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Hong Zhu, Eric P. Beers, and Rongcai Yuan

and activity of enzymes associated with cell wall degradation such as β -1,4-glucanase (cellulase or EG) and polygalacturonase (PG) ( Bonghi et al., 2000 ; Roberts et al., 2002 ), which causes the middle lamellae of abscission zone cells to dissolve

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Jianguo Li, Hong Zhu, and Rongcai Yuan

; Patterson and Bleecker, 2004 ). Concomitant with increased ethylene production is increased expression of genes and activity of enzymes associated with cell wall degradation such as β-1,4-glucanase (cellulase or EG) and polygalacturonase (PG) ( Bonghi et al

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Rhiana F. Jones, Paul W. Bosland, Robert L. Steiner, Richard W. Jones, and Mary A. O’Connell

-specific endo β-1,4 glucanase (XEG), which specifically attacks the xyloglucan bonds in plant cell walls ( Yoshizawa et al., 2012 ) by hydrolyzing the xyloglucan and loosening the cross-linkages ( Hayashi, 1989 ). XEGIPs inhibit XEGs from breaking down the

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Hong-Wei Zhou, Lilian Sonego, Andrai Khalchitski, Ruth Ben-Arie, Amnon Lers, and Susan Lurie

Most `Flavortop' nectarines [Prunus persica (L.) Batsch (Nectarine Group)] that were placed directly into 0 °C storage developed chilling injury after removal, while preconditioning fruit for 2 days at 20 °C (delayed storage) reduced chilling injury substantially. Chilling injury was expressed as the development of a dry, woolly flesh texture during ripening. Delayed-storage fruit were as firm as control fruit when placed in storage, but softened more during storage. Analysis of cell wall components showed that in woolly fruit a higher percentage of pectin was retained in the sodium carbonate fraction, although during ripening polymers in this fraction decreased in molecular mass (Mr). In the guanidine thiocyanate hemicellulose fraction of woolly fruit, the associated pectin and hemicellulose remained as large polymers, while in delayed-storage fruit they decreased in Mr during ripening. Endo-polygalacturonase (PG), pectin esterase (PE), and endo-glucanase (EGase) activities of delayed-storage fruit were the same as control fruit at the beginning of storage, although exo-PG was higher. However, differences were observed at the end of storage. Endo-PG activity was lower in control than delayed-storage fruit at the end of storage while PE activity was higher, and exo-PG and EGase activities were similar. These differences in activity were not reflected in the mRNA abundance of the respective enzymes. Endo-PG and PE message was similar in all fruit at the end of storage and increased during ripening, while EGase message was low at all times except in control fruit after storage and development of woolliness. Prevention of chilling injury by delayed storage appears to be due to the ability of the fruit to continue a progressive, slow cell wall degradation in storage which allows normal ripening to proceed when the fruit are rewarmed. Regulation of the softening process did not appear to be by enzyme synthesis, since mRNA levels of the enzymes did not correspond with enzyme activity.

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Adirek Rugkong, Jocelyn K.C. Rose, and Chris B. Watkins

Tomato fruit (Solanum lycopersicum L.) can develop mealiness and enhanced softening when exposed to chilling temperatures during storage, but the involvement of cell wall-associated enzymes in chilling injury development is not well understood. To study this aspect of injury development, we have exposed breaker-stage `Trust' tomato fruit to a chilling temperature of 3 °C for 0, 7, 14, and 21 days followed by storage at 20 °C for 12 days. Ethylene production was not affected by storage except after 21 days where production was greater at 20 °C. Exposure of fruit to chilling temperatures delayed the ripening-related color change (chroma and hue) and initially increased compression values, but percent extractable juice was not affected consistently. Increased polygalacturonase (PG) activity during ripening was reduced by about 50% after 7 days at 3 °C, and further inhibited with increasing storage periods. In contrast, the activities of pectin methylesterase (PME) and α-galactosidase were not significantly affected by the cold treatments. β-Galactosidase activity was greater in all chilled fruit compared with fruit ripened at harvest, whereas endo-β-1,4-glucanase activity was lower after 21 days at 3 °C. In chilled fruits, transcript accumulations for PG, PME (PME1.9), and expansin (Expt.1) were lower during storage at 20 °C compared with those of nonchilled fruits. Transcript accumulation for β-galactosidase (TBG4) was affected only at 14 days of cold storage, when transcript accumulation decreased. Cold treatment increased transcript accumulation of endo-β-1,4-glucanase (Cel1) after 12 days at 20 °C and decreased transcript accumulation after 7 days and 21 days at 21 °C. Cell wall analyses to investigate relationships among enzyme activities and cell wall disassembly are ongoing.

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A. Rugkong, J.K.C. Rose, and C.B. Watkins

Tomato fruit (Solanum lycopersicon L.) can develop mealiness and enhanced softening when exposed to chilling temperatures during storage, but the involvement of cell wall-associated enzymes in chilling injury development is not well understood. To study this aspect of injury development, we have exposed breaker stage tomato cv. Trust fruit to a chilling temperature of 3 °C for 0, 7, 14, and 21 days followed by storage at 20 °C for 12 days. Ethylene production was not affected by storage except after 21 days, where production was greater at 20 °C. Exposure of fruit to chilling temperatures delayed the ripening-related color change (chroma and hue) and initially increased compression values, but percentage of extractable juice was not affected consistently. Increased polygalacturonase activity during ripening was reduced by about 50% after 7 days at 3 °C, and further inhibited with increasing storage periods. In contrast, the activities of pectin methylesterase and α-galactosidase were not significantly affected by the cold treatments. β-Galactosidase activity was greater in all chilled fruit compared with fruit ripened at harvest, whereas endo-β-1,4-glucanase activity was lower after 21 days at 3 °C. These results will be compared with equivalent changes in the activities of cell wall enzymes that are associated with wooliness development in chilling-injured peach fruit.

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William C. Kazokas and Jacqueline K. Burns

Mature and immature `Valencia' orange [Citrus sinensis (L.) Osbeck] and immature `Valencia' orange and `Tahiti' lime (Citrus latifolia Tan.) fruit with attached pedicels were treated with 8 μL·L-1 ethylene for periods up to 24 hours. Endo-β-1,4-glucanase (cellulase) activity and gene expression were determined in fruit abscission zones during and after ethylene exposure. Cellulase activities were not detected in mature `Valencia' orange and immature `Tahiti' lime fruit abscission zones immediately following harvest and after 6 hours of ethylene treatment. After 12 hours of ethylene treatment, cellulase activity increased and was highest after 24 hours. Cellulase gene expression preceded the rise in cellulase activity and was detectable after 6 hours of ethylene treatment, but then declined after 12 hours. Following transfer to air storage, abscission zone cellulase activity in mature `Valencia' fruit remained high, whereas activity in immature `Tahiti' fruit declined. After 168 hours air storage, activity in abscission zones of mature `Valencia' fruit decreased slightly, but activity in abscission zones of immature `Tahiti' lime fruit increased to the highest level. Expression of abscission zone cellulase gene Cel-a1 in abscission zones of mature `Valencia' fruit markedly increased after transfer to air and was highest after 48 hours air storage. Cel-a1 expression returned to low levels after 168 hours of air storage, but expression of cellulase gene Cel-b1 remained at low levels throughout the air storage period. Expression of Cel-a1 and Cel-b1 declined in fruit abscission zones of immature `Valencia' and `Tahiti' lime fruit upon transfer to air. After 168 hours of air storage, expression of Cel-a1 again rose to high levels but Cel-b1 remained low. The results suggest that differences in cellulase activity and gene expression measured in mature and immature fruit abscission zones during ethylene treatment and subsequent air storage may, in part, explain the differential response of mature and immature fruit to abscission agents.

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Jiwon Jeong and Donald J. Huber

Pre-ripe `Booth 7' avocado (Persea americana Mill.) fruit, a cross of West Indian and Guatemalan strains, were treated with 0.9 μL·L-1 1-methylcyclopropene (1-MCP) for 12 hours at 20 °C. After storage for 18 days in air at 13 °C, at which time whole fruit firmness values averaged about 83 N, half of the 1-MCP-treated fruit were treated with 100 μL·L-1 ethylene for 12 hours and then transferred to 20 °C. 1-MCP delayed softening, and fruit treated with 1-MCP retained more green color than air-treated fruit when full ripe (firmness 10 to 15 N). 1-MCP affected the activities of pectinmethylesterase (EC 3.2.1.11), α-(EC 3.2.1.22) and β-galactosidases (EC 3.2.1.23), and endo-β-1,4-glucanase (EC 3.2.1.4). The appearance of polygalacturonase (EC 3.2.1.15) activity was completely suppressed in 1-MCP-treated fruit for up to 24 days, at which time the firmness of 1-MCP-treated fruit had declined nearly 80% compared with initial values. The effect of exogenous ethylene applied to partially ripened 1-MCP-treated fruit differed for different ripening parameters. Ethylene applied to mid-ripe avocado exerted no effect on the on-going rate or final extent of softening of 1-MCP-treated fruit, even though polygalacturonase and endo-1,4-β-glucanase activities increased in response to ethylene. β-galactosidase decreased in 1-MCP-treated fruit in response to ethylene treatment. 1-MCP delayed the increase in solubility and depolymerization of water- and CDTA (1,2-cyclohexylenedinitrilotetraacetic acid)-soluble polyuronides, likely due to reduced polygalacturonase activity. At the full-ripe stage, the levels of arabinose, galactose, glucose, mannose, rhamnose, and xylose associated with the CDTA-soluble polyuronide fraction were similar among all treatments. In contrast, the galactose levels of water-soluble polyuronides declined 40% and 17% in control and 1-MCP treated fruit, respectively. Hemicellulose neutral sugar composition was unaffected by 1-MCP or ethylene treatment. The data indicate that the capacity of avocado fruit to recover from 1-MCP-mediated suppression of ripening can be only partially amended through short-term ethylene application and differs significantly for different ripening parameters.

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Hong Zhu, Rongcai Yuan, Duane W. Greene, and Eric P. Beers

lycopersicum L. ( Bleecker, 1999 )] and five have been isolated and characterized in apple ( Dal Cin et al., 2005 ; Li and Yuan, 2008 ). Among the ethylene-inducible transcriptional cascade are the genes for hydrolases such as β-1,4-glucanase (cellulase or EG

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Yu-Xiong Zhong, Jian-Ye Chen, Hai-Ling Feng, Jian-Fei Kuang, Ruo Xiao, Min Ou, Hui Xie, Wang-Jin Lu, Yue-Ming Jiang, and He-Tong Lin

storage life or maintain quality of longan fruit. It has been considered that cell-wall components are important for fruit texture (Manganaris et al., 2005, 2006). Cellular-wall modification-related enzymes, including endo-β-1,4-glucanase (EGcase