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.