Distribution of cell wall matrix polysaccharides in the epidermis of flax hypocotyl seedlings : Calcium induced-acidification of pectins

Abstract

We have investigated the effects of exogenously applied calcium on the nature and distribution of calcium and pectic polysaccharides in cortical tissues of flax (Linum usitatissimum L.) hypocotyls, using a variety of microscopical techniques, different polysaccharide staining methods and antibody probes. SIMS analysis of calcium distribution show an increase in the Ca 2+ level, especially in the walls of epidermal cells when the hypocotyls are grown in the presence of calcium. The nickel staining method reveals that polygalacturonans are mainly located in the epidermis. Different anti-pectin antibodies, such as anti-PGA/RG I, JIM 5, JIM 7 and CCRCM 2, were also used to determine the specific location of pectic polysaccharides within the cortical tissues. In the presence of calcium, immunofluorescence labelling shows an increase in JIM 5 and PGA/RG I-recognized epitopes, especially in the epidermal cells. In contrast, under the same conditions, JIM 7 labelling decreases. In addition, the increase in JIM 5 labelling is also observed using immunogold labelling. These data indicate that the plantlets grown in the presence of calcium exhibit high amounts of acidic pectins in their walls. Moreover, in these plantlets, chelation of calcium causes a clear increase of the cationic gold labelling in the epidermal walls indicating that the anionic sites of acidic pectins are largely compensated by calcium cations. Parallely, the combination of subtractive treatment and PATAg staining demonstrates an increase of calcium bridges that hold acidic pectins within the epidermal walls. Together, these findings suggest that calcium controls not only the acidification of pectic polysaccharides but also the linkages holding them in the cell wall structure.