Ethylene-based technologies for controlling ripening in climacteric fruit have been in widespread use for a number of years. Likewise, using chemicals that block ethylene synthesis or perception have been widely used to extend shelf life of a variety of horticultural commodities. In the last few years, our understanding of the molecular mechanisms for ethylene synthesis and perception has greatly expanded. Genes encoding the ethylene biosynthetic enzymes and the ethylene receptor have been cloned from many plant species, which has meant that molecular approaches to engineering reduced ethylene synthesis or perception are now reality. Scientists have been examining the feasibility of using molecular approaches to control ethylene in a variety of horticultural and ornamental species. They have shown that it is relatively easy to produce plants that are reduced in either synthesis or response to ethylene. However, scientists have uncovered some issues associated with commercial-level use of these transgenic plants. Overall, my results illustrate the great potential of the technology to control the rate of climacteric fruit ripening, abscission, and ethylene-induced senescence in multiple species, but using transgenes in many cases needs to be directed to target tissues through the use of tissue-specific transcriptional promoters. With that caveat in mind, there should be a strong future for improving the quality of a range of agronomic and horticultural species.
Recent advances in molecular genetics and genomics technologies have had a significant impact on tomato research over the last decade and are likely to have considerable influence on the nature and outcome of research activities related to tomato in the future. Specific applications of genomics technologies in our laboratory include positional cloning of genes associated with fruit ripening and quality (rin and nor), localization of ripening-related genes on the molecular-marker map to assist candidate gene discovery related to fruit ripening and quality, and characterization of mutants influencing fruit quality and nutritional value with the goal of identifying candidate genes for said mutants and alternative molecular tools for modification of fruit quality and nutrition. Isolation of the rin and nor genes has been verified via complementation of corresponding mutant tomato plants via insertion of the appropriate CaMV35s-driven wild-type sense cDNA. Both the rin and nor genes have sequence characteristics suggestive of transcription factors. Preliminary evidence suggests the role of similar genes in the ripening of additional climacteric and non-climacteric fruit species. Additional efforts in the laboratory include molecular analyses of light signal transduction as related to 1) regulation of carotenoid and flavonoid accumulation, and 2) potential manipulation of corresponding pathways for modification of fruit quality and nutrient value.
The application of ultraviolet light on fruit and vegetables is a promising new method to control storage diseases and to delay the onset of senescence. In this investigation, we studied the effects of hormic dose (1,4 Merg•cm-2) of UV-radiation on the ripening of tomato pericarp discs by measuring different characteristics of ripening and senescence during storage. We observed that UV-treatment induced significant delays of the red color development, chlorophyll degradation, and lycopene production compared to control discs. UV-treatment also retarded the decline of the chlorophyll-a fluorescence ratios Fv: Fm and *F : Fm′, two characteristics related, respectively, to the maximum and operational quantum yield of photosystem II electron transport. Furthermore, the climacteric ethylene peak was delayed in the treated discs. However, UV-treatment did not alter textural changes, and the respiratory climacteric peaks were observed concomitantly for both treated and untreated tomato discs. However, the respiratory rate was consistently higher in treated discs. These results indicate that UV irradiation of tomato pericarp discs delays some processes of ripening associated with chloroplast to chromoplast transition whereas other ripening processes seem unaffected.
`Shinko' and `Shinsui' Asian pears were kept in air, 2 kPa O2, 2 kPa O2 + 2.5 kPa CO2, and 2 kPa O2 + 5 kPa CO2 (balance N2 in each treatment) at 0 °C or 5 °C for up to 24 weeks. The three CA treatments reduced respiration (O2 consumption) and ethylene production rates relative to air control pears; these rates were higher at 5 °C than at 0 °C and higher for `Shinsui' than for `Shinko' pears. While `Shinsui' pears had a climacteric pattern of respiration and ethylene production rates, `Shinko' pears produced very small quantities of ethylene and exhibited a non-climacteric respiratory pattern. `Shinko' pears had a much longer postharvest life than `Shinsui' pears (24 weeks vs. 12 weeks at 0 °C). CA treatments had a greater effect on delaying deterioration of `Shinsui' than `Shinko' pears, which were more sensitive to CO2 injury and associated accumulation of fermentative metabolites (acetaldehyde, ethanol, ethyl acetate). `Shinko' pears did not benefit from CA storage and were best kept in air at 0 °C. An atmosphere of 2 kPa O2 with or without up to 5 kPa CO2 delayed flesh breakdown of `Shinsui' pears during storage 0 °C.
Mei (Prunus mume `Daqinghe') fruit were immersed in 20 °C (control), 47 °C (HWT47), 50 °C (HWT50), or 53°C (HWT53) water for 3 min after harvest, then stored at 20 °C. Firmness, peel color, chlorophyll, chlorophyllase activity, soluble solids content (SSC), titratable acidity (TA), respiration, ethylene production, and pectinmethylesterase (PME) and polygalacturonase (PG) activity were monitored to determine the effects of hot water treatment in delaying fruit ripening. Control fruit displayed a typical climacteric pattern of respiration and ethylene production. Peak CO2 production and ethylene production were observed 6 days after harvest. Fruit softening was accompanied by decreases in hue angle, chlorophyll content, SSC, and TA and increases in chlorophyllase and PME and PG activity. Hot water treatment delayed the onset of the climacteric peaks of CO2 and ethylene production. The delays were associated with delays in fruit softening, consistent with lags in the rise of PME and PG activity; delays in yellowing and chlorophyll breakdown, consistent with lags in the rise of chlorophyllase activity; and delays in loss of SSC and TA. The shelf life of fruit increased by 6 days, or 60%, with HWT47, and by 8 days, or 80%, with HWT50 or HWT53.
5'-methylthioadenosine (MTA) nucleosidase (EC.2.2.2.28) and 5-methylthioribose (MTR) kinase (EC.2.7.1.100) activities were evaluated in `rin', `nor', and `Rutgers' tomato fruit during development and ripening. Changes in the activities of these enzymes were compared to ethylene biosynthesis. MTA nucleosidase and MTR kinase activities in `rin' and `nor' were ≈30% and 22%, respectively, lower than `Rutgers' during the first 2 weeks of fruit development. In `Rutgers', activities of these enzymes declined sharply until fruit maturity. Shortly before climacteric rise in ethylene synthesis, MTA nucleosidase, and MTR kinase activities increased, reaching a maximum level before peak ethylene synthesis then declined when fruit started to approach senescence. Whereas, `rin' and `nor' mutants exhibited no climacteric rise in ethylene synthesis and no change in MTA nucleosidase or MTR kinase activities, following their decline after 2 weeks of growth. A rapid increase in ethylene synthesis was observed when mature green `rin' and `nor' fruit were wounded. This increase in ethylene was paralleled by an increase in MTA nucleosidase and MTR kinase activities. However, increase in wound ethylene, MTA nucleosidase, and MTR kinase activities in `rin' and `nor' was ≈40% less than what we had previously reported in `Rutgers'. Relationship of MTA and MTR kinase activities to fruit growth, development, ripening, and natural and wound ethylene biosynthesis will be described.
Abstract
To help clarify the mode of action of solutions containing a combination of 8-hydroxyquinoline citrate (QC), N-dimethylamino succinamic acid (B-Nine), and sucrose in delaying senescence of certain cut flowers, the effect of these solutions on flower respiration and on water flow in stems of ‘Red Sim’ cut carnations was measured. The respiratory curve exhibited a very pronounced climacteric rise, peaking between 6 (control) and 8 days following treatment. All treatments delayed the climacte peak, those treatments containing sucrose delaying it most.
QC used alone or in combination with B-Nine and sucrose promoted water flow through a 4 cm basal stem section. Sucrose alone caused a decrease in water flow. B-Nine used alone promoted water flow somewhat but when used with sucrose, water flow was similar to that with sucrose used alone. Water flow tended to be inversely correlated with micro-organism growth at the stem base as observed microscopically. Where microorganism growth was inhibited most by the treatments, water flow followed a pattern similar to the respiratory curve, with a peak occurring at about the same time as the respiratory climacteric peak, suggesting that there is a relationship between flower respiration and stem water flow. When micro-organism growth was not greatly inhibited, water flow did not follow the same pattern as respiration, but tended to decrease steadily with time.
Treating `Elliott's White' cut carnations with 50 or 100 mm aminotriazole for 4 days inhibits the respiratory climacteric and significantly extends vase life. Aminotriazole induced time- and concentration-dependent inhibition of ethylene evolution and onset of the ethylene climacteric by inhibiting ACC synthase activity. Flowers treated with 50 or 100 mm aminotriazole for 2 days exhibited concentration-dependent increases in ethylene evolution, respiratory activity, ACC synthase activity, and petal ACC content in response to the application of exogenous ethylene at 10 μl·liter-1. Senescence-associated morphological changes, increased ACC synthase activity, ACC content, and ethylene evolution were completely inhibited in flowers treated for 4 days with 100 mm aminotriazole. Although treatment with 50 mm aminotriazole for 4 days did not completely inhibit components of the ethylene biosynthetic pathway, no morphological or respiratory responses to the application of exogenous ethylene at 10 μl·liter-1 were observed, a result indicating that prolonged aminotriazole treatment inhibited ethylene action. Chemical names used: 3-1H-amino-1,2,4-triazole-1-yl (aminotriazole), 1-aminocyclopropane-1-carboxylic acid (ACC).
`Bartlett' pears (Pyrus communis L.) at two physiological stages, climacteric minimum or approaching the climacteric peak as achieved via storage for 2 or 8 weeks in air at 0C, respectively, were either ripened at 20C in air immediately or after exposure to 0.25% 02 for 4 days at 20C. Fruit stored for 2 weeks had relatively stable phosphofructokinase (PFK), pyrophosphate: fru-6-P phosphotransferase (PFP), and pyruvate kinase (PK) activities but decreasing succinate dehydrogenase (SDH) activities during ripening in air. Similar fruit treated with 0.25% O2 had slightly increased PFK, PFP, and SDH activities and decreased PK activity. Fruit stored for 8 weeks exhibited higher levels of PFK and PFP activity upon transfer to 20C, in accordance with their more advanced physiological state. In general, the enzymic changes in these fruit upon exposure to 0.25% O2 and subsequent ripening in air were similar to those observed in the less-mature counterparts, most notable being an increase in mitochondrial SDH. Exposure of suspension-cultured pear fruit cells to hypoxia resulted in an accentuated rise in phosphoenolpyruvate carboxykinase activity and a dramatic rise in SDH activity upon transfer to air. Taken in concert, the enzymic analysis supports the hypothesis that the rise in succinate levels observed in hypoxic fruit tissues is the result of a partial reductive tricarboxylic acid cycle. Cytochrome oxidase activity did not change during hypoxia whereas soluble peroxidase decreased somewhat, perhaps a reflection of their Michaelis constants for O2.
Changes in cytosolic and vacuolar pH, ATP, ADP, and the ATP : ADP ratio were measured in whole fruit or mesocarp disks of avocado [Persea americana (Mill.) cv. Hass] during brief exposures to elevated CO2. Intact climacteric fruit exposed to air (21% O2), 20% CO2 (17% O2, balance N2), or 40% CO2 (13% O2, balance N2) had cytosolic pH values of 7.0, 6.6, and 6.4, respectively, while mesocarp disks had cytosolic pH values of 6.9, 6.7, and 6.4, respectively. The ß-ATP levels of intact climacteric fruit exposed to 20% CO2 or 40% CO2 for 2 h were reduced by 25% or 43%, respectively, relative to air-exposed fruit. HPLC analysis of nucleotide phosphates from preclimacteric avocados revealed that ATP levels and the ATP : ADP ratio increased in 40% compared to the air-stored fruit. However, 1 day after transfer to air, the effects of elevated CO2 had dissipated. These modifications in cellular state could alter the activity of respiratory enzymes in fruit exposed to elevated CO2 atmospheres.