PECTIN METHYLESTERASES FROM POPLAR CAMBIUM AND INNER BARK - LOCALIZATION, PROPERTIES AND SEASONAL-CHANGES

In the course of a study on the early events of cambial derivative dif ferentiation in Populus x euramericana, seasonal changes in the patter n of pectin methylesterase (PME, EC 3.1.1.11) isoforms were followed. During the resting season, cell wall extracts contained mainly alkalin e isoforms with an M-r around 55 kDa and optimal PPI between 5.6 and 6 .0, Neutral isoforms with an M-r around 35 kDa and optimal pH between 6.0 and 6.6 predominated in the extracts during the period of high mer istematic activity. In the active cambial initials and in their immedi ate derivatives, the enzymes were immunolocalized exclusively in the d ictyosomes. In older cells, they were present both in dictyosomes and in wall junctions. These results indicate that exportation of neutral PMEs towards the walls might be considered as an early marker of diffe rentiation in cambial derivatives.