Responses to aluminum and cadmium of a RNAi sorghum line with decreased levels of phosphoenolpyruvate carboxylase 3 (PPC3)

Phosphoenolpyruvate carboxylase (PEPC) is an enzyme family with central roles in carbon and nitrogen metabolisms. In order to obtain knowledge about the function of specific PEPC isoenzymes, we have characterized RNAi sorghum lines with decreased level of PPC3 (Ppc3 lines). PPC3 is the main root PEPC, and participates in responses to salinity and ammonium stress. This prompted us to study the responses of Ppc3 lines to Al and Cd stress. Both Al and Cd treatments decreased germination rate, growth rate, and shoot and root biomass production. These detrimental effects were higher in Ppc3 lines than in Wt. Root PPC3 expression and PEPC activity was increased by Al and Cd in Wt, but not in Ppc3 lines. The treatments also increased PPC2 and PPCK gene expression, and PEPCk activity, but these changes did not substitute the lack of PPC3. The production and secretion of exudates by plant roots play an important role in plant tolerance to Al and Cd toxicity. Al (2 days of treatment) caused a 40-fold increase of the citrate content of root exudates, without differences between Wt and Ppc3 lines. On the contrary, Cd (7 days of treatment) caused a 400-fold increase of citrate in Wt exudates, but only a 30-fold increase in Ppc3 lines. These results indicate that although PPC3 is not necessary for early production of organic acids in root exudates, it is crucial for maintaining high level of synthesis and accumulation of citrate, and that PPC3 is the main PEPC isoenzyme responsible for this response to heavy metal stress.