Pericarp Browning Of Longan Fruit Research Articles

Integrated function of proanthocyanidin and lignin polymerization mediated by LAC/PRXs in pericarp browning of longan fruit

Longan can be regarded as a distinct dry fruit with fleshly aril, whose pericarp browning may follow an unclear browning mechanism as dry fruits. Here, we integrated metabolome and transcriptome analysis, validated the function of the candidate metabolites and genes, to dissect the underlying mechanism. With the pericarp water content of longan fruit decreased from 51% to 20%, the pericarp completely turned brown after 8 days of storage at 25 ℃ and relative humidity 70%. Instead of the decline of soluble proanthocyanidins (PAs), such as (−)-epicatechin (EC), (+)-catechin (CT), and procyanidin A2/B2 etc., as well as lignin monomers, coniferyl alcohol (Con_A) and sinapyl alcohol (Sin_A), the contents of their polymers, insoluble PAs and lignin, increased. Gene expression of three Laccases (LAC14–4, 14–8, 7) and four Class III Peroxidase (PRX52–1, 52–2, 53–2, 4–15) were highly negatively correlated to metabolites EC and Con_A. Exogenously expressed LAC7 and LAC14–8, PRX4–15 and PRX52–1 could catalyze the polymerization and browning of EC, CT, Con_A or Sin_A in vitro. Compared to single substrate (EC or Con_A), the polymerization and browning were dramatically accelerated when double substrates (EC and Con_A) were present, by either LACs or PRXs. In addition, these LACs and PRXs were found to be located in endoplasmic reticulum (ER), correlated to their ‘secretion protein’ signal peptides. Based on these results, we propose that the water-loss in pericarp induces the polymerization reaction of PAs and lignin monomers mediated by LACs/PRXs, leading to the browning of longan pericarp. The hypothesis is distinct to the ‘polyphenol oxidases-phenols’ classical browning mechanism established for fleshly fruits.

Characterization of a pericarp browning related LACCASE 14-4 from longan fruit with a focus on (epi)catechin oxidative polymerization

Polyphenol oxidase (PPO) has long been considered as the key enzyme responsible for the pericarp browning of longan fruit after harvest. However, due to the overlapping substrate ranges of PPOs and laccases, the contribution of laccases in longan pericarp browning is unclear. In this study, we found that the contents of flavan-3-ols and procyanidin A2/B2 in longan pericarp decreased during browning after harvest. It might be caused by high activity of laccases rather than PPOs. A 100-kDa laccase was purified from the pericarp and its protein sequence was identical to the deduced sequence of LAC14-4. The laccase showed more favored to (−)-epicatechin (EC) than other flavan-3-ols and procyanidin A2/B2 identified in the pericarp. Using EC and (+)-catechin (CT) as substrates, brown products were generated by LAC14-4 and exogenously expressed LAC14-4-3 × HA in vitro; isomers of procyanidin polymers were detected in the products. Transient expression of LAC14-4-GFP in tobacco leaves showed that LAC14-4 was located in the vacuoles and endoplasmic reticulum. In situ hybridization showed that LAC14-4 mainly expressed in the outer part of mesocarp closed to the brachysclereid layer. The transcript, protein and activity levels of LAC14-4 in pericarp were upregulated during browning. In summary, longan pericarp contains high levels of LAC14-4 that catalyzes the procyanidin polymerization leading to longan pericarp browning.

Transient H2O2 Induction by ClO2 Fumigation Alters Prx-Trx System and Causes MAPK Accumulation Attenuating Browning in Harvested Longan Fruit

Peroxiredoxin-thioredoxin (Prx-Trx) system plays a critical role in both redox sensing and controlling of signal transduction pathways. This study aimed to demonstrate the effects of chlorine dioxide (ClO2) fumigation on the alteration of redox status of peroxiredoxin (Prx) and thioredoxin (Trx) in hydrogen peroxide (H2O2) sensing and the activation of mitogen-activated protein kinase (MAPK) involved in signal transduction and pericarp browning of longan fruit. After fresh longan fruit were fumigated with 10 mg L-1 ClO2 for 10 min, a transient peak of H2O2 was found at 6 h. It also led to strong accumulation of mitogen-activated protein kinases 3 and 6 (MPK3 and MPK6) that persisted up to 3-5 d after fumigation. Furthermore, the oxidation state as well as reductase activities of both Prx and Trx increased concurrently, reaching their peaks at 12-24 h after fumigation. Both redox state and redox potential of nicotinamide adenine dinucleotide phosphate were significantly affected throughout the time of storage (7 d). The altered Prx and Trx redox state and accumulation of MPK3 and MPK6 were closely associated with the reduction in excessive H2O2 accumulation and pericarp browning after 24 h of fumigation. These findings indicated that ClO2 may trigger the H2O2-induced Prx/Trx redox cycling and MAPK activity, resulting in the reduction of pericarp browning of longan fruit during storage.

STUDY ON THE EFFECTS OF LONGAN FRUIT DIPPING IN CHLORINE DIOXIDE ON PERICARP BROWNING AND QUALITY DURING COLD STORAGE

Pericarp browning is the main factor to reduce the storage life and marketability of longan fruit. In the present study evaluated chlorine dioxide (ClO2) dipping on reducing pericarp browning. Fresh longan fruit was dipped in 1%, 1.25% and 1.5% ClO2 for 5 min and stored at 5±2 oC, 85±5% RH for 42 days, investigated every 7 days. Evaluation by browning index (BI) as pericarp browning, outer pericarp color by a chromameter expressed as L*and b* value and hue angle, disease incidence, ClO2 residue and sensory acceptance was determined. The result showed that dipping in 1%, 1.25% and 1.5% ClO2 could reduce pericarp browning (lower BI, higher L* and b* value and hue angle). However, they showed no significant differences with commercial SO2 fumigation but they were significant differences with untreated fruit. ClO2treatments could reduce disease incidence and low ClO2 residue in flesh. The sensory acceptance such as outer pericarp color, flesh and flavor treated with 1.5% ClO2 had shown great potential. The results suggested that 1.5% ClO2 dipping could reduce pericarp browning of longan fruit as same as the commercial SO2 fumigation.

Gaseous chlorine dioxide increases energy status and energy metabolism-related enzyme activities leading to reduction in pericarp browning of longan fruit during storage

Senescence in plants is governed by intrinsic developmental programs which is often under the regulation of their cellular energy status. Changes in energy production and energy metabolism-related enzyme activities contribute to senescence in plants. This study aimed to investigate the effects of gaseous chlorine dioxide (ClO2) in reducing pericarp browning related to energy metabolism. Fresh longan fruit were fumigated with 0 (control) and 10 mg L−1 ClO2 for 10 min and then stored in a cardboard box at 25 ± 1 °C with 82 % relative humidity for 7 d. It was shown that the activities of energy metabolism-related enzymes, including nicotinamide adenine dinucleotide dehydrogenase, succinate dehydrogenase, cytochrome c oxidase, succinyl-CoA synthetase and ATPase (H+-adenosine triphosphatase, Ca2+-adenosine triphosphatase and Mg2+-adenosine triphosphatase) decreased during storage. The decreases in the enzyme activities coincided with the decreases in adenosine triphosphate content and energy charge, as well as an increase in pericarp browning of longan. ClO2 treatment increased the energy status significantly and the activities of energy metabolism-related enzymes during the first 2–5 d of storage. Furthermore, ClO2 increased succinyl-CoA and succinate contents as well as respiration rate in the first day of storage. The increases in cellular energy and the activities of energy metabolism-related enzymes by ClO2 were closely associated with the reduction in pericarp browning. It can be concluded that the decline in cellular energy and the activities of enzymes related to energy metabolism involved in browning development of longan fruit and ClO2 could promote and maintain energy production by enhancing the energy metabolism-related enzyme activities, leading to the reduction in pericarp browning of longan fruit during storage.

ICE-TEMPERATURE STORAGE COMBINED WITH STAGE-WISE SO2 RELEASER TREATMENT EXTENDED SHELF LIFE OF LONGAN FRUIT

Longan fruit are highly perishable when stored at ambient temperature. SO 2 treatment has been widely used to inhibit pericarp browning of longan fruit yet entails the risk of high SO 2 residue in aril. Our previous study indicated that stage-wise SO 2 releaser treatment reduced SO 2 residue in aril of longan fruit. Ice-temperature storage has the potential to maintain the quality of the fruit aril, but results in severe pericarp browning. In the present study, ice-temperature combined with stage-wise SO 2 releaser treatment (ITS) was applied to the storage of longan fruit. The pericarp color, physiological change and fruit quality were investigated. Compared to control fruit in traditional 3°C storage, the pericarp color of the ITS-treated fruit maintained a fresh-green appearance, which was represented by the high level of b, C and L values and a high content of yellow pigments. After 48 days, a higher good fruit rate of 91.15% was achieved for ITS-treated fruit, when compared to the control fruit with 0% good fruit rate. ITS remarkably inhibited the activity of PPO and POD, and decreased pH value of fruit pericarp. A lower SO 2 residue with 29.2 mg/kg in aril was detected in ITS-treated fruit compared to SO 2 -treated fruit. Taken together, these results suggested that ITS is a practical technique for extending shelf life of longan fruit.

Effect of ozone in combination with some organic acids on the control of postharvest decay and pericarp browning of longan fruit

The effects of ozone, in combination with citric, ascorbic and oxalic acid, on postharvest decay and pericarp browning of longan fruit cv. “Daw” were studied. Fruits were exposed to ozone at a concentration of 200 μl l −1 for 0 (control), 15, 30, 60 and 120 min; then stored at a 25 °C. Exposing fruits to ozone for 60 and 120 min significantly reduced the microorganism population on the longan fruit surfaces immediately after fumigation and for 3 d in storage. However, when exposure time was increased to 120 min, disease incidence increased. In a second experiment, longan fruits were dipped in citric, ascorbic and oxalic acid solutions at concentrations of 0%, 5% and 10% w/v. Oxalic acid at 5% was a more potent anti-browning agent compared with other acids. Fruit treated with ozone in combination with oxalic or citric acid had less browning and a reduction of polyphenol oxidase. Therefore, ozone in combination with oxalic or citric acid could be a partial alternative to sulphur dioxide fumigation as a control of postharvest decay and browning.