKGROUND

Complete submergence of Rumex palustris leads to hyponastic (upward) petiole growth followed by enhanced petiole elongation. Previous pharmacological experiments have provided insights into the signal transduction pathway leading to this combined 'escape' response. It will, however, be difficult to gain further knowledge using these methods. Consequently, new approaches are required.

SCOPE

Here we propose that different environmental signals resulting in similar phenotypes can help to understand better the submergence response. In this review, we show that both ethylene and shade induce similar growth responses in R. palustris and Arabidopsis thaliana. We illustrate how this can be exploited to unravel novel signalling components in submergence-induced elongation growth. Furthermore, we illustrate the potential of arabidopsis as a useful model in submergence research based on similarities with submergence-tolerant species such as R. palustris and the molecular opportunities it presents. This is illustrated by examples of current work exploring this concept.

CONCLUSIONS

Incorporating different model systems, such as arabidopsis and shade avoidance, into submergence research can be expected to create powerful tools to unravel signal transduction routes determining submergence tolerance.