Xinhua Zhang

Methyl Salicylate-Induced Arginine Catabolism Is Associated with Up-regulation of Polyamine and Nitric Oxide Levels and Improves Chilling Tolerance in Cherry Tomato Fruit

The effects of methyl salicylate (MeSA) on chilling injury (CI) and gene expression levels, enzyme activities, and metabolites related to arginine catabolism in cherry tomato fruit were investigated. Freshly harvested fruits were treated with 0.05 mM MeSA vapor at 20 °C for 12 h and then stored at 2 °C for up to 28 days. MeSA reduced CI and enhanced the accumulation of putrescine, spermidine, and spermine, which was associated with increased gene expression levels and activities of arginase, arginine decarboxylase, and ornithine decarboxylase at most sampling times. MeSA also increased nitric oxide synthase activity, which at least partly contributed to the increased nitric oxide content. The results indicate that MeSA activates the different pathways of arginine catabolism in cold-stored fruit and that the reduction in CI by MeSA may be due to the coordinated metabolism of arginine and the increase in polyamines and nitric oxide levels.

Up-regulating arginase contributes to amelioration of chilling stress and the antioxidant system in cherry tomato fruits

BACKGROUND Arginase, which plays an important role in regulating the metabolism of L-arginine (Arg) in mammalian cells, has been reported to be involved in stress responses in higher plants. In view of the well-established roles of polyamines, nitric oxide and proline in plant tolerance to chilling stress, arginase may play an important role in fruit chilling tolerance by regulating the metabolism of Arg. However, the current information available on this is very limited. RESULTS Cherry tomatoes (Lycopersicon esculentum cv.Messina) at the mature green stage were treated with Arg or N(ω)--hydroxy-nor-L-arginine (nor-NOHA) and analysed for chilling injury, electrolyte leakage, malondialdehyde and proline contents and arginase and antioxidant enzyme (superoxide dismutase, catalase, peroxidaseand ascorbate peroxidase) activities during cold storage. The effects of low temperature on the transcriptional levels of two arginase genes (LeARG1 and LeARG2) were also evaluated. LeARG1 and LeARG2 expression and arginase activity were significantly induced by low temperature. Compared with the control, Arg treatment alleviated fruit chilling injury, reduced electrolyte leakage, malondialdehyde content and peroxidaseactivity and increased other parameters. Treatment with nor-NOHA caused the opposite effects. CONCLUSION Up-regulation of arginase activity and gene expression may be a chilling tolerance strategy in cherry tomato fruit. Inhibition of chilling-induced arginase activity could aggravate chilling injury and oxidation damage. Arginase appears to play an important role in the chilling resistance process of cherry tomato fruit induced by Arg.

Postharvest Application of Methyl Jasmonate for Improving Quality Retention of Agaricus bisporus Fruit Bodies

The influence of methyl jasmonate (MeJA) on postharvest quality and enzyme activities, gene expression level, and the functional component content linked to postharvest deterioration in Agaricus bisporus (J.E. Lange) Imbach fruit bodies was investigated. Freshly harvested fruit bodies were treated with 0 (control), 10 and 100 μM MeJA vapor at 20 °C for 12 h and then stored at 10 °C for up to 7 days. The results indicated that treatments with 100 μM MeJA vapor maintained a high level of soluble protein and total sugar, delayed browning, promoted the accumulation of phenolics and flavonoids, and inhibited the increase of respiratory rate and membrane leakage. Furthermore, 100 μM MeJA inhibited the activities of polyphenoloxidase, increased the antioxidant enzymes activities of catalase and superoxide dismutase, and lowered relative expression levels of three genes encoding polyphenol oxidase (AbPPO1, AbPPO2, and AbPPO3) throughout the storage period. Comparatively, 10 μM MeJA also had a clear beneficial effect on postharvest mushroom quality maintenance but was not as effective as 100 μM MeJA treatment. These findings suggest that application of MeJA could have potential in maintaining the quality of harvested A. bisporus fruit bodies.

Hot air treatment-induced arginine catabolism is associated with elevated polyamines and proline levels and alleviates chilling injury in postharvest tomato fruit

BACKGROUND To understand whether arginine catabolism might be involved in hot air (HA)-induced chilling tolerance mechanism in tomato fruit, we investigated the effect of HA treatment on endogenous arginine catabolism in relation to chilling injury. RESULTS Tomato fruit were harvested at mature green stage and treated with HA at 38°C for 12 h and then stored at 2°C for 21 days. The effects of HA treatment on fruit chilling injury and gene expression levels or enzyme activity, and metabolites related to arginine catabolism were evaluated. HA treatment reduced the chilling injury symptoms of tomato fruit and enhanced the accumulation of endogenous polyamines, especially putrescine and proline. This accumulation is associated with the increased transcript levels of genes encoding arginase (LeARG1 and LeARG2), arginine decarboxylase (LeADC), ornithine decarboxylase (LeODC) and ornithine aminotransferase (LeOAT) at most sampling times. However, HA treatment had little effect on nitric oxide synthase activity and nitric oxide concentration. CONCLUSION These results revealed that the reduction in chilling injury by HA treatment may be due to the accumulation of putrescine and proline induced primarily by activating the catabolism of endogenous arginine.

Synergistic Effects of l-Arginine and Methyl Salicylate on Alleviating Postharvest Disease Caused by Botrysis cinerea in Tomato Fruit

The effects of l-arginine (Arg, 1 mM) and/or methyl salicylate (MeSA, 0.05 mM) treatment on gray mold caused by Botrytis cinerea in tomato fruit were studied. Results indicated that Arg or MeSA alleviated the incidence and severity of fruit disease caused by B. cinerea, and that both Arg and MeSA (Arg + MeSA) further inhibited the development of fruit decay. Treatment with Arg + MeSA not only enhanced the activities of superoxide dismutase, catalase, and peroxidase but also promoted the expression levels of pathogenesis-related protein 1 gene and the activities of defense-related enzymes of phenylalanine ammonia-lyase, polyphenol oxidase, β-1,3-glucanase, and chitinase during most of the storage periods, which were associated with lower disease incidence and disease index. In addition, the combined treatment elevated the levels of total phenolics, polyamines, especially putrescine, and nitric oxide. These observations suggest that treatment of fruit with Arg + MeSA is an effective and promising way to alleviate postharvest decays on a commercial scale.

Identification and active site analysis of the 1-aminocyclopropane-1-carboxylic acid oxidase catalysing the synthesis of ethylene in Agaricus bisporus.

BACKGROUND 1-Aminocyclopropane-1-carboxylate oxidase (ACO) is a key enzyme that catalyses the final step in the biosynthesis of the plant hormone ethylene. Recently, the first ACO homologue gene was isolated in Agaricus bisporus, whereas information concerning the nature of the ethylene-forming activity of this mushroom ACO is currently lacking. METHODS Recombinant ACO from A. bisporus (Ab-ACO) was purified and characterised for the first time. Molecular modelling combined with site-directed mutagenesis and kinetic and spectral analysis were used to investigate the property of Ab-ACO. RESULTS Ab-ACO has eight amino acid residues that are conserved in the Fe (II) ascorbate family of dioxygenases, including four catalytic residues in the active site, but Ab-ACO lacks a key residue, S289. In comparison to plant ACOs, Ab-ACO requires ACC and Fe (II) but does not require ascorbate. In addition, Ab-ACO had relatively low activity and was completely dependent on bicarbonate, which could be ascribed to the replacement of S289 by G289. Moreover, the ferrous ion could induce a change in the tertiary, but not the secondary, structure of Ab-ACO. CONCLUSIONS These results provide crucial experimental support for the ability of Ab-ACO to catalyse ethylene formation in a similar manner to that of plant ACOs, but there are differences between the biochemical and catalytic characteristics of Ab-ACO and plant ACOs. GENERAL SIGNIFICANCE This work enhances the understanding of the ethylene biosynthesis pathways in fungi and could promote profound physiological research of the role of ethylene in the regulation of mushroom growth and development.

Ornithine decarboxylase is involved in methyl jasmonate-regulated postharvest quality retention in button mushrooms (Agaricus bisporus): The involvement of ODC in MeJA-regulated quality retention in button mushrooms

BACKGROUND In the present study, we investigated the role of ornithine decarboxylase (ODC) in the methyl jasmonate (MeJA)-regulated postharvest quality maintenance of Agaricus bisporus (J. E. Kange) Imbach button mushrooms by pretreating mushrooms with a specific irreversible inhibitor called α-difluoromethylornithine (DFMO) before exposure to MeJA vapor. RESULTS Mushrooms were treated with 0 or 100 µmol L-1 MeJA or a combination of 120 µmol L-1 DFMO and 100 µmol L-1 MeJA, respectively, before storage at 4 °C for 21 days. Treatment with MeJA alone induced the increase in ODC activity whereas this effect was greatly suppressed by pretreatment with DFMO. α-Difluoromethylornithine strongly attenuated the effect of MeJA on decreasing cap opening, slowing the decline rate of soluble protein and total sugar, and accumulating total phenolics and flavonoids. α-Difluoromethylornithine pretreatment also counteracted the ability of MeJA to inhibit polyphenol oxidase and lipoxygenase activities, and malondialdehyde production, and to stimulate superoxide dismutase and catalase activities. It also largely downregulated MeJA-induced accumulation of free putrescine (Put). CONCLUSION These results reveal that ODC is involved in MeJA-regulated postharvest quality retention of button mushrooms, and this involvement is likely to be associated with Put levels.

SlMYC2 Involved in Methyl Jasmonate-Induced Tomato Fruit Chilling Tolerance

MYC2, a basic helix-loop-helix transcription factor, is a master regulator in Jasmonic acid (JA) signaling pathway. However, the functions of SlMYC2 in methyl jasmonate (MeJA)-mediated fruit chilling tolerance are far from being clearly understood. Thus, in the present work, we constructed SlMYC2-silenced tomato fruit by virus-induced gene silencing (VIGS) and investigated the function of SlMYC2 in MeJA-induced tomato fruit chilling tolerance. The results showed that MeJA treatment markedly induced the SlMYC2 expression; increased proline content, lycopene content, and antioxidant enzyme activities, including superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase; inhibited the increase of electrical conductivity and malondialdehyde content; and effectively reduced the chilling injury (CI) incidence and CI index. However, these effects of MeJA treatment were partially counteracted in SlMYC2-silenced tomato fruit, and the CI incidence and CI index in ( SlMYC2-silenced + MeJA)-treated fruit were higher than those in MeJA-treated fruit. Our results indicated that SlMYC2 might be involved in MeJA-induced chilling tolerance, possibly by ameliorating the antioxidant enzyme system of fruit and increasing proline and lycopene levels.