FAIR-CT97-5023 Roles of polyphenol oxidases and peroxidases in wood colour quality of walnut (Juglans spp)

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FAIR Area 1.3 - Forestry-Wood Chain : FAIR Marie Curie Research Training Grants

Objectives:

Concerning the production of high quality timber of walnut (Juglans sp.) in Europe. The general objective of the research concerns one of the main characters of the walnut wood quality, namely its colour. Heartwood formation is related to the process of ageing of xylem cells. The ultimate stage of this process is the death of the last living sapwood cells (parenchyma cells). The coloured compounds produced during this stage are the result of a series of oxidations and polymerizations by the way of chemical and enzymatic reactions. The choice of studying peroxidases and polyphenoloxidases (PPO) is linked to the role of these enzymes in different browning processes of tissues in particular physiological conditions (defense against pathogens, fruit maturation, etc.).

There are three scientific objectives to understand the role of peroxidases and PPO in the browning of wood. These objectives are: the determination of which oxidase and which substrate of this enzyme plays a role in browning; the precise localisation of oxidase activities; (iii) to understand the mechanisms of interaction between enzymes and substrates leading to the colouring of wood.

Activities and Results:

With respect to the localisation of peroxidases and polyphenoloxidases activity, the project found that classic histology and immunohistolocalisation yielded detailed information about the localisation of two families of oxidative enzymes.

Due to where they were found, PPOS do not seem to have a direct function In the Process of heartwood formation. On the other hand, the localisatioti of pcroxidase activity and appearance of colouring at the sapwood/heartwood interface seems highly significant. It was also shown that the same xylem band is made up of different "phenol cells" and "peroxidase cells". This result is only slightly surprising given the strong inhibiting capacity of many phenolic compounds, in particular with respect to peroxidase activity. At the cellular level (banded parenchyma, paratracheal parenchyma), the peroxidase giffacol- dependent activity appeared to be confined to the vacuole, although the 3,3'- diaminobenzidine test reveals more that one cytoplasi-nic form. The isoelectrofocalisation gels revealed the presence of many peroxidase activity bands which were detected in the transition zone, including one band indicating high activity with an isoelectric point of 7.7. This band is also present in the cambial zone of Juglans nigra, but with even higher activity.

This project forms part of FAIR-CT96-1887 (W-Brains).

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Scientific Supervisor

• Universite de Montpellier II Biotechnologie et Physiologie Vegetale / FRANCE