Differential localization of arabinan and galactan side chains of rhamnogalacturonan 1 in cambial derivatives

The development of pectin structural features during the differentiation of cambial derivatives was investigated in aspen (Populus tremula L. x P. tre muloides Michx.) using biochemical and immunocytochemical methods. Comparis ons were also made between active and resting tissues. Active tissues, in p articular cambial cells and phloem derivatives, were characterized by a hig h pectin content. Use of antibodies raised against arabinan side chains of rhamnogalacturonan 1 (LM6), as well as biochemical analysis, revealed an ob vious decrease from the cortex to the differentiating xylem. Galactan side chains, detected with LM5 antibodies, were present mainly in the cambial zo ne and enlarging xylem cells. In contrast, they were totally absent from si eve-tube cell walls. Image analysis of LM5 immunogold labelling in the camb ial zone showed a clustered distribution of galactan epitopes in the radial walls, a distribution which might result from the association of two diffe rent periodic processes, namely the exocytosis of galactan and wall expansi on. Cessation of cambial activity was characterized by cell wall thickening accompanied by a sharp decrease in the relative amount of pectin and a low ering of the degree of methylesterification. The data provide evidence that the walls of phloem and xylem cells differ in their pectin composition eve n at a very early stage of commitment. These differences offer useful tools for identifying the initial cells among their immediate neighbours.