Ethylene Receptor Research Articles

Hydrogen gas alleviates postharvest senescence of cut rose 'Movie star' by antagonizing ethylene.

H2 prolonged the vase life and improved the vase quality of cut roses through repressing endogenous ethylene production and alleviating ethylene signal transduction during the entire senescing period. Recently, the application of hydrogen gas (H2) was shown to improve postharvest quality and longevity in perishable horticultural products, but the specific regulation mechanism remains obscure. Here, endogenous ethylene production and the expression of genes in ethylene biosynthesis and signalling pathway were investigated to explore the crosstalk between H2 and ethylene during the senescence of cut roses. Our results revealed that addition of exogenous ethylene by ethephon accelerated the senescence of cut roses, in which 100mg L-1 ethephon displayed the most obvious senescent phenotype. While the applied different concentrations (1%, 10%, 50% and 100%) of hydrogen-rich water (HRW) conducted different affects in alleviating the senescence of cut roses, and 1% HRW displayed the best ornamental quality and the longest vase life by reducing ethylene production, supported by the decrease of 1-aminocyclopropene-1-carboxylate (ACC) accumulation, ACC synthase (ACS) and ACC oxidase (ACO) activities, and Rh-ACS3 and Rh-ACO1 expressions in ethylene biosynthesis. In addition, HRW increased the transcripts of ethylene receptor genes Rh-ETR1 at blooming period from day 4 to day 6 and suppressed Rh-ETR3 at senescence phase at day 8 after harvest. Furthermore, the relevant affection of HRW on Rh-ETR1 and Rh-ETR3 expressions still existed when the ethylene production was compromised by adequate addition of exogenous ethylene in HRW-treated cut rose petals, and HRW directly repressed the protein level of Rh-ETR3 in a transient expression assay. Overall, the results suggested that H2 is involved in neutralizing ethylene-mediated postharvest in cut flowers.

Ethanol, at physiological concentrations, affects ethylene sensing in tomato germinating seeds and seedlings

Ethanol is known to accumulate in various plant organs under various environmental conditions. However, there are very scarce data about ethanol sensing by plants. We observed that ethanol accumulates up to 3.5 mM during tomato seed imbibition, particularly when seeds were stacked. Stacked seeds germinated less than spread out seeds suggesting ethanol inhibits germination. In support of this, exogenous ethanol at physiological concentrations, ranging from 1 to 10 mM, inhibited germination of wild type tomato seeds. However, the germination pattern over the whole ethanol concentration range tested was modified in an ethylene insensitive mutant, never-ripe (nr). The effects of exogenous ethanol were not linked to differences in ethylene production by imbibed seeds. But, we observed that exogenous ethanol at a concentration as low as 0.01 mM down regulated the expression of some ethylene receptors. Moreover, the triple response induced by ethylene in tomato seedlings was partially alleviated by 1 mM ethanol. Similar observations were made on Arabidopsis seeds. These results show there are interactions between ethylene sensing and ethanol in plants.

Alleviating dormancy in Brassica oleracea seeds using NO and KAR1 with ethylene biosynthetic pathway, ROS and antioxidant enzymes modifications

BackgroundSeed dormancy is a prevailing condition in which seeds are unable to germinate, even under favorable environmental conditions. Harvested Brassica oleracea (Chinese cabbage) seeds are dormant and normally germinate (poorly) at 21 °C. This study investigated the connections between ethylene, nitric oxide (NO), and karrikin 1 (KAR1) in the dormancy release of secondary dormant Brassica oleracea seeds.ResultsNO and KAR1 were found to induce seed germination, and stimulated the production of ethylene and 1-aminocyclopropane-1-carboxylic acid (ACC), and both ethylene biosynthesis enzyme ACC oxidase (ACO) [1] and ACC synthase (ACS) [2]. In the presence of NO and KAR1, ACS and ACO activity reached maximum levels after 36 and 48 h, respectively. The inhibitor of ethylene 2,5-norbornadiene (NBD) had an adverse effect on Brassica oleracea seed germination (inhibiting nearly 50% of germination) in the presence of NO and KAR1. The benefits from NO and KAR1 in the germination of secondary dormant Brassica oleracea seeds were also associated with a marked increase in reactive oxygen species (ROS) (H2O2 and O2˙ˉ) and antioxidant enzyme activity at early germination stages. Catalase (CAT) and glutathione reductase (GR) activity increased 2 d and 4 d, respectively, after treatment, while no significant changes were observed in superoxide dismutase (SOD) activity under NO and KAR1 applications. An increase in H2O2 and O2˙ˉ levels were observed during the entire incubation period, which increasing ethylene production in the presence of NO and KAR1. Abscisic acid (ABA) contents decreased and glutathione reductase (GA) contents increased in the presence of NO and KAR1. Gene expression studies were carried out with seven ethylene biosynthesis ACC synthases (ACS) genes, two ethylene receptors (ETR) genes and one ACO gene. Our results provide more evidence for the involvement of ethylene in inducing seed germination in the presence of NO and KAR1. Three out of seven ethylene biosynthesis genes (BOACS7, BOACS9 and BOACS11), two ethylene receptors (BOETR1 and BOETR2) and one ACO gene (BOACO1) were up-regulated in the presence of NO and KAR1.ConclusionConsequently, ACS activity, ACO activity and the expression of different ethylene related genes increased, modified the ROS level, antioxidant enzyme activity, and ethylene biosynthesis pathway and successfully removed (nearly 98%) of the seed dormancy of secondary dormant Brassica olereace seeds after 7 days of NO and KAR1 application.

24-epibrassinolide triggers cadmium stress mitigation in Cucumis sativus through intonation of antioxidant system

The core objective of current study was to find out the potential of 24-epibrassinolide (24-EBL) in alleviation of cadmium (Cd) induced stress in cucumber (Cucumis sativus L.). C. sativus seeds were primed with 5 μM 24-EBL and allowed to grow in Cd spiked petri dishes for 15 days. The Cd-driven stress exhibited noticeable injurious effects on growth parameters of C. sativus seedlings. The oxidative stress pledged due to overproduction of malondialdehyde (MDA) and reactive oxygen species (ROS) severely affected morphological features besides gas exchange parameters, leaf relative water content (LRWC) and amount of photosynthetic pigments in C. sativus seedlings under Cd stress. Nevertheless, 24-EBL reduced Cd contents in cucumber foliage and elucidated the vital role of this biomolecule in Cd translocation. The exogenous application of 24-EBL improved shoot length, root length and biomass production of Cd-stressed plants. Alternatively, 24-EBL triggered reduction in lipid peroxidation besides increased synthesis of ethylene and indole acetic acid (IAA). Furthermore, 24-EBL modulated the expression level of ethylene receptor genes including CS-ERS, 1-aminocyclopropane-1-carboxylate oxidase2 (CsAOX), 1-aminocyclopropane-1-carboxylate oxidase1 (CSACO1), 1-aminocyclopropane-1-carboxylate oxidase2 (CSACO2), and indole acetic related CS 453. Altogether, these physiological vicissitudes advocate that 24-EBL augmented antioxidative capability contributes in the advancement of Cd tolerance in cucumber seedlings.

Binding mode of 1-methylcyclopropene, an ethylene antagonist in various copper models

1-Methylcyclopropene is a commercially available ethylene inhibitor, which has been proposed to bind with the copper active site in Ethylene Receptor 1 involved in the fruit ripening. Various copper complexes have been proposed as models for ethylene receptors which are used to study the interactions with 1-Methylcyclopropene and the respective ethylene counterparts. 1-Methylcyclopropene binds with copper complexes in a side-on orientation (via the π bond) with a larger negative binding energy due to the observed back bonding from the metal. The computed binding energy values evidently show that the same has a higher preference towards the complexes with copper in its +1 oxidation state owing to the tetrahedral geometry. The copper complexes with 1-Methylcyclopropene absorb light at 235–304 nm with a high intensity attributed to the Ligand to Metal Charge Transfer. Among the models studied, the binding of 1-Methylcyclopropene get highly stabilized by the presence of aromatic rings as ligands (OC6H5, N2C3H4).

Identification and Expression Analysis of Gretchen Hagen 3 (GH3) in Kiwifruit (Actinidia chinensis) During Postharvest Process.

In plants, the Gretchen GH3 (GH3) protein is involved in free auxin (IAA) and amino acid conjugation, thus controlling auxin homeostasis. To date, many GH3 gene families have been identified from different plant species. However, the GH3 gene family in kiwifruit (Actinidia chinensis) has not been reported. In this study, 12 AcGH3 genes were identified, phylogenetic analysis of AtGH3 (Arabidopsis), SlGH3 (Solanum lycopersicum), and AcGH3 provided insights into various orthologous relationships among these proteins, which were categorized into three groups. Expression analysis of AcGH3 genes at different postharvest stages suggested limited or no role for most of the AcGH3 genes at the initiation of fruit ripening. AcGH3.1 was the only gene exhibiting ripening-associated expression. Further study showed that the expression of AcGH3.1 gene was induced by NAA (1-naphthylacetic acid, auxin analogue) and inhibited by 1-MCP (1-methylcyclopropene, ethylene receptor inhibitor), respectively. AcGH3.1 gene silencing inhibited gene expression and delayed fruit softening in kiwifruit. The results indicate that AcGH3.1 may play an important role in the softening process of fruits. Analysis of the AcGH3.1 promoter revealed the presence of many cis-elements related to hormones, light, and drought. The determination of GUS (β-Galactosidase) enzyme activity revealed that promoter activity increased strikingly upon abscisic acid (ABA), ethylene, or NAA treatment, and significantly decreased with salicylic acid (SA) treatment. The present study could help in the identification of GH3 genes and revelation of AcGH3.1 gene function during postharvest stages, which pave the way for further functional verification of the AcGH3.1 gene.

Isolation and expression analysis of an ethylene receptor gene Ps-ETR2 in tree peony cut flowers

Isolation and expression analysis of an ethylene receptor gene <i>Ps-ETR2</i> in tree peony cut flowers

AgroFresh wins food fight

A federal jury in Delaware has determined that the Indian agrochemical maker UPL and its Decco US Post-Harvest subsidiary stole technology from AgroFresh that keeps apples, pears, and other fruit fresh long after harvest. The jury awarded AgroFresh $31 million; UPL says it will appeal. The dispute concerns the use of 1-methylcyclopropene (1-MCP), a synthetic gas used to delay fruit spoilage. Fruit starts the ripening process by releasing ethylene gas, and growers can slow ripening by adding 1-MCP to storage facilities, where the gas binds to fruit’s ethylene receptors and blocks the effects of ethylene. Researchers at North Carolina State University discovered 1-MCP in 1994. AgroFresh, a 2015 spin-off of Dow Chemical, pioneered development of the molecule. According to court documents, a onetime AgroFresh consultant developed a new generation of 1-MCP technology for fruit preservation and licensed patents to Decco in 2016. AgroFresh, however, successfully argued that it owned the

Amplification and Adaptation in the Ethylene Signaling Pathway

AbstractPlants are exquisitely sensitive to the ethylene signal and also respond to a much wider range of ethylene concentrations than would seem possible based on the simple circuitry of its primary signal transduction pathway, suggesting the existence of mechanisms for amplification and adaptation to ethylene signals. Here, such regulatory systems are considered within the context of what is known about the plant ethylene signaling pathway as well as signaling by the animal G‐protein coupled receptors, and the bacterial methyl‐accepting chemotaxis proteins. Magnitude amplification and sensitivity amplification mechanisms are considered as strategies for amplification of the ethylene signal. Several families of negative feedback regulators that desensitize plants to ethylene and thereby facilitate the ethylene adaptation response of plants are described. These negative feedback regulators include the ethylene receptors themselves, the RTE1/GR family, and the ARGOS family, all of which function at the level of the ethylene receptors to desensitize plants to ethylene. These negative regulators also include the EBF family of F‐box proteins, which target the EIN3/EIL family of transcription factors for degradation. Ethylene signal amplification and adaptation employ both transcriptional and post‐transcriptional regulation.

Hydrogen Sulfide Inhibits Fruit Softening by Regulating Ethylene Synthesis and Signaling Pathway in Tomato (Solanum lycopersicum)

Hydrogen sulfide (H2S) has been proven to be a multifunctional signaling molecule in plants. In this study, we attempted to explore the effects of H2S on the climacteric fruit tomato during postharvest storage. H2S fumigation for 1 d was found to delay the peel color transition from green to red and decreased fruit firmness induced by ethylene. Further investigation showed that H2S fumigation downregulated the activities and gene expressions of cell wall–degrading enzymes pectin lyase (PL), polygalacturonase (PG), and cellulase. Furthermore, H2S fumigation downregulated the expression of ethylene biosynthesis genes SlACS2 and SlACS3. Ethylene treatment for 1 d was found to induce the expression of SlACO1, SlACO3, and SlACO4 genes, whereas the increase was significantly inhibited by H2S combined with ethylene. Furthermore, H2S decreased the transcript accumulation of ethylene receptor genes SlETR5 and SlETR6 and ethylene transcription factors SlCRF2 and SlERF2. The correlation analysis suggested that the fruit firmness was negatively correlated with ethylene biosynthesis and signaling pathway. The current study showed that exogenous H2S could inhibit the synthesis of endogenous ethylene and regulate ethylene signal transduction, thereby delaying fruit softening and the ripening process of tomato fruit during postharvest storage.

The ethylene receptors CpETR1A and CpETR2B cooperate in the control of sex determination in Cucurbita pepo

High-throughput screening of an ethyl methanesulfonate-generated mutant collection of Cucurbita pepo using the ethylene triple-response test resulted in the identification of two semi-dominant ethylene-insensitive mutants: etr1a and etr2b. Both mutations altered sex determination mechanisms, promoting conversion of female into bisexual or hermaphrodite flowers, and monoecy into andromonoecy, thereby delaying the transition to female flowering and reducing the number of pistillate flowers per plant. The mutations also altered the growth rate and maturity of petals and carpels in pistillate flowers, lengthening the time required for flowers to reach anthesis, as well as stimulating the growth rate of ovaries and the parthenocarpic development of fruits. Whole-genome sequencing allowed identification of the causal mutation of the phenotypes as two missense mutations in the coding region of CpETR1A and CpETR2B, each one corresponding to one of the duplicates of ethylene receptor genes highly homologous to Arabidopsis ETR1 and ETR2. The phenotypes of homozygous and heterozygous single- and double-mutant plants indicated that the two ethylene receptors cooperate in the control of the ethylene response. The level of ethylene insensitivity, which was determined by the strength of each mutant allele and the dose of wild-type and mutant etr1a and etr2b alleles, correlated with the degree of phenotypic changes in the mutants.

1-Methylcyclopropene affects ethylene synthesis and chlorophyll degradation during cold storage of ‘Comice’ pears

Loss of green skin color and the onset of senescent scald (scald) in ‘Comice’ pears affect fruit quality and reduce the market value. To understand the molecular mechanisms of chlorophyll degradation in the skins of ‘Comice’ pears, fruit were exposed to 1-methylcyclopropene (1-MCP) at 0.15 and 0.3 μL L−1 and stored at -1.1 °C for up to six months. The results showed ethylene production rate, and respiration rate were significantly inhibited in pears treated with 1-MCP at 0.15 and 0.3 μL L−1. Senescent scald was eliminated, and the amount of extractable juice (EJ) remained constant, and chlorophyll degradation was retarded. However, the ripening capacity was inhibited. Ripening capacity recovered (by 14 days following warming to room temperature) in pears treated with 1-MCP at the lower concentration (0.15 μ L L−1) but the fruit treated with 1-MCP at the higher concentration (0.3 μL L−1) failed to ripen. The expressions of the ethylene synthesis genes (PcACS1, PcACS4, PcACS5, and PcACO1) and receptor genes (PcETR1, PcETR2, PcETR5, and PcERS1) were upregulated during storage in the control fruit but were significantly repressed by the 1-MCP treatments. In contrast, the expression of PcACS2 was upregulated by the 1-MCP treatments. The expressions of the chlorophyll degradation genes (PcPPH, PcNOL, PcSGR1, and PcPAO) were significantly higher in the control fruit but suppressed in the 1-MCP-treated fruit. The transcription levels of PcRCCR and PcCLH1 decreased during the six months of cold storage period and 1-MCP had little effect on their expressions. The correlations between chlorophyll degradation and the ethylene synthesis genes and signal-related genes indicated that 1-MCP delayed chlorophyll degradation by inhibiting the ethylene production rate and suppressing the expressions of the PcPPH, PcNOL, PcSGR1, and PcPAO genes, which were closely associated with the chlorophyll catabolic pathway in ‘Comice’ pears.

Different regulatory modules of two mango ERS1 promoters modulate specific gene expression in response to phytohormones in transgenic model plants

Ethylene is a key element of plant physiology, thus ethylene research is important for both, fundamental research and agriculture. Previous work on ethylene receptors focused on expression level and protein interaction, but knowledge on regulation of gene transcription is scarce. Promoters of mango ethylene receptor genes (pMiERS1a, pMiERS1b) were analysed particularly regarding responsiveness to hormones. The promoter sequences reveal some variation and they were characterized by identifying functional regulatory candidate modules via truncated-promoter approach. Based on ectopic gene expression studies in transgenic Arabidopsis and Nicotiana it is demonstrated that both promoters are positively responsive to ethylene. For pMiERS1a the AHBP/DOFF1 module is linked to ethylene responsiveness, while for pMiERS1b it is the module MYBL/OPAQ1. A negative gene regulation in response to abscisic acid (ABA) is linked to MYBL/DOFF2. A positive response to indole-3-acetic acid (IAA) was found for GTBX/MYCL1, containing the motifs IBOX/IDDF/TEFB, which are present in this combination only in pMiERS1b, but not in pMiERS1a. Conclusively, the general response of the ethylene receptor genes is conserved, but similar regulation can be linked to different modules. Further, a minor variation in a transcription factor binding site (TFBS) motif within an overall conserved module type can lead to a different expression.

The Effect of 1-Methylcyclopropene Treatment on Shallot Physiological Process and Qeight Loss

Fresh-handling treatment using 1- Methylcyclopropene (1-MCP) has been proven effective in controlling respiration process of various horticultural commodities, helping to extend shelf-life for storage and transportation purpose. The effect of 1 -MCP treatment on shallot (A llium ascalonicum L. ) had yet been studied. It was expected to delay maturity, prevent negative effects of ethylene, and inhibit senescence by blocking ethylene receptor. The objective of this research was to study the effects of 1-MCP on the physiological process and weight loss of shallot. The research was preceded by the determination of 1 -MCP dosage, i.e. 0.5, 1, 1.5, 2, and 2.5 g. The best dosage was then used in the 1-MCP treatment on shallot. The duration of treatment was 0, 6, 18, 30, and 42 hours. Observation of physiological process was done to respiration rate (CO2) and ethylene production rate. Weight loss was also observed. The best dosage of 1-MCP was 2.5g with the content of 437.78 ppm. On the main experiment, 18 hours of treatment had the lowest but statistically the same respiration rate compared to other duration, and statistically different with the shallot without treatment. For ethylene production rate, 18 hours of treatment had the best effect and statistically different with the ones without treatment. Shallot with different treatments also showed insignificant different on weight loss, but were significantly different with shallot without treatment. Without treatment, the quality of shallot was dropped after 70 days of storage, while all the quality of treated shallot was maintained up to 84 days. It was concluded that 1 -MCP treatment was effective in controlling respiration rate and ethylene production rate. However, different duration of 1-MCP treatment had not showed different effects on shallot physiological process and weight loss.

Targeted Proteomics Allows Quantification of Ethylene Receptors and Reveals SlETR3 Accumulation in Never-Ripe Tomatoes.

Ethylene regulates fruit ripening and several plant functions (germination, plant growth, plant-microbe interactions). Protein quantification of ethylene receptors (ETRs) is essential to study their functions, but is impaired by low resolution tools such as antibodies that are mostly nonspecific, or the lack of sensitivity of shotgun proteomic approaches. We developed a targeted proteomic method, to quantify low-abundance proteins such as ETRs, and coupled this to mRNAs analyses, in two tomato lines: Wild Type (WT) and Never-Ripe (NR) which is insensitive to ethylene because of a gain-of-function mutation in ETR3. We obtained mRNA and protein abundance profiles for each ETR over the fruit development period. Despite a limiting number of replicates, we propose Pearson correlations between mRNA and protein profiles as interesting indicators to discriminate the two genotypes: such correlations are mostly positive in the WT and are affected by the NR mutation. The influence of putative post-transcriptional and post-translational changes are discussed. In NR fruits, the observed accumulation of the mutated ETR3 protein between ripening stages (Mature Green and Breaker + 8 days) may be a cause of NR tomatoes to stay orange. The label-free quantitative proteomics analysis of membrane proteins, concomitant to Parallel Reaction Monitoring analysis, may be a resource to study changes over tomato fruit development. These results could lead to studies about ETR subfunctions and interconnections over fruit development. Variations of RNA-protein correlations may open new fields of research in ETR regulation. Finally, similar approaches may be developed to study ETRs in whole plant development and plant-microorganism interactions.

Influence of drop shock on physiological responses and genes expression of apple fruit

The aim of our study was to investigate the effect of drop shock on physiological responses and genes expression in harvested apple fruit stored at 20 ± 2 °C. Ethylene production, respiratory rate, firmness, soluble solid content, relative electrical conductivity, LOX (lipoxygenase) activity, MDA (malondialdehyde) content, variation of volatile compounds, ethylene biosynthetic genes, and ethylene receptor genes of apple fruit were examined. The results indicated that drop shock observably resulted in the increase of ethylene production, respiratory rate, soluble solid content, relative electrical conductivity, LOX activity, MDA content and gene expression levels in apples. Furthermore, drop shock significantly decreased firmness and high-intensitive drop shock stimulated the accumulation of aldehydes and esters in harvested apples. Overall, the greater impact on apple quality is the effect of larger amplitude of shock during truck transportation, which seriously reduced storage life and quality of postharvest apples.

Application of Cas12a and nCas9-activation-induced cytidine deaminase for genome editing and as a non-sexual strategy to generate homozygous/multiplex edited plants in the allotetraploid genome of tobacco.

Protoplasts can be used for genome editing using several different CRISPR systems, either separately or simultaneously, and that the resulting mutations can be recovered in regenerated non-chimaeric plants. Protoplast transfection and regeneration systems are useful platforms for CRISPR/Cas mutagenesis and genome editing. In this study, we demonstrate the use of Cpf1 (Cas12a) and nCas9-activation-induced cytidine deaminase (nCas9-Target-AID) systems to mutagenize Nicotiana tabacum protoplasts and to regenerate plants harboring the resulting mutations. We analyzed 20 progeny plants of Cas12a-mediated phytoene desaturase (PDS) mutagenized regenerants, as well as regenerants from wild-type protoplasts, and confirmed that their genotypes were inherited in a Mendelian manner. We used a Cas9 nickase (nCas9)-cytidine deaminase to conduct C to T editing of the Ethylene receptor 1 (ETR1) gene in tobacco protoplasts and obtained edited regenerates. It is difficult to obtain homozygous edits of polyploid genomes when the editing efficiency is low. A second round of mutagenesis of partially edited regenerants (a two-step transfection protocol) allowed us to derive ETR1 fully edited regenerants without the need for sexual reproduction. We applied three different Cas systems (SaCas9, Cas12a, and nCas9-Traget AID) using either a one-step or a two-step transfection platform to obtain triply mutated and/or edited tobacco regenerants. Our results indicate that these three Cas systems can function simultaneously within a single cell.

Ethylene and 1-MCP treatments affect leaf abscission and associated metabolism of Chinese cabbage

Leaf abscission during storage of Chinese cabbage (Brassica rapa, subspecies pekinensis and chinensis) can result in serious losses. To uncover the effects of the plant hormone ethylene on leaf abscission, harvested cabbages were treated with ethylene and its competitive inhibitor, 1-methylcyclopropene (1-MCP), and with 1-MCP followed by ethylene. Ethylene treatment accelerated leaf abscission, altered cell structure of the abscission zones, and increased activity and gene expression of cell wall-degrading enzymes. Expression of genes related to ethylene receptors and signaling pathways including BcERS1, BcERS2, BcETR2, BcCTR1, BcEIL1, BcEIL2, and BcEIL3 were also up-regulated. In 1-MCP-treated samples, leaf breakstrength was higher, and the increase of cell wall-degrading enzyme activity and the expression of enzyme-related genes were reduced. Notably, ethylene sensitivity recovered upon subsequent ethylene treatment following 1-MCP treatment. These results indicated that ethylene may constitute an important factor in leaf abscission of Chinese cabbage.

Cyanobacteria Respond to Low Levels of Ethylene.

Ethylene is a gas that has long been known to act as a plant hormone. We recently showed that a cyanobacterium, Synechocystis sp. PCC 6803 (Synechocystis) contains an ethylene receptor (SynEtr1) that regulates cell surface and extracellular components leading to altered phototaxis and biofilm formation. To determine whether other cyanobacteria respond to ethylene, we examined the effects of exogenous ethylene on phototaxis of the filamentous cyanobacterium, Geitlerinema sp. PCC 7105 (Geitlerinema). A search of the Geitlerinema genome suggests that two genes encode proteins that contain an ethylene binding domain and Geitlerinema cells have previously been shown to bind ethylene. We call these genes GeiEtr1 and GeiEtr2 and show that in air both are expressed. Treatment with ethylene decreases the abundance of GeiEtr1 transcripts. Treatment of Geitlerinema with 1000 nL L–1 ethylene affected the phototaxis response to white light as well as monochromatic red light, but not blue or green light. This is in contrast to Synechocystis where we previously found ethylene affected phototaxis to all three colors. We also demonstrate that application of ethylene down to 8 nL L–1 stimulates phototaxis of both cyanobacteria as well as biofilm formation of Synechocystis. We formerly demonstrated that the transcript levels of slr1214 and CsiR1 in Synechocystis are reduced by treatment with 1000 nL L–1 ethylene. Here we show that application of ethylene down to 1 nL L–1 causes a reduction in CsiR1 abundance. This is below the threshold for most ethylene responses documented in plants. By contrast, slr1214 is unaffected by this low level of ethylene and only shows a reduction in transcript abundance at the highest ethylene level used. Thus, cyanobacteria are very sensitive to ethylene. However, the dose-binding characteristics of ethylene binding to Geitlerinema and Synechocystis cells as well as to the ethylene binding domain of SynEtr1 heterologously expressed in yeast, are similar to what has been reported for plants and exogenously expressed ethylene receptors from plants. These data are consistent with a model where signal amplification is occurring at the level of the receptors.

Ethylene Receptors in Nonplant Species

AbstractThe effects of ethylene and ethylene signal transduction have been long studied in plants. It is only recently reported that a single‐celled cyanobacterium, Synechocystis, contains a functional ethylene receptor. However, it remains to be determined if functional ethylene receptors are more widespread. This review summarizes what is known about responses to ethylene, ethylene receptors, and signal transduction in nonplant species. A summary is provided of information gleaned from searches of sequenced genomes and the possibility that ethylene receptors are widespread in microbes is discussed.