Nancy Jung Chen

Heat treatment and fruit ripening

Abstract Postharvest heat treatments lead to an alteration of gene expression and fruit ripening can sometimes be either delayed or disrupted. The extent of the alternation of fruit ripening is a function of the exposure temperature and duration and how quickly the commodity is cooled following the heat treatment. The most commonly measured components of fruit ripening affected by heat treatments include fruit softening, membrane and flavor changes, respiration rate, ethylene production, and volatile production. Cell wall degrading enzymes and ethylene production are frequently the most disrupted and are sometimes not produced or their appearance is delayed following heating. Other processes associated with ripening are not altered to the same extent or soon recover. Fruit sensitivity to heat treatments is modified by preharvest weather conditions, cultivar, rate of heating, and subsequent storage conditions. The amount of sensitivity or tolerance to heat stress of a commodity is related to the level of heat protective proteins at harvest and the postharvest production of heat shock proteins. Two types of heat responses are seen. The first is a normal cellular response ( 45°C and is modified by the pre-stress environmental conditions, the cellular response to stress and cellular recovery. Loss of membrane integrity appears to be an effect and not a cause of injury. The site of the injury lesion is still unknown and could be associated with transcription, translation and cellular recovery capacity after an injury threshold has been exceeded.

Endoxylanase expressed during papaya fruit ripening: purification, cloning and characterization

Papaya (Carica papaya L.) softening during fruit ripening is correlated with the activities of an endoxylanase (EC 3.2.1.8). A 32.5-kDa xylanase (CpaEXY1) from ripening fruit mesocarp was purified 45 871-fold on enzymatic activity and to homogeneity by SDS electrophoresis. The enzyme had endo- and not exo-xylanase activity, a pH optimum of 5-7 and was inhibited by Ca2+, Co2+, and Zn2+. Degenerate primers were constructed from five peptides obtained from the purified enzyme, and a full-length cDNA clone (AY138968) was isolated from a library constructed from ripening mesocarp. CpaEXY1 coded for a 64.96-kDa protein that had up to 61% identity with the 12 predicted Arabidopsis Family 10 endoxylanase-like sequences and 40% to the barley aleurone xylanase. The peptide sequences, obtained from the trypsin-digested purified protein, were all found between amino acid 267and 426 out of the predicted 584 amino acids. The N-terminal 27 amino acids were hydrophobic and formed a predicted secretory signal peptide. A predicted carbohydrate-binding module was located between amino acids 60and 182, distinct from the C-terminal endoxylanase catalytic center. CpaEXY1 was developmentally expressed during fruit ripening and the expression correlated with the variation in softening patterns of different varieties. The findings are consistent with the hypothesis that CpaEXY1 was expressed during fruit ripening; the expression was correlated with softening and was modified by post-translational proteolysis. This modification may take place in the cell wall, and regulate enzyme activity and cell-wall-microdomain-specific hydrolysis.

Effect of storage temperature and wrapping on quality characteristics of litchi fruit

Abstract The post-harvest changes in two litchi (Litchi chinensis Sonn.) cultivars ‘Hei Ye’ and ‘Chen Zi’ were studied. Fruit was stored in a paper bag or a closed polyethylene bag at 22 or 2° C. ‘Hei Ye’ pericarp browning was reduced by storage at 22° C in a closed polyethylene bag. Storage at 2° C delayed the onset of pericarp browning, but decay became a problem 20 days after harvest. There was a continued decline in respiration rate after harvest with no ethylene being detected, and a decline in titratable acidity associated with changes in malic acid content. Ascorbic acid declined in storage from 1 to 0.4 mg g−1 in 4 days at 22° C, irrespective of storage method or temperature. Ascorbic acid changes in ‘Chen Zi’ were more marked than in ‘Hei Ye’, declining from 1.2 to 0.2 mg g−1 at 22° C after 8 days. Both cultivars showed similar qualitative changes in total soluble solids, titratable acidity and ascorbic acid content. Low-temperature storage decreased the rate and extent of changes. Storage in a polyethylene bag delayed the onset of pericarp browning at both storage temperatures. However, pericarp browning did not significantly affect aril quality as measured by sugar, acids and phenol parameters. There was a decline in total soluble solids from 18.5 to 15.5% in 8 days at 22° C due to a decrease in the amount of sucrose. There was no pattern to the minor changes in glucose and fructose content.

Fruit Development, Ripening and Quality Related Genes in the Papaya Genome

Papaya (Carica papaya L.) is the first fleshy fruit with a climacteric ripening pattern to be sequenced. As a member of the Rosids superorder in the order Brassicales, papaya apparently lacks the genome duplication that occurred twice in Arabidopsis. The predicted papaya genes that are homologous to those potentially involved in fruit growth, development, and ripening were investigated. Genes homologous to those involved in tomato fruit size and shape were found. Fewer predicted papaya expansin genes were found and no Expansin Like-B genes were predicted. Compared to Arabidopsis and tomato, fewer genes that may impact sugar accumulation in papaya, ethylene synthesis and response, respiration, chlorophyll degradation and carotenoid synthesis were predicted. Similar or fewer genes were found in papaya for the enzymes leading to volatile production than so far determined for tomato. The presence of fewer papaya genes in most fruit development and ripening categories suggests less subfunctionalization of gene action. The lack of whole genome duplication and reductions in most gene families and biosynthetic pathways make papaya a valuable and unique tool to study the evolution of fruit ripening and the complex regulatory networks active in fruit ripening.

Papaya Fruit Softening: Role of Hydrolases

Papaya (Carica papaya L.) cultivars show a wide variation in fruit softening rates, a character that determines fruit quality and shelf life, and thought to be the result of cell wall degradation. The activity of pectin methylesterase, β-galactosidase, endoglucanase, endoxylanase and xylosidase were correlated with normal softening, though no relationship was found between polygalacturonase activity and softening. When softening was modified by 1-MCP treatment, a delay occurred before the normal increase in activities of all cell wall activities except endoxylanase which was completely suppressed. Significant cell wall mass loss occurred in the mesocarp tissue during normal softening, but did not occur to the same extent following 1-MCP treatment. During normal softening, pectin polysaccharides and loosely bound matrix polysaccharides were solubilized and the release of xylosyl and galactosyl residues occurred. Cell wall changes in galactosyl residues after 1-MCP treatment were comparable to those of untreated fruit but 1-MCP treated fruit did not soften completely. The changes in the cell wall fractions containing xylosyl residues in 1-MCP treated fruit showed less solubilization and a higher association of xylosyl residues with the pectic polysaccharides. The results indicated that normal modification of cell wall xylosyl components during ripening did not occur following 1-MCP treatment at the color-break stage, this was associated with the failure of these fruit to fully soften and a selective suppression of endoxylanase activity. The results support a role for endoxylanase in normal papaya fruit softening and its suppression by 1-MCP lead to a failure to fully soften. Normal papaya ripening related softening was dependent upon the expression and activity of endoglucanase, β-galactosidase and endoxylanase.

Heat treatment prevents postharvest geotropic curvature of asparagus spears (Asparagus officinalis L.)

Negative geotropic curvature of fresh asparagus spears after harvest was prevented by a brief heat treatment. The heat treatment was immersion in heated water at 47.5 degrees C for 2-5 min and cooling to storage temperature as soon as possible after heat treatment. The heat treatment can be varied from 45 degrees C for longer than 5 min to 50 degrees C for 2.5 min or less to avoid significant loss of spear appearance. The treatment temperature and time needed to be adjusted for spear diameter, small diameter spears required a shorter exposure time or lower temperature.

Genome of papaya, a fast growing tropical fruit tree

Papaya is a major fruit crop in tropical and subtropical regions worldwide. It has long been recognized as a nutritious and healthy fruit rich in vitamins A and C. Its small genome, unique aspects of nascent sex chromosomes, and agricultural importance are justifications for sequencing the genome. A female plant of the transgenic variety SunUp was selected for sequencing its genome to avoid the complication of assembling the XY chromosomes in a male or hermaphrodite plant. The draft genome revealed fewer genes than sequenced genomes of flowering plants, partly due to its lack of genome wide duplication since the ancient triplication event shared by eudicots. Most gene families have fewer members in papaya, including significantly fewer disease resistance genes. However, striking amplifications in gene number were found in some functional groups, including MADS-box genes, starch synthases, and volatiles that might affect the speciation and adaptation of papaya. The draft genome was used to clone a gene controlling fruit flesh color and to accelerate the construction of physical maps of sex chromosomes in papaya. Finishing the papaya genome and re-sequencing selected genomes in the family will further facilitate papaya improvement and the study of genome and sex chromosome evolution in angiosperms, particularly in Caricaceae.

Carbon Flux and Carbohydrate Gene Families in Pineapple

The recently sequenced pineapple genome was used to identify and analyze some of the key gene families involved in carbohydrate biosynthesis, breakdown and modification. Gene products were grouped into glycosyltransferases (GT), glycoside hydrolases (GH), carbohydrate esterases (CE), and polysaccharide lyases (PL) based upon predicted catalytic activity. Non-catalytic carbohydrate-binding modules (CBM) and enzymes involved in lignification were also identified. The pineapple genes were compared with those from two and five monocot and eudicots species, respectively. The complement of pineapple sugar and cell wall metabolism genes is similar to that found in rice and sorghum, though the numbers of GTs and GHs is often fewer. This applies to a lesser extent to the genes involved in nucleotide-sugar interconversion, with both pineapple and papaya having a minimum complement. Interestingly, pineapple does not appear to contain mixed linkage β-glucan in its walls while possessing cellulose synthase-like (Csl), J and H genes. Pineapple and papaya have less than half the number of GT1 genes involved in small molecule glycosylation compared to Arabidopsis and tomato, and fewer members in GH families than Arabidopsis. The ratio of rice and sorghum to pineapple genes in GH families was more variable than in the case of GTs and it is unclear why pineapple GH gene numbers are so low. Rice, sorghum and pineapple have far fewer CE8, PL1 and GH28 genes related to pectin metabolism than most eudicots. The general lower number of cell wall genes in pineapple possibly reflects the absence of a genome duplication event. The data also suggests that pineapple straddles the boundary between grasses (family Poaceae) and eudicots in terms of genes involved in carbohydrate metabolism, which is also reflected in its cell wall composition.

Genomics of Papaya Fruit Development and Ripening

Papaya (Carica papaya L.) is the first fleshy fruit with a climacteric ripening pattern to be sequenced. Many of the predicted papaya genes potentially involved in fruit growth, development, and ripening have homology to those involved in tomato fruit size and shape. Fewer genes that may impact sugar accumulation in papaya, ethylene synthesis and response, respiration, chlorophyll degradation, and carotenoid synthesis are predicted than in tomato. Similar or fewer genes were found in papaya for the enzymes leading to volatile production than so far determined for tomato. Similar numbers or fewer genes are found in papaya for the enzymes leading to volatile production than so far determined for tomato. The presence of fewer papaya genes in most fruit development and ripening categories suggests less subfunctionalization of gene action. The lack of whole genome duplication and reductions in most gene families and biosynthetic pathways make papaya a valuable and unique tool to study the convergent evolution of fruit ripening. The available data suggest that few phylogenetic constraints exist in the evolution of fruit type with fleshy fruits appearing independently in different plant families. The evolutionary origin and fundamental molecular mechanisms that lead to the development of the fruit ripening syndrome are unknown. Abscission-like cell separation processes that occur in leaves, petals, flowers, and pollen show parallel to fruit ripening in terms of physiology and biochemistry and potentially in the network and clusters of genes expressed. A global understanding of the evolution of the complex regulatory networks controlling the modules active during fruit ripening could lead to new regulatory controls that could limit postharvest losses in fruits.

Lotus Cell Walls and the Genes Involved in its Synthesis and Modification

The lotus genome (Nelumbo nucifera (Gaertn.)) lacks the paleo-triplication found in other eudicots and has evolved remarkably slowly with fewer nucleotide mutations. It is thought to have greater retention of duplicated genes than other angiosperms. We evaluated the potential genes involved in cell wall synthesis and its modification, and ethylene synthesis and response. In many cell wall transferases and hydrolases families, lotus had fewer members in most families when compared to Arabidopsis. Lotus had similar or fewer members in each family as found in poplar, grape and papaya. The exceptions were in the sialyl and beta-glucuronsyl transferases where similar number were found as in the core eudicots. Lotus had similar numbers of polygalacturonase and pectin methyl esterases as found in Arabidopsis but fewer in all other hydrolases families. For starch degradation, lotus had only two alpha amylases predicted genes versus eight to ten in other eudicots, with similar numbers of beta amylase genes predicted. Lotus also had less than half the number of genes predicted for the enzymes involved in lignin and tannin synthesis compared to Arabidopsis. The stress plant growth regulator ethylene’s synthesis, reception and response predicted genes were fewer in lotus than other eudicots. Only two ethylene receptor genes were predicted in lotus with five reported for Arabidopsis and six for tomato. Our analysis does not supports the conclusion that this species has greater retention of duplicated genes though our data does support the conclusion that lotus split occurred at the base of the eudicots.