Characterization of two putative ethylene receptor genes expressed during peach fruit development and abscission

Editing of the starch branching enzyme gene SBE2 generates high-amylose storage roots in cassava

The production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme gene SBE2 was firstly achieved. High-amylose cassava (Manihot esculenta Crantz) is desirable for starch industrial applications and production of healthier processed food for human consumption. In this study, we report the production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme 2 (SBE2). Mutations in two targeted exons of SBE2 were identified in all regenerated plants; these mutations, which included nucleotide insertions, and short or long deletions in the SBE2 gene, were classified into eight mutant lines. Three mutants, M6, M7 and M8, with long fragment deletions in the second exon of SBE2 showed no accumulation of SBE2 protein. After harvest from the field, significantly higher amylose (up to 56% in apparent amylose content) and resistant starch (up to 35%) was observed in these mutants compared with the wild type, leading to darker blue coloration of starch granules after quick iodine staining and altered starch viscosity with a higher pasting temperature and peak time. Further ¹H-NMR analysis revealed a significant reduction in the degree of starch branching, together with fewer short chains (degree of polymerization [DP] 15-25) and more long chains (DP>25 and especially DP>40) of amylopectin, which indicates that cassava SBE2 catalyzes short chain formation during amylopectin biosynthesis. Transition from A- to B-type crystallinity was also detected in the starches. Our study showed that CRISPR/Cas9-mediated mutagenesis of starch biosynthetic genes in cassava is an effective approach for generating novel varieties with valuable starch properties for food and industrial applications.

The LcKNAT1-LcEIL2/3 Regulatory Module Is Involved in Fruitlet Abscission in Litchi

Large and premature organ abscission may limit the industrial development of fruit crops by causing serious economic losses. It is well accepted that ethylene (ET) is a strong inducer of organ abscission in plants. However, the mechanisms underlying the control of organ abscission by ET are largely unknown. We previously revealed that LcKNAT1, a KNOTTED-LIKE FROM ARABIDOPSIS THALIANA1 (KNAT1)-like protein, acted as a negative regulator in control of fruitlet abscission through suppressing the expression of ET biosynthetic genes in litchi. In this study, we further reported that LcKNAT1 could also directly repress the transcription of LcEIL2 and LcEIL3, two ETHYLENE INSENSITIVE 3-like (EIL) homologs in litchi, which functioned as positive regulators in ET-activated fruitlet abscission by directly promoting the expression of genes responsible for ET biosynthesis and cell wall degradation. The expression level of LcKNAT1 was downregulated, while LcEIL2/3 was upregulated at the abscission zone (AZ) accompanying the fruitlet abscission in litchi. The results of electrophoretic mobility shift assays (EMSAs) and transient expression showed that LcKNAT1 could directly bind to the promoters of LcEIL2 and LcEIL3 and repress their expression. Furthermore, the genetic cross demonstrated that the β-glucuronidase (GUS) expression driven by the promoters of LcEIL2 or LcEIL3 at the floral AZ was obviously suppressed by LcKNAT1 under stable transformation in Arabidopsis. Taken together, our findings suggest that the LcKNAT1-LcEIL2/3 regulatory module is likely involved in the fruitlet abscission in litchi, and we propose that LcKNAT1 could suppress both ET biosynthesis and signaling to regulate litchi fruit abscission.

Transcriptomics of Improved Fruit Retention by Hexanal in 'Honeycrisp' Reveals Hormonal Crosstalk and Reduced Cell Wall Degradation in the Fruit Abscission Zone

Apples (Malus domestica Borkh) are prone to preharvest fruit drop, which is more pronounced in 'Honeycrisp'. Hexanal is known to improve fruit retention in several economically important crops. The effects of hexanal on the fruit retention of 'Honeycrisp' apples were assessed using physiological, biochemical, and transcriptomic approaches. Fruit retention and fruit firmness were significantly improved by hexanal, while sugars and fresh weight did not show a significant change in response to hexanal treatment. At commercial maturity, abscisic acid and melatonin levels were significantly lower in the treated fruit abscission zone (FAZ) compared to control. At this stage, a total of 726 differentially expressed genes (DEGs) were identified between treated and control FAZ. Functional classification of the DEGs showed that hexanal downregulated ethylene biosynthesis genes, such as S-adenosylmethionine synthase (SAM2) and 1-aminocyclopropane-1-carboxylic acid oxidases (ACO3, ACO4, and ACO4-like), while it upregulated the receptor genes ETR2 and ERS1. Genes related to ABA biosynthesis (FDPS and CLE25) were also downregulated. On the contrary, key genes involved in gibberellic acid biosynthesis (GA20OX-like and KO) were upregulated. Further, hexanal downregulated the expression of genes related to cell wall degrading enzymes, such as polygalacturonase (PG1), glucanases (endo-β-1,4-glucanase), and expansins (EXPA1-like, EXPA6, EXPA8, EXPA10-like, EXPA16-like). Our findings reveal that hexanal reduced the sensitivity of FAZ cells to ethylene and ABA. Simultaneously, hexanal maintained the cell wall integrity of FAZ cells by regulating genes involved in cell wall modifications. Thus, delayed fruit abscission by hexanal is most likely achieved by minimizing ABA through an ethylene-dependent mechanism.

Transcriptome Analysis of Pre-Storage 1-MCP and High CO2-Treated 'Madoka' Peach Fruit Explains the Reduction in Chilling Injury and Improvement of Storage Period by Delaying Ripening

Cold storage of peach fruit at low temperatures may induce chilling injury (CI). Pre-storage 1-MCP and high CO₂ treatments were reported among the methods to ameliorate CI and reduce softening of peach fruit. However, molecular data indicating the changes associated with pre-storage 1-MCP and high CO₂ treatments during cold storage of peach fruit are insufficient. In this study, a comparative analysis of the difference in gene expression and physico-chemical properties of fruit at commercial harvest vs. stored fruit for 12 days at 0 °C (cold-stored (CS), pre-storage 1-MCP+CS, and pre-storage high CO₂+CS) were used to evaluate the variation among treatments. Several genes were differentially expressed in 1-MCP+CS- and CO₂+CS-treated fruits as compared to CS. Moreover, the physico-chemical and sensory data indicated that 1-MCP+CS and CO₂+CS suppressed CI and delayed ripening than the CS, which could lead to a longer storage period. We also identified the list of genes that were expressed commonly and exclusively in the fruit treated by 1-MCP+CS and CO₂+CS and compared them to the fruit quality parameters. An attempt was also made to identify and categorize genes related to softening, physiological changes, and other ripening-related changes. Furthermore, the transcript levels of 12 selected representative genes from the differentially expressed genes (DEGs) in the transcriptome analysis were confirmed via quantitative real-time PCR (qRT-PCR). These results add information on the molecular mechanisms of the pre-storage treatments during cold storage of peach fruit. Understanding the genetic response of susceptible cultivars such as ‘Madoka’ to CI-reducing pre-storage treatments would help breeders release CI-resistant cultivars and could help postharvest technologists to develop more CI-reducing technologies.

Isolation of the ZmERS4 Gene From Maize and Its Functional Analysis in Transgenic Plants

A gene encoding a protein similar to ethylene receptor was isolated from maize (Zea mays L.), which was named as ZmERS4.The gene was 1,905 bp in length with an open reading frame that encoded a protein consisting of 634 amino acids. The homologous analysis showed that ZmERS4 shared high similarity with the ethylene receptor protein, OsERS1, from rice (Oryza sativa L.). ZmERS4 grouped into the ETR1 subfamily of ethylene receptors based on its conserved domain and phylogenetic status. Tissue-specific and induced expression analyses indicated that ZmERS4 was differentially expressed in maize tissues, predominantly in the stems and leaves, and was induced by salicylic acid (SA). Overexpression of ZmERS4 in Arabidopsis improved resistance against the bacterial pathogen, PstDC3000, by inducing the expression of SA signaling-related genes. Moreover, treatment with flg22 induced the expression of the defense-related gene, PR1, in maize protoplasts that transiently expressed ZmERS4. Furthermore, the ultra-high-performance liquid chromatography (UPLC) analysis showed that the SA contents in ZmERS4-overexpressing Arabidopsis lines were significantly higher than the control lines. Additionally, the improved resistance of ZmERS4-overexpressing Arabidopsis against PstDC3000 was blocked after pretreatment with the SA biosynthetic inhibitor, ABT. Based on the collective findings, we hypothesize that ZmERS4 plays an important role in disease resistance through SA-mediated signaling pathways.

Varied Expression of Senescence-Associated and Ethylene-Related Genes during Postharvest Storage of Brassica Vegetables

The genus Brassica comprises a highly diverse range of vegetable crops varying in morphology, harvestable crop product, and postharvest shelf-life that has arisen through domestication, artificial selection and plant breeding. Previous postharvest studies on the shelf-life of Brassica species has mainly focused on the variable rates of physiological changes including respiration and transpiration. Therefore, further understanding of the molecular basis of postharvest senescence in Brassica vegetables is needed to understand its progression in improving their postharvest shelf-life. The aim of this study was to better understand the trajectory of molecular responses in senescence-associated genes but not induced by ethylene and ethylene-induced genes towards altered postharvest storage conditions. After storage at different temperatures, the expression levels of the key senescence-associated genes (SAGs) and the ethylene biosynthesis, perception, and signaling genes were quantitatively analyzed in cabbage, broccoli and kale. The expression levels of these genes were tightly linked to storage temperature and phase of senescence. Expression of ORE15, SAG12, and NAC29 were continuously increased during the twelve days of postharvest storage at room temperature. Prolonged exposure of these three vegetables to cold temperature reduced the variation in the expression levels of ORE15 and SAG12, observed as mostly decreased which resulted in limiting senescence. The transcript levels of the ethylene receptor were also decreased at lower temperature, further suggesting that decreased ethylene biosynthesis and signaling in cabbage during postharvest storage would delay the senescence mechanism. These results enhanced our understanding of the transcriptional changes in ethylene-independent SAGs and ethylene-related genes in postharvest senescence, as well as the timing and temperature sensitive molecular events associated with senescence in cabbage, broccoli and kale and this knowledge can potentially be used for the improvement of postharvest storage in Brassica vegetables.

Molecular Bases of Fruit Quality in Prunus Species: An Integrated Genomic, Transcriptomic, and Metabolic Review with a Breeding Perspective

In plants, fruit ripening is a coordinated developmental process that requires the change in expression of hundreds to thousands of genes to modify many biochemical and physiological signal cascades such as carbohydrate and organic acid metabolism, cell wall restructuring, ethylene production, stress response, and organoleptic compound formation. In Prunus species (including peaches, apricots, plums, and cherries), fruit ripening leads to the breakdown of complex carbohydrates into sugars, fruit firmness reductions (softening by cell wall degradation and cuticle properties alteration), color changes (loss of green color by chlorophylls degradation and increase in non-photosynthetic pigments like anthocyanins and carotenoids), acidity decreases, and aroma increases (the production and release of organic volatile compounds). Actually, the level of information of molecular events at the transcriptional, biochemical, hormonal, and metabolite levels underlying ripening in Prunus fruits has increased considerably. However, we still poorly understand the molecular switch that occurs during the transition from unripe to ripe fruits. The objective of this review was to analyze of the molecular bases of fruit quality in Prunus species through an integrated metabolic, genomic, transcriptomic, and epigenetic approach to better understand the molecular switch involved in the ripening process with important consequences from a breeding point of view.

Meta-analysis of the effects of 1-methylcyclopropene (1-MCP) treatment on climacteric fruit ripening

1-Methylcyclopropene (1-MCP) is an inhibitor of ethylene perception that is widely used to maintain the quality of several climacteric fruits during storage. A large body of literature now exists on the effects of 1-MCP on climacteric fruit ripening for different species and environmental conditions, presenting an opportunity to use meta-analysis to systematically dissect these effects. We classified 44 ripening indicators of climacteric fruits into five categories: physiology and biochemistry, quality, enzyme activity, color, and volatiles. Meta-analysis showed that 1-MCP treatment reduced 20 of the 44 indicators by a minimum of 22% and increased 6 indicators by at least 20%. These effects were associated with positive effects on delaying ripening and maintaining quality. Of the seven moderating variables, species, 1-MCP concentration, storage temperature and time had substantial impacts on the responses of fruit to 1-MCP treatment. Fruits from different species varied in their responses to 1-MCP, with the most pronounced responses observed in rosaceous fruits, especially apple, European pear fruits, and tropical fruits. The effect of gaseous 1-MCP was optimal at 1 μl/l, with a treatment time of 12-24 h, when the storage temperature was 0 °C for temperate fruits or 20 °C for tropical fruits, and when the shelf temperature was 20 °C, reflecting the majority of experimental approaches. These findings will help improve the efficacy of 1-MCP application during the storage of climacteric fruits, reduce fruit quality losses and increase commercial value.

LcEIL2/3 are involved in fruitlet abscission via activating genes related to ethylene biosynthesis and cell wall remodeling in litchi

Fruit crops are subject to precocious fruit abscission, during which the phytohormone ethylene (ET) acts as a major positive regulator. However, the molecular basis of ET-induced fruit abscission remains poorly understood. Here, we show that two ETHYLENE INSENSITIVE 3-like (EIL) homologs in litchi, LcEIL2 and LcEIL3, play a role in ET-activated fruitlet abscission. LcEIL2/3 were significantly upregulated in the fruit abscission zone (AZ) during the ET-induced fruitlet abscission in litchi. The presence of LcEIL2/3 in wild-type Arabidopsis and ein3 eil1 mutants can accelerate the floral organ abscission. Moreover, the electrophoretic mobility shift assay and dual luciferase reporter analysis illustrated that LcEIL2/3 directly interacted with the gene promoters to activate the expression of cell wall remodeling genes LcCEL2/8 and LcPG1/2, and ET biosynthetic genes LcACS1/4/7 and LcACO2/3. Furthermore, we showed that LcPG1/2 were expressed in the floral abscission zone of Arabidopsis, and constitutive expression of LcPG2 in Arabidopsis promoted the floral organ abscission. In conclusion, we propose that LcEIL2/3 are involved in ET-induced fruitlet abscission via controlling expression of genes related to ET biosynthesis and cell wall remodeling in litchi.

The Role of 1-Methylcyclopropene in the regulation of ethylene biosynthesis and ethylene receptor gene expression in Mangifera indica L. (Mango Fruit)

Mango (Mangifera indica L.) is respiratory climacteric fruit that ripens and decomposes quickly following their harvest. 1-methylcyclopropene (1-MCP) is known to affect the ripening of fruit, delaying the decay of mango stored under ambient conditions. The objective of this study was to clarify the role of 1-MCP in the regulation of ethylene biosynthesis and ethylene receptor gene expression in mango. 1-MCP significantly inhibited the 1-aminocyclopropane-1-carboxylic acid (ACC) content. The activity of ACC oxidase (ACO) increased on days 6, 8, and 10 of storage, whereas delayed ACC synthase (ACS) activity increased after day 4. The two homologous ethylene receptor genes, ETR1 and ERS1 (i.e., MiETR1 and MiERS1), were obtained and deposited in GenBank® (National Center for Biotechnology Information-National Institutes of Health [NCBI-NIH]) (KY002681 and KY002682). The MiETR1 coding sequence was 2,220 bp and encoded 739 amino acids (aa). The MiERS1 coding sequence was 1,890 bp and encoded 629 aa, similar to ERS1 in other fruit. The tertiary structures of MiETR1 and MiERS1 were also predicted. MiERS1 lacks a receiver domain and shares a low homology with MiETR1 (44%). The expression of MiETR1 and MiERS1 mRNA was upregulated as the storage duration extended and reached the peak expression on day 6. Treatment with 1-MCP significantly reduced the expression of MiETR1 on days 4, 6, and 10 and inhibited the expression of MiETR1 on days 2, 4, 6, and 10. These results indicated that MiETR1 and MiERS1 had important functions in ethylene signal transduction. Treatment with 1-MCP might effectively prevent the biosynthesis of ethylene, as well as ethylene-induced ripening and senescence. This study presents an innovative method for prolonging the storage life of mango after their harvest through the regulation of MiETR1 and MiERS1 expression.

Analysis of Metabolites and Gene Expression Changes Relative to Apricot (Prunus armeniaca L.) Fruit Quality During Development and Ripening

Apricot (Prunus armeniaca L.) is a valuable worldwide agronomical crop, with a delicious fruit highlighted as a functional food with both nutritional and bioactive properties, remarkably beneficial to human health. Apricot fruit ripening is a coordinated developmental process which requires change in the expression of hundreds to thousands of genes to modify many biochemical and physiological processes arising from quality characteristics in ripe fruit. In addition, enhancing fruit and nutraceutical quality is one of the central objectives to be improved in the new varieties developed by breeding programs. In this study we analyzed the contents of main metabolites linked to the nutraceutical value of apricot fruits, together with the most important pomological characteristics and biochemical contents of fruit during the ripening process in two contrasted apricot genotypes. Additionally, the gene expression changes were analyzed using RNA-Seq and real time qPCR. Results showed that genes with differential expression in the biosynthetic pathways, such as phenylpropanoids, flavonoids, starch and sucrose and carotenoid metabolism, could be possible candidates as molecular markers of fruit quality characteristics for fruit color and soluble solid content. The gene involves in carotenoid metabolism carotenoid cleavage dioxygenase 4, and the gene sucrose synthase in starch and sucrose metabolism were identified as candidate genes in the ripening process for white skin ground color and flesh color and high soluble sugar content. The application of these candidate genes on marker-assisted selection in apricot breeding programs may contribute to the early selection of high-quality fruit genotypes with suitable nutraceutical values.

Double-sided battle: The role of ethylene during Monilinia spp. infection in peach at different phenological stages

Controversy exists on whether ethylene is involved in determining fruit resistance or susceptibility against biotic stress. In this work, the hypothesis that ethylene biosynthesis in peaches at different phenological stages may be modulated by Monilinia spp. was tested. To achieve this, at 49 and 126 d after full bloom (DAFB), ethylene biosynthesis of healthy and infected 'Merryl O'Henry' peaches with three strains of Monilinia spp. (M. fructicola (CPMC6) and M. laxa (CPML11 and ML8L) that differ in terms of aggressiveness) was analysed at the biochemical and molecular level along the course of infection in fruit stored at 20 °C. At 49 DAFB, results evidenced that infected fruit showed inhibition of ethylene production in comparison with non-inoculated fruit, suggesting that the three Monilinia strains were somehow suppressing ethylene biosynthesis to modify fruit defences to successfully infect the host. On the contrary, at 126 DAFB ethylene production increased concomitantly with brown rot spread, and values for non-inoculated fruit were almost undetectable throughout storage at 20 °C. The expression of several target genes involved in the ethylene biosynthetic pathway confirmed that they were differentially expressed upon Monilinia infection, pointing to a strain-dependent regulation. Notably, Prunus persica 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) (PpACS) family was the most over-expressed over time, demonstrating a positive ethylene regulation, especially at 126 DAFB. At this phenological stage it was demonstrated the ability of Monilinia spp. to alter ethylene biosynthesis through PpACS1 and benefit from the consequences of an ethylene burst likely on cell wall softening. Overall, our results put forward that infection not only among different strains but also at each stage is achieved by different mechanisms, with ethylene being a key factor in determining peach resistance or susceptibility to brown rot.

Hormone balance in a climacteric plum fruit and its non-climacteric bud mutant during ripening

Hormone balance plays a crucial role in the control of fruit ripening. We characterized and compared hormone balance in two Japanese plum cultivars (Prunus salicina Lindl.), namely Santa Rosa, a climacteric type, and Sweet Miriam, its non-climacteric bud-sport mutant. We assessed hormonal changes in gene expression associated with hormone biosynthesis, perception and signaling during ripening on-the tree and throughout postharvest storage and in response to ethylene treatments. Non-climacteric fruit displayed lower ethylene levels than climacteric fruit at all stages and lower auxin levels during the initiation of ripening on-the-tree and during most of post-harvest storage. Moreover, 1-MCP-induced ethylene decrease also resulted in low auxin contents in Santa Rosa, supporting the role of auxin in climacteric fruit ripening. The differences in auxin contents between Santa Rosa and Sweet Miriam fruit could be the consequence of different routed auxin biosynthesis pathways as indicated by the significant negative correlations between clusters of auxin metabolism-associated genes. Ethylene induced increased ABA levels throughout postharvest storage in both ripening types. Overall, ripening of Santa Rosa and Sweet Miriam fruit are characterized by distinct hormone accumulation pathways and interactions.

Differential and reciprocal regulation of ethylene pathway genes regulates petal abscission in fragrant and non-fragrant roses

The fragrant rose, Rosa bourboniana, is highly sensitive to ethylene and shows rapid petal abscission (within 16-18 h) while the non-fragrant hybrid rose, R. hybrida, shows delayed abscission (50-52 h) due to reduced ethylene sensitivity. To understand the molecular basis governing these differences, all components of the ethylene pathway (biosynthesis/ receptor/signalling) were studied for expression during abscission. Transcript accumulation of most ethylene biosynthesis genes (ACS/ACO families) increased rapidly in petal abscission zones of R. bourboniana within 4-8 h of ethylene treatment. The expression of most receptor and signalling genes encoding CTRs, EIN2 and EIN3/EIL homologues also followed similar kinetics. Under natural field conditions where abscission takes longer, there was a temporal delay in transcript accumulation of most ethylene pathway genes while some biosynthesis genes (showing reduced ethylene sensitivity) were more strongly up-regulated by abscission cues. In contrast, in R. hybrida where even ethylene-induced abscission is considerably delayed, transcript accumulation of most ethylene biosynthesis and signalling genes was, surprisingly, reduced by ethylene and showed an opposite regulation compared to R. bourboniana. The results suggest that differential and reciprocal regulation of ethylene pathway is one of the major reasons for differences in petal abscission and vase-life between Rosa bourboniana and R. hybrida.

Characterization of the expression of the stress-responsive PpERS1 gene from peach and analysis of its promoter using transgenic tomato

The PpERS1 gene, which encodes an ethylene receptor and responds to abiotic and biotic stresses, was cloned from peach (Prunus persica L. Batsch cv Okubao). The genomic DNA sequence of PpERS1 comprises seven exons which are separated by six introns, interestingly alternative splicing of the first intron produced three different PpERS1 transcripts. In addition, a 2.8-kb sequence including the promoter of PpERS1 was isolated and analyzed by placing expressing of the GUS reporter gene under its control. Several putative cis-elements were identified in the promoter of PpERS1, including two ethylene-responsive elements (EREs), five W boxes, and four putative binding sites for MYB-type transcription factors. Deletion analysis indicated the presence of an enhancer element in the PpERS1 promoter. Temporal and spatial expression analysis of the PpERS1 promoter using histochemical GUS staining showed GUS activity in all tissues examined throughout the development of transgenic tomato plants. Exposure to various stresses caused similar changes in expression patterns in peach and transgenic tomato plants. Overall, our results suggested that PpERS1 gene might play important roles in response to multiple stresses via signal transduction mediated by ethylene receptors. The characterization of the PpERS1 promoter contributes to our understanding of the transcriptional regulation of this ethylene receptor in peach.

Expression and interaction of the mango ethylene receptor MiETR1 and different receptor versions of MiERS1

Different versions of the mango ethylene receptor MiERS1 were identified and the analysis indicates that, in addition to MiERS1, two short versions of this receptor (MiERS1m, MiERS1s), representing truncated proteins with central deletions of functional domains, are present in mango. The short receptor versions reveal a different expression pattern compared to MiERS1, and they are highly variably transcribed. With transient expression assays using fluorescent fusion proteins, the localisation and the interaction of the receptors were determined in leaf cells of the tobacco model. MiERS1, MiETR1, and the short MiERS1 receptor versions are anchored in the endoplasmic reticulum (ER) membrane and co-localise with each other and with an ER-marker. Furthermore, ectopic expression of the mango receptors appears to induce a re-organisation of the ER resulting in accumulation of ER bodies. Interaction assays suggest that both short MiERS1 receptor versions can bind to proteins located in the ER. Bi-molecular fluorescence complementation (BiFC) assays indicate, that MiERS1m may dimerise with itself and can also interact with MiERS1, but not with MiETR1. Further, it as found that MiETR1 can interact with MiERS1. Interaction of MiERS1s with the other ethylene receptors could not be detected, although it was located in the ER membrane system.

On the role of ethylene, auxin and a GOLVEN-like peptide hormone in the regulation of peach ripening

BACKGROUND

In melting flesh peaches, auxin is necessary for system-2 ethylene synthesis and a cross-talk between ethylene and auxin occurs during the ripening process. To elucidate this interaction at the transition from maturation to ripening and the accompanying switch from system-1 to system-2 ethylene biosynthesis, fruits of melting flesh and stony hard genotypes, the latter unable to produce system-2 ethylene because of insufficient amount of auxin at ripening, were treated with auxin, ethylene and with 1-methylcyclopropene (1-MCP), known to block ethylene receptors. The effects of the treatments on the different genotypes were monitored by hormone quantifications and transcription profiling.

RESULTS

In melting flesh fruit, 1-MCP responses differed according to the ripening stage. Unexpectedly, 1-MCP induced genes also up-regulated by ripening, ethylene and auxin, as CTG134, similar to GOLVEN (GLV) peptides, and repressed genes also down-regulated by ripening, ethylene and auxin, as CTG85, a calcineurin B-like protein. The nature and transcriptional response of CTG134 led to discover a rise in free auxin in 1-MCP treated fruit. This increase was supported by the induced transcription of CTG475, an IAA-amino acid hydrolase. A melting flesh and a stony hard genotype, differing for their ability to synthetize auxin and ethylene amounts at ripening, were used to study the fine temporal regulation and auxin responsiveness of genes involved in the process. Transcriptional waves showed a tight interdependence between auxin and ethylene actions with the former possibly enhanced by the GLV CTG134. The expression of genes involved in the regulation of ripening, among which are several transcription factors, was similar in the two genotypes or could be rescued by auxin application in the stony hard. Only GLV CTG134 expression could not be rescued by exogenous auxin.

CONCLUSIONS

1-MCP treatment of peach fruit is ineffective in delaying ripening because it stimulates an increase in free auxin. As a consequence, a burst in ethylene production speeding up ripening occurs. Based on a network of gene transcriptional regulations, a model in which appropriate level of CTG134 peptide hormone might be necessary to allow the correct balance between auxin and ethylene for peach ripening to occur is proposed.

Gene Expression Analysis of Plum pox virus (Sharka) Susceptibility/Resistance in Apricot (Prunus armeniaca L.)

RNA-Seq has proven to be a very powerful tool in the analysis of the Plum pox virus (PPV, sharka disease)/Prunus interaction. This technique is an important complementary tool to other means of studying genomics. In this work an analysis of gene expression of resistance/susceptibility to PPV in apricot is performed. RNA-Seq has been applied to analyse the gene expression changes induced by PPV infection in leaves from two full-sib apricot genotypes, “Rojo Pasión” and “Z506-7”, resistant and susceptible to PPV, respectively. Transcriptomic analyses revealed the existence of more than 2,000 genes related to the pathogen response and resistance to PPV in apricot. These results showed that the response to infection by the virus in the susceptible genotype is associated with an induction of genes involved in pathogen resistance such as the allene oxide synthase, S-adenosylmethionine synthetase 2 and the major MLP-like protein 423. Over-expression of the Dicer protein 2a may indicate the suppression of a gene silencing mechanism of the plant by PPV HCPro and P1 PPV proteins. On the other hand, there were 164 genes involved in resistance mechanisms that have been identified in apricot, 49 of which are located in the PPVres region (scaffold 1 positions from 8,050,804 to 8,244,925), which is responsible for PPV resistance in apricot. Among these genes in apricot there are several MATH domain-containing genes, although other genes inside (Pleiotropic drug resistance 9 gene) or outside (CAP, Cysteine-rich secretory proteins, Antigen 5 and Pathogenesis-related 1 protein; and LEA, Late embryogenesis abundant protein) PPVres region could also be involved in the resistance.

Ethephon induced abscission in mango: physiological fruitlet responses

Fruitlet abscission of mango is typically very severe, causing considerable production losses worldwide. Consequently, a detailed physiological and molecular characterization of fruitlet abscission in mango is required to describe the onset and time-dependent course of this process. To identify the underlying key mechanisms of abscission, ethephon, an ethylene releasing substance, was applied at two concentrations (600 and 7200 ppm) during the midseason drop stage of mango. The abscission process is triggered by ethylene diffusing to the abscission zone where it binds to specific receptors and thereby activating several key physiological responses at the cellular level. The treatments reduced significantly the capacity of polar auxin transport through the pedicel at 1 day after treatment and thereafter when compared to untreated pedicels. The transcript levels of the ethylene receptor genes MiETR1 and MiERS1 were significantly upregulated in the pedicel and pericarp at 1, 2, and 3 days after the ethephon application with 7200 ppm, except for MiETR1 in the pedicel, when compared to untreated fruitlet. In contrast, ethephon applications with 600 ppm did not affect expression levels of MiETR1 in the pedicel and of MiERS1 in the pericarp; however, MiETR1 in the pericarp at day 2 and MiERS1 in the pedicel at days 2 and 3 were significantly upregulated over the controls. Moreover, two novel short versions of the MiERS1 were identified and detected more often in the pedicel of treated than untreated fruitlets at all sampling times. Sucrose concentration in the fruitlet pericarp was significantly reduced to the control at 2 days after both ethephon treatments. In conclusion, it is postulated that the ethephon-induced abscission process commences with a reduction of the polar auxin transport capacity in the pedicel, followed by an upregulation of ethylene receptors and finally a decrease of the sucrose concentration in the fruitlets.

Cross-talk between environmental stresses and plant metabolism during reproductive organ abscission

In plants, flowering is a crucial process for reproductive success and continuity of the species through time. Fruit production requires the perfect development of reproductive structures. Abscission, a natural process, can occur to facilitate shedding of no longer needed, infected, or damaged organs. If stress occurs during flower development, abscission can intervene at flower level, leading to reduced yield. Flower abscission is a highly regulated developmental process simultaneously influenced and activated in response to exogenous (changing environmental conditions, interactions with microorganisms) and endogenous (physiological modifications) stimuli. During climate change, plant communities will be more susceptible to environmental stresses, leading to increased flower and fruit abscission, and consequently a decrease in fruit yield. Understanding the impacts of stress on the reproductive phase is therefore critical for managing future agricultural productivity. Here, current knowledge on flower/fruit abscission is summarized by focusing specifically on effects of environmental stresses leading to this process in woody plants. Many of these stresses impair hormonal balance and/or carbohydrate metabolism, but the exact mechanisms are far from completely known. Hormones are the abscission effectors and the auxin/ethylene balance is of particular importance. The carbohydrate pathway is the result of complex regulatory processes involving the balance between photosynthesis and mobilization of reserves. Hormones and carbohydrates together participate in complex signal transduction systems, especially in response to stress. The available data are discussed in relation to reproductive organ development and the process of abscission.

Analysis of gene expression changes in peach leaves in response to Plum pox virus infection using RNA-Seq

Differences in gene expression were studied after Plum pox virus (PPV, sharka disease) infection in peach GF305 leaves with and without sharka symptoms using RNA-Seq. For each sample, more than 80% of 100-nucleotide paired-end (PE) Illumina reads were aligned on the peach reference genome. In the symptomatic sample, a significant proportion of reads were mapped to PPV reference genomes (1.04% compared with 0.00002% in non-symptomatic leaves), allowing for the ultra-deep assembly of the complete genome of the PPV isolate used (9775 nucleotides, missing only 11 nucleotides at the 5' genome end). In addition, significant alternative splicing events were detected in 359 genes and 12 990 single nucleotide polymorphisms (SNPs) were identified, 425 of which could be annotated. Gene ontology annotation revealed that the high-ranking mRNA target genes associated with the expression of sharka symptoms are mainly related to the response to biotic stimuli, to lipid and carbohydrate metabolism and to the negative regulation of catalytic activity. A greater number of differentially expressed genes were observed in the early asymptomatic phase of PPV infection in comparison with the symptomatic phase. These early infection events were associated with the induction of genes related to pathogen resistance, such as jasmonic acid, chitinases, cytokinin glucosyl transferases and Lys-M proteins. Once the virus had accumulated, the overexpression of Dicer protein 2a genes suggested a gene silencing plant response that was suppressed by the virus HCPro and P1 proteins. These results illustrate the dynamic nature of the peach-PPV interaction at the transcriptome level and confirm that sharka symptom expression is a complex process that can be understood on the basis of changes in plant gene expression.

Identification and expression analysis of ethylene biosynthesis and signaling genes provides insights into the early and late coffee cultivars ripening pathway

The plant hormone ethylene is involved in the regulation of a multitude of plant processes, ranging from seed germination to organ senescence. Ethylene induces fruit ripening in climacteric fruits, such as coffee, being directly involved in fruit ripening time and synchronization. Coffee early cultivars usually show a more uniform ripening process although little is known about the genetic factors that promote the earliness of ripening. Thus, this work aimed to characterize the putative members of the coffee (Coffea arabica) ethylene biosynthesis and signaling pathways, as well as to analyze the expression patterns of these members during fruit ripening of early (Catucaí 785-15) and late (Acauã) coffee cultivars. Reverse Transcription-qPCR analysis of the four biosynthesis genes (CaACS1-like; CaACO1-like; CaACO4-like e CaACO5-like) analyzed in this study showed that CaACO1-like and CaACO4-like displayed an expression pattern typically observed in climacteric fruits, being up-regulated during ripening. CaACS1-like gene expression was also up-regulated during fruit ripening of both cultivars, although in a much lesser extent when compared to the changes in CaACO1-like and CaACO4-like gene expression. CaACO5-like was only induced in raisin fruit and may be related to senescence processes. On the other hand, members of the ethylene signaling pathway (CaETR1-like, CaETR4-like, CaCTR2-like, CaEIN2-like, CaEIN3-like, CaERF1) showed slightly higher expression levels during the initial stages of development (green and yellow-green fruits), except for the ethylene receptors CaETR1-like and CaETR4-like, which were constitutively expressed and induced in cherry fruits, respectively. The higher ethylene production levels in Catucaí 785-15 fruits, indicated by the expression analysis of CaACO1-like and CaACO4-like, suggest that it promotes an enhanced CaETR4-like degradation, leading to an increase in ethylene sensitivity and consequently to an earliness in the ripening process of this cultivar. Ethylene production in Acauã fruits may not be sufficient to inactivate the CaETR4-like levels and thus ripening changes occur in a slower pace. Thus, the expression analysis of the ethylene biosynthesis and signaling genes suggests that ethylene is directly involved in the determination of the ripening time of coffee fruits, and CaACO1-like, CaACO4-like and CaETR4-like may display essential roles during coffee fruit ripening.

Transcriptomic events involved in melon mature-fruit abscission comprise the sequential induction of cell-wall degrading genes coupled to a stimulation of endo and exocytosis

BACKGROUND

Mature-fruit abscission (MFA) in fleshy-fruit is a genetically controlled process with mechanisms that, contrary to immature-fruit abscission, has not been fully characterized. Here, we use pyrosequencing to characterize the transcriptomes of melon abscission zone (AZ) at three stages during AZ-cell separation in order to understand MFA control at an early stage of AZ-activation.

PRINCIPAL FINDINGS

The results show that by early induction of MFA, the melon AZ exhibits major gene induction, while by late induction of MFA, melon AZ shows major gene repression. Although some genes displayed similar regulation in both early and late induction of abscission, such as EXT1-EXT4, EGase1, IAA2, ERF1, AP2D15, FLC, MADS2, ERAF17, SAP5 and SCL13 genes, the majority had different expression patterns. This implies that time-specific events occur during MFA, and emphasizes the value of characterizing multiple time-specific abscission transcriptomes. Analysis of gene-expression from these AZs reveal that a sequential induction of cell-wall-degrading genes is associated with the upregulation of genes involved in endo and exocytosis, and a shift in plant-hormone metabolism and signaling genes during MFA. This is accompanied by transcriptional activity of small-GTPases and synthaxins together with tubulins, dynamins, V-type ATPases and kinesin-like proteins potentially involved in MFA signaling. Early events are potentially controlled by down-regulation of MADS-box, AP2/ERF and Aux/IAA transcription-factors, and up-regulation of homeobox, zinc finger, bZIP, and WRKY transcription-factors, while late events may be controlled by up-regulation of MYB transcription-factors.

SIGNIFICANCE

Overall, the data provide a comprehensive view on MFA in fleshy-fruit, identifying candidate genes and pathways associated with early induction of MFA. Our comprehensive gene-expression profile will be very useful for elucidating gene regulatory networks of the MFA in fleshy-fruit.

Ethylene and auxin biosynthesis and signaling are impaired by methyl jasmonate leading to a transient slowing down of ripening in peach fruit

Peach (Prunus persica) was chosen as a model to further clarify the physiological role of jasmonates (JAs) during fruit ripening. To this aim, the effect of methyl jasmonate (MJ, 0.88 mM), applied at a late stage (S3) of fruit development under field conditions (in planta), on the time-course of fruit ripening over a 14-day period was evaluated. As revealed by a non-destructive device called a DA-meter, exogenously applied MJ impaired the progression of ripening leading to less ripe fruit at harvest. To better understand the molecular basis of MJ interference with ripening, the time-course changes in the expression of ethylene-, cell wall-, and auxin-related genes as well as other genes (LOX, AOS and bZIP) was evaluated in the fruit mesocarp. Real-time PCR analyses revealed that transcript levels of ethylene-related genes were strongly affected. In a first phase (days 2 and/or 7) of the MJ response, mRNAs of the ethylene biosynthetic genes ACO1, ACS1 and the receptor gene ETR2 were strongly but transiently down-regulated, and then returned to or above control levels in a second phase (days 11 and/or 14). Auxin biosynthetic, conjugating, transport and perception gene transcripts were also affected. While biosynthetic genes (TRPB and IGPS) were up-regulated, auxin-conjugating (GH3), perception (TIR1) and transport (PIN1) genes were transiently but strongly down-regulated in a first phase, but returned to control levels subsequently. Transcript levels of two JA-related genes (LOX, AOS) and a developmentally regulated transcription factor (bZIP) were also affected, suggesting a shift ahead of the ripening process. Thus, in peach fruit, the transient slowing down of ripening by exogenous MJ was associated with an interference not only with ethylene but also with auxin-related genes.

Cloning and expression of ethylene receptor ERS1 at various developmental and ripening stages of mango fruit

Ethylene induces characteristic ripening reactions in climacteric fruits through its binding to histidine-kinase (HK) receptors, activating the expression of ripening genes. Ethylene receptors have been found in Arabidopsis thaliana (Brassicaceae) and some fruits; number and expression patterns differ among species. In mango, only ethylene receptor ETR1 was known. We cloned ERS1 cDNA from mango, and evaluated the expression of Mi-ERS1 and Mi-ETR1 by qPCR in developmental and ripening stages of this fruit. The Mi-ERS1 coding sequence is 1890 bp long and encodes 629 amino acids, similar to ERS1 from other fruits. Also, the amino acid sequence of ERS1 C-terminal HK domain shows the cognate fold after molecular modeling. Mi-ERS1 expression levels increased as mangoes ripened, showing the highest levels at the climacteric stage, while Mi-ETR1 levels did not change during development and ripening. We conclude that the patterns of expression of Mi-ERS1 and Mi-ETR1 differ in mango fruit.

Spermidine application to young developing peach fruits leads to a slowing down of ripening by impairing ripening-related ethylene and auxin metabolism and signaling

Peach (Prunus persica var. laevis Gray) was chosen to unravel the molecular basis underlying the ability of spermidine (Sd) to influence fruit development and ripening. Field applications of 1 mM Sd on peach fruit at an early developmental stage, 41 days after full bloom (dAFB), i.e. at late stage S1, led to a slowing down of fruit ripening. At commercial harvest (125 dAFB, S4II) Sd-treated fruits showed a reduced ethylene production and flesh softening. The endogenous concentration of free and insoluble conjugated polyamines (PAs) increased (0.3-2.6-fold) 1 day after treatment (short-term response) butsoon it declined to control levels; starting from S3/S4, when soluble conjugated forms increased (up to five-fold relative to controls at ripening), PA levels became more abundant in treated fruits, (long-term response). Real-time reverse transcription-polymerase chain reaction analyses revealed that peaks in transcript levels of fruit developmental marker genes were shifted ahead in accord with a developmental slowing down. At ripening (S4I-S4II) the upregulation of the ethylene biosynthetic genes ACO1 and ACS1 was dramatically counteracted by Sd and this led to a strong downregulation of genes responsible for fruit softening, such as PG and PMEI. Auxin-related gene expression was also altered both in the short term (TRPB) and in the long term (GH3, TIR1 and PIN1), indicating that auxin plays different roles during development and ripening processes. Messenger RNA amounts of other hormone-related ripening-regulated genes, such as NCED and GA2-OX, were strongly downregulated at maturity. Results suggest that Sd interferes with fruit development/ripening by interacting with multiple hormonal pathways.

Role of ethylene receptors during senescence and ripening in horticultural crops

The past two decades have been rewarding in terms of deciphering the ethylene signal transduction and functional validation of the ethylene receptor and downstream genes involved in the cascade. Our knowledge of ethylene receptors and its signal transduction pathway provides us a robust platform where we can think of manipulating and regulating ethylene sensitivity by the use of genetic engineering and making transgenic. This review focuses on ethylene perception, receptor mediated regulation of ethylene biosynthesis, role of ethylene receptors in flower senescence, fruit ripening and other effects induced by ethylene. The expression behavior of the receptor and downstream molecules in climacteric and non climacteric crops is also elaborated upon. Possible strategies and recent advances in altering the ethylene sensitivity of plants using ethylene receptor genes in an attempt to modulate the regulation and sensitivity to ethylene have also been discussed. Not only will these transgenic plants be a boon to post-harvest physiology and crop improvement but, it will also help us in discovering the mechanism of regulation of ethylene sensitivity.

Expression of ethylene response genes during persimmon fruit astringency removal

Thirteen ethylene signaling related genes were isolated and studied during ripening of non-astringent 'Yangfeng' and astringent 'Mopan' persimmon fruit. Some of these genes were characterized as ethylene responsive. Treatments, including ethylene and CO(2), had different effects on persimmon ripening, but overlapping roles in astringency removal, such as increasing the reduction in levels of soluble tannins. DkERS1, DkETR2, and DkERF8, may participate in persimmon fruit ripening and softening. The expression patterns of DkETR2, DkERF4, and DkERF5 had significant correlations with decreases in soluble tannins in 'Mopan' persimmon fruit, suggesting that these genes might be key components in persimmon fruit astringency removal and be the linkage between different treatments, while DkERF1 and DkERF6 may be specifically involved in CO(2) induced astringency removal. The possible roles of ethylene signaling genes in persimmon fruit astringency removal are discussed.

Polyamine-induced modulation of genes involved in ethylene biosynthesis and signalling pathways and nitric oxide production during olive mature fruit abscission

After fruit ripening, many fruit-tree species undergo massive natural fruit abscission. Olive (Olea europaea L.) is a stone-fruit with cultivars such as Picual (PIC) and Arbequina (ARB) which differ in mature fruit abscission potential. Ethylene (ET) is associated with abscission, but its role during mature fruit abscission remains largely uncharacterized. The present study investigates the possible roles of ET and polyamine (PA) during mature fruit abscission by modulating genes involved in the ET signalling and biosynthesis pathways in the abscission zone (AZ) of both cultivars. Five ET-related genes (OeACS2, OeACO2, OeCTR1, OeERS1, and OeEIL2) were isolated in the AZ and adjacent cells (AZ-AC), and their expression in various olive organs and during mature fruit abscission, in relation to interactions between ET and PA and the expression induction of these genes, was determined. OeACS2, OeACO2, and OeEIL2 were found to be the only genes that were up-regulated in association with mature fruit abscission. Using the inhibition of ET and PA biosynthesis, it is demonstrated that OeACS2 and OeEIL2 expression are under the negative control of PA while ET induces their expression in AZ-AC. Furthermore, mature fruit abscission depressed nitric oxide (NO) production present mainly in the epidermal cells and xylem of the AZ. Also, NO production was differentially responsive to ET, PA, and different inhibitors. Taken together, the results indicate that PA-dependent ET signalling and biosynthesis pathways participate, at least partially, during mature fruit abscission, and that endogenous NO and 1-aminocyclopropane-1-carboxylic acid maintain an inverse correlation, suggesting an antagonistic action of NO and ET in abscission signalling.

Ethylene and 1-methylcyclopropene differentially regulate gene expression during onion sprout suppression

Onion (Allium cepa) is regarded as a nonclimacteric vegetable. In onions, however, ethylene can suppress sprouting while the ethylene-binding inhibitor 1-methylcyclopropene (1-MCP) can also suppress sprout growth; yet, it is unknown how ethylene and 1-MCP elicit the same response. In this study, onions were treated with 10 μL L(-1) ethylene or 1 μL L(-1) 1-MCP individually or in combination for 24 h at 20°C before or after curing (6 weeks) at 20°C or 28°C and then stored at 1°C. Following curing, a subset of these same onions was stored separately under continuous air or ethylene (10 μL L(-1)) at 1°C. Onions treated with ethylene and 1-MCP in combination after curing for 24 h had reduced sprout growth as compared with the control 25 weeks after harvest. Sprout growth following storage beyond 25 weeks was only reduced through continuous ethylene treatment. This observation was supported by a higher proportion of down-regulated genes characterized as being involved in photosynthesis, measured using a newly developed onion microarray. Physiological and biochemical data suggested that ethylene was being perceived in the presence of 1-MCP, since sprout growth was reduced in onions treated with 1-MCP and ethylene applied in combination but not when applied individually. A cluster of probes representing transcripts up-regulated by 1-MCP alone but down-regulated by ethylene alone or in the presence of 1-MCP support this suggestion. Ethylene and 1-MCP both down-regulated a probe tentatively annotated as an ethylene receptor as well as ethylene-insensitive 3, suggesting that both treatments down-regulate the perception and signaling events of ethylene.

New approaches to Prunus transcriptome analysis

The recent sequencing of the complete genome of the peach offers new opportunities for further transcriptomic studies in Prunus species in the called post-genomics era. First works on transcriptome analysis in Prunus species started in the early 2000s with the development of ESTs (expressed sequence tags) and the analysis of several candidate genes. Later, new strategies of massive analysis (high throughput) of transcriptomes have been applied, producing larger amounts of data in terms of expression of a large number of genes in a single experiment. One of these systems is massive transcriptome analysis using cDNA biochips (microarrays) to analyze thousands of genes by hybridization of mRNA labelled with fluorescence. However, the recent emergence of a massive sequencing methodology ("deep-sequencing") of the transcriptome (RNA-Seq), based on lowering the costs of DNA (in this cases complementary, cDNA) sequencing, could be more suitable than the application of microarrays. Recent papers have described the tremendous power of this technology, both in terms of profiling coverage and quantitative accuracy in transcriptomic studies. Now this technology is being applied to plant species, including Prunus. In this work, we analyze the potential in using this RNA-Seq technology in the study of Prunus transcriptomes and the development of genomic tools. In addition, the strengths and limitations of RNA-Seq relative to microarray profiling have been discussed.

Non-climacteric fruit ripening in pepper: increased transcription of EIL-like genes normally regulated by ethylene

Only limited information has been published to date on the similarities and differences between climacteric and non-climacteric fruit ripening on transcriptional level. To address this issue, we performed a direct comparative transcriptome analysis between tomato and pepper fruits using heterologous microarray hybridization. Given the significant differences in the morphological, physiological, and biochemical characteristics of pepper and tomato fruits, the existence of extensive common regulons is surprising. This finding suggests the conservation of ripening mechanisms in climacteric and non-climacteric fruits. However, disparate expression profiles were also observed in both fruits. This study revealed that a gene that encodes an enzyme that converts lycopene to downstream carotenoids is induced in pepper but not in tomato. Most of the genes that encode ribosomal proteins are only induced in early fruit-stage pepper fruit and show rapidly diminishing expression in the later developmental stages. The genes involved in ethylene biosynthesis were not induced in pepper fruit. However, the EIL-like genes, ethylene-mediated signaling components, were induced in pepper fruit. Divergent types of transcription factors were expressed in ripening tomato and pepper fruits, suggesting they may be key factors that differentiate these distinct ripening processes.

Transcriptome profiling of ripening nectarine (Prunus persica L. Batsch) fruit treated with 1-MCP

A large-scale transcriptome analysis has been conducted using microPEACH1.0 microarray on nectarine (Prunus persica L. Batsch) fruit treated with 1-methylcyclopropene (1-MCP). 1-MCP maintained flesh firmness but did not block ethylene biosynthesis. Compared with samples at harvest, only nine genes appeared to be differentially expressed when fruit were sampled immediately after treatment, while a total of 90 targets were up- or down-regulated in untreated fruit. The effect of 1-MCP was confirmed by a direct comparison of transcript profiles in treated and untreated fruit after 24 h of incubation with 106 targets differentially expressed. About 30% of these targets correspond to genes involved in primary metabolism and response processes related to ethylene, auxin, and other hormones. In treated fruit, altered transcript accumulation was detected for some genes with a role in ripening-related events such as softening, colour development, and sugar metabolism. A rapid decrease in flesh firmness and an increase in ethylene production were observed in treated fruit maintained for 48 h in air at 20 degrees C after the end of the incubation period. Microarray comparison of this sample with untreated fruit 24 h after harvest revealed that about 45% of the genes affected by 1-MCP at the end of the incubation period changed their expression during the following 48 h in air. Among these genes, an ethylene receptor (ETR2) and three ethylene-responsive factors (ERF) were present, together with other transcription factors and ethylene-dependent genes involved in quality parameter changes.

Effect of exogenous ethylene on ACC content and ACC oxidase activity during ripening of Manila mangoes subjected to hot water treatment

Mangoes (Mangifera indica L.) 'Manila' were subjected to the USDA-approved hot water treatment and then exposed to synthetic air mixtures containing 0.5, 0.75 or 1 ml l(-1) of ethylene for 6, 12 or 18 h at 25 degrees C, to induce accelerated ripening. After treatment the mangoes were allowed to ripen in air at 24-25 degrees C. The content of 1-aminocyclopropane-1-carboxylic acid (ACC) and ACC oxidase (ACO) activity increased in fruit treated with 0.5 and 0.75 ml l(-1) of ethylene for 6 or 12 h. Ethylene production was reduced in fruit treated with 1 ml l(-1) of ethylene. This was due to the decreased of ACC synthesis rather than to lower ACC oxidase activity. Treatment with 0.5 ml l(-1) of ethylene for 12 h was found best for accelerate ripening; fruits were fully ripened and edible 3 days after treatment, compared to 6-7 days for untreated mangoes.

The involvement of auxin in the ripening of climacteric fruits comes of age: the hormone plays a role of its own and has an intense interplay with ethylene in ripening peaches

Ethylene has long been regarded as the main regulator of ripening in climacteric fruits. The characterization of a few tomato mutants, unable to produce climacteric ethylene and to ripen their fruits even following treatments with exogenous ethylene, has shown that other factors also play an important role in the control of climacteric fruit ripening. In climacteric peach and tomato fruits it has been shown that, concomitant with ethylene production, increases in the amount of auxin can also be measured. In this work a genomic approach has been used in order to understand if such an auxin increase is functional to an independent role played by the hormone during ripening of the climacteric peach fruits. Besides the already known indirect activity on ripening due to its up-regulation of climacteric ethylene synthesis, it has been possible to show that auxin plays a role of its own during ripening of peaches. In fact, the hormone has shown the ability to regulate the expression of a number of different genes. Moreover, many genes involved in biosynthesis and transport and, in particular, the signalling (receptors, Auxin Response Factors and Aux/IAA) of auxin had increased expression in the mesocarp during ripening, thus strengthening the idea that this hormone is actively involved in the ripening of peaches. This study has also demonstrated the existence of an important cross-talk between auxin and ethylene, with genes in the auxin domain regulated by ethylene and genes in the ethylene domain regulated by auxin.

A novel ethylene receptor homolog gene isolated from ethylene-insensitive flowers of gladiolus (Gladiolus grandiflora hort.)

Gladiolus is an ethylene insensitive flower whose exogenous ethylene and ethylene inhibitors have no effect on the petal senescence process. To study which processes in gladiolus are associated with changes in ethylene perception, two types of gladiolus genes, named GgERS1a and GgERS1b, respectively, homologous to the Arabidopsis ethylene receptor gene ERS1 were isolated. GgERS1a is conserved in terms of exon numbers and intron positions, whereas GgERS1b is almost same with GgERS1a except lacking 636 nucleotide encoding first and second histidine kinase (HisKA) motifs. The sequence data on full length genomic DNA indicated that both GgERS1a and b were spliced from different genomic DNA. As the result of mRNA expression study, in spite of lacking the two significant motifs, the expression of GgERS1b dramatically changed with advance in petal senescence, whereas the level of GgERS1a expressed highly and constitutively. The result suggests that both the genes possess a significant role for the subfunctionalization process to provide ethylene insensitivity in gladiolus flowers.

How ethylene works in the reproductive organs of higher plants: a signaling update from the third millennium

Ethylene (ET) is a notable signaling molecule in higher plants. In the year 1993 the ET receptor gene, ETR1, was identified; this ETR1 receptor protein being the first plant hormone receptor to be isolated. It is striking that there are six ET receptors in tomato instead of five in Arabidopsis, the two best-known signaling-model systems. Even though over the last few years great progress has been made in elucidating the genes and proteins involved in ET signaling, the complete pathway remains to be established. The present review examines the most representative successive advances that have taken place in this millennium in terms of the signaling pathway of ET, as well as the implications of the signaling in the reproductive organs of plants (i.e., flowers, fruits, seeds and pollen grains). A detailed comparative study is made on the advances in knowledge in the last decade, showing how the characterization of ET signaling provides clues for understanding how higher plants regulate their ET sensitivity. Also, it is indicated that ET signaling is at present sparking interest within phytohormonal molecular physiology and biology, and it is explained why several socio-economic aspects (flowering and fruit ripening) are undoubtedly involved in ET physiology.

The use of 1-methylcyclopropene (1-MCP) on fruits and vegetables

The recent availability of the inhibitor of ethylene perception, 1-methylcyclopropene (1-MCP), has resulted in an explosion of research on its effects on fruits and vegetables, both as a tool to further investigate the role of ethylene in ripening and senescence, and as a commercial technology to improve maintenance of product quality. The commercialization of 1-MCP was followed by rapid adoption by many apple industries around the world, and strengths and weaknesses of the new technology have been identified. However, use of 1-MCP remains limited for other products, and therefore it is still necessary to speculate on its commercial potential for most fruits and vegetables. In this review, the effects of 1-MCP on fruits and vegetables are considered from two aspects. First, a selected number of fruit (apple, avocado, banana, pear, peaches and nectarines, plums and tomato) are used to illustrate the range of responses to 1-MCP, and indicate possible benefits and limitations for commercialization of 1-MCP-based technology. Second, an outline of general physiological and biochemical responses of fruits and vegetables to the chemical is provided to illustrate the potential for use of 1-MCP to better understand the role of ethylene in ripening and senescence processes.

Transcription of ethylene perception and biosynthesis genes is altered by putrescine, spermidine and aminoethoxyvinylglycine (AVG) during ripening in peach fruit (Prunus persica)

The time course of ethylene biosynthesis and perception was investigated in ripening peach fruit (Prunus persica) following treatments with the polyamines putrescine (Pu) and spermidine (Sd), and with aminoethoxyvinylglycine (AVG). Fruit treatments were performed in planta. Ethylene production was measured by gas chromatography, and polyamine content by high-performance liquid chromatography; expression analyses were performed by Northern blot or real-time polymerase chain reaction. Differential increases in the endogenous polyamine pool in the epicarp and mesocarp were induced by treatments; in both cases, ethylene production, fruit softening and abscission were greatly inhibited. The rise in 1-aminocyclopropane-1-carboxylate oxidase (PpACO1) mRNA was counteracted and delayed in polyamine-treated fruit, whereas transcript abundance of ethylene receptors PpETR1 (ethylene receptor 1) and PpERS1 (ethylene sensor 1) was enhanced at harvest. Transcript abundance of arginine decarboxylase (ADC) and S-adenosylmethionine decarboxylase (SAMDC) was transiently reduced in both the epicarp and mesocarp. AVG, here taken as a positive control, exerted highly comparable effects to those of Pu and Sd. Thus, in peach fruit, increasing the endogenous polyamine pool in the epicarp or in the mesocarp strongly interfered, both at a biochemical and at a biomolecular level, with the temporal evolution of the ripening syndrome.

Ethylene biosynthesis and perception in apple fruitlet abscission (Malus domestica L. Borck)

Abscission was studied in immature apple fruits (cv. Golden Delicious) during the physiological drop. Fruitlet populations, characterized by different abscission potential, were analysed. Non-abscising fruitlets (NAF) were obtained from central flowers borne in clusters where all the lateral flowers had been removed at bloom while abscising fruitlets (AF) were derived from lateral fruitlets of trees sprayed with benzylaminopurine (BAP) at 200 ppm, 17 d after petal fall (APF), when the fruit cross diameter was about 10-12 mm. Fruit shedding, monitored at the end of the June drop, was significantly different in the two populations, being less than 10%, and more than 90%, in NAF and AF, respectively. In AF, fruit drop peaked around 33 d after petal fall (APF) and was preceded by an increase in ethylene around 20 d APF. Transcript analysis was performed from 17-24 d APF, since preliminary experiments pointed out that major changes in expression of abscission related genes occurred within this period. Transcript accumulation of genes involved in ethylene biosynthesis (MdACS5B and MdACO) and action (MdERS1, MdETR1, and MdCTR1) was studied in the seed, cortex, peduncle, and abscission zone (AZ) of the two fruit populations. MdACS5B and MdACO transcripts accumulated along the experimental period in AF population, even though at a different magnitude, while ethylene evolution declined after peaking at day three. MdETR1, MdERS1, and MdCTR1 expression patterns depended on tissue and/or population. The ERS/ETR ratio was higher in AF than in NAF populations. Overall results pointed out that apple fruitlet drop is preceded by a stimulation of ethylene biosynthesis and a gain in sensitivity to the hormone.

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?

Notwithstanding the economic importance of non-climacteric fruits like grape and strawberry, little is known about the mechanisms that regulate their ripening. Up to now no growth regulator has emerged with a primary role similar to that played by ethylene in the ripening of the climacteric fruits. Strawberries can produce ethylene, although in limited amounts. Two cDNAs coding for enzymes of the ethylene biosynthetic pathway (i.e. FaACO1 and FaACO2), and three cDNAs encoding different ethylene receptors have been isolated. Two receptors (i.e. FaEtr1 and FaErs1) belong to the type-I while the third (i.e. FaEtr2) belongs to the type-II group. The expression of both the ACO and the receptor-encoding genes has been studied in fruits at different stages of development and in fruits treated with hormones (i.e. ethylene and the auxin analogue NAA). All the data thus obtained have been correlated to the known data about ethylene production by strawberry fruits. Interestingly, a good correlation has resulted between the expression of the genes described in this work and the data of ethylene production. In particular, similarly to what occurs during climacteric fruit ripening, there is an increased synthesis of receptors concomitant with the increased synthesis of ethylene in strawberries as well. Moreover, the receptors mostly expressed in ripening strawberries are the type-II ones, that is those with a degenerate histidine-kinase domain. Since the latter domain is thought to establish a weaker link to the CTR1 proteins, even the little ethylene produced by ripening strawberries might be sufficient to trigger ripening-related physiological responses.

Ethylene signal transduction

BACKGROUND

The phytohormone ethylene is a key regulator of plant growth and development. Components of the pathway for ethylene signal transduction were identified by genetic approaches in Arabidopsis and have now been shown to function in agronomically important plants as well.

SCOPE

This review focuses on recent advances in our knowledge on ethylene signal transduction, in particular on recently proposed components of the pathway, on the interaction between the pathway components and on the roles of transcriptional and post-transcriptional regulation in ethylene signalling.

CONCLUSIONS

Data indicate that the site of ethylene perception is at the endoplasmic reticulum and point to the importance of protein complexes in mediating the initial steps in ethylene signal transduction. The expression level of pathway components is regulated by both transcriptional and post-transcriptional mechanisms, degradation of the transcription factor EIN3 being a primary means by which the sensitivity of plants to ethylene is regulated. EIN3 also represents a control point for cross-talk with other signalling pathways, as exemplified by the effects of glucose upon its expression level. Amplification of the initial ethylene signal is likely to play a significant role in signal transduction and several mechanisms exist by which this may occur based on properties of known pathway components. Signal output from the pathway is mediated in part by carefully orchestrated changes in gene expression, the breadth of these changes now becoming clear through expression analysis using microarrays.