Ethylene biosynthesis in detached young persimmon fruit is initiated in calyx and modulated by water loss from the fruit

Physiological Changes and Transcriptomic Analysis throughout On-Tree Fruit Ripening Process in Persimmon (Diospyros kaki L.)

The involvement of effectors and transcriptional regulators in persimmon fruit maturation has been mostly approached by the literature under postharvest conditions. In order to elucidate the participation of these genes in the on-tree fruit maturation development, we have collected samples from seven persimmon germplasm accessions at different developmental stages until physiological maturation. This study has focused on the expression analysis of 13 genes involved in ethylene biosynthesis and response pathways, as well as the evolution of important agronomical traits such as skin colour, weight, and firmness. Results revealed different gene expression patterns, with genes up- and down-regulated during fruit development progression. A principal component analysis was performed to correlate gene expression with agronomical traits. The decreasing expression of the ethylene biosynthetic genes DkACO1, DkACO2, and DkACS2, in concordance with other sensing (DkERS1) and transduction genes (DkERF18), provides a molecular mechanism for the previously described high production of ethylene in immature detached fruits. On the other side, DkERF8 and DkERF16 are postulated to induce fruit softening and skin colour change during natural persimmon fruit ripening via DkXTH9 and DkPSY activation, respectively. This study provides valuable information for a better understanding of the ethylene signalling pathway and its regulation during on-tree fruit ripening in persimmon.

Spatiotemporal dynamics of the tomato fruit transcriptome under prolonged water stress

Water availability influences all aspects of plant growth and development; however, most studies of plant responses to drought have focused on vegetative organs, notably roots and leaves. Far less is known about the molecular bases of drought acclimation responses in fruits, which are complex organs with distinct tissue types. To obtain a more comprehensive picture of the molecular mechanisms governing fruit development under drought, we profiled the transcriptomes of a spectrum of fruit tissues from tomato (Solanum lycopersicum), spanning early growth through ripening and collected from plants grown under varying intensities of water stress. In addition, we compared transcriptional changes in fruit with those in leaves to highlight different and conserved transcriptome signatures in vegetative and reproductive organs. We observed extensive and diverse genetic reprogramming in different fruit tissues and leaves, each associated with a unique response to drought acclimation. These included major transcriptional shifts in the placenta of growing fruit and in the seeds of ripe fruit related to cell growth and epigenetic regulation, respectively. Changes in metabolic and hormonal pathways, such as those related to starch, carotenoids, jasmonic acid, and ethylene metabolism, were associated with distinct fruit tissues and developmental stages. Gene coexpression network analysis provided further insights into the tissue-specific regulation of distinct responses to water stress. Our data highlight the spatiotemporal specificity of drought responses in tomato fruit and indicate known and unrevealed molecular regulatory mechanisms involved in drought acclimation, during both vegetative and reproductive stages of development.

Comparative Metabolite and Gene Expression Analyses in Combination With Gene Characterization Revealed the Patterns of Flavonoid Accumulation During Cistus creticus subsp. creticus Fruit Development

Cistus creticus L. subsp. creticus (rockrose) is a shrub widespread in Greece and the Mediterranean basin and has been used in traditional medicine as herb tea for colds, for healing and digestive hitches, for the treatment of maladies, as perfumes, and for other purposes. Compounds from its flavonoid fraction have recently drawn attention due to antiviral action against influenza virus and HIV. Although several bioactive metabolites belonging to this group have been chemically characterized in the leaves, the genes involved in their biosynthesis in Cistus remain largely unknown. Flavonoid metabolism during C. creticus fruit development was studied by adopting comparative metabolomic and transcriptomic approaches. The present study highlights the fruit of C. creticus subsp. creticus as a rich source of flavonols, flavan-3-ols, and proanthocyanidins, all of which displayed a decreasing trend during fruit development. The majority of proanthocyanidins recorded in Cistus fruit are B-type procyanidins and prodelphinidins, while gallocatechin and catechin are the dominant flavan-3-ols. The expression patterns of biosynthetic genes and transcription factors were analyzed in flowers and throughout three fruit development stages. Flavonoid biosynthetic genes were developmentally regulated, showing a decrease in transcript levels during fruit maturation. A high degree of positive correlations between the content of targeted metabolites and the expression of biosynthetic genes indicated the transcriptional regulation of flavonoid biosynthesis during C. creticus fruit development. This is further supported by the high degree of significant positive correlations between the expression of biosynthetic genes and transcription factors. The results suggest that leucoanthocyanidin reductase predominates the biosynthetic pathway in the control of flavan-3-ol formation, which results in catechin and gallocatechin as two of the major building blocks for Cistus proanthocyanidins. Additionally, there is a decline in ethylene production rates during non-climacteric Cistus fruit maturation, which coincides with the downregulation of the majority of flavonoid- and ethylene-related biosynthetic genes and corresponding transcription factors as well as with the decline in flavonoid content. Finally, functional characterization of a Cistus flavonoid hydroxylase (F3'5'H) was performed for the first time.

Comparative transcriptome analysis reveals the mechanism involving ethylene and cell wall modification related genes in Diospyros kaki fruit firmness during ripening

Rapid loss of firmness is a major handicap for persimmon (Diospyros kaki Thunb.) transportation and retail. The present study employed a comparative transcriptomic approach to elucidate the mechanism involving ethylene and cell wall modification related genes in fruit firmness control of two cultivars during post harvest ripening. In contrast to the short shelf life cultivar (Mopan), the long shelf life cultivar (Yoho) kept high firmness during ripening. Extensive loss of firmness in Mopan drove an intense transcriptional activity. Globally, Mopan and Yoho shared very few common differentially expressed structural genes and regulators. Yoho strongly repressed the expression of ACC synthase and several classes of cell wall degradation genes at the onset of ripening and only induced them during late ripening period. Various ERF, WRKY, MYB, bHLH transcription factors were found highly active during fruit ripening. Overall, this study generates novel gene resources as important tools for extending persimmon shelf life.

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.

Influences of 1-methylcyclopropene-containing papers on the metabolisms of membrane lipids in Anxi persimmons during storage

Objectives

The aim of this work was to analyse the effects of 1-methylcyclopropene (1-MCP) treatment on the metabolisms of membrane lipids in postharvest Anxi persimmons during storage.

Materials and methods

Anxi persimmon (Diospyros kaki L. f. cv. Anxi) fruits were treated by paper containing 1-MCP with a concentration of 1.35 μl/l. The cellular membrane permeability was analysed by the electric conductivity meter. The activities of lipoxygenase (LOX), phospholipase (PLD) and lipase were determined by spectrophotometry. The component and relative amounts of membrane fatty acids were determined using gas chromatograph (GC).

Results

The 1-MCP-treated Anxi persimmons manifested a lower electrolyte leakage rate, lower LOX, PLD and lipase activities, higher levels of unsaturated fatty acids (USFAs), higher ratio of USFAs to saturated fatty acids (SFAs) (U/S), higher index of USFAs (IUFA), but lower levels of SFAs.

Conclusions

The degradation and the metabolisms of membrane lipids could be suppressed by 1-MCP treatment, which might be accountable for the delaying softening of postharvest Anxi persimmons during storage.

Disruption of the Auxin Gradient in the Abscission Zone Area Evokes Asymmetrical Changes Leading to Flower Separation in Yellow Lupine

How auxin transport regulates organ abscission is a long-standing and intriguing question. Polar auxin transport across the abscission zone (AZ) plays a more important role in the regulation of abscission than a local concentration of this hormone. We recently reported the existence of a spatiotemporal sequential pattern of the indole-3-acetic acid (IAA) localization in the area of the yellow lupine AZ, which is a place of flower detachment. In this study, we performed analyses of AZ following treatment with an inhibitor of polar auxin transport (2,3,5-triiodobenzoic acid (TIBA)). Once we applied TIBA directly onto the AZ, we observed a strong response as demonstrated by enhanced flower abscission. To elucidate the molecular events caused by the inhibition of auxin movement, we divided the AZ into the distal and proximal part. TIBA triggered the formation of the IAA gradient between these two parts. The AZ-marker genes, which encode the downstream molecular components of the inflorescence deficient in abscission (IDA)-signaling system executing the abscission, were expressed in the distal part. The accumulation of IAA in the proximal area accelerated the biosynthesis of abscisic acid and ethylene (stimulators of flower separation), which was also reflected at the transcriptional level. Accumulated IAA up-regulated reactive oxygen species (ROS) detoxification mechanisms. Collectively, we provide new information regarding auxin-regulated processes operating in specific areas of the AZ.

Role of Brassinosteroids in Persimmon ( Diospyros kaki L.) Fruit Ripening

Brassinosteroids (BRs) are phytohormones that regulate numerous processes including fruit ripening. In this study, persimmon ( Diospyros kaki L.) fruits were treated with 24-epibrassinolide (EBR) or brassinazole (Brz, a BR biosynthesis inhibitor) and then stored at ambient temperature. The results show that endogenous BR contents gradually increased during persimmon fruit ripening. EBR treatment significantly increased both the content of water-soluble pectin and the activities of polygalacturonase, pectate lyase, and endo-1,4-beta-glucanase but significantly reduced the content of acid-soluble pectin and cellulose, resulting in rapid fruit softening. The EBR treatment also promoted ethylene production and respiration rate. In contrast, Brz treatment delayed persimmon fruit ripening. qRT-PCR analysis showed that DkPG1, DkPL1, DkPE2, DkEGase1, DkACO2, DkACS1, and DkACS2 were up-regulated (especially a 38-fold increase in DkEGase1) in the fruit of the EBR-treated group. These results suggest that BRs are involved in persimmon fruit ripening by influencing cell-wall-degrading enzymes and ethylene biosynthesis.

A Transcriptome Approach Toward Understanding Fruit Softening in Persimmon

Persimmon (Diospyros kaki Thunb.), which is a climacteric fruit, softens in 3-5 weeks after harvest. However, little is known regarding the transcriptional changes that underlie persimmon ripening. In this study, high-throughput de novo RNA sequencing was performed to examine differential expression between freshly harvested (FH) and softened (ST) persimmon fruit peels. Using the Illumina HiSeq platform, we obtained 259,483,704 high quality reads and 94,856 transcripts. After the removal of redundant sequences, a total of 31,258 unigenes were predicted, 1,790 of which were differentially expressed between FH and ST persimmon (1,284 up-regulated and 506 down-regulated in ST compared with FH). The differentially expressed genes (DEGs) were further subjected to KEGG pathway analysis. Several pathways were found to be up-regulated in ST persimmon, including "amino sugar and nucleotide sugar metabolism." Pathways down-regulated in ST persimmon included "photosynthesis" and "carbon fixation in photosynthetic organisms." Expression patterns of genes in these pathways were further confirmed using quantitative real-time RT-PCR. Ethylene gas production during persimmon softening was monitored with gas chromatography and found to be correlated with the fruit softening. Transcription involved in ethylene biosynthesis, perception and signaling was up-regulated. On the whole, this study investigated the key genes involved in metabolic pathways of persimmon fruit softening, especially implicated in increased sugar metabolism, decreased photosynthetic capability, and increased ethylene production and other ethylene-related functions. This transcriptome analysis provides baseline information on the identity and modulation of genes involved in softening of persimmon fruits and can underpin the future development of technologies to delay softening in persimmon.

Exploring the Functions of 9-Lipoxygenase (DkLOX3) in Ultrastructural Changes and Hormonal Stress Response during Persimmon Fruit Storage

Lipoxygenase (LOX) initiates the hydroperoxidation of polyunsaturated fatty acids and is involved in multiple physiological processes. In this study, investigation of various microscopic techniques showed that the fruit peel cellular microstructure of the two persimmon cultivars differed after 12 days of storage, resulting in fruit weight loss and an increased number and depth of microcracks. Analysis of subcellular localization revealed that greater amounts of DkLOX3-immunolabelled gold particles accumulated in “Fupingjianshi” than in “Ganmaokui” during storage. In addition, the expression of DkLOX3 was positively up-regulated by abscisic acid (ABA), concomitant with the promotion of ethylene synthesis and loss of firmness, and was suppressed by salicylic acid (SA), concomitant with the maintenance of fruit firmness, inhibition of ethylene production and weight loss. In particular, the expression of DkLOX3 differed from the ethylene trajectory after methyl jasmonate (MeJA) treatment. Furthermore, we isolated a 1105 bp 5′ flanking region of DkLOX3 and the activity of promoter deletion derivatives was induced through various hormonal treatments. Promoter sequence cis-regulatory elements were analysed, and two conserved hormone-responsive elements were found to be essential for responsiveness to hormonal stress. Overall, these results will provide us with new clues for exploring the functions of DkLOX3 in fruit ripening and hormonal stress response.

RhMKK9, a rose MAP KINASE KINASE gene, is involved in rehydration-triggered ethylene production in rose gynoecia

BACKGROUND

Flower opening is an important process in the life cycle of flowering plants and is influenced by various endogenous and environmental factors. Our previous work demonstrated that rose (Rosa hybrida) flowers are highly sensitive to dehydration during flower opening and the water recovery process after dehydration induced ethylene production rapidly in flower gynoecia. In addition, this temporal- and spatial-specific ethylene production is attributed to a transient but robust activation of the rose MAP KINASE6-ACC SYNTHASE1 (RhMPK6-RhACS1) cascade in gynoecia. However, the upstream component of RhMPK6-RhACS1 is unknown, although RhMKK9 (MAP KINASE KINASE9), a rose homologue of Arabidopsis MKK9, could activate RhMPK6 in vitro. In this study, we monitored RhMKK2/4/5/9 expression, the potential upstream kinase to RhMPK6, in rose gynoecia during dehydration and rehydration.

RESULTS

We found only RhMKK9 was rapidly and strongly induced by rehydration. Silencing of RhMKK9 significantly decreased rehydration-triggered ethylene production. Consistently, the expression of several ethylene-responsive genes was down regulated in the petals of RhMKK9-silenced flowers. Moreover, we detected the DNA methylation level in the promoter and gene body of RhMKK9 by Chop-PCR. The results showed that rehydration specifically elevated the DNA methylation level on the RhMKK9 gene body, whereas it resulted in hypomethylation in its promoter.

CONCLUSIONS

Our results showed that RhMKK9 possibly acts as the upstream component of the RhMKK9-RhMPK6-RhACS1 cascade and is responsible for water recovery-triggered ethylene production in rose gynoecia, and epigenetic DNA methylation is involved in the regulation of RhMKK9 expression by rehydration.

Factors affecting ethylene and carbon dioxide concentrations during ripening: Incidence on final dry matter, total soluble solids content and acidity of mango fruit

Ripening of climacteric fruits is associated with pronounced changes in fruit gas composition caused by a concomitant rise in respiration and ethylene production. There is a discrepancy in the literature since some authors reported that changes in fruit gas compositions differ in attached and detached fruits. This study presents for the first time an overview of pre- and post-harvest factors that lead to variations in the climacteric respiration and ethylene production, and attempts to determine their impacts on fruit composition, i.e., dry matter, total soluble solids content and acidity. The impact of growing conditions such as the fruit position in the canopy and the fruit carbon supply; fruit detachment from the tree, including the maturity stage at harvest; and storage conditions after harvest, i.e., relative humidity and temperature were considered as well as changes in fruit skin resistance to gas diffusion during fruit growth and storage. Results showed that fruit gas composition vary with all pre and post-harvest factors studied. Although all mangoes underwent a respiratory climacteric and an autocatalytic ethylene production, whatever pre and post-harvest factors studied, large differences in ethylene production, climacteric respiration and fruit quality were measured. Results suggested that the ripening capacity is not related to the fruit ability to produce great amount of ethylene. In agreement with precedent studies, this work provided several lines of evidence that gas composition of fruit is related to its water balance. Our measurements indicated that skin resistance to gas diffusion increased after the harvest and during storage. It was so suggested that the faster ripening of detached fruit may be explained in part by changes in fruit water balance and skin resistance to gas diffusion caused by fruit detachment.

Chloride stress triggers maturation and negatively affects the postharvest quality of persimmon fruit. Involvement of calyx ethylene production

In recent years many hectares planted with persimmon trees in E Spain have been diagnosed with chloride toxicity. An effect of this abiotic stress on fruit quality has been reported in different crops. However, the impact of chloride stress on persimmon fruit quality is unknown. The harvest and postharvest quality of persimmons harvested from trees that manifest different intensities of chloride toxicity foliar symptoms was evaluated herein. Our results revealed that fruits from trees under chloride stress conditions underwent chloride accumulation in the calyx, which was more marked the greater the salt stress intensity trees were exposed to. Increased chloride concentrations in the calyx stimulated ethylene production in this tissue. In the fruits affected by slight and moderate chloride stress, calyx ethylene production accelerated the maturity process, as reflected by increased fruit colour and diminished fruit firmness. In the fruits under severe chloride stress, the high ethylene levels in the calyx triggered autocatalytic ethylene production in other fruit tissues, which led fruit maturity to drastically advance. In these fruits effectiveness of CO2 deastringency treatment was not complete and fruit softening enhanced during the postharvest period. Moreover, chloride stress conditions had a marked effect on reducing fruit weight, even in slightly stressed trees.

Isolation and Characterization of Two Persimmon Xyloglucan Endotransglycosylase/Hydrolase (XTH) Genes That Have Divergent Functions in Cell Wall Modification and Fruit Postharvest Softening

Fruit cell wall modification is the primary factor affecting fruit softening. Xyloglucan endotransglycosylase/hydrolase (XTH), a cell wall-modifying enzyme, is involved in fruit softening. In this study, two novel XTH genes (DkXTH6 and DkXTH7) were identified from persimmon fruit. Transcriptional profiles of both of the two genes were analyzed in different tissues of persimmon, and in response to multiple hormonal and environmental treatments [gibberellic acid (GA3), abscisic acid (ABA), propylene, and low temperature]. Expression of DkXTH6 was positively up-regulated during ethylene production and by propylene and ABA treatments, and suppressed by GA3 and cold treatment. In contrast, DkXTH7 exhibited its highest transcript levels in GA3-treated fruit and cold-treated fruit, which had higher fruit firmness. We found that DkXTH6 protein was localized in cell wall by its signal peptide, while cytoplasmic DkXTH7 protein contained no signal peptide. When expressed in vitro, the recombinant proteins of both DkXTH6 and DkXTH7 exhibited strict xyloglucan endotransglycosylase (XET) activity but no xyloglucan endohydrolase (XEH) activity. The recombinant protein of DkXTH6 showed a higher affinity with small acceptor molecules than the recombinant DkXTH7. Taken together with their opposing expression patterns and subcellular localizations, these results suggested that DkXTH6 might take part in cell wall restructuring and DkXTH7 was likely to be involved in cell wall assembly, indicating their special roles in persimmon fruit softening.

Precise spatio-temporal modulation of ACC synthase by MPK6 cascade mediates the response of rose flowers to rehydration

Drought is a major abiotic stress that affects the development and growth of most plants, and limits crop yield worldwide. Although the response of plants to drought has been well documented, much less is known about how plants respond to the water recovery process, namely rehydration. Here, we describe the spatio-temporal response of plant reproductive organs to rehydration using rose flowers as an experimental system. We found that rehydration triggered rapid and transient ethylene production in the gynoecia. This ethylene burst serves as a signal to ensure water recovery in flowers, and promotes flower opening by influencing the expression of a set of rehydration-responsive genes. An in-gel kinase assay suggested that the rehydration-induced ethylene burst resulted from transient accumulation of RhACS1/2 proteins in gynoecia. Meanwhile, RhMPK6, a rose homolog of Arabidopsis thaliana MPK6, is rapidly activated by rehydration within 0.5 h. Furthermore, RhMPK6 was able to phosphorylate RhACS1 but not RhACS2 in vitro. Application of the kinase inhibitor K252a suppressed RhACS1 accumulation and rehydration-induced ethylene production in gynoecia, and the protein phosphatase inhibitor okadaic acid had the opposite effect, confirming that accumulation of RhACS1 was phosphorylation-dependent. Finally, silencing of RhMPK6 significantly reduced ethylene production in gynoecia when flowers were subjected to rehydration. Taken together, our results suggest that temporal- and spatial-specific activation of an RhMPK6-RhACS1 cascade is responsible for rehydration-induced ethylene production in gynoecia, and that the resulting ethylene-mediated signaling pathway is a key factor in flower rehydration.

The role of abscisic acid in fruit ripening and responses to abiotic stress

The phytohormone abscisic acid (ABA) plays a crucial role not only in fruit development and ripening, but also in adaptive responses to biotic and abiotic stresses. In these processes, the actions of ABA are under the control of complex regulatory mechanisms involving ABA metabolism, signal transduction, and transport. The endogenous ABA content is determined by the dynamic balance between biosynthesis and catabolism, processes which are regulated by 9-cis-epoxycarotenoid dioxygenase (NCED) and ABA 8'-hydroxylase (CYP707A), respectively. ABA conjugation by cytosolic UDP-glucosyltransferases, or release by β-glucosidases, is also important for maintaining ABA homeostasis. Recently, multiple putative ABA receptors localized at different subcellular sites have been reported. Among these is a major breakthrough in the field of ABA signalling-the identification of a signalling cascade involving the PYR/PYL/RCAR protein family, the type 2C protein phosphatases (PP2Cs), and subfamily 2 of the SNF1-related kinases (SnRK2s). With regard to transport, two ATP-binding cassette (ABC) proteins and two ABA transporters in the nitrate transporter 1/peptide transporter (NRT1/PTR) family have been identified. In this review, we summarize recent research progress on the role of ABA in fruit ripening, stress response, and transcriptional regulation, and also the functional verification of both ABA-responsive and ripening-related genes. In addition, we suggest possible commercial applications of genetic manipulation of ABA signalling to improve fruit quality and yields.

An organ-specific role for ethylene in rose petal expansion during dehydration and rehydration

Dehydration is a major factor resulting in huge loss from cut flowers during transportation. In the present study, dehydration inhibited petal cell expansion and resulted in irregular flowers in cut roses, mimicking ethylene-treated flowers. Among the five floral organs, dehydration substantially elevated ethylene production in the sepals, whilst rehydration caused rapid and elevated ethylene levels in the gynoecia and sepals. Among the five ethylene biosynthetic enzyme genes (RhACS1-5), expression of RhACS1 and RhACS2 was induced by dehydration and rehydration in the two floral organs. Silencing both RhACS1 and RhACS2 significantly suppressed dehydration- and rehydration-induced ethylene in the sepals and gynoecia. This weakened the inhibitory effect of dehydration on petal cell expansion. β-glucuronidase activity driven by both the RhACS1 and RhACS2 promoters was dramatically induced in the sepals, pistil, and stamens, but not in the petals of transgenic Arabidopsis. This further supports the organ-specific induction of these two genes. Among the five rose ethylene receptor genes (RhETR1-5), expression of RhETR3 was predominantly induced by dehydration and rehydration in the petals. RhETR3 silencing clearly aggravated the inhibitory effect of dehydration on petal cell expansion. However, no significant difference in the effect between RhETR3-silenced flowers and RhETR-genes-silenced flowers was observed. Furthermore, RhETR-genes silencing extensively altered the expression of 21 cell expansion-related downstream genes in response to ethylene. These results suggest that induction of ethylene biosynthesis by dehydration proceeds in an organ-specific manner, indicating that ethylene can function as a mediator in dehydration-caused inhibition of cell expansion in rose petals.

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.

Targeted systems biology profiling of tomato fruit reveals coordination of the Yang cycle and a distinct regulation of ethylene biosynthesis during postclimacteric ripening

The concept of system 1 and system 2 ethylene biosynthesis during climacteric fruit ripening was initially described four decades ago. Although much is known about fruit development and climacteric ripening, little information is available about how ethylene biosynthesis is regulated during the postclimacteric phase. A targeted systems biology approach revealed a novel regulatory mechanism of ethylene biosynthesis of tomato (Solanum lycopersicum) when fruit have reached their maximal ethylene production level and which is characterized by a decline in ethylene biosynthesis. Ethylene production is shut down at the level of 1-aminocyclopropane-1-carboxylic acid oxidase. At the same time, 1-aminocyclopropane-1-carboxylic acid synthase activity increases. Analysis of the Yang cycle showed that the Yang cycle genes are regulated in a coordinated way and are highly expressed during postclimacteric ripening. Postclimacteric red tomatoes on the plant showed only a moderate regulation of 1-aminocyclopropane-1-carboxylic acid synthase and Yang cycle genes compared with the regulation in detached fruit. Treatment of red fruit with 1-methylcyclopropane and ethephon revealed that the shut-down mechanism in ethylene biosynthesis is developmentally programmed and only moderately ethylene sensitive. We propose that the termination of autocatalytic ethylene biosynthesis of system 2 in ripe fruit delays senescence and preserves the fruit until seed dispersal.

Altered chloroplast development and delayed fruit ripening caused by mutations in a zinc metalloprotease at the lutescent2 locus of tomato

The chloroplast is the site of photosynthesis in higher plants but also functions as the center of synthesis for primary and specialized metabolites including amino acids, fatty acids, starch, and diverse isoprenoids. Mutants that disrupt aspects of chloroplast function represent valuable tools for defining structural and biochemical regulation of the chloroplast and its interplay with whole-plant structure and function. The lutescent1 (l1) and l2 mutants of tomato (Solanum lycopersicum) possess a range of chlorophyll-deficient phenotypes including reduced rates of chlorophyll synthesis during deetiolation and enhanced rates of chlorophyll loss in leaves and fruits as they age, particularly in response to high-light stress and darkness. In addition, the onset of fruit ripening is delayed in lutescent mutants by approximately 1 week although once ripening is initiated they ripen at a normal rate and accumulation of carotenoids is not impaired. The l2 locus was mapped to the long arm of chromosome 10 and positional cloning revealed the existence of a premature stop codon in a chloroplast-targeted zinc metalloprotease of the M50 family that is homologous to the Arabidopsis (Arabidopsis thaliana) gene ETHYLENE-DEPENDENT GRAVITROPISM DEFICIENT AND YELLOW-GREEN1. Screening of tomato germplasm identified two additional l2 mutant alleles. This study suggests a role for the chloroplast in mediating the onset of fruit ripening in tomato and indicates that chromoplast development in fruit does not depend on functional chloroplasts.

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.

The fading distinctions between classical patterns of ripening in climacteric and non-climacteric fruit and the ubiquity of ethylene-An overview

The process of fruit ripening is normally viewed distinctly in climacteric and non-climacteric fruits. But, many fruits such as guava, melon, Japanese plum, Asian pear and pepper show climacteric as well as non-climacteric behaviour depending on the cultivar or genotype. Investigations on in planta levels of CO2 and ethylene at various stages of fruits during ripening supported the role and involvement of changes in the rate of respiration and ethylene production in non-climacteric fruits such as strawberry, grapes and citrus. Non-climacteric fruits are also reported to respond to the exogenous application of ethylene. Comparative analysis of plant-attached and plant-detached fruits did not show similarity in their ripening behaviour. This disparity is being explained in view of 1. Hypothetical ripening inhibitor, 2. Differences in the production, release and endogenous levels of ethylene, 3. Sensitivity of fruits towards ethylene and 4. Variations in the gaseous microenvironment among fruits and their varieties. Detailed studies on genetic and inheritance patterns along with the application of '-omics' research indicated that ethylene-dependent and ethylene-independent pathways coexist in both climacteric and non-climacteric fruits. Auxin levels also interact with ethylene in regulating ripening. These findings therefore reveal that the classification of fruits based on climacteric rise and/or ethylene production status is not very distinct or perfect. However, presence of a characteristic rise in CO2 levels and a burst in ethylene production in some non-climacteric fruits as well as the presence of system 2 of ethylene production point to a ubiquitous role for ethylene in fruit ripening.

Temperature and Ultra Low Oxygen Effects and Involvement of Ethylene in Chilling Injury of ‘Rojo Brillante’ Persimmon Fruit

The effects of storage temperature, inhibition of ethylene action by treatment with 1-methylcyclopropene (1-MCP) and ultra low oxygen (ULO) atmosphere on chilling injury (CI), fruit firmness and ethylene production in the astringent ‘Rojo Brillante’ persimmon fruit were investigated. CI symptoms were manifested as a very dramatic loss of firmness after fruit transfer from cold storage to shelf-life conditions (18 °C). During cold storage, fruit softening appeared more rapidly in fruit stored at the intermediate temperature of 10 °C than at 1°C or 14.5 °C. Ethylene production increased with storage time at the chilling temperature (1 °C) but a sharp increase took place upon fruit transfer from 1 °C to ambient temperature. This ethylene increase was accompanied by a loss of fruit firmness associated with chilling damage development. A pre-treatment with the competitive inhibitor of ethylene action 1-MCP, at 1 μL/L, reduced firmness loss and mitigated CI damage but considerably increased ethylene production in fruit transferred to shelf-life conditions after a prolonged cold storage period. Collectively, these results suggest a role of ethylene in the reduction of flesh firmness and consequently in the induction of CI in persimmon fruit. Moreover, ethylene exerts a negative feedback regulation of cold-induced ethylene biosynthesis. Storage of ‘Rojo Brillante’ persimmon fruit under ULO (1.3—1.8% O2, v/v) atmosphere did not affect the incidence of CI but reduced fruit astringency, suggesting that ULO may be an alternative postharvest storage system for ‘Rojo Brillante’ persimmon fruit.

Solar UV-B radiation influences carotenoid accumulation of tomato fruit through both ethylene-dependent and -independent mechanisms

The effect of UV-B shielding on ethylene production in ripening tomato fruits and the contribution of ethylene and UV-B radiation on carotenoid accumulation and profile during ripening were assessed to get more insight about the interplay between these two regulatory factors. To this aim, rin and nor tomato mutants, unable to produce ripening ethylene, and cv Ailsa Craig were cultivated under control or UV-B depleted conditions until full fruit ripening. The significantly decreased ethylene evolution following UV-B depletion, evident only in Ailsa Craig, suggested the requirement of functional rin and nor genes for UVB-mediated ethylene production. Carotenoid content and profile were found to be controlled by both ethylene and UV-B radiation. This latter influenced carotenoid metabolism either in an ethylene-dependent or -independent way, as indicated by UVB-induced changes also in nor and rin carotenoid content and confirmed by correlation plots between ethylene evolution and carotenoid accumulation performed separately for control and UV-B shielded fruits. In conclusion, natural UV-B radiation influences carotenoid metabolism in a rather complex way, involving ethylene-dependent and -independent mechanisms, which seem to act in an antagonistic way.

Induction of ethylene in avocado fruit in response to chilling stress on tree

Chilling of avocado fruit (Persea americana cv. Arad) in the orchard caused a dramatic induction of fruit ripening and a parallel increase in ethylene biosynthesis and receptor genes' expression during shelf life. In-orchard chilling stress stimulated ethylene and CO(2) production already in fruit attached to the tree, and these reduced thereafter during 20 degrees C storage. In non-chilled control fruit, ethylene and CO(2) production started after 3d at 20 degrees C and exhibited a climacteric peak. In-orchard chilling stress also led to membrane destruction expressed as higher electrical conductivity (EC) in chilling stressed (CS) fruit and accelerated softening compared with control fruit. The increase in ethylene production on the day of harvest in CS fruit was accompanied by high expression of two 1-aminocyclopropane-1-carboxylic aCSd (ACC) synthase genes: PaACS1 and PaACS2, and ACC oxidase PaACO. The initial gene expressions of PaACS1, PaACS2, and PaACO in the CS fruit at the day of harvest was similar to the levels reached by the control fruit after 4d at 20 degrees C. The expression levels of both PaETR and PaERS1 in CS fruit on tree were 25 times higher than the control. In control fruit, expression of ethylene receptor genes was very low at harvest and increased in parallel to the onset of the climacteric ethylene peak. PaCTR1 transcript levels were less affected by chilling stress, and small changes (less than 3-fold) were observed in CS fruit on the day of harvest. Together, our results suggest that ethylene biosynthesis and ethylene response-pathway genes are involved in regulation of ethylene responsiveness in response to in-orchard chilling stress and during ripening.

Ripening-associated ethylene biosynthesis in tomato fruit is autocatalytically and developmentally regulated

To investigate the regulatory mechanism(s) of ethylene biosynthesis in fruit, transgenic tomatoes with all known LeEIL genes suppressed were produced by RNA interference engineering. The transgenic tomato exhibited ethylene insensitivity phenotypes such as non-ripening and the lack of the triple response and petiole epinasty of seedlings even in the presence of exogenous ethylene. Transgenic fruit exhibited a low but consistent increase in ethylene production beyond 40 days after anthesis (DAA), with limited LeACS2 and LeACS4 expression. 1-Methylcyclopropene (1-MCP), a potent inhibitor of ethylene perception, failed to inhibit the limited increase in ethylene production and expression of the two 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) genes in the transgenic fruit. These results suggest that ripening-associated ethylene (system 2) in wild-type tomato fruit consists of two parts: a small part regulated by a developmental factor through the ethylene-independent expression of LeACS2 and LeACS4 and a large part regulated by an autocatalytic system due to the ethylene-dependent expression of the same genes. The results further suggest that basal ethylene (system 1) is less likely to be involved in the transition to system 2. Even if the effect of system 1 ethylene is eliminated, fruit can show a small increase in ethylene production due to unknown developmental factors. This increase would be enough for the stimulation of autocatalytic ethylene production, leading to fruit ripening.

Expression of ethylene biosynthetic and receptor genes in rose floral tissues during ethylene-enhanced flower opening

Ethylene production, as well as the expression of ethylene biosynthetic (Rh-ACS1-4 and Rh-ACO1) and receptor (Rh-ETR1-5) genes, was determined in five different floral tissues (sepals, petals, stamens, gynoecia, and receptacles) of cut rose (Rosa hybrida cv. Samantha upon treatment with ethylene or the ethylene inhibitor 1-methylcyclopropene (1-MCP). Ethylene-enhanced ethylene production occurred only in gynoecia, petals, and receptacles, with gynoecia showing the greatest enhancement in the early stage of ethylene treatment. However, 1-MCP did not suppress ethylene production in these three tissues. In sepals, ethylene production was highly decreased by ethylene treatment, and increased dramatically by 1-MCP. Ethylene production in stamens remained unchanged after ethylene or 1-MCP treatment. Induction of certain ethylene biosynthetic genes by ethylene in different floral tissues was positively correlated with the ethylene production, and this induction was also not suppressed by 1-MCP. The expression of Rh-ACS2 and Rh-ACS3 was quickly induced by ethylene in gynoecia, but neither Rh-ACS1 nor Rh-ACS4 was induced by ethylene in any of the five tissues. In addition, Rh-ACO1 was induced by ethylene in all floral tissues except sepals. The induced expression of ethylene receptor genes by ethylene was much faster in gynoecia than in petals, and the expression of Rh-ETR3 was strongly suppressed by 1-MCP in all floral tissues. These results indicate that ethylene biosynthesis in gynoecia is regulated developmentally, rather than autocatalytically. The response of rose flowers to ethylene occurs initially in gynoecia, and ethylene may regulate flower opening mainly through the Rh-ETR3 gene in gynoecia.

Involvement of negative feedback regulation in wound-induced ethylene synthesis in 'Saijo' persimmon

Wounding is one of the most effective stress signals to induce ethylene synthesis in persimmon (Diospyros kaki Thunb.). We found that wound-induced ethylene biosynthesis is subjected to negative feedback regulation in mature 'Saijo' persimmon fruit since ethylene production was enhanced by 1-methylcyclopropene (1-MCP) (an inhibitor of ethylene perception) pretreatment, which was approximately 1.8 fold of that in control tissues (without 1-MCP pretreatment). Wound-induced 1-aminocyclopropane-1-carboxylate (ACC) synthase activity and DK-ACS2 gene expression were substantially increased by 1-MCP pretreatment after 12 h, which resulted in much higher ACC content in 1-MCP pretreated tissues than that in a control after 24 h. These results indicated that wound-induced DK-ACS2 gene expression was negatively regulated by ethylene in mature persimmon fruit. However, 1-MCP pretreatment had no effect on DK-ACO1 gene expression, suggesting the independence of wound-induced DK-ACO1 on ethylene. Out of accord with DK-ACO1 gene expression, ACC oxidase activity was enhanced 48 h after wounding in 1-MCP pretreated tissues, reaching a peak 1.5-fold higher than that in control tissues at 60 h.

G-protein-coupled alpha2A-adrenoreceptor agonists differentially alter citrus leaf and fruit abscission by affecting expression of ACC synthase and ACC oxidase

Temporal and spatial expression patterns of genes encoding 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS1 and ACS2) and ACC oxidase (ACO), ACC concentration, and ethylene production in leaves and fruit of 'Valencia' orange (Citrus sinensis [L.] Osbeck) were examined in relation to differential abscission after treatment with 2-chloroethylphosphonic acid (ethephon) alone or in combination with guanfacine or clonidine, two G-protein-coupled alpha(2A)-adrenoreceptor selective agonists. Guanfacine and clonidine markedly reduced ethephon-enhanced leaf abscission, but had little effect on ethephon-enhanced fruit loosening. Ethephon-enhanced fruit and leaf ethylene production, and ACC concentration in fruit abscission zones, fruit peel, leaf abscission zones, and leaf blades were decreased by guanfacine. Guanfacine reduced ethephon-enhanced expression of ACS1 and ACO genes in leaf abscission zones and blades, but to a lesser extent in fruit abscission zones. The expression pattern of the ACS2 gene, however, was not associated with abscission. The results demonstrate that differential expression of ACS1 and ACO genes is associated with reduction of ethephon-enhanced leaf abscission by guanfacine, and suggest a link between G-protein-related signalling and abscission.

Molecular and physiological evidence suggests the existence of a system II-like pathway of ethylene production in non-climacteric Citrus fruit

Mature citrus fruits, which are classified as non-climacteric, evolve very low amounts of ethylene during ripening but respond to exogenous ethylene by ripening-related pigment changes and accelerated respiration. In the present study we show that young citrus fruitlets attached to the tree produce high levels of ethylene, which decrease dramatically towards maturation. Upon harvest, fruitlets exhibited a climacteric-like rise in ethylene production, preceded by induction of the genes for 1-aminocyclopropane-1-carboxylate (ACC) synthase 1 (CsACS1), ACC oxidase 1 (CsACO1) and the ethylene receptor CsERS1. This induction was advanced and augmented by exogenous ethylene or propylene, indicating an autocatalytic system II-like ethylene biosynthesis. In mature, detached fruit, very low rates of ethylene production were associated with constitutive expression of the ACC synthase 2 (CsACS2) and ethylene receptor CsETR1 genes (system I). CsACS1 gene expression was undetectable at this stage, even following ethylene or propylene treatment, and CsERS1 gene expression remained constant, indicating that no autocatalytic response had occurred. The transition from system II-like behavior of young fruitlets to system I behavior appears to be under developmental control.