UPPER LIMITS FOR ENDOGENOUS OLIGOGALACTURONIDES AND FREE GALACTURONICACID IN ROSE CELL-SUSPENSION CULTURES - IMPLICATIONS FOR THE ACTION OF EXO-POLYGALACTURONASES AND ENDO-POLYGALACTURONASES IN-VIVO

To test for the possible production of biologically-active oligogalact uronides by healthy plant cells, we fed D-[U-C-14] glucose to rose (Ro sa sp., cv. (Paul's Scarlet)) cell-suspension cultures and analysed th e anionic metabolites that accumulated in the medium. Acidic C-14-poly saccharides and a novel acidic C-14-oligosaccharide accumulated, but s teady-state concentrations of [C-14] oligogalacturonides (degree of po lymerisation 2-12) were undetectable (less than or equal to 50 mu g/L) . The product of [C-14] oligogalacturonide turnover would be free [C-1 4]galacturonic acid; however, this too failed to accumulate in the ros e culture medium (steady-state concentration less than or equal to 200 nM), even though the cultures lacked the ability to take up or metabo lise exogenous D-galacturonic acid. Therefore, healthy rose cell cultu res lacked sufficient endo-polygalacturonase activity (EC 3.2.1.15) to generate detectable oligogalacturonides from endogenous pectic polysa ccharides. Also, the absence of detectable [C-14]galacturonic acid sho ws that exo-polygalacturonase (EC 3.2.1.67), known to be present in th e apoplast of these rose cells, failed to act at an appreciable rate o n any endogenous apoplastic substrate, either soluble or wall-bound. W e argue that the main natural substrate of plant exo-polygalacturonase in vegetative tissues is microbially-generated pectic fragments after infection.