Role Of Ethylene Research Articles

Aerenchyma formation in the wetland plant Juncus effusus is independent of ethylene

Flooded plant roots commonly form aerenchyma, which allows gas diffusion between shoots and roots. The programmed cell death involved in this induced aerenchyma formation is controlled by the plant hormone ethylene, as has been shown for maize (Zea mays). However, the role of ethylene is uncertain in wetland species that form constitutive aerenchyma (also under nonflooded conditions). The aim of this study is to shed light on the involvement of ethylene in constitutive aerenchyma formation in Juncus effusus. Plants of J. effusus and maize were treated with ethylene and inhibitors of ethylene action to determine the consequences for aerenchyma formation. Neither treatment with high ethylene concentrations nor with ethylene inhibitors resulted in changes in root aerenchyma in J. effusus. By contrast, ethylene increased aerenchyma development in maize unless ethylene action inhibitors were applied simultaneously. Similarly, root elongation was insensitive to ethylene treatment in J. effusus, but was affected negatively in maize. The data show that aerenchyma in J. effusus is highly constitutive and, in contrast to the inducible aerenchyma in maize, is not obviously controlled by ethylene.

A role for ethylene in low-oxygen signaling in rice roots

Inhibitors of action and synthesis of ethylene (Ag(+), norbornadien, Co(2+)) were able to reduce the level of gamma-aminobutyric acid (Gaba) in rice roots during the development of an anaerobic environment. The inhibitory effect was reversed by the addition of the G protein activator 5'-guanylylimidodiphosphate. Gaba accumulation was modulated by the presence of CO(2) (inhibitor of ethylene action and synthesis) and stimulated by 2-chloroethylphosphonic acid (ethefon). These findings are consistent with a role of ethylene during a low-oxygen stress.

Ethylene Signaling Mediates a Maize Defense Response to Insect Herbivory

The signaling pathways that enable plants to mount defenses against insect herbivores are known to be complex. It was previously demonstrated that the insect-resistant maize (Zea mays L.) genotype Mp708 accumulates a unique defense cysteine proteinase, Mirl-CP, in response to caterpillar feeding. In this study, the role of ethylene in insect defense in Mp708 and an insect-susceptible line Tx601 was explored. Ethylene synthesis was blocked with either cobalt chloride or aminoethoxyvinylglycine. Alternatively, ethylene perception was inhibited with 1-methylcyclopropene. Blocking ethylene synthesis and perception resulted in Mp708 plants that were more susceptible to caterpillar feeding. In addition, fall armyworm (Spodoptera frugiperda) larvae that fed on inhibitor-treated Mp708 plants had signifycantly higher growth rates than those reared on untreated plants. In contrast, these responses were not significantly altered in Tx601. The ethylene synthesis and perception inhibitors also reduced the accumulation of Mirl-CP and its transcript mir1 in response to herbivory. These results indicate that ethylene is a component of the signal transduction pathway leading to defense against insect herbivory in the resistant genotype Mp708.

Ethylene effect on kochia (Kochia scoparia) and emission following dicamba application

The role of ethylene in kochia injury following the application of dicamba and other auxinic herbicides is not known. Experiments were conducted to evaluate kochia response to ethylene generated within plant tissue and to quantify ethylene emission from kochia following dicamba application to determine if ethylene production could be used to differentiate between dicamba-susceptible (S) and dicamba-resistant (R) kochia accessions. Dicamba at 0.28 kg ha−1and fluroxypyr at 0.28 kg ha−1caused severe stem and petiole twisting within 6 h of application, with S kochia exhibiting a greater level of epinasty than R kochia and fluroxypyr causing more epinasty than dicamba. Fluroxypyr provided similar control of S and R kochia, but dicamba provided less control of R kochia than S kochia. Ethylene evolution in S kochia from ethephon at 1.1 kg ha−1and dicamba at 0.28 kg ha−1was similar, approximately 70 ng (g fresh weight)−1h−1. Elevated ethylene concentration from as much as 2 kg ha−1ethephon did not produce epinastic symptoms or other plant response in S or R kochia. Ethylene evolution following dicamba application increased with time up to 48 h after application and with increasing dicamba rate up to 0.56 kg ha−1, which were the end points of the research. Ethylene evolution from the S accession was 2.5 times that of the R accession averaged across dicamba rates and harvest times. The greatest difference in ethylene evolution between S and R accessions occurred 24 h after treatment at 0.14 kg ha−1dicamba, with S kochia producing ethylene at 470%, compared with R kochia.

Molecular mechanisms of ethylene signaling in Arabidopsis

Ethylene is a gaseous plant hormone involved in several important physiological processes throughout a plant's life cycle. Decades of scientific research devoted to deciphering how plants are able to sense and respond to this key molecule have culminated in the establishment of one of the best characterized signal transduction pathways in plants. The ethylene signaling pathway starts with the perception of this gaseous hormone by a family of membrane-anchored receptors followed by a Raf-like kinase CTR1 that is physically associated with the receptors and actively inhibits downstream components of the pathway. A major gap is represented by the mysterious plant protein EIN2 that genetically works downstream of CTR1 and upstream of the key transcription factor EIN3. Transcriptional regulation by EIN3 and EIN3-family members has emerged as a key aspect of ethylene responses. The major components of this transcriptional cascade have been characterized and the involvement of post-transcriptional control by ubiquitination has been determined. Nevertheless, many aspects of this pathway still remain unknown. Recent genomic studies aiming to provide a more comprehensive view of modulation of gene expression have further emphasized the ample role of ethylene in a myriad of cellular processes and particularly in its crosstalk with other important plant hormones. This review aims to serve as a guide to the main scientific discoveries that have shaped the field of ethylene biology in the recent years.

Ethylene-sensitivity regulates proteolytic activity and cysteine protease gene expression in petunia corollas*

To investigate ethylene's role in petal senescence, a comparative analysis of age-related changes in total protein, protease activity, and the expression of nine cysteine protease genes in the corollas of ethylene-sensitive Petuniaxhybrida cv. Mitchell Diploid (MD) and ethylene-insensitive (35S:etr1-1; line 44568) transgenic petunias was conducted. The later stages of corolla senescence in MD flowers were associated with decreased fresh weight, decreased total protein, and increased proteolytic activity. Corolla senescence was delayed by approximately 8 d in etr-44568 transgenic petunias, and decreases in corolla fresh weight, protein content, and maximum proteolytic activity were similarly delayed. Protease inhibitor studies indicated that the majority of the protease activity in senescing petals was due to cysteine proteases. Nine cysteine proteases expressed in petals were subsequently identified. Northern blot analysis indicated that six of the nine cysteine proteases showed increased transcript abundance during petal senescence. One of these cysteine proteases, PhCP10, was detected only in senescing tissues. Expression of four of the senescence-associated cysteine proteases was delayed, but not prevented in etr-44568 flowers. The other two senescence associated cysteine proteases had high levels of transcript accumulation in etr-44568 corollas at 8 d after flower opening, when MD flowers were senescing. These patterns suggest that age-related factors, other than ethylene, were regulating the up-regulation of these genes during flower ageing. The delay in visible symptoms and biochemical and molecular indicators of senescence in ethylene-insensitive flowers is consistent with the concept that ethylene modulates the timing of senescence pathways in petals.

Ethylene regulates fast apoplastic acidification and expansin A transcription during submergence‐induced petiole elongation in Rumex palustris

The semi-aquatic dicot Rumex palustris responds to complete submergence by enhanced elongation of young petioles. This elongation of petiole cells brings leaf blades above the water surface, thus reinstating gas exchange with the atmosphere and increasing survival in flood-prone environments. We already know that an enhanced internal level of the gaseous hormone ethylene is the primary signal for underwater escape in R. palustris. Further downstream, concentration changes in abscisic acid (ABA), gibberellin (GA) and auxin are required to gain fast cell elongation under water. A prerequisite for cell elongation in general is cell wall loosening mediated by proteins such as expansins. Expansin genes might, therefore, be important target genes in submergence-induced and plant hormone-mediated petiole elongation. To test this hypothesis we have studied the identity, kinetics and regulation of expansin A mRNA abundance and protein activity, as well as examined pH changes in cell walls associated with this adaptive growth. We found a novel role of ethylene in triggering two processes affecting cell wall loosening during submergence-induced petiole elongation. First, ethylene was shown to promote fast net H(+) extrusion, leading to apoplastic acidification. Secondly, ethylene upregulates one expansin A gene (RpEXPA1), as measured with real-time RT-PCR, out of a group of 13 R. palustris expansin A genes tested. Furthermore, a significant accumulation of expansin proteins belonging to the same size class as RpEXPA1, as well as a strong increase in expansin activity, were apparent within 4-6 h of submergence. Regulation of RpEXPA1 transcript levels depends on ethylene action and not on GA and ABA, demonstrating that ethylene evokes at least three, parallel operating pathways that, when integrated at the whole petiole level, lead to coordinated underwater elongation. The first pathway involves ethylene-modulated changes in ABA and GA, these acting on as yet unknown downstream components, whereas the second and third routes encompass ethylene-induced apoplastic acidification and ethylene-induced RpEXPA1 upregulation.

Changes in Proteolytic Activity and Cysteine Proteinase Gene Expression during the Senescence of etr1-1 Transgenic Petunias

Corolla senescence in petunias was accompanied by a decrease in total proteins and a corresponding increase in proteolytic activity. Transgenic petunias that contain the mutated ethylene receptor (35S:etr1-1) have reduced sensitivity to ethylene and delayed flower senescence. Declines in total protein levels and increases in proteolytic activity were also delayed in etr1-1 flowers and corresponded with corolla wilting. Experiments using class-specific proteinase inhibitors indicated that proteolytic activity in petunia corollas was largely due to cysteine proteinases. Total nitrogen levels within the corollas of both wild type and etr1-1 flowers also decreased during senescence. Nine cDNAs encoding putative cysteine proteinases (CPs) were identified from a petunia EST database developed at the University of Florida. Six of these cysteine proteinases showed increased transcript abundance during corolla senescence (senescence-associated CPs) while three decreased in abundance. Of the six senescence-associated cysteine proteinases, only five showed delayed up regulation in etr1-1 flowers that corresponded with corolla wilting. The role of ethylene in the regulation of protein degradation during flower senescence will be discussed.

ROLE OF ETHYLENE IN OVARY GROWTH AND VASE LIFE OF DENDROBIUM ´POMPADOUR´ FLOWERS

We studied the role of ethylene and sugar (sucrose) supply in ovary growth of Dendrobium 'Pompadour'. Ethylene synthesis was inhibited using aminooxyacetic acid (AOA). Sucrose was supplied in a holding solution also containing two antimicrobial compounds (8-hydroxyquinoline sulfate and silver nitrate). Control flowers were left unpollinated. Ovary growth only occurred after pollination, and was completely suppressed by AOA treatment. AOA was not toxic in unpollinated control flowers. Sucrose (5%) with the holding solution had no effect on ovary growth. Preliminary experiments with pollinated Dendrobium Sonia Bom # 28 flowers also showed complete suppression of ovary growth by AOA, whilst ovary growth was stimulated following treatment with exogenous ethylene. Flower wilting was the main determinant of the length of vase life; from day 8 some flowers also started to fall. In unpollinated flowers, AOA delayed the time to wilting of 50% of the flowers, but did not affect the time to flower abscission. Sucrose increased vase life of unpollinated orchid flowers by delaying wilting and a suppressing flower abscission. In contrast, the sucrose solution did not affect the (much shorter) vase life of pollinated flowers. In contrast to unpollinated climacteric flowers, like carnation, where sugars reduce sensitivity to endogenously produced ethylene, such a decrease in sensitivity apparently does not occur in pollinated Dendrobium flowers.

POSTHARVEST CALYX RETENTION OF CITRUS FRUIT

Abscission is an active developmental process occurring at the abscission zone of the fruit peduncle. The target cells in the abscission zone, which are involved in the process of separation, are located in the separation layer, which is organized in a few cellular layers. In general, it is accepted that the increase in ethylene production in the fruit is followed by increased sensitivity of these cells to ethylene, which would lead to abscission. The abscission process is divided into two phases with respect to sensitivity to auxin and ethylene. The auxin, 2,4-D (2,4-dichlorophenoxy acetic acid), has been used as a postharvest packhouse treatment to retard calyx abscission (to repress postharvest decay). Commercially the sodium salt 2,4-D (Deccomone®) is applied to the fruit in a dip treatment at 500 ppm. The aim of this experiment was to test three agrochemicals (aminoethoxyvinylglycine (AVG), 1-naphthylacetic acid (NAA) and 1-methylcyclopropene (1-MCP)) that could possibly replace the postharvest application of the auxin 2,4-D. AVG had a fruit firming effect but no significant effect on calyx retention. NAA resulted in a high percentage of abscission, probably due to auxin overdose. The 1-MCP at low concentrations of 100 ppb and 250 ppb resulted in calyx retention but had a desiccating effect on the calyx whereas at a high concentration of 500 ppb there was a significant increase in calyx abscission. There was no loss of internal quality or colour. To conclude, according to what is known about the role of ethylene in abscission, 1-MCP and AVG should have prevented abscission of the calyx. Unfortunately, the results show that this complex plant mechanism is not so readily manipulated and 2,4-D remains the best product to inhibit calyx abscission of citrus fruit.

Aroma production from cut sweet pea flowers (Lathyrus odoratus): the role of ethylene

The major components of the scent of cut sweet pea flowers (Lathyrus odoratus L. cv Royal Wedding) are (E) and (Z)‐ocimene, linalool, nerol, geraniol and phenylacetaldehyde. The aroma is almost exclusively produced by the standard and wing petals, with very little emanating from the keel petals and other floral structures. Only traces of these volatiles were detected in the liquid excreted by glandular trichomes on the surface of the scented petals. Once flowers are cut for display they produce increasing amounts of ethylene which induces wilting after 48 h and petal abscission 24 h later. The rate of linalool and ocimene emission declines over the first 48 h to approximately 10% of that directly after harvest. Ethylene production is not saturating during the first 24 h of vase life and exogenous ethylene further accelerates the senescence processes and loss of fragrance. Addition of the ethylene antagonists 1‐methylcyclopropene (1‐MCP) and silver thiosulphate (STS) delayed wilting and abscission for several days and similarly inhibits the decline in terpenoid emission.

TRANSGENES, ETHYLENE AND POSTHARVEST APPLICATIONS

The role of ethylene in controlling many aspects of plant development is well established. Many of these ethylene-controlled processes are fundamentally important to postharvest biology and much effort has been devoted to decreasing the effects of ethylene on processes such as fruit ripening and decay, flower senescence and abscission. In recent years, technology to produce transgenic plants has emerged. Our understanding of the mechanisms controlling ethylene synthesis and perception has expanded rapidly in the last several years. Using the available molecular tools, there have been demonstrations of effective control of fruit ripening and flower senescence in multiple species. However, unintended side effects of interfering with ethylene signaling have proven to be major challenges to bringing biotechnology-based products to the market. These challenges and ways to circumvent them are discussed.

EFFECT OF DIFFERENT GAS TREATMENTS (1-MCP AND N2O) AND MA ON ETHYLENE BIOSYNTHESIS, RIPENING AND QUALITY OF PEACHES

Postharvest storage of peach fruits is limited by the appearance of low temperature induced disorders, mainly internal browning and woolliness. Besides cultivar, growing and climate conditions, other factors are relevant for the appearance of these disorders. Previous studies, using different CA conditions, heat treatments, storage temperatures, were not successful for the 'Merry O'Henry' cultivar. Other treatments with 1-MCP, N 2 O and MA have been used with the purpose to see the possible role of ethylene on the appearance of these disorders. Fruits were selected according to size, colour and flesh firmness and placed at 1°C for 45 days. Part of the fruits were treated with 1000 ppb of 1-MCP for 24 hours, other were treated continuously with 50% N 2 O and other kept under MA with plastic bags.[...]

REGULATION AND ROLE OF DIFFERENTIAL ETHYLENE BIOSYNTHESIS IN GRAVISTIMULATED ANTIRRHINUM MAJUS L. CUT FLOWER STEMS

Gravistimulation induced differential ethylene production in Antirrhinum majus L. cut flower stems with highest levels in the lower halves of the gravistimulated stems. Expression levels of three different 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) genes, an ACC oxidase (ACO) and an ethylene receptor (ETR/ERS homolog) gene were studied in the bending zone after 9 h of gravistimulation. One of the ACS genes (Am-ACS3) was abundantly expressed in the lower halves but not in the upper halves of gravistimulated stems. This strongly suggests that Am-ACS3 is responsible for the observed differential ethylene production in gravistimulated stems. Am-ACO and Am-ETR/ERS gene expression was increased in both the lower and upper halves of gravistimulated stems, suggesting that they play no role in differential ethylene production. When gravistimulation was performed in an environment enriched with either 20 µL/L ethylene or 100 nL/L 1-methylcyclopropene (1-MCP), a slight stimulation of bending by 1-MCP and a slight inhibition of bending by ethylene were observed. The regulation and role of ethylene in gravitropism of cut snapdragon flowering stems is discussed.

Ethylene and flower longevity in Alstroemeria: relationship between tepal senescence, abscission and ethylene biosynthesis

Senescence of floral organs is broadly divided into two groups: those that exhibit sensitivity to exogenous ethylene and those that do not. Endogenous ethylene production from the former group is via a well-characterized biochemical pathway and is either due to developmental or pollination-induced senescence. Many flowers from the order Liliales are characterized as ethylene-insensitive since they do not appear to produce endogenous ethylene, or respond to exogenous ethylene treatments, however, the majority of cases studied are wilting flowers, rather than those where life is terminated by perianth abscission. The role of ethylene in the senescence and abscission of Alstroemeria peruviana cv. Rebecca and cv. Samora tepals was previously unclear, with silver treatments recommended for delaying leaf rather than flower senescence. In the present paper the effects of exogenous ethylene, 2-chloroethylphosphonic acid (CEPA) and silver thiosulphate (STS) treatments on tepal senescence and abscission have been investigated. Results indicate that sensitivity to ethylene develops several days after flower opening such that STS only has a limited ability to delay tepal abscission. Detachment force measurements indicate that cell separation events are initiated after anthesis. Endogenous ethylene production was measured using laser photoacoustics and showed that Alstroemeria senesce independently of ethylene production, but that an extremely small amount of ethylene (0.15 nl flower(-1) h(-1)) is produced immediately prior to abscission. Investigation of the expression of genes involved in ethylene biosysnthesis by semi-quantitative RT-PCR indicated that transcriptional regulation is likely to be at the level of ACC oxidase, and that the timing of ACC oxidase gene expression is coincident with development of sensitivity to exogenous ethylene.

Apple aroma: alcohol acyltransferase, a rate limiting step for ester biosynthesis, is regulated by ethylene

The role of ethylene in aroma biosynthesis of apple fruits was investigated using transgenic ‘Greensleeves’ apple trees suppressed for ACC-oxidase or ACC-synthase enzyme activity, and an ethylene action inhibitor (1-methycyclopropene, 1-MCP). In the transgenic lines and 1-MCP treated fruit, reductions higher than 90% in ethylene biosynthesis and respiration rates were observed in apples held at 20 °C for 14 days. We observed a major reduction in ester production in the ethylene-suppressed lines and in the 1-MCP treated fruit, with only slight differences in the levels of alcohol and aldehyde volatiles under these conditions. The activity of alcohol acyl-CoA transferase (AAT), a key enzyme in ester biosynthesis, showed an ethylene dependent pattern of regulation. Additionally, gene expression levels of specifically an AAT clone were highly regulated by ethylene. In contrast, activity and expression levels of alcohol dehydrogenase (ADH) were not affected by changes in the levels of endogenous ethylene. These results suggest that ethylene is involved in ester biosynthesis in apple via regulation of AAT.

Physiological and biochemical aspects and molecular mechanisms of plant adaptation to the elevated concentration of atmospheric CO2

The review of publications concerning the impact of increasing CO2 concentration in the Earth’s atmosphere (Ca) on higher terrestrial plants. The physiological changes in plants induced by increasing Ca, including growth and biochemical composition, the characteristics of photosynthesis and respiration, as well as the molecular mechanisms of the regulation of the activity of most important biosynthetic enzymes at early and late stages of the exposure to elevated Ca are under consideration. Various concepts of metabolic regulation during acclimation to increasing CO2 concentration are critically reviewed. The pathways of possible involvement of carbonic anhydrase-mediated systems of CO2 transport and concentration during C3 photosynthesis of higher plants, the metabolic and signal mechanisms of photosynthesis inhibition by carbohydrates and the role of ethylene at elevated Ca are presented. The effect of elevated Ca on plant development and source-sink relations, as well as its interaction with other environmental factors, such as mineral, primarily nitrogen nutrition, light, temperature, and water regime, are discussed in with the context of potential forecasting of the consequences of increase in Ca and temperature for the activities of various higher plant forms in the rapidly changing climate.

Assessment of inequality of root hair density in Arabidopsis thaliana using the Gini coefficient: a close look at the effect of phosphorus and its interaction with ethylene.

Root hair density (i.e. the number of root hairs per unit root length) in Arabidopsis thaliana varies among individual plants in response to different nutrient stresses. The degree of such variation, defined as inequality, serves as a unique indicator of the uniformity of response within a plant population to nutrient availability. Using the Gini coefficient (G) as an inequality index, the inequality of root hair density in Arabidopsis thaliana 'Columbia' was evaluated under conditions of nutrient stresses; in particular the effect of phosphorus and its interaction with ethylene. With decreasing phosphorus concentration, root hair density increased while inequality decreased logarithmically. The addition of the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) under high phosphorus increased root hair density and decreased inequality by 7-fold. Inhibition of ethylene action with 1-methylcyclopropene (MCP) and silver thiosulphate (STS) under low phosphorus decreased root hair density, and increased inequality by 9-fold and 4-fold, respectively. The ethylene action inhibitors had little effect on root hair density under high phosphorus, but inequality increased 3-fold in the presence of MCP and decreased 2-fold in the presence of STS. Compared with the control, deficiencies in S, N and K increased inequality of root hair density, whereas deficiencies in P, Ca, B, Mn, Fe, Zn, Cu and Mg decreased inequality. In particular, the inequality of root hair density increased by over 2-fold under deficiencies of N or K, but decreased 14-fold under phosphorus deficiency. The inequality analysis indicates a strong correlation between prevalent signals from the environment (i.e. phosphorus stress) and the response of the plant, and the role of ethylene in this response. As the environmental signals become stronger, an increasing proportion of individuals respond, resulting in a decrease in variation in responsiveness among individual plants as indicated by reduced inequality.

Ethylene Synthesis and Auxin Augmentation in Pistil Tissues are Important for Egg Cell Differentiation after Pollination in Maize

The role of ethylene and auxin in stigma-to-ovule signalling was investigated in maize (Zea mays L.). Maturation of the egg cells in an ear was stimulated before actual fertilization by the application of fresh pollen grains or quartz sand to fully receptive stigmas. Ethylene emission by maize ears increased in response to those treatments. Silks and ovaries were involved in ethylene synthesis after pollen or sand was shed over the silks. The content of ethylene precursor [1-aminocyclopropane-1-carboxylic acid (ACC)] increased in both pistil parts soon after pollination. ACC rise was delayed by 4 h in the ovaries, and by 8 h in the silks after mock-pollination with sand. The auxin level increased rapidly in the silks and ovaries after pollination, and it was very high in the pollinated silks due to the high indole-3-acetic acid (IAA) content of pollen grains. IAA rise also appeared in the silks and ovaries after treatment with sand but it was delayed by 8 h. Application of ACC (10 microM) or IAA (6 microM) solutions to non-pollinated silks stimulated maturation of the egg cells. Moreover, the response of the egg cells to pollination was cancelled by l-alpha-(2-aminoethoxyvinyl)-glycine, alpha-aminoisobutyric acid or 2,3,5-triiodobenzoic acid applied to the silks before pollination. Thus ethylene synthesis and polar auxin transport in the silks pollinated with fresh pollen were necessary to evoke accelerated differentiation of the egg cells in maize ovules. Differences in pistil responses found between true- and mock-pollination suggest that signalling pathways are at least partially different for the reception of pollen grains and sand crystals on maize stigma.

The roles of ethylene, auxin, abscisic acid, and gibberellin in the hyponastic growth of submerged Rumex palustris petioles.

Rumex palustris responds to complete submergence with upward movement of the younger petioles. This so-called hyponastic response, in combination with stimulated petiole elongation, brings the leaf blade above the water surface and restores contact with the atmosphere. We made a detailed study of this differential growth process, encompassing the complete range of the known signal transduction pathway: from the cellular localization of differential growth, to the hormonal regulation, and the possible involvement of a cell wall loosening protein (expansin) as a downstream target. We show that hyponastic growth is caused by differential cell elongation across the petiole base, with cells on the abaxial (lower) surface elongating faster than cells on the adaxial (upper) surface. Pharmacological studies and endogenous hormone measurements revealed that ethylene, auxin, abscisic acid (ABA), and gibberellin regulate different and sometimes overlapping stages of hyponastic growth. Initiation of hyponastic growth and (maintenance of) the maximum petiole angle are regulated by ethylene, ABA, and auxin, whereas the speed of the response is influenced by ethylene, ABA, and gibberellin. We found that a submergence-induced differential redistribution of endogenous indole-3-acetic acid in the petiole base could play a role in maintenance of the response, but not in the onset of hyponastic growth. Since submergence does not induce a differential expression of expansins across the petiole base, it is unlikely that this cell wall loosening protein is the downstream target for the hormones that regulate the differential cell elongation leading to submergence-induced hyponastic growth in R. palustris.