Journal of Experimental Botany, Vol. 53, No. 379, pp. 2333-2339,
December 1, 2002
© 2002 Oxford University Press
Characterization of two putative ethylene receptor genes expressed during peach fruit development and abscission
Received 7 May 2002; Accepted 19 July 2002
Department of Environmental Agronomy and Crop Science, University of Padova, Via Romea, 16-Agripolis, Legnaro (Padova), 35020 Italy
1 To whom correspondence should be addressed. Fax: +39 049 8272850. e-mail: angelo.ramina{at}unipd.it
Abbreviations: 1-MCP, 1-methylcyclopropene, AZ, abscission zone; DNZ, distal non-zone; NZ, non-zone; PNZ, proximal non-zone.
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Two peach genes homologous to the Arabidopsis ethylene receptor genes ETR1 and ERS1, named Pp-ETR1 and Pp-ERS1 respectively, have been isolated and characterized. Pp-ETR1 and Pp-ERS1 are conserved in terms of exon numbers and intron positions, although the first and fifth introns of Pp-ETR1 have an unusual length. In addition, two putative polyadenylation sites, that may cause an incomplete splicing at the 3' terminus, are present in the fifth intron. A motif of 28 nt, which shows high homology with ethylene responsive elements found in promoters of genes up-regulated by ethylene, is present in the promoter region of Pp-ERS1. Expression analysis, carried out by quantitative RT-PCR, was performed during fruit development and ripening, and leaf and fruitlet abscission. The level of Pp-ETR1 transcripts remained unchanged in all the tissues and developmental stages examined, whereas Pp-ERS1 mRNA abundance increased in ripening mesocarp, in leaf and fruitlet activated abscission zones, and following propylene application. 1-methylcyclopropene (1-MCP), an inhibitor of ethylene action, did not affect Pp-ETR1 transcription, while it down-regulated Pp-ERS1. A rise in ethylene evolution, accompanied by an increase of Pp-ERS1 transcript accumulation occurred within 24 h from the end of 1-MCP treatment. These results indicate that Pp-ERS1 might play a role in abscission and ripening.
Key words: Abscission zone, ERS1, ETR1, 1-methylcyclopropene (1-MCP), quantitative RT-PCR, ripening, Prunus persica.
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Introduction |
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Ethylene plays an important role in the initiation and continuation of ripening in all climacteric fruits, including peach. Genes encoding 1-aminocyclo-1-propane carboxylate (ACC) synthase and ACC oxidase, the two key enzymes of the biosynthetic pathway, have recently been isolated and characterized in peach (Mathooko et al., 2001; Ruperti et al., 2001), However, in peach, little molecular information is available about ethylene perception and signal transduction. Progress made in understanding ethylene signal perception has been achieved by molecular genetic approaches using Arabidopsis mutants, in which At-ETR1 (Ethylene Receptor 1) was first isolated by map-based cloning (Chang et al., 1993). The amino terminal hydrophobic region of the ETR1 protein contains three putative membrane-spanning subdomains, which form the ethylene binding site (Rodriguez et al., 1999). The carboxy terminus of ETR1 is likely to be involved in transmitting the ethylene signal, as this region contains both a histidine protein kinase domain and a receiver domain. Using an At-ETR1 cDNA as probe, Hua et al. (1995) isolated the At-ERS1 (Ethylene Sensor 1) gene. This gene encodes a protein, which has sequence similarity with the amino-terminal and histidine kinase domain of ETR1, but is lacking the receiver domain. Two-hybrid and in vitro binding assays have shown that the carboxy-terminal region of both At-ETR1 and At-ERS1 interacts with CTR1 (Clark et al., 1998), a RAF kinase-like protein that acts as a negative regulator of the ethylene transduction pathway (Kieber et al., 1993).
At-ETR1 and At-ERS1 show differential expression in relation to the type of tissue and to the exogenous application of ethylene. The expression of At-ETR1 is ubiquitous and ethylene independent, while that of At-ERS1 is tissue specific, ethylene dependent and developmentally regulated (Hua et al., 1998).
Three additional genes (ETR2, EIN4 and ERS2) encoding ethylene receptors have been isolated from Arabidopsis (Hua et al., 1998; Sakai et al., 1998). Genes coding for ETR- and ERS-type polypeptides have been identified in several important fruit species, including tomato, muskmelon, and kiwi (reviewed in Giovannoni, 2001). Exhaustive research has been carried out in tomato, in which six putative receptors, named Le-ETR1, Le-ETR2, NR, Le-ETR4, Le-ETR5, and Le-ETR6 (accession number AY079426) have been so far identified (reviewed in Bleecker, 1999). All the deduced proteins encoded by these genes are of the ETR-type with the exception of NR. The expression pattern of these genes is different: in fact, NR transcripts increase in abundance during fruit ripening (Payton et al., 1996; Tieman and Klee, 1999), whereas those of Le-ETR1 and Le-ETR2 show a more or less constitutive pattern of expression (Lashbrook et al., 1998). Le-ETR4 and Le-ETR5 are mainly expressed in reproductive tissues (Tieman and Klee, 1999). A mutation in the ethylene-binding domain of the NR receptor is responsible for the mutant phenotype Never-ripe (Nr), in which fruits are impaired in their colour change and softening (Wilkinson et al., 1995). The effect of the NR mutation, together with the increase in expression of NR observed at the onset of ripening in wild-type tomatoes, indicated a specific role for this gene in ripening.
In this paper the characterization of two peach genes encoding putative ETR- and ERS-type proteins, named Pp-ETR1 and Pp-ERS1, respectively, is reported. Pp-ETR1 appeared to be constitutively expressed and ethylene independent, whilst the Pp-ERS1 transcription rate dramatically increased in activated leaf and fruitlet abscission zones (AZ) as well as in ripening fruit. The effect of 1-methylcyclopropene (1-MCP), a competitive inhibitor for the ethylene binding sites (Sisler et al., 1996), on transcription of these genes was analysed.
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Materials and methods |
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Plant material
Fruits were harvested from peach trees (Prunus persica L. Batsch, cvs Springcrest and Maria Marta) grown at the experimental farm of the University of Padova (Agripolis, Legnaro, Italy). Fruits from Springcrest were picked at stages S1 (first exponential growth phase), S2 (pit hardening), S3 (second exponential growth phase), and S4 (climacteric stage), as defined by Tonutti et al. (1997). Some of the S1 and S4 fruits were flushed with air and air+500 µl l–1 propylene (an ethylene analogue) for 12 h.
Leaf and fruit explants including the abscission zone (AZ) and surrounding areas (non-zone, NZ) were prepared and treated with air and air+propylene (500 µl l–1) for 12 h as described by Ruperti et al. (2002).
Maria Marta fruits, harvested at the pre-climacteric stage (ethylene production <0.4 nl g–1 FW h–1 and firmness value of about 80 N), were used for the 1-MCP studies. Fruits were enclosed in 10 l jars, injected with 1 µl l–1 of 1-MCP gas, generated by aqueous neutralization of EthylBloc® (Rohm and Haas), and incubated at 25 °C for 12 h. At the end of the treatment fruits were transferred to air and their ripening kinetics were evaluated by monitoring ethylene evolution and firmness loss as described by Tonutti et al. (1997).
Screening of genomic library and sequencing of DNA
The construction of a peach genomic library has been previously described by Ruperti et al. (2001). The genomic library was screened using as a probe cDNAs encoding Arabidopsis ETR1 (At-ETR1) and strawberry ERS. The strawberry ERS cDNA was used because all the clones isolated using the At-ETR1 cDNA encoded ETR1 type proteins.
The genomic DNA fragments containing putative peach ETR1 and ERS1 genes were sequenced using a ABI PRISM Rhodomine Terminator kit (Roche, Branchburg, NJ, USA) and both M13 universal primers and sequence specific primers. Sequence assembly was performed using SeqMan II and MEGALIGN programs from DNASTAR (Madison, WI, USA). Homology searches were carried out using the BLASTP algorithm (Altschul et al., 1997) on GenBank databases. Identity percentages were calculated with the ALIGN program (Pearson and Lipman, 1988). Amino acid sequences were deduced using the Six Frame Translation program by BCM Search Laucher (Human Genome Center, Baylor College of Medicine, Houston, TX, USA).
Southern analysis
Genomic DNA (10 µg) was digested with BamHI, EcoRI and PstI, blotted, and hybridized with the specific probes for Pp-ETR1 and Pp-ERS1 (Fig. 1) at high stringency (50% formamide, 42 °C) and with the whole Pp-ETR1 gene at low stringency (20% formamide, 42 °C) condition as indicated by Ruperti et al. (2001).
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RNA isolation and quantitative RT-PCR
Total RNA was obtained following the protocol described by Ruperti et al. (2001). Five µg of total RNA were treated with 5 units of (amplification grade) DNAse I (GibcoBRL-Life Technologies). The cDNA was obtained from 0.8 µg of DNAse-treated RNA- using the Super Script First strand Synthesis system kit (GibcoBRL-Life Technologies) with the oligo-dT12–18 as primer.
The quantitative RT-PCR was carried out using the SYBR® Green RT-PCR master mix kit (PE Applied Biosystem). Specific primers for Pp-ERS1 (sense 5'-GATTGAGAGTGAGGGCATTG-3', antisense 5'-GCTGCTGTTGTATCACAAGG-3'), Pp-ETR1 (sense 5'-ATGATAACGGGTCAGTGACT-3'; antisense 5'-AAA TAACGTGCAAGAACTC ATC-3') and peach 18S rRNA (Nickrent and Soltis, 1995) (sense 5'-GTTACT TTTAGGACTCCGCC-3'; antisense 5'-TTCCTTTAAGTTTCAGCCTTG-3') were designated to amplify fragments of 90 bp with a melting point around 56 °C. For each sample, three replicates were performed in a final volume of 50 µl containing 1 µl of the cDNA, 15 pmol of Pp-ETR1 or Pp-ERS1 specific primers and 25 µl of 2x SYBR Green PCR Master Mix according to the manufacturer’s instruction. All PCRs were carried out using the Gene Amp® 5700 Sequence Detection System for 10 min at 95 °C (initial denaturation) and then for 40 cycles consisting of 30 s at 95 °C; 30 s at 54 °C and 30 s at 72 °C. At the end of the PCR for each sample, on the basis of the fluorescence logarithmic graph, the appropriate threshold was chosen and the Ct, the fractional cycle number at which a significant increase above the threshold can be first detected, was calculated using the Gene Amp® 5700 Sequence Detection System software. The data were organized as the comparative method described in the User Bulletin No 2 (PE Applied Biosystems). The amount of the target is given by 2–
Ct, where Ct is the difference in threshold cycles for target (Ct sample) and reference (Ct 18S) so the Ct of each sample was normalized to 18S rRNA to account for variability in the initial concentration and quality of the total RNA and in the conversion efficiency of the reverse transcription reaction.
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Characterization and genomic organization of Pp-ETR1 and Pp-ERS1
Gene organization of Pp-ETR1 (accession number AF396830) is identical, in terms of exon number and intron position, to that reported for At-ETR1 (Chang et al., 1993), although the first and fifth introns are 5 and 20 times longer than those present at the corresponding positions in the At-ETR1 gene (Fig. 1). These lengths are unusual for plant genes, although introns of the same size have also been found in tomato (Le-ETR1, (Lashbrook et al., 1998) and Rosa hybrida (Rh-ETR2, (Müller et al., 2000). In addition, two polyadenylation sites that might be responsible for an incomplete splicing at the 3' terminus, are present within the fifth intron. Translation of such a truncated transcript would lead to a product missing a large portion of the receiver domain (Fig. 1).
The coding region of Pp-ERS1 (accession number AF316534) is organized into five exons interrupted by four introns, and no marked difference in length of introns between At-ERS1 and Pp-ERS1 has been observed. Within the Pp-ERS1 promoter a motif showing 70.4% identity with a regulatory element found in the promoter of ERF1 (Ethylene Response Factor), a gene known to be involved in the regulation of primary responses to ethylene (Solano et al., 1998), is present. This sequence has also been observed in the promoter regions of tomato E4 and carnation GST1 genes, which are both up-regulated by ethylene (Montgomery et al., 1993; Itzhaki and Woodson, 1993).
The open reading frame of Pp-ETR1 and Pp-ERS1 encode a polypeptide of 738 and 634 amino acids, respectively. The deduced polypeptides are 63.3% identical to each other. When Pp-ETR1 and Pp-ERS1 polypeptides are compared with the deduced amino acid sequences of the known ethylene receptor genes, they show the highest level of identity with Md-ETR1 (93.8%; Malus domestica, AF03244) and Cp-ERS (81.4%, Carica papaya, AF311942), respectively. The low identity between Pp-ETR1 and Pp-ERS1 is mainly due to the lack of the receiver domain in the latter. Both proteins are conserved at residues Ala31, Ile62, Cys65, and Ala102, thought to be important for the normal function of ethylene receptors (Bleecker and Schaller, 1996), Cys4 and Cys6, which are required for disulphide-linked dimer formation (Schaller and Bleeker, 1995), and His354 and Asp659, presumptive sites of auto-phosphorylation in the histidine kinase domain and receiver domain (Chang et al., 1993).
Genomic organization of ethylene receptors
Five strong hybridization signals and seven faint bands were detected using the whole Pp-ETR1 genomic clone as the probe in the Southern analysis carried out on genomic DNA digested with BamHI, EcoRI and PstI at low stringency conditions (Fig. 2A). The presence of multiple bands suggests that, in peach, ethylene receptors are encoded by a multigene family. Southern analysis carried out at high stringency conditions, using 3' Pp-ERS1 or Pp-ETR1 specific probes (Fig. 2B, C), showed that, in peach, a second gene with high homology to Pp-ERS1 might be present. These data are supported by Pp-ERS1 restriction analysis in which only one site for BamHI and EcoRI is present, while two hybridization bands for both enzymes were detected in Southern analysis.
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Expression of Pp-ETR1 and Pp-ERS1 during fruit development and ripening, and leaf and fruitlet abscission
To understand the possible role of Pp-ETR1 and Pp-ERS1 in fruit physiology, gene expression was analysed throughout development and ripening. The expression analysis was carried out by quantitative PCR as both transcripts were undetectable by northern analysis.
The expression of Pp-ETR1 remained at the basal levels and did not show significant changes in the four stages of fruit development, while the level of Pp-ERS1, always higher than that of Pp-ETR1, showed a marked increase during ripening (Fig. 3A). In addition, propylene treatment did not affect the transcript level of Pp-ETR1, while Pp-ERS1 appeared to be up-regulated by the gas in both the S1 and S4 stages (Fig. 3B).
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The stimulatory effect of propylene on Pp-ERS1 transcription was confirmed by expression analysis carried out in leaf and fruit abscission zones. In fact, a marked increase in Pp-ERS1 transcripts was observed in AZ and, to a lesser extent, in NZ, 12 h after the induction of abscission, whereas no significant differences were detected in explants maintained in air (Fig. 4B). No effects were determined by the gas on Pp-ETR1 transcription (Fig. 4A).
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Effect of 1-MCP on Pp-ETR1 and Pp-ERS1 expression during fruit ripening
The application of 1-MCP effectively delayed fruit ripening. In fact, while the firmness of untreated fruits dropped from about 80 N to 6 N within 36 h after picking, in 1-MCP-treated fruits a similar value was reached only at 84 h (Fig. 5A). Expression analysis of Pp-ERS1 corroborated data on firmness evolution. In fact, Pp-ERS1 transcript accumulation appeared to be significantly delayed in 1-MCP-treated fruits compared with the untreated ones (Fig. 5C). The Pp-ETR1 transcript level was unaffected by treatment (Fig. 5D). The inhibitory effect of 1-MCP on Pp-ERS1 transcription was abolished 24 h after the end of treatment (36 h after picking). This correlates with a dramatic stimulation of ethylene evolution occurring 24 h after the end of the treatment (Fig. 5B, C). The full recovery of ripening that occurred at the end of the 1-MCP treatment was concomitant with the increase of Pp-ERS1 transcription, while that of Pp-ETR1 remained unaffected (Fig. 5A, C).
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Discussion |
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Two peach genes, Pp-ETR1 and Pp-ERS1, were isolated showing a similar organization to that of the corresponding gene in Arabidopsis (Chang et al., 1993; Hua et al., 1995). As observed in other species (Bleecker, 1999), these two genes belong to a multigene family. Southern analysis carried out with the specific probe for Pp-ERS1 at high stringency conditions (minimum required level of homology >85%) suggests that, in peach, another gene related to Pp-ERS1 exists. The presence of two genes encoding ERS-type protein has only been reported in Arabidopsis (Hua et al., 1998). The deduced proteins of both genes contained a sensor domain and a histidine kinase domain, in which residues thought to be important for the normal function of ETR- and ERS-type protein as ethylene receptors were conserved (Bleecker, 1999). These results indicate that Pp-ETR1 and Pp-ERS1 could be putative ethylene receptors with the ability to bind ethylene. The main difference between Pp-ETR1 and Pp-ERS1, as already observed for other ETR1- and ERS1-type proteins, is due to the lack of the receiver response domain on Pp-ERS1. Clark et al. (1998) pointed out that if the receiver domain is deleted from AT-ETR1 protein, binding affinity between this protein and At-CTR1 is weakened. Studies carried out in tomato, using a construct containing an antisense for the receiver domain and 3' untranslated region portion of Le-ETR1 demonstrated that down-regulation of this gene transcript results in altered phenotypes determined by the activation of alternative transductive pathways (Whitelaw et al., 2002). Our data showed that, inside the fifth intron of Pp-ETR1, located before the exon encoding the receiver domain, two polyadenylation sites that are potentially responsible for an incomplete splicing at the 3' terminus are present. Normal and truncated transcripts of Pp-ETR1 have been reported by Bassett and Carole (1999), using an RT-PCR technique throughout peach fruit development and ripening. Thus, two different ETR1 polypeptides having different interactions with the downstream effectors might be present in peach fruit.
Quantitative RT-PCR data indicated that Pp-ETR1 and Pp-ERS1 transcripts are differentially expressed in leaf and fruitlet AZ, as well as in immature and ripe fruit. As reported by Bassett and Carole (1999) Pp-ETR1 appeared to be constitutive and ethylene-independent during fruit development and ripening. A similar behaviour has been observed for an ETR-type gene in tomato fruit (Zhou et al., 1996) and in passion fruit (Mita et al., 1998), while transcripts increased in ripe mango (Martínez et al., 2001), muskmelon (Sato-Nara et al., 1999) and tomato (Le-ETR4 and Le-ETR5, (Tieman and Klee, 1999) during ripening. These results probably reflect the fact that different ETR-type genes were used as probes for expression analysis. Pp-ERS1 transcripts increased during fruit ripening and its expression appeared to be up-regulated by propylene treatment. The propylene effect was dramatic in fruitlet and leaf AZs: the gas treatment induced a 4-fold increase of Pp-ERS1 transcription compared with that detected in air-treated AZs. This correlates with the acceleration of abscission induced by propylene treatment as observed by Ruperti et al. (1998). Similarly to Pp-ERS1, increases of ERS-type transcripts have been observed during ripening in tomato (NR), muskmelon (Cm-ERS1) and passion (Pe-ERS2) fruits (Payton et al., 1996; Sato-Nara et al., 1999; Mita et al., 2002): these genes are up-regulated by ethylene and the accumulation of specific mRNA parallels the increase in tissue sensitivity to ethylene. These data are in agreement with the ethylene receptor-inhibition model proposed by Hackett et al. (2000) using an Nr antisense inhibition approach to restore normal ripening in the tomato Never ripe mutant. The correlation between ripening and expression of Pp-ERS1 has been confirmed by the experiment carried out with 1-MCP. The application of the inhibitor delayed fruit ripening, evaluated in terms of firmness decay and ethylene evolution, and concurrently down-regulated Pp-ERS1, while Pp-ETR1 transcription was unaffected. 1-MCP action was rapidly abolished after moving fruits to air, when a rapid stimulation of ethylene evolution and a concurrent increase of Pp-ERS1 mRNAs were observed. The full recovery of peach fruit ripening occurring at the end of the 1-MCP treatment might be imputed to the fast regeneration of the ERS1-type receptors as well as to the stimulation of hormone biosynthesis. This might explain why peach fruits, differently from those of other species, need continuous or intermittent exposure to 1-MCP for the complete suppression of the ripening syndrome, as reported by Mathooko et al. (2001).
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Acknowledgements |
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We are grateful to Dr Caren Chang (College of Life Sciences, University of Maryland) and Professor Giorgio Casadoro and Dr Livio Trainotti (Department of Biology, University of Padova) for the gift of Arabidopsis ETR1 cDNA and strawberry ERS cDNA, respectively. We also thank Professor Mario Pezzotti and Dr Giambattista Tornielli (Department of Science and Technology, University of Verona) for the assistance in the quantitative RT-PCR assay. The present study was supported by the Italian Ministry of University and Scientific and Technological Research (MURST-ex 40%).
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I. El-Sharkawy, W. S. Kim, A. El-Kereamy, S. Jayasankar, A. M. Svircev, and D. C. W. Brown Isolation and characterization of four ethylene signal transduction elements in plums (Prunus salicina L.) J. Exp. Bot., October 1, 2007; 58(13): 3631 - 3643. [Abstract] [Full Text] [PDF]
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H. Hayama, T. Shimada, H. Fujii, A. Ito, and Y. Kashimura Ethylene-regulation of fruit softening and softening-related genes in peach J. Exp. Bot., December 1, 2006; 57(15): 4071 - 4077. [Abstract] [Full Text] [PDF]
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V. D. Cin, M. Danesin, A. Boschetti, A. Dorigoni, and A. Ramina Ethylene biosynthesis and perception in apple fruitlet abscission (Malus domestica L. Borck) J. Exp. Bot., November 1, 2005; 56(421): 2995 - 3005. [Abstract] [Full Text] [PDF]
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L. Trainotti, A. Pavanello, and G. Casadoro Different ethylene receptors show an increased expression during the ripening of strawberries: does such an increment imply a role for ethylene in the ripening of these non-climacteric fruits? J. Exp. Bot., August 1, 2005; 56(418): 2037 - 2046. [Abstract] [Full Text] [PDF]
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 Y.-F. CHEN, N. ETHERIDGE, and G. E. SCHALLER Ethylene Signal Transduction Ann. Bot., May 1, 2005; 95(6): 901 - 915. [Abstract] [Full Text] [PDF]
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C. P. Yau, L. Wang, M. Yu, S. Y. Zee, and W. K. Yip Differential expression of three genes encoding an ethylene receptor in rice during development, and in response to indole-3-acetic acid and silver ions J. Exp. Bot., March 1, 2004; 55(397): 547 - 556. [Abstract] [Full Text] [PDF]
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