Upper limits for endogenous oligogalacturonides and free galacturonic acid in rose cell-suspension cultures: Implications for the action of Exo- and Endo-polygalacturonases in vivo

Abstract

Summary To test for the possible production of biologically-active oligogalacturonides by healthy plant cells, we fed d-[U- 14 C] glucose to rose ( Rosa sp., cv. ) cell-suspension cultures and analysed the anionic metabolites that accumulated in the medium. Acidic 14 C-polysaccharides and a novel acidic 14 C-oligosaccharide accumulated, but steady-state concentrations of [ 14 C] oligogalacturonides (degree of polymerisation 2–12) were undetectable (≤50μg/L). The product of [ 14 C]oligogalacturonide turnover would be free [ 14 C] galacturonic acid; however, this too failed to accumulate in the rose culture medium (steady-state concentration ≤200nM), even though the cultures lacked the ability to take up or metabolise exogenous d-galacturonic acid. Therefore, healthy rose cell cultures lacked sufficient endo-polygalacturonase activity (EC 3.2.1.15) to generate detectable oligogalacturonides from endogenous pectic polysaccharides. Also, the absence of detectable [ 14 C]galacturonic acid shows that exo-polygalacturonase (EC 3.2.1.67), known to be present in the apoplast of these rose cells, failed to act at an appreciable rate on any endogenous apoplastic substrate, either soluble or wall-bound. We argue that the main natural substrate of plant exo-polygalacturonase in vegetative tissues is microbially-generated pectic fragments after infection.