Identification of biochemical differences between the sapwood and transition zone in Robinia pseudoacacia L. by differential display of proteins

The predominant proteins and enzymes in the sapwood and transition zones of Robinia pseudoacacia L. were identified and expressed by two methods: 2D SDS-PAGE (two-dimensional sodium dodecyl sulphate polyacrylamide gel electrophoresis) and electrospray ionisation tandem mass spectrometry (ESI-MS/MS). Large differences in the amount of proteins extracted were observed between the bark, sapwood and transition zones. Soluble proteins strongly expressed in sapwood have been identified, and the results interpreted to mean that these proteins are responsible for carbohydrate metabolism and flavonoid turnover. By contrast, proteins strongly expressed in the transition zone are mainly responsible for flavonoid biosynthesis. Lectins were found in protein fractions of both sapwood and the transition zone, and heat-stress proteins were detected only in the transition zone. The results are a further proof that flavonoids are synthesised directly at the transition zone between sapwood and heartwood, and that materials deposited in the sapwood are the source for synthesis of metabolites in heartwood, such as flavonoids and tannins.