Fruit development of the diploid kiwifruit, Actinidia chinensis 'Hort16A'

Short-term anaerobic treatment maintained the quality of Actinidia arguta by activating the antioxidant defense system

BACKGROUND

This study aimed to investigate the effect of 6, 12, and 24 h short-term anaerobic treatment on kiwiberry quality and antioxidant properties at 5 °C.

RESULTS

Short-term anaerobic treatment was found to delay ripening and softening in kiwiberries, evident from changes in ethylene release, total soluble solids, starch, protopectin, and fruit texture. The 24 h treatment group exhibited the lowest decay rate of 12% on day 49, a 38% reduction compared with the control group. Anaerobic treatment reduced flesh translucency and decay in the fruit. The 12 h and 24 h treatments enhanced the activities of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase, and increased the level of total phenolics, flavonoids, anthocyanins, and ascorbic acid. Moreover, it lowered oxidative damage in cell membranes, evidenced by reduced malondialdehyde content and relative conductivity.

CONCLUSION

These results indicate that anaerobic treatment maintains the fruit quality by stimulating its antioxidant defense system. Therefore, short-term anaerobic treatment emerges as a promising method for kiwiberry storage. © 2024 Society of Chemical Industry.

The kiwifruit amyloplast proteome (kfALP): a resource to better understand the mechanisms underlying amyloplast biogenesis and differentiation

The biogenesis and differentiation (B&D) of amyloplasts contributes to fruit flavor and color. Here, remodeling of starch granules, thylakoids and plastoglobules was observed during development and ripening in two kiwifruit (Actinidia spp.) cultivars - yellow-fleshed 'Hort16A' and green-fleshed 'Hayward'. A protocol was developed to purify starch-containing plastids with a high degree of intactness, and amyloplast B&D was studied using label-free-based quantitative proteomic analyses in both cultivars. Over 3000 amyloplast-localized proteins were identified, of which >98% were quantified and defined as the kfALP (kiwifruit amyloplast proteome). The kfALP data were validated by Tandem-Mass-Tag (TMT) labeled proteomics in 'Hort16A'. Analysis of the proteomic data across development and ripening revealed: 1) a conserved increase in the abundance of proteins participating in starch synthesis/degradation during both amyloplast B&D; 2) up-regulation of proteins for chlorophyll degradation and of plastoglobule-localized proteins associated with chloroplast breakdown and plastoglobule formation during amyloplast differentiation; 3) constitutive expression of proteins involved in ATP supply and protein import during amyloplast B&D. Interestingly, two different pathways of amyloplast B&D were observed in the two cultivars. In 'Hayward', significant increases in abundance of photosynthetic- and tetrapyrrole metabolism-related proteins were observed, but the opposite trend was observed in 'Hort16A'. In conclusion, analysis of the kfALP provides new insights into the potential mechanisms underlying amyloplast B&D with relevance to key fruit quality traits in contrasting kiwifruit cultivars.

Combination of Sodium Nitroprusside and Controlled Atmosphere Maintains Postharvest Quality of Chestnuts through Enhancement of Antioxidant Capacity

The purpose of this study was to clarify the effect of CA (controlled atmosphere, 2-3% O2 + 3% CO2) and NO (nitric oxide, generated by 0.4 nM sodium nitroprusside), alone or combined (CA + NO), on the physio-chemical properties, enzyme activities and antioxidant capacities of chestnuts during storage at 0 °C for 180 d. Compared with control (CT), CA and CA+NO both improved the storage quality of the samples, but only CA resulted in more ethanol production. Moreover, these improvements were further enhanced and ethanol synthesis was inhibited by the addition of NO. A spectrometer was used to assess the production of phenolic content (TPC) and activities of phenylalanine ammonia-lyase (PAL), superoxide dismutas (SOD), peroxidase (POD), catalase (CAT) and polyphenol oxidase (PPO) as influenced by CA or CA+NO treatments. Higher TPC, PAL, SOD, POD, CAT, and lower PPO were observed in CA alone, and more so in the combination with NO group. The increased antioxidant production and enhanced antioxidant activities contributed to scavenging reactive oxygen species (ROS) and reducing malondialdehyde (MDA). This study unveiled the correlations and differences between the effects of CA and CA+NO on storage quality, providing valuable insights into postharvest preservation and suggesting that the combination (CA+NO) was more beneficial for quality maintenance in chestnuts.

Transcriptome Analysis Reveals Fruit Quality Formation in Actinidia eriantha Benth

Actinidia chinensis Planch. is a fruit tree originating from China that is abundant in the wild. Actinidia eriantha Benth. is a type of A. chinensis that has emerged in recent years. The shape of A. eriantha is an elongated oval, and the skin is covered with dense, non-shedding milk-white hairs. The mature fruit has flesh that is bright green in colour, and the fruit has a strong flavour and a grass-like smell. It is appreciated for its rich nutrient content and unique flavour. Vitamin C, sugar, and organic acids are key factors in the quality and flavour composition of A. eriantha but have not yet been systematically analysed. Therefore, we sequenced the transcriptome of A. eriantha at three developmental stages and labelled them S1, S2, and S3, and comparisons of S1 vs. S2, S1 vs. S3, and S2 vs. S3 revealed 1218, 4019, and 3759 upregulated differentially expressed genes and 1823, 3415, and 2226 downregulated differentially expressed genes, respectively. Furthermore, the upregulated differentially expressed genes included 213 core genes, and Gene Ontology enrichment analysis showed that they were enriched in hormones, sugars, organic acids, and many organic metabolic pathways. The downregulated differentially expressed genes included 207 core genes, which were enriched in the light signalling pathway. We further constructed the metabolic pathways of sugars, organic acids, and vitamin C in A. eriantha and identified the genes involved in vitamin C, sugar, and organic acid synthesis in A. eriantha fruits at different stages. During fruit development, the vitamin C content decreased, the carbohydrate compound content increased, and the organic acid content decreased. The gene expression patterns were closely related to the accumulation patterns of vitamin C, sugars, and organic acids in A. eriantha. The above results lay the foundation for the accumulation of vitamin C, sugars, and organic acids in A. eriantha and for understanding flavour formation in A. eriantha.

Artificial pollination of kiwifruit (Actinidia chinensis Planch. var. chinensis) (Ericales: Actinidiaceae) results in greater fruit set compared to flowers pollinated by managed bees (Apis mellifera L. (Hymenoptera: Apidae) and Bombus impatiens Cresson (Hymenoptera: Apidae))

Due to a lack of knowledge on the pollination requirements of kiwifruit cultivars grown within the United States, farmers simultaneously implement multiple pollination methods, like the rental of managed bee species or artificial pollination to achieve high fruit yields. However, implementing multiple pollination methods is costly and possibly an inefficient use of resources. We assessed the contribution of two managed bees (Apis mellifera and Bombus impatiens) to the pollination of kiwifruit by i) determining the relative abundance of kiwifruit pollen collected by foragers of each bee species, and ii) comparing fruit set and fruit quality among insect and artificially pollinated flowers through an insect exclusion experiment. A significant difference was observed between the mean relative abundance of kiwifruit pollen carried in the corbicula of A. mellifera and B. impatiens, with B. impatiens carrying on average 46% more kiwifruit pollen than A. mellifera. Artificially pollinated kiwifruit flowers set significantly greater numbers of fruit per flower at four weeks post-bloom and at harvest compared to insect pollination, wind pollination, and pollen exclusion treatment. Artificial pollination produced fruits of greater weight, size, and seed number compared to insect-pollinated flowers, and few fruits were produced in the pollen exclusion and wind pollination treatments. Kiwifruit producers experiencing similar conditions to ours should focus on artificially pollinating their crops rather than relying on managed or wild insects for kiwifruit pollination. Future research should evaluate other methods of artificial pollination to determine their effectiveness, efficiency, and economics in the pollination of kiwifruit grown within the United States.

The Pollen Donor Affects Seed Development, Taste, and Flavor Quality in 'Hayward' Kiwifruit

To investigate how different species or ploidy level of pollen donors affects the fruit quality of kiwifruit, flowers of 'Hayward' kiwifruit (a hexaploid Actinidia deliciosa cultivar, 6x) were hand-pollinated with pollen from ten different male donors. Kiwifruit plants pollinated with four distant species-M7 (2x, A. kolomikta), M8 (4x, A. arguta), M9 (4x, A. melanandra), and M10 (2x, A. eriantha)-had a low fruit-setting rate and therefore were not investigated further. Of the other six treatments, kiwifruit plants pollinated with M4 (4x, A. chinensis), M5 (6x, A. deliciosa) M6 (6x, A. deliciosa) had a larger fruit size and weight than those pollinated with M1 (2x, A. chinensis) and M2 (2x, A. chinensis). However, pollination with M1 (2x) and M2 (2x) resulted in seedless fruits, having few small and aborted seeds. Notably, these seedless fruits had higher fructose, glucose, and total sugar and lower citric acid content. This resulted in a higher sugar to acid ratio compared to fruits from plants pollinated with M3 (4x, A. chinensis), M4 (4x), M5 (6x), and M6 (6x). Most volatile compounds increased in the M1 (2x)- and M2 (2x)-pollinated fruit. A combination of principal component analysis (PCA), electronic tongue, and electronic nose suggested that the different pollen donors significantly affected the kiwifruit's overall taste and volatiles. Specifically, two diploid donors had the most positive contribution. This was in agreement with the findings from the sensory evaluation. In conclusion, the present study showed that the pollen donor affected the seed development, taste, and flavor quality of 'Hayward' kiwifruit. This provides useful information for improving the fruit quality and breeding of seedless kiwifruit.

Chromosome-scale genome assembly of a natural diploid kiwifruit (Actinidia chinensis var. deliciosa)

The most commercialized kiwifruit, Actinidia chinensis var. deliciosa (Acd), is an allohexaploid (2n = 6x = 174), making high-quality assemblage genome challenging. We previously discovered a rare naturally occurring diploid Acd plant. Here, chromosome-level de novo genome assembly for this diploid Acd was reported, reaching approximately 621.98 Mb in length with contig and scaffold N50 values of 10.08 and 21.09 Mb, respectively, 99.66% of the bases anchored to 29 pseudochromosomes, and 38,990 protein-coding genes and 42.29% repetitive elements annotated. The divergence time of A. chinensis cv. 'Red5' and 'Hongyang' (11.1-27.7 mya) was more recent compared with the divergence time of them and Acd (19.9-41.2 mya), with the divergence time of A. eriantha cv. 'White' being the earliest (22.9-45.7 mya) among that of the four Actinidia species. The 4DTv distance distribution highlighted three recent whole-genome duplication events in Acd. This is the first high-quality diploid Acd genome, which lays an important foundation for not only kiwifruit functional genomics studies but also further elucidating genome evolution of allohexaploid Acd.

Atypical Climacteric and Functional Ethylene Metabolism and Signaling During Fruit Ripening in Blueberry (Vaccinium sp.)

Climacteric fruits display an increase in respiration and ethylene production during the onset of ripening, while such changes are minimal in non-climacteric fruits. Ethylene is a primary regulator of ripening in climacteric fruits. The ripening behavior and role of ethylene in blueberry (Vaccinium sp.) ripening is controversial. This work aimed to clarify the fruit ripening behavior and the associated role of ethylene in blueberry. Southern highbush (Vaccinium corymbosum hybrids) and rabbiteye (Vaccinium ashei) blueberry displayed an increase in the rate of respiration and ethylene evolution, both reaching a maxima around the Pink and Ripe stages of fruit development, consistent with climacteric fruit ripening behavior. Increase in ethylene evolution was associated with increases in transcript abundance of its biosynthesis genes, AMINOCYCLOPROPANE CARBOXYLATE (ACC) SYNTHASE1 (ACS1) and ACC OXIDASE2 (ACO2), implicating them in developmental ethylene production during ripening. Blueberry fruit did not display autocatalytic system 2 ethylene during ripening as ACS transcript abundance and ACC concentration were not enhanced upon treatment with an ethylene-releasing compound (ethephon). However, ACO transcript abundance was enhanced in response to ethephon, suggesting that ACO was not rate-limiting. Transcript abundance of multiple genes associated with ethylene signal transduction was upregulated concomitant with developmental increase in ethylene evolution, and in response to exogenous ethylene. As these changes require ethylene signal transduction, fruit ripening in blueberry appears to involve functional ethylene signaling. Together, these data indicate that blueberry fruit display atypical climacteric ripening, characterized by a respiratory climacteric, developmentally regulated but non-autocatalytic increase in ethylene evolution, and functional ethylene signaling.

Identification of genes and metabolic pathways involved in wounding-induced kiwifruit ripening

Fruit is constantly challenged by wounding events, inducing accelerated ripening and irreversible metabolic changes. However, cognate mechanisms that regulate this process are little known. To expand our knowledge of ripening metabolism induced by wounding, an artificial-wound global transcriptome investigation combined with metabolite profiling study was conducted in postharvest kiwifruit (Actinidia chinensis var. deliciosa (A. Chev.) A. Chev. 'Hayward'). Wounding treatment promoted fruit ripening, as demonstrated by changes in fruit firmness, ethylene production and respiration activity determined periodically during a ripening period of 8 d at room temperature. Calcium imaging using fluorescent probe Fluo-3 AM revealed spatial dynamics of Ca²⁺ signaling in the wounding area following 8d ripening. Several sugars including fructose, glucose, and sucrose as well as organic acids such as citric, succinic and galacturonic acid were increased by wounding. Changes of various amino acids in wounded-treated fruit, especially 5-oxoproline and valine along with alternations of soluble alcohols, like myo-inositol were detected. Gene expression analysis of the wounded fruit showed increased expression of genes that are mainly involved in defense response (e.g., AdTLP.1-3, AdPP2C.1-2, AdMALD1), calcium ion binding (e.g., AdCbEFh, AdCLR, AdANX), TCA cycle (e.g., AdMDH.1, AdMDH.2, AdCS), sugars (e.g., AdSUSA.1, AdSPS4, AdABFr), secondary metabolism (e.g., AdPAL.1-3, AdCCR, AdHCT.1-2), lipid processing (e.g., AdGELP.1-4, AdGELP) and pectin degradation (e.g., AdPE.1-2, AdPAE.1-2, AdPG.1-2) as well as in ethylene (AdERF7, AdERF1B, AdACO.1-4) and auxin (AdICE, AdAEFc, AdASII) synthesis and perception. Moreover, genes related to aquaporins, such as AdAQP2, AdAQP4 and AdAQP7 were down-regulated in fruit exposed to wounding. These results demonstrate multiple metabolic points of wounding regulatory control during kiwifruit ripening and provide insights into the molecular basis of wounding-mediated ripening.

A Data Driven Approach to Assess Complex Colour Profiles in Plant Tissues

The ability to quantify the colour of fruit is extremely important for a number of applied fields including plant breeding, postharvest assessment, and consumer quality assessment. Fruit and other plant organs display highly complex colour patterning. This complexity makes it challenging to compare and contrast colours in an accurate and time efficient manner. Multiple methodologies exist that attempt to digitally quantify colour in complex images but these either require a priori knowledge to assign colours to a particular bin, or fit the colours present within segment of the colour space into a single colour value using a thresholding approach. A major drawback of these methodologies is that, through the process of averaging, they tend to synthetically generate values that may not exist within the context of the original image. As such, to date there are no published methodologies that assess colour patterning using a data driven approach. In this study we present a methodology to acquire and process digital images of biological samples that contain complex colour gradients. The CIE (Commission Internationale de l'Eclairage/International Commission on Illumination) ΔE2000 formula was used to determine the perceptually unique colours (PUC) within images of fruit containing complex colour gradients. This process, on average, resulted in a 98% reduction in colour values from the number of unique colours (UC) in the original image. This data driven procedure summarised the colour data values while maintaining a linear relationship with the normalised colour complexity contained in the total image. A weighted ΔE2000 distance metric was used to generate a distance matrix and facilitated clustering of summarised colour data. Clustering showed that our data driven methodology has the ability to group these complex images into their respective binomial families while maintaining the ability to detect subtle colour differences. This methodology was also able to differentiate closely related images. We provide a high quality set of complex biological images that span the visual spectrum that can be used in future colorimetric research to benchmark colourimetric method development.

The Effect of 1-MCP on the Expression of Carotenoid, Chlorophyll Degradation, and Ethylene Response Factors in 'Qihong' Kiwifruit

The development of yellow color is an important aspect of fruit quality in yellow fleshed kiwifruit during fruit ripening, and it has a large influence on consumer preference. The yellow color is determined by carotenoid accumulation and chlorophyll degradation and is likely affected by ethylene production. This study investigates the expression of carotenoid, chlorophyll degradation, and ethylene response factors in ‘Qihong’ fruit, which had reached the near ripening stage (firmness ≈ 20 N) and were either left untreated (controls) or treated with 0.5 μL L⁻¹ of 1-MCP for 12 h. Both the accumulation of β-carotene (not lutein) and degradation of chlorophyll a and b increased in response to the 1-MCP treatment, resulting in more yellow colored flesh in the 1-MCP treated fruit with higher carotenoid and lower chlorophyll contents. 1-MCP up-regulated AcLCY-β, AcSGR1, and AcPAO2, but reduced the expression of AcCCD1. These four genes were correlated with the concentrations of β-carotene and the chlorophylls. The expression of three ethylene response factors, including Acc29730, Acc25620, and Acc23763 were delayed and down-regulated in 1-MCP treated fruit, showing the highest correlation with the expression of AcLCY-β, AcSGR1, AcPAO2, and AcCCD1. Dual-Luciferase assays showed that 1-MCP treatment not only eliminated the inhibition of Acc23763 on the promoters of both AcPAO2 and AcLCY-β, but also reduced the activation of Acc29730 and Acc25620 on the AcCCD1 promoter. Our findings indicate that Acc29730, Acc25620, and Acc23763 may play an important role in the response to 1-MCP treatment during the fruit eating ripe stage, which likely altered the promoter activities of carotenoid and chlorophyll-related genes (AcPAO2, AcLCY-β and AcCCD1) to regulate their transcripts, resulting in more yellow color in the fruit flesh of ‘Qihong’.

Regulation of wound ethylene biosynthesis by NAC transcription factors in kiwifruit

BACKGROUND

The phytohormone ethylene controls many processes in plant development and acts as a key signaling molecule in response to biotic and abiotic stresses: it is rapidly induced by flooding, wounding, drought, and pathogen attack as well as during abscission and fruit ripening. In kiwifruit (Actinidia spp.), fruit ripening is characterized by two distinct phases: an early phase of system-1 ethylene biosynthesis characterized by absence of autocatalytic ethylene, followed by a late burst of autocatalytic (system-2) ethylene accompanied by aroma production and further ripening. Progress has been made in understanding the transcriptional regulation of kiwifruit fruit ripening but the regulation of system-1 ethylene biosynthesis remains largely unknown. The aim of this work is to better understand the transcriptional regulation of both systems of ethylene biosynthesis in contrasting kiwifruit organs: fruit and leaves.

RESULTS

A detailed molecular study in kiwifruit (A. chinensis) revealed that ethylene biosynthesis was regulated differently between leaf and fruit after mechanical wounding. In fruit, wound ethylene biosynthesis was accompanied by transcriptional increases in 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS), ACC oxidase (ACO) and members of the NAC class of transcription factors (TFs). However, in kiwifruit leaves, wound-specific transcriptional increases were largely absent, despite a more rapid induction of ethylene production compared to fruit, suggesting that post-transcriptional control mechanisms in kiwifruit leaves are more important. One ACS member, AcACS1, appears to fulfil a dominant double role; controlling both fruit wound (system-1) and autocatalytic ripening (system-2) ethylene biosynthesis. In kiwifruit, transcriptional regulation of both system-1 and -2 ethylene in fruit appears to be controlled by temporal up-regulation of four NAC (NAM, ATAF1/2, CUC2) TFs (AcNAC1-4) that induce AcACS1 expression by directly binding to the AcACS1 promoter as shown using gel-shift (EMSA) and by activation of the AcACS1 promoter in planta as shown by gene activation assays combined with promoter deletion analysis.

CONCLUSIONS

Our results indicate that in kiwifruit the NAC TFs AcNAC2-4 regulate both system-1 and -2 ethylene biosynthesis in fruit during wounding and ripening through control of AcACS1 expression levels but not in leaves where post-transcriptional/translational regulatory mechanisms may prevail.

Modifying Carbohydrate Supply to Fruit during Development Changes the Composition and Flavour of Actinidia chinensis var. chinensis 'Zesy002' Kiwifruit

Consumer acceptance of fruit is determined by size, flavour and ripeness. In this study we investigated how altering the carbohydrate supply to Actinidia chinensis var. chinensis 'Zesy002' kiwifruit altered the balance between growth and accumulation of metabolites. Canes were phloem girdled and fruit thinned to a leaf-to-fruit ratio (L:F) of either 2 (Low carbohydrate) or 6 (High carbohydrate) at either 38 (Early) or 86 (Late) days after anthesis (DAA) and compared with ungirdled control canes with a L:F of 3. Fruit growth, metabolite accumulation, cytokinin concentrations and maturation were monitored and the sensory attributes of ripe fruit were assessed. The final weight of Early-High and Late-High carbohydrate fruit was 38% and 16% greater compared with control fruit. High carbohydrate fruit had increased starch, soluble sugar and cytokinin concentrations and fruit began to mature earlier and those with a Low carbohydrate had decreased concentrations and matured later compared with control fruit. Control fruit were described by consumers as more acidic and under-ripe compared with those from Early-High carbohydrate canes, but as sweeter than those from Low carbohydrate canes. This study showed that carbohydrate supply can have a major impact on the growth, sugar accumulation and maturity of 'Zesy002' fruit sinks.

Comparison of fruit morphology and nutrition metabolism in different cultivars of kiwifruit across developmental stages

Kiwifruit (Actinidia) is becoming increasingly popular worldwide due to its favorable flavour and high vitamin C content. However, quality parameters vary among cultivars. To determine the differences in quality and metabolic parameters of kiwifruit, we monitored the growth processes of 'Kuilv' (Actinidia arguta), 'Hongyang' (Actinidia chinensis) and 'Hayward' (Actinidia deliciosa). We found that 'Kuilv' required the shortest time for fruit development, while 'Hayward' needed the longest time to mature. The fruit size of 'Hayward' was the largest and that of 'Kuilv' was the smallest. Furthermore, 'Hongyang' showed a double-S shape of dry matter accumulation, whereas 'Kuilv' and 'Hayward' showed a linear or single-S shape pattern of dry matter accumulation during development. The three cultivars demonstrated the same trend for total soluble solids accumulation, which did not rise rapidly until 90-120 days after anthesis. However, the accumulation of organic acids and soluble sugars varied among the cultivars. During later fruit development, the content of glucose, fructose and quinic acid in 'Kuilv' fruit was far lower than that in 'Hongyang' and 'Hayward'. On the contrary, 'Kuilv' had the highest sucrose content among the three cultivars. At maturity, the antioxidative enzymatic systems were significantly different among the three kiwifruit cultivars. 'Hongyang' showed higher activities of superoxide dismutase than the other cultivars, while the catalase content of 'Hayward' was significantly higher than that of 'Hongyang' and 'Kuilv'. These results provided knowledge that could be implemented for the marketing, handling and post-harvest technologies of the different kiwifruit cultivars.

The Pollination and Fruit Quality of Two Kiwifruit Cultivars (Actinidia chinensis var. chinensis 'AU Golden Sunshine' and 'AU Gulf Coast Gold') (Ericales: Actinidiaceae) Grown in the Southeastern United States

Kiwifruit is a new emerging crop for the southeastern United States that requires cross-pollination to set fruit. However, the pollination requirements for varieties grown in the southeastern United States are unknown. Through insect surveys and a bagging experiment, we assessed the pollination requirements of two female kiwifruit cultivars (Actinidia chinensis var. chinensis 'AU Golden Sunshine' and A. chinensis var. chinensis 'AU Gulf Coast Gold'). For each, fruit quantity (fruit set) and fruit quality (weight, size, seed count, firmness, soluble solid content, and dry matter) were compared among three pollination treatments (wind, insect, and artificial pollination). Low abundances of insects were observed visiting female flowers of both kiwifruit cultivars, and therefore likely minimally influenced kiwifruit pollination. Artificial pollination resulted in the greatest percentages of fruit set and marketable fruits, followed by insect and wind pollination. Artificial pollination resulted in fruits that were greater in weight, size, and contained more seeds, than insect- and wind-pollinated fruits. Firmness and soluble solid content did not vary greatly between pollination treatments, yet were greater in 'AU Golden Sunshine'. Dry matter content did not vary greatly between pollination treatments or between each cultivar. To maximize yields and optimize fruit quality, these results suggest that kiwifruit producers should place more effort into artificial pollination compared to wind and insect pollination. Future research should explore the use of managed bees (e.g., honey bees and bumble bees) within kiwifruit orchards to determine ways to utilize them as a secondary source for pollination needs.

A gene expression atlas for kiwifruit (Actinidia chinensis) and network analysis of transcription factors

BACKGROUND

Transcriptomic studies combined with a well annotated genome have laid the foundations for new understanding of molecular processes. Tools which visualise gene expression patterns have further added to these resources. The manual annotation of the Actinidia chinensis (kiwifruit) genome has resulted in a high quality set of 33,044 genes. Here we investigate gene expression patterns in diverse tissues, visualised in an Electronic Fluorescent Pictograph (eFP) browser, to study the relationship of transcription factor (TF) expression using network analysis.

RESULTS

Sixty-one samples covering diverse tissues at different developmental time points were selected for RNA-seq analysis and an eFP browser was generated to visualise this dataset. 2839 TFs representing 57 different classes were identified and named. Network analysis of the TF expression patterns separated TFs into 14 different modules. Two modules consisting of 237 TFs were correlated with floral bud and flower development, a further two modules containing 160 TFs were associated with fruit development and maturation. A single module of 480 TFs was associated with ethylene-induced fruit ripening. Three "hub" genes correlated with flower and fruit development consisted of a HAF-like gene central to gynoecium development, an ERF and a DOF gene. Maturing and ripening hub genes included a KNOX gene that was associated with seed maturation, and a GRAS-like TF.

CONCLUSIONS

This study provides an insight into the complexity of the transcriptional control of flower and fruit development, as well as providing a new resource to the plant community. The Actinidia eFP browser is provided in an accessible format that allows researchers to download and work internally.

Proteomics and Metabolomics Reveal the Regulatory Pathways of Ripening and Quality in Post-Harvest Kiwifruits

Understanding the metabolic modulation of major quality traits during ripening is critical for fruit quality improvement in kiwifruits. Here, integrated proteomic and metabolomic profiling was undertaken to comprehensively examine the dynamics of kiwifruit ripening. This data set presents a global view of the critical pathways involved in fruit ripening, and the contributions of key events to the regulation of kiwifruit ripening and softening, amino acid metabolism, balance in sugar accumulation and organic acid metabolism, glycolysis, and tricarboxylic acid (TCA) pathways were discussed. We suggested key enzymes for starch synthesis and degradation, including AGPase, SS, and SBE, especially for BMY, which was considered a key enzyme for starch degradation. In addition, our analysis implicated the key enzymes ACO4 and ACS9 in ethylene synthesis and the aspartate aminotransferase ASP3 in the conversion of amino acids. These results provide new insights into the modulation of fruit ripening, metabolism, and quality in post-harvest kiwifruits.

Genome-wide identification and comprehensive analysis of NAC family genes involved in fruit development in kiwifruit (Actinidia)

BACKGROUND

NAC transcription factors (TFs) are plant-specific proteins encoded by a large gene family. They play important roles in diverse biological processes, such as plant growth and development, leaf senescence, and responses to biotic or abiotic stresses. Functions of a number of NAC TFs have been identified mainly in model plants. However, very few studies on NAC TFs have been conducted in the fruit tree of kiwifruit.

RESULTS

Genome-wide NAC genes were identified and their phylogeny, genomic structure, chromosomal location, synteny relationships, protein properties and conserved motifs were analyzed. In addition, the fruit developmental process was evaluated in a new kiwifruit cultivar of Actinidia eriantha 'Ganlu 1'. And expressions for all those NAC genes were analyzed by quantitative real-time PCR method in fruits of 'Ganlu 1' during its developmental process. Our research identified 142 NAC TFs which could be phylogenetically divided into 23 protein subfamilies. The genomic structures of those NAC genes indicated that their exons were between one and ten. Analysis of chromosomal locations suggested that 116 out of 142 NACs distributed on all the 29 kiwifruit chromosomes. In addition, genome-wide gene expression analysis showed that expressions of 125 out of 142 NAC genes could be detected in fruit samples.

CONCLUSION

Our comprehensive study provides novel information on NAC genes and expression patterns in kiwifruit fruit. This research would be helpful for future functional identification of NAC genes involved in kiwifruit fruit development.

Correlation between fruit weight and nutritional metabolism during development in CPPU-treated Actinidia chinensis 'Hongyang'

Forchlorfenuron, N-(2-chloro-4-pyridyl)-N-phenylurea (CPPU), is often used to promote fruit growth and improve production. The role of CPPU in kiwifruit growth has been established. However, the correlation between fruit weight and nutritional metabolism during development after CPPU treatments remains largely undetermined. Here, we surveyed the variations in weight and nutrient components of the 'Hongyang' kiwifruit (Actinidia chinensis) when CPPU was sprayed on fruit 25 days after anthesis. The CPPU application did not significantly influence the dry matter, soluble solids, starch, vitamin C or protein concentrations. However, the fresh weight, length and maximum diameter were significantly increased compared with the control. Moreover, in fruit of the same developmental stage, the fructose, glucose and soluble sugar levels increased after the CPPU treatment, compared with the control. On the contrary, citric, quinic and titratable acid concentrations decreased. However, a correlation analysis between fresh weight and the nutritional contents revealed that CPPU did not affect the concentrations of the most abundant organic acids (quinic and citric) and sugars (glucose, fructose and sucrose), compared with control fruit of the same weight. Therefore, CPPU applications enhance 'Hongyang' kiwifruit weight/size. However, there were no significant differences in the nutritional qualities of treated and untreated fruit having the same weights.

Fruit From Two Kiwifruit Genotypes With Contrasting Softening Rates Show Differences in the Xyloglucan and Pectin Domains of the Cell Wall

Fruit softening is controlled by hormonal and developmental cues, causing an upregulation of cell wall-associated enzymes that break down the complex sugar matrices in the cell wall. The regulation of this process is complex, with different genotypes demonstrating quite different softening patterns, even when they are closely related. Currently, little is known about the relationship between cell wall structure and the rate of fruit softening. To address this question, the softening of two Actinidia chinensis var. chinensis (kiwifruit) genotypes (a fast 'AC-F' and a slow 'AC-S' softening genotype) was examined using a range of compositional, biochemical, structural, and molecular techniques. Throughout softening, the cell wall structure of the two genotypes was fundamentally different at identical firmness stages. In the hemicellulose domain, xyloglucanase enzyme activity was higher in 'AC-F' at the firm unripe stage, a finding supported by differential expression of xyloglucan transglycosylase/hydrolase genes during softening. In the pectin domain, differences in pectin solubilization and location of methyl-esterified homogalacturonan in the cell wall between 'AC-S' and 'AC-F' were shown. Side chain analyses and molecular weight elution profiles of polyuronides and xyloglucans of cell wall extracts revealed fundamental differences between the genotypes, pointing towards a weakening of the structural integrity of cell walls in the fast softening 'AC-F' genotype even at the firm, unripe stage. As a consequence, the polysaccharides in the cell walls of 'AC-F' may be easier to access and hence more susceptible to enzymatic degradation than in 'AC-S', resulting in faster softening. Together these results suggest that the different rates of softening between 'AC-F' and 'AC-S' are not due to changes in enzyme activities alone, but that fundamental differences in the cell wall structure are likely to influence the rates of softening through differential modification and accessibility of specific cell wall polysaccharides during ripening.

Nutritional component analyses of kiwifruit in different development stages by metabolomic and transcriptomic approaches

BACKGROUND

Metabolites in kiwifruit greatly influence nutritional values; however, the dynamic changes in nutrient composition and the gene expression level of yellow kiwifruit have not been studied so far. To investigate the types and accumulation patterns of metabolites, a metabolomics approach utilizing liquid chromatography-electrospray ionization mass spectrometry and transcriptomics were used to analyze the yellow flesh of kiwifruit cultivar 'jinshi 1' collected at different stages of days after full bloom.

RESULTS

In total, 285 metabolites were identified over the kiwifruit developmental stages. The composition of the metabolites of kiwifruit at different stages of development was different. The organic acids contents and their derivatives were higher at the initial stage of development and then gradually decreased. The lipids and amino acids contents fluctuated at different stages of development but did not change significantly. Transcript profiles throughout yellow kiwifruit development were constructed and analyzed, with a focus on the biosynthesis and metabolism of compounds such as sugars, organic acids and ascorbic acid, which are indispensable for the development and formation of quality fruit. The transcript levels of genes involved in sucrose and starch metabolism were consistent with the change in soluble sugar and starch content throughout kiwifruit development. The metabolism of ascorbic acid was primarily through the l-galactose pathway.

CONCLUSION

Our metabolome and transcriptome approach identified dynamic changes in five types of nutrient metabolite levels, and correlations among such levels, in developing fruit. The results provide information that can be used by metabolic engineers and molecular breeders to improve kiwifruit quality. © 2020 Society of Chemical Industry.

Carbon starvation reduces carbohydrate and anthocyanin accumulation in red-fleshed fruit via trehalose 6-phosphate and MYB27

Kiwifruit (Actinidia spp.) is a recently domesticated fruit crop with several novel-coloured cultivars being developed. Achieving uniform fruit flesh pigmentation in red genotypes is challenging. To investigate the cause of colour variation between fruits, we focused on a red-fleshed Actinidia chinensis var. chinensis genotype. It was hypothesized that carbohydrate supply could be responsible for this variation. Early in fruit development, we imposed high or low (carbon starvation) carbohydrate supplies treatments; carbohydrate import or redistribution was controlled by applying a girdle at the shoot base. Carbon starvation affected fruit development as well as anthocyanin and carbohydrate metabolite concentrations, including the signalling molecule trehalose 6-phosphate. RNA-Seq analysis showed down-regulation of both gene-encoding enzymes in the anthocyanin and carbohydrate biosynthetic pathways. The catalytic trehalose 6-phosphate synthase gene TPS1.1a was down-regulated, whereas putative regulatory TPS7 and TPS11 were strongly up-regulated. Unexpectedly, under carbon starvation MYB10, the anthocyanin pathway regulatory activator was slightly up-regulated, whereas MYB27 was also up-regulated and acts as a repressor. To link these two metabolic pathways, we propose a model where trehalose 6-phosphate and the active repressor MYB27 are involved in sensing the carbon starvation status. This signals the plant to save resources and reduce the production of anthocyanin in fruits.

Genome-Wide Identification and Expression Analysis of Polygalacturonase Gene Family in Kiwifruit (Actinidia chinensis) during Fruit Softening

Polygalacturonase (PG) is an essential hydrolytic enzyme responsible for pectin degradation and thus plays an important role in fruit softening and other cell separation processes. PG protein is encoded by a multigene family, however, the members of PG gene family in kiwifruit (Actinidia chinensis) have not been extensively identified. In this study, a total of 51 AcPG genes in kiwifruit genome were identified. They are phylogenetically clustered into seven clades, and of them AcPG4 and AcPG18 with other known PG genes involved in fruit softening from peach, pear, papaya and melon form a small cluster together. The members of kiwifruit PG gene family consist of three to nine exons and two to eight introns, and their exon/intron structures are generally conserved in all clades except the clade D and E. During fruit softening of kiwifruit 'Donghong' under ambient temperature, cell wall modifying enzymes, including PG, PL (pectate and pectin lyases), and PE (pectinesterase, also known as pectin methylesterase, PME) showed a different activity profile, and of them, PG and PE activities largely correlated with the change of pectin content and firmness. Moreover, only 11 AcPG genes were highly or moderately expressed in softening fruit, and of which three AcPG genes (AcPG4, AcPG18, and AcPG8, especially the former two) has been found to strongly correlate with the profile of PG activity and pectin content, as well as fruit firmness, suggesting that they maybe play an important role in fruit softening. Thus, our findings not only benefit the functional characterization of kiwifruit PG genes, but also provide a subset of potential PG candidate genes for further genetic manipulation.

Phytohormone and Transcriptomic Analysis Reveals Endogenous Cytokinins Affect Kiwifruit Growth under Restricted Carbon Supply

Following cell division, fruit growth is characterized by both expansion through increases in cell volume and biomass accumulation in cells. Fruit growth is limited by carbon starvation; however, the mechanism controlling fruit growth under restricted carbohydrate supply is poorly understood. In a previous study using red-fleshed kiwifruit, we showed that long-term carbon starvation had detrimental effects on carbohydrate, anthocyanin metabolism, and fruit growth. To elucidate the mechanisms underlying the reduction in fruit growth during kiwifruit development, we integrated phytohormone profiling with transcriptomic and developmental datasets for fruit under high or low carbohydrate supplies. Phytohormone profiling of the outer pericarp tissue of kiwifruit showed a 6-fold reduction in total cytokinin concentrations in carbon-starved fruit, whilst other hormones were less affected. Principal component analysis visualised that cytokinin composition was distinct between fruit at 16 weeks after mid bloom, based on their carbohydrate supply status. Cytokinin biosynthetic genes (IPT, CYP735A) were significantly downregulated under carbon starvation, in agreement with the metabolite data. Several genes that code for expansins, proteins involved in cell wall loosening, were also downregulated under carbon starvation. In contrast to other fleshy fruits, our results suggest that cytokinins not only promote cell division, but also drive fruit cell expansion and growth in kiwifruit.

Fruit Salad in the Lab: Comparing Botanical Species to Help Deciphering Fruit Primary Metabolism

Although fleshy fruit species are economically important worldwide and crucial for human nutrition, the regulation of their fruit metabolism remains to be described finely. Fruit species differ in the origin of the tissue constituting the flesh, duration of fruit development, coordination of ripening changes (climacteric vs. non-climacteric type) and biochemical composition at ripeness is linked to sweetness and acidity. The main constituents of mature fruit result from different strategies of carbon transport and metabolism. Thus, the timing and nature of phloem loading and unloading can largely differ from one species to another. Furthermore, accumulations and transformations of major soluble sugars, organic acids, amino acids, starch and cell walls are very variable among fruit species. Comparing fruit species therefore appears as a valuable way to get a better understanding of metabolism. On the one hand, the comparison of results of studies about species of different botanical families allows pointing the drivers of sugar or organic acid accumulation but this kind of comparison is often hampered by heterogeneous analysis approaches applied in each study and incomplete dataset. On the other hand, cross-species studies remain rare but have brought new insights into key aspects of primary metabolism regulation. In addition, new tools for multi-species comparisons are currently emerging, including meta-analyses or re-use of shared metabolic or genomic data, and comparative metabolic flux or process-based modeling. All these approaches contribute to the identification of the metabolic factors that influence fruit growth and quality, in order to adjust their levels with breeding or cultural practices, with respect to improving fruit traits.

Copy number variants in kiwifruit ETHYLENE RESPONSE FACTOR/APETALA2 (ERF/AP2)-like genes show divergence in fruit ripening associated cold and ethylene responses in C-REPEAT/DRE BINDING FACTOR-like genes

The ETYHLENE RESPONSE FACTOR/APETALA2 (ERF/AP2) transcription factors have been shown to control a wide range of developmental and environmental responses in plants. These include hormonal responses to ethylene and Abscisic Acid (ABA) as well as to cold and drought. In Actinidia chinensis (kiwifruit), ripening is unusual: although it is sometimes classed as a climacteric fruit (ethylene-associated ripening), much of fruit ripening occurs independently from autocatalytic ethylene production. Initiation of ripening appears to be strongly developmentally controlled and modulated by low temperature. In this study, fruit treated with different temperatures showed an increase in soluble sugar accumulation, and a corresponding increase in ß-AMYLASE (BAM) genes (predominantly BAM3.2 and BAM9) with lower temperatures. To investigate the potential role of the AP2/ERF gene family in the control of fruit ripening in kiwifruit this family was investigated further. Using the new genome annotation and further genome sequence analysis we identified 226 ERF-like genes, 10 AP2L/RAV-like genes and 32 AP2-like genes. An RNA-seq screen from kiwifruit of different maturities, and following treatment with ethylene and temperatures between 0 and 16°C, revealed 4%, 26% and 18% of the ERF-like genes were upregulated by maturation, ethylene and cold temperatures, respectively. Focusing on the C-REPEAT/DRE BINDING FACTOR (CBF) cold master regulators, nine potential genes were identified based on sequence similarity. Five of these CBF-like genes were found in a copy number variant (CNV) cluster of six genes on chromosome 14. Expression analysis showed that two homeologous genes (ERF41 and ERF180) increased in abundance with cold and ethylene, while the cluster of CNV CBF-like genes had lost the ability to respond to cold and increased only with ethylene, suggesting an evolutionary progression of function of these genes.

Untargeted and Targeted Metabolomics and Tryptophan Decarboxylase In Vivo Characterization Provide Novel Insight on the Development of Kiwifruits (Actinidia deliciosa)

Kiwifruit (Actinidia deliciosa cv. Hayward) is a commercially important crop with highly nutritional green fleshy fruits. The post-harvest maturation of the fruits is well characterized, but little is known about the metabolic changes that occur during fruit development. Here we used untargeted metabolomics to characterize the non-volatile metabolite profile of kiwifruits collected at different time points after anthesis, revealing profound metabolic changes before the onset of ripening including the depletion of many classes of phenolic compounds. In contrast, the phytohormone abscisic acid accumulated during development and ripening, along with two indolamines (serotonin and its precursor tryptamine), and these were monitored in greater detail by targeted metabolomics. The role of indolamines in kiwifruit development is completely unknown, so we also characterized the identity of genes encoding tryptophan decarboxylase in A. deliciosa and its close relative A. chinensis to provide insight into the corresponding biological processes. Our results indicate that abscisic acid and indolamines fulfill unrecognized functions in the development and ripening of kiwifruits.

Unveiling Kiwifruit Metabolite and Protein Changes in the Course of Postharvest Cold Storage

Actinidia deliciosa cv. Hayward fruit is renowned for its micro- and macronutrients, which vary in their levels during berry physiological development and postharvest processing. In this context, we have recently described metabolic pathways/molecular effectors in fruit outer endocarp characterizing the different stages of berry physiological maturation. Here, we report on the kiwifruit postharvest phase through an integrated approach consisting of pomological analysis combined with NMR/LC-UV/ESI-IT-MSⁿ- and 2D-DIGE/nanoLC-ESI-LIT-MS/MS-based proteometabolomic measurements. Kiwifruit samples stored under conventional, cold-based postharvest conditions not involving the use of dedicated chemicals were sampled at four stages (from fruit harvest to pre-commercialization) and analyzed in comparison for pomological features, and outer endocarp metabolite and protein content. About 42 metabolites were quantified, together with corresponding proteomic changes. Proteomics showed that proteins associated with disease/defense, energy, protein destination/storage, cell structure and metabolism functions were affected at precise fruit postharvest times, providing a justification to corresponding pomological/metabolite content characteristics. Bioinformatic analysis of variably represented proteins revealed a central network of interacting species, modulating metabolite level variations during postharvest fruit storage. Kiwifruit allergens were also quantified, demonstrating in some cases their highest levels at the fruit pre-commercialization stage. By lining up kiwifruit postharvest processing to a proteometabolomic depiction, this study integrates previous observations on metabolite and protein content in postharvest berries treated with specific chemical additives, and provides a reference framework for further studies on the optimization of fruit storage before its commercialization.

AcFT promotes kiwifruit in vitro flowering when overexpressed and Arabidopsis flowering when expressed in the vasculature under its own promoter

Kiwifruit (Actinidia chinensis) has three FLOWERING LOCUS T (FT) genes, AcFT, AcFT1, and AcFT2, with differential expression and potentially divergent roles. AcFT was previously shown to be expressed in source leaves and induced in dormant buds by winter chilling. Here, we show that AcFT promotes flowering in A. chinensis, despite a short sequence insertion not present in other FT-like genes. A 3.5-kb AcFT promoter region contained all the regulatory elements required to mediate vascular expression in transgenic Arabidopsis thaliana (Arabidopsis). The promoter activation was initially confined to the veins in the distal end of the leaf, before extending to the veins in the base of the leaf, and was detected in inductive and noninductive photoperiods. The 3-kb and 2.7-kb promoter regions of AcFT1 and AcFT2, respectively, demonstrated different activation patterns in Arabidopsis, corresponding to differential expression in kiwifruit. Expression of AcFT cDNA from the AcFT promoter was capable to induce early flowering in transgenic Arabidopsis in noninductive photoperiods. Further, expression of AcFT cDNA fused to the green fluorescent protein was detected in the vasculature and was also capable to advance flowering in noninductive photoperiods. Taken together, these studies implicate AcFT in regulation of kiwifruit flowering time and as a candidate for kiwifruit florigen.

Comparative transcriptome analysis reveals distinct ethylene-independent regulation of ripening in response to low temperature in kiwifruit

BACKGROUND

Kiwifruit are classified as climacteric since exogenous ethylene (or its analogue propylene) induces rapid ripening accompanied by ethylene production under positive feedback regulation. However, most of the ripening-associated changes (Phase 1 ripening) in kiwifruit during storage and on-vine occur largely in the absence of any detectable ethylene. This ripening behavior is often attributed to basal levels of system I ethylene, although it is suggested to be modulated by low temperature.

RESULTS

To elucidate the mechanisms regulating Phase 1 ripening in kiwifruit, a comparative transcriptome analysis using fruit continuously exposed to propylene (at 20 °C), and during storage at 5 °C and 20 °C was conducted. Propylene exposure induced kiwifruit softening, reduction of titratable acidity (TA), increase in soluble solids content (SSC) and ethylene production within 5 days. During storage, softening and reduction of TA occurred faster in fruit at 5 °C compared to 20 °C although no endogenous ethylene production was detected. Transcriptome analysis revealed 3761 ripening-related differentially expressed genes (DEGs), of which 2742 were up-regulated by propylene while 1058 were up-regulated by low temperature. Propylene exclusively up-regulated 2112 DEGs including those associated with ethylene biosynthesis and ripening such as AcACS1, AcACO2, AcPL1, AcXET1, Acβ-GAL, AcAAT, AcERF6 and AcNAC7. Similarly, low temperature exclusively up-regulated 467 DEGS including AcACO3, AcPL2, AcPMEi, AcADH, Acβ-AMY2, AcGA2ox2, AcNAC5 and AcbZIP2 among others. A considerable number of DEGs such as AcPG, AcEXP1, AcXET2, Acβ-AMY1, AcGA2ox1, AcNAC6, AcMADS1 and AcbZIP1 were up-regulated by either propylene or low temperature. Frequent 1-MCP treatments failed to inhibit the accelerated ripening and up-regulation of associated DEGs by low temperature indicating that the changes were independent of ethylene. On-vine kiwifruit ripening proceeded in the absence of any detectable endogenous ethylene production, and coincided with increased expression of low temperature-responsive DEGs as well as the decrease in environmental temperature.

CONCLUSIONS

These results indicate that kiwifruit possess both ethylene-dependent and low temperature-modulated ripening mechanisms that are distinct and independent of each other. The current work provides a foundation for elaborating the control of these two ripening mechanisms in kiwifruit.

Elucidating the Mechanisms of the Tomato ovate Mutation in Regulating Fruit Quality Using Proteomics Analysis

The ovate mutation has frequently been used to study changes in fruit shape but not fruit quality. A deterioration in fruit quality associated with the ovate mutation was discovered in this study. To elucidate how ovate influences the quality of fruit, we performed a proteomics analysis of the fruits of the ovate mutant (LA3543) and wild-type ("Ailsa Craig", LA2838A) using tandem mass tag analysis. The results indicated that the ovate mutation significantly influences fruit quality in a number of ways, including by reducing the expression of 1-aminocyclopropane-1-carboxylic acid oxidase 3 (ACO3) in ethylene biosynthesis, improving firmness by reducing the amount of pectinesterase and polygalacturonase, reducing sugar accumulation by downregulating the abundance of mannan endo-1,4-β-mannosidase 4, β-galactosidase, and β-amylase, and reducing the malic acid content by downregulating the accumulation of malic enzymes and malate synthase. These findings could inform future improvements in fruit quality.

Postharvest application of brassica meal-derived allyl-isothiocyanate to kiwifruit: effect on fruit quality, nutraceutical parameters and physiological response

The use of natural compounds to preserve fruit quality and develop high value functional products deserves attention especially in the growing industry of processing and packaging ready-to-eat fresh-cut fruit. In this work, potential mechanisms underlying the effects of postharvest biofumigation with brassica meal-derived allyl-isothiocyanate on the physiological responses and quality of 'Hayward' kiwifruits were studied. Fruits were treated with 0.15 mg L-1 of allyl-isothiocyanate vapours for 5 h and then stored in controlled atmosphere (2% O2, 4.5% CO2) at 0 °C and 95% relative humidity, maintaining an ethylene concentration <0.02 μL L-1. The short- and long-term effects of allyl-isothiocyanate on fruit quality traits, nutraceutical attributes, glutathione content, antiradical capacity and the activity of antioxidant enzymes were investigated. The treatment did not influence the overall fruit quality after 120 days of storage, but interestingly it enhanced the ascorbic acid, polyphenols and flavan-3-ol content, improving the antioxidant potential of kiwifruit. The short-term effect of allyl-isothiocyanate was evidenced by an increase of superoxide dismutase activity and of oxidative glutathione redox state, which were restored 24 h after the treatment. The expression levels of genes involved in detoxification functions, ethylene, ascorbate and phenylpropanoid biosynthesis, were also significantly affected upon allyl-isothiocyanate application. These results suggest that allyl-isothiocyanate treatment probably triggered an initial oxidative burst, followed by an induction of protective mechanisms, which finally increased the nutraceutical and technological value of treated kiwifruits.

Comparative Transcriptional Analysis of Loquat Fruit Identifies Major Signal Networks Involved in Fruit Development and Ripening Process

Loquat (Eriobotrya japonica Lindl.) is an important non-climacteric fruit and rich in essential nutrients such as minerals and carotenoids. During fruit development and ripening, thousands of the differentially expressed genes (DEGs) from various metabolic pathways cause a series of physiological and biochemical changes. To better understand the underlying mechanism of fruit development, the Solexa/Illumina RNA-seq high-throughput sequencing was used to evaluate the global changes of gene transcription levels. More than 51,610,234 high quality reads from ten runs of fruit development were sequenced and assembled into 48,838 unigenes. Among 3256 DEGs, 2304 unigenes could be annotated to the Gene Ontology database. These DEGs were distributed into 119 pathways described in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. A large number of DEGs were involved in carbohydrate metabolism, hormone signaling, and cell-wall degradation. The real-time reverse transcription (qRT)-PCR analyses revealed that several genes related to cell expansion, auxin signaling and ethylene response were differentially expressed during fruit development. Other members of transcription factor families were also identified. There were 952 DEGs considered as novel genes with no annotation in any databases. These unigenes will serve as an invaluable genetic resource for loquat molecular breeding and postharvest storage.

Chlorophyll metabolism in pollinated vs. parthenocarpic fig fruits throughout development and ripening

Expression of 13 genes encoding chlorophyll biosynthesis and degradation was evaluated. Chlorophyll degradation was differentially regulated in pollinated and parthenocarpic fig fruits, leading to earlier chlorophyll degradation in parthenocarpic fruits. Varieties of the common fig typically yield a commercial summer crop that requires no pollination, although it can be pollinated. Fig fruit pollination results in larger fruit size, greener skin and darker interior inflorescence color, and slows the ripening process compared to non-pollinated fruits. We evaluated the effect of pollination on chlorophyll content and levels of transcripts encoding enzymes of the chlorophyll metabolism in fruits of the common fig 'Brown Turkey'. We cloned and evaluated the expression of 13 different genes. All 13 genes showed high expression in the fruit skin, inflorescences and leaves, but extremely low expression in roots. Pollination delayed chlorophyll breakdown in the ripening fruit skin and inflorescences. This was correlated with the expression of genes encoding enzymes in the chlorophyll biosynthesis and degradation pathways. Expression of pheophorbide a oxygenase (PAO) was strongly negatively correlated with chlorophyll levels during ripening in pollinated fruits; along with its high expression levels in yellow leaves, this supports a pivotal role for PAO in chlorophyll degradation in figs. Normalizing expression levels of all chlorophyll metabolism genes in the pollinated and parthenocarpic fruit skin and inflorescences showed three synthesis (FcGluTR1, FcGluTR2 and FcCLS1) and three degradation (FcCLH1, FcCLH2 and FcRCCR1) genes with different temporal expression in the pollinated vs. parthenocarpic fruit skin and inflorescences. FcCAO also showed different expressions in the parthenocarpic fruit skin. Thus, chlorophyll degradation is differentially regulated in the pollinated and parthenocarpic fruit skin and inflorescences, leading to earlier and more sustained chlorophyll degradation in the parthenocarpic fruit.

Identification and validation of reference genes for accurate normalization of real-time quantitative PCR data in kiwifruit

Identification and validation of reference genes are required for the normalization of qPCR data. We studied the expression stability produced by eight primer pairs amplifying four common genes used as references for normalization. Samples representing different tissues, organs and developmental stages in kiwifruit (Actinidia chinensis var. deliciosa (A. Chev.) A. Chev.) were used. A total of 117 kiwifruit samples were divided into five sample sets (mature leaves, axillary buds, stigmatic arms, fruit flesh and seeds). All samples were also analysed as a single set. The expression stability of the candidate primer pairs was tested using three algorithms (geNorm, NormFinder and BestKeeper). The minimum number of reference genes necessary for normalization was also determined. A unique primer pair was selected for amplifying the 18S rRNA gene. The primer pair selected for amplifying the ACTIN gene was different depending on the sample set. 18S 2 and ACT 2 were the candidate primer pairs selected for normalization in the three sample sets (mature leaves, fruit flesh and stigmatic arms). 18S 2 and ACT 3 were the primer pairs selected for normalization in axillary buds. No primer pair could be selected for use as the reference for the seed sample set. The analysis of all samples in a single set did not produce the selection of any stably expressing primer pair. Considering data previously reported in the literature, we validated the selected primer pairs amplifying the FLOWERING LOCUS T gene for use in the normalization of gene expression in kiwifruit.

Comprehensive Transcriptome Profiling Reveals Long Noncoding RNA Expression and Alternative Splicing Regulation during Fruit Development and Ripening in Kiwifruit (Actinidia chinensis)

Genomic and transcriptomic data on kiwifruit (Actinidia chinensis) in public databases are very limited despite its nutritional and economic value. Previously, we have constructed and sequenced nine fruit RNA-Seq libraries of A. chinensis "Hongyang" at immature, mature, and postharvest ripening stages of fruit development, and generated over 66.2 million paired-end and 24.4 million single-end reads. From this dataset, here we have identified 7051 long noncoding RNAs (lncRNAs), 29,327 alternative splicing (AS) events and 2980 novel protein-coding genes that were not annotated in the draft genome of "Hongyang." AS events were demonstrated in genes involved in the synthesis of nutritional metabolites in fruit, such as ascorbic acids, carotenoids, anthocyanins, and chlorophylls, and also in genes in the ethylene signaling pathway, which plays an indispensable role in fruit ripening. Additionally, transcriptome profiles and the contents of sugars, organic and main amino acids were compared between immature, mature, and postharvest ripening stages in kiwifruits. A total of 5931 differentially expressed genes were identified, including those associated with the metabolism of sugar, organic acid, and main amino acids. The data generated in this study provide a foundation for further studies of fruit development and ripening in kiwifruit, and identify candidate genes and regulatory elements that could serve as targets for improving important agronomic traits through marker assisted breeding and biotechnology.

The hybrid non-ethylene and ethylene ripening response in kiwifruit (Actinidia chinensis) is associated with differential regulation of MADS-box transcription factors

BACKGROUND

Ripening in tomato is predominantly controlled by ethylene, whilst in fruit such as grape, it is predominantly controlled by other hormones. The ripening response of many kiwifruit (Actinidia) species is atypical. The majority of ripening-associated fruit starch hydrolysis, colour change and softening occurs in the apparent absence of ethylene production (Phase 1 ripening) whilst Phase 2 ripening requires autocatalytic ethylene production and is associated with further softening and an increase in aroma volatiles.

RESULTS

To dissect the ripening response in the yellow-fleshed kiwifruit A. chinensis ('Hort16A'), a two dimensional developmental stage X ethylene response time study was undertaken. As fruit progressed through maturation and Phase 1 ripening, fruit were treated with different concentrations of propylene and ethylene. At the start of Phase 1 ripening, treated fruit responded to ethylene, and were capable of producing endogenous ethylene. As the fruit progressed through Phase 1 ripening, the fruit became less responsive to ethylene and endogeneous ethylene production was partially repressed. Towards the end of Phase 1 ripening the fruit were again able to produce high levels of ethylene. Progression through Phase 1 ripening coincided with a developmental increase in the expression of the ethylene-unresponsive MADS-box FRUITFUL-like gene (FUL1). The ability to respond to ethylene however coincided with a change in expression of another MADS-box gene SEPALLATA4/RIPENING INHIBITOR-like (SEP4/RIN). The promoter of SEP4/RIN was shown to be transactivated by EIN3-like transcription factors, but unlike tomato, not by SEP4/RIN itself. Transient over-expression of SEP4/RIN in kiwifruit caused an increase in ethylene production.

CONCLUSIONS

These results suggest that the non-ethylene/ethylene ripening response observed in kiwifruit is a hybrid of both the tomato and grape ripening progression, with Phase 1 being akin to the RIN/ethylene inhibitory response observed in grape and Phase 2 akin to the RIN-associated autocatalytic ethylene response observed in tomato.

Role for the banana AGAMOUS-like gene MaMADS7 in regulation of fruit ripening and quality

MADS-box transcription factors play important roles in organ development. In plants, most studies on MADS-box genes have mainly focused on flower development and only a few concerned fruit development and ripening. A new MADS-box gene named MaMADS7 was isolated from banana fruit by rapid amplification of cDNA ends (RACE) based on a MADS-box fragment obtained from a banana suppression subtractive hybridization (SSH) cDNA library. MaMADS7 is an AGAMOUS-like MADS-box gene that is preferentially expressed in the ovaries and fruits and in tobacco its protein product localizes to the nucleus. This study found that MaMADS7 expression can be induced by exogenous ethylene. Ectopic expression of MaMADS7 in tomato resulted in broad ripening phenotypes. The expression levels of seven ripening and quality-related genes, ACO1, ACS2, E4, E8, PG, CNR and PSY1 in MaMADS7 transgenic tomato fruits were greatly increased while the expression of the AG-like MADS-box gene TAGL1 was suppressed. Compared with the control, the contents of β-carotene, lycopene, ascorbic acid and organic acid in transformed tomato fruits were increased, while the contents of glucose and fructose were slightly decreased. MaMADS7 interacted with banana 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase gene 1 (MaACO1) and tomato phytoene synthase gene (LePSY1) promoters. Our results indicated that MaMADS7 plays an important role in initiating endogenous ethylene biosynthesis and fruit ripening.

Transcriptome analysis of ripe and unripe fruit tissue of banana identifies major metabolic networks involved in fruit ripening process

BACKGROUND

Banana is one of the most important crop plants grown in the tropics and sub-tropics. It is a climacteric fruit and undergoes ethylene dependent ripening. Once ripening is initiated, it proceeds at a fast rate making postharvest life short, which can result in heavy economic losses. During the fruit ripening process a number of physiological and biochemical changes take place and thousands of genes from various metabolic pathways are recruited to produce a ripe and edible fruit. To better understand the underlying mechanism of ripening, we undertook a study to evaluate global changes in the transcriptome of the fruit during the ripening process.

RESULTS

We sequenced the transcriptomes of the unripe and ripe stages of banana (Musa accuminata; Dwarf Cavendish) fruit. The transcriptomes were sequenced using a 454 GSFLX-Titanium platform that resulted in more than 7,00,000 high quality (HQ) reads. The assembly of the reads resulted in 19,410 contigs and 92,823 singletons. A large number of the differentially expressed genes identified were linked to ripening dependent processes including ethylene biosynthesis, perception and signalling, cell wall degradation and production of aromatic volatiles. In the banana fruit transcriptomes, we found transcripts included in 120 pathways described in the KEGG database for rice. The members of the expansin and xyloglucan transglycosylase/hydrolase (XTH) gene families were highly up-regulated during ripening, which suggests that they might play important roles in the softening of the fruit. Several genes involved in the synthesis of aromatic volatiles and members of transcription factor families previously reported to be involved in ripening were also identified.

CONCLUSIONS

A large number of differentially regulated genes were identified during banana fruit ripening. Many of these are associated with cell wall degradation and synthesis of aromatic volatiles. A large number of differentially expressed genes did not align with any of the databases and might be novel genes in banana. These genes can be good candidates for future studies to establish their role in banana fruit ripening. The datasets developed in this study will help in developing strategies to manipulate banana fruit ripening and reduce post harvest losses.

Ethylene regulates Apple (Malus x domestica) fruit softening through a dose x time-dependent mechanism and through differential sensitivities and dependencies of cell wall-modifying genes

In fleshy fruit species that have a strong requirement for ethylene to ripen, ethylene is synthesized autocatalytically, producing increasing concentrations as the fruits ripen. Apple fruit with the ACC OXIDASE 1 (ACO1) gene suppressed cannot produce ethylene autocatalytically at ripening. Using these apple lines, an ethylene sensitivity dependency model was previously proposed, with traits such as softening showing a high dependency for ethylene as well as low sensitivity. In this study, it is shown that the molecular control of fruit softening is a complex process, with different cell wall-related genes being independently regulated and exhibiting differential sensitivities to and dependencies on ethylene at the transcriptional level. This regulation is controlled through a dose × time mechanism, which results in a temporal transcriptional response that would allow for progressive cell wall disassembly and thus softening. This research builds on the sensitivity dependency model and shows that ethylene-dependent traits can progress over time to the same degree with lower levels of ethylene. This suggests that a developmental clock measuring cumulative ethylene controls the fruit ripening process.

Metabolic analysis of kiwifruit (Actinidia deliciosa) berries from extreme genotypes reveals hallmarks for fruit starch metabolism

Tomato, melon, grape, peach, and strawberry primarily accumulate soluble sugars during fruit development. In contrast, kiwifruit (Actinidia Lindl. spp.) and banana store a large amount of starch that is released as soluble sugars only after the fruit has reached maturity. By integrating metabolites measured by gas chromatography-mass spectrometry, enzyme activities measured by a robot-based platform, and transcript data sets during fruit development of Actinidia deliciosa genotypes contrasting in starch concentration and size, this study identified the metabolic changes occurring during kiwifruit development, including the metabolic hallmarks of starch accumulation and turnover. At cell division, a rise in glucose (Glc) concentration was associated with neutral invertase (NI) activity, and the decline of both Glc and NI activity defined the transition to the cell expansion and starch accumulation phase. The high transcript levels of β-amylase 9 (BAM9) during cell division, prior to net starch accumulation, and the correlation between sucrose phosphate synthase (SPS) activity and sucrose suggest the occurrence of sucrose cycling and starch turnover. ADP-Glc pyrophosphorylase (AGPase) is identified as a key enzyme for starch accumulation in kiwifruit berries, as high-starch genotypes had 2- to 5-fold higher AGPase activity, which was maintained over a longer period of time and was also associated with enhanced and extended transcription of the AGPase large subunit 4 (APL4). The data also revealed that SPS and galactinol might affect kiwifruit starch accumulation, and suggest that phloem unloading into kiwifruit is symplastic. These results are relevant to the genetic improvement of quality traits such as sweetness and sugar/acid balance in a range of fruit species.

Homologs of FT, CEN and FD respond to developmental and environmental signals affecting growth and flowering in the perennial vine kiwifruit

FLOWERING LOCUS T (FT) and CENTRORADIALIS (CEN) homologs have been implicated in regulation of growth, determinacy and flowering. The roles of kiwifruit FT and CEN were explored using a combination of expression analysis, protein interactions, response to temperature in high-chill and low-chill kiwifruit cultivars and ectopic expression in Arabidopsis and Actinidia. The expression and activity of FT was opposite from that of CEN and incorporated an interaction with a FLOWERING LOCUS D (FD)-like bZIP transcription factor. Accumulation of FT transcript was associated with plant maturity and particular stages of leaf, flower and fruit development, but could be detected irrespective of the flowering process and failed to induce precocious flowering in transgenic kiwifruit. Instead, transgenic plants demonstrated reduced growth and survival rate. Accumulation of FT transcript was detected in dormant buds and stem in response to winter chilling. In contrast, FD in buds was reduced by exposure to cold. CEN transcript accumulated in developing latent buds, but declined before the onset of dormancy and delayed flowering when ectopically expressed in kiwifruit. Our results suggest roles for FT, CEN and FD in integration of developmental and environmental cues that affect dormancy, budbreak and flowering in kiwifruit.

Metabolic profiling and outer pericarp water state in Zespri, CI.GI, and Hayward kiwifruits

The metabolic profiling of aqueous extracts of Zespri Gold ( Actinidia chinensis ) and CI.GI (a controlled crossbreed from different species of Actinidia deliciosa ) kiwifruits and the water state of the outer pericarp of entire fruits were monitored over the season by means of high-field NMR spectroscopy and T(2) relaxation time measurements, respectively, and compared with the corresponding ones of Hayward kiwifruits previously investigated. A more complete assignment of the (1)H spectrum with respect to that obtained previously was reported: histidine, phenylalanine, quercetin 3-rhamnoside, and epicatechin were identified. Metabolic profiling confirmed Zespri's earlier maturation compared with the two other varieties. The water state of entire kiwifruits was measured nondestructively on fruits attached to the plants or detached from the plants. T(2) relaxation times were found to be sensitive to the kiwifruit developmental stage.

Kiwifruit proteins and enzymes: actinidin and other significant proteins

Protein is a minor but significant component of kiwifruit. Crude protein is typically measured at about 1% of fresh weight; however, soluble protein is much less, around 0.3%. The difference can be accounted for by nonprotein nitrogen and insoluble protein, such as polypeptide chains forming part of the cell wall. Kiwifruit soluble protein is mostly accounted for by the proteolytic enzyme actinidin and its inactive forms, a so-called thaumatin-like protein and an unusual protein called kiwellin, which has no known function. Actinidin is the predominant enzyme in kiwifruit and can play a role in aiding the digestive process. There is also a wide range of enzymes involved in the ripening of kiwifruit, particularly enzymes involved in polysaccharide and oligosaccharide metabolism and in the development of flavor and aroma compounds. Whether the enzymatic actions of these have any effect during the consumption and digestion of kiwifruit is not known, although any noticeable effect is unlikely. Some enzymes are likely to have an effect on flavor, texture, and nutritional values, during storage, processing, and preparation of kiwifruit.