ARABINOGALACTAN-PROTEIN EPITOPES IN SOMATIC EMBRYOGENESIS OF DAUCUS-CAROTA L

Two monoclonal antibodies (ZUM 15 and ZUM 18) directed against carrot (Daucus carota L.) seed arabinogalactan proteins (ACPs) were used to i solate specific AGP fractions. For both carrot and tomato (Lycopersico n esculentum Mill.) seed ACPs analyzed by crossed-electrophoresis, the ZUM 15 and ZUM 18 AGP fractions showed one identical peak. However, t he Rf values for the two species were different: 0.82 for carrot seed AGPs and 0.52 for tomato seed ACPs. When the fractionated AGPs (carrot or tomato) were added to carrot cell lines they had a dramatic effect on the culture. One AGP fraction (ZUM 15 AGPs) was able to induce vac uolation of embryogenic cells. Those cells failed to produce embryos. The other AGP fraction (ZUM 18 AGPs) increased the percentage of embry ogenic cells from about 40% up to 80% within one week and this subsequ ently resulted in the formation of more embryos on hormone-free medium . This activity was higher than that of unfractionated carrot seed AGP s, while the optimum concentration was 50-fold lower. Since both ZUM 1 8 ACPs (carrot or tomato) yielded identical responses it can be conclu ded that neither the Rf value nor the source are essential for biologi cal activity. The dose-response curve of ZUM 18 AGPs showed a sharp op timum. When the ACPs that also bound to the antibody ZUM 15 were remov ed, the dose-response curve of the remaining ACPs (containing only the ZUM 18 epitope, not the ZUM 15 epitope) resembled a saturation curve. Regardless of its concentration, the fraction in which AGP molecules contained both epitopes showed no appreciable embryogenesis-promoting activity. The biological activity of AGPs was therefore determined by the presence of embryogenesis-enhancing and-inhibiting epitopes. The i nhibiting and enhancing epitopes can be located on separate molecules or one single ACP molecule.