Carbon metabolism in in vitro cultures of date palm: the role of carboxylases (PEPC and RubisCO)

While describing major trends of carbon metabolism during the initiation an d expression of somatic embryogenesis in date palm (Phoenix dactylifera L., cv. Deglet Nour), we have investigated the role of two carboxylases, namel y PEPC (Phosphoenolpyruvate carboxylase, EC 4.1.1.31) and RubisCO (Ribulose 1,5-bisphosphate carboxylase/oxygenase, EC 4.1.1.39), in embryogenic and n on-embryogenic cultures. The detection of PEPC activity on polyacrylamide n ative gels after electrophoresis revealed the presence of 3 active isoforms in crude extracts from the embryogenic (E) callus strain, whereas only a s ingle band was present in the non-embryogenic (NE) one. The level of PEPC s pecific capacity was of the same order (3.9 +/- 1.2 mu mol CO2 h(-1) mg(-1) TSP) in both types of cultures. Further changes in carboxylase (PEPC and R ubisCO) activities during the growth and development of somatic embryo-deri ved plantlets were also analysed. The PEPC/RubisCO ratio was found to progr essively decrease (from 17.7 to 0.2) throughout the in vitro development of plantlets, due to a substantial depletion of PEPC activity, which decrease d from 5.3 to 1.2 mu mol CO2 h(-1) mg(-1) TSP. Concomitantly, RubisCO assum ed greater importance (from 0.3 to 5.3 mu mol CO2 h(-1) mg(TSP)(-1)) and be came the main route for inorganic carbon fixation. Western blot analysis us ing polyclonal antibodies raised against PEPC and RubisCO purified from tob acco leaves confirmed this trend in terms of relative enzyme abundance.