Submitted to: Biologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2000
Publication Date: N/A
Interpretive Summary: The ability of muskmelon fruit to resist fungal decay is, in part, related to compounds found in the tissues. Historically, peroxidase enzymes have been associated with numerous plant functions including disease resistance. The outer (exocarp) and middle (mesocarp) sections of the muskmelon fruit were assayed, at 5 day intervals, for peroxidases from 5-50 days post- pollination (DPP). The exocarp is analogous to the rind and the mesocarp analogous to the edible portion of the fruit. Exocarp peroxidase increased through 30 DPP, and subsequently decreased. Research has demonstrated that muskmelon fruit are more susceptible to fungal attack as they reach maturity (40 DPP). Peroxidase levels in the mesocarp were much lower than in the exocarp which corresponds to tissue that is very susceptible to fungal colonization. Analysis demonstrated that up to eight forms, isozymes, of peroxidase present in the exocarp through 50 DPP. However, the number and location within the tissues changed during fruit development. Changes in peroxidase activity corresponded to fruit net formation and may be associated with susceptibility to fruit rot. Additional research will be required to determine the possible role of peroxidase enzymes in muskmelon fruit rot resistance.
Technical Abstract: Muskmelon fruit exocarp provides a physical and chemical barrier to fruit rot pathogens. Peroxidase has been implicated in disease resistance in plants. Nitrocellulose blots and total peroxidase activity assays of muskmelon fruit tissue demonstrated presence of peroxidases in both exocarp and mesocarp tissues from 5-50 days post-pollination (DPP). Exocarp peroxidase increased at 5-30 DPP, peaked at 30 DPP, and decreased from 40 to 50 DPP in fruit. Total mesocarp peroxidase activity was lower than the exocarp at all developmental stages. Mesocarp peroxidase activity decreased from outer, to middle, to inner tissue at all developmental stages. Total mesocarp activity peaked in fruit at 20 DPP. Native-PAGE of exocarp tissue showed at least two basic and two acidic peroxidases. Number and intensity of peroxidase isozymes were greatest in fruit exocarp at 30 DPP. IEF-PAGE of 5-50 DPP fruit exocarp showed at least 8 peroxidase eisozymes (pI 4.6-9.6). Peroxidase isozymes increased in fruit from 5-30 DPP, and bands were most intense in the exocarp at 30 DPP. Anion exchange chromatography of the exocarp from 5-50 DPP showed one peak of anionic peroxidase activity which was not present until 15 DPP. This peak was greatest in 30 DPP fruit and less in 40 and 50 DPP fruit. Cationic peroxidase isozymes appeared to be the predominant and most intense isoforms throughout fruit development. Exocarp anionic peroxidase isoforms increased in intensity and number in 15-30 DPP fruit, and decreased in 40 and 50 DPP fruit. Total peroxidase activity and specific isozyme activity shifted during fruit development. Changes in peroxidase activity corresponded to fruit net formation and may be associated with susceptibility to fruit rot.