Effect of LiCl on phosphoenolpyruvate carboxylase kinase and the phosphorylation of phosphoenolpyruvate carboxylase in leaf disks and leaves of Sorghum vulgare

In the present work, the effect of LiCl on phosphoenolpyruvate carboxylase kinase (PEPCase-k), C-4 phosphoenolpyruvate carboxylase (PEPCase: EC 4.1.1.31) and its phosphorylation process has been investigated in illuminated leaf disks and leaves of the C-4 plant Sorghum vulgare. Although this salt induced severe damages to older leaves, it did not significantly alter the physiological parameters (photosynthesis, transpiration rate, intercellular CO2 concentration) of young leaves. An immunological approach was used to demonstrate that the PEPCase-k protein accumulated rapidly in illuminated leaf tissues, consistent with the increase in its catalytic activity. In vivo, LiCl was shown to strongly enhance the light effect on PEPCase-k protein content, this process being dependent on protein synthesis. In marked contrast, the salt was found to inhibit the PEPCase-k activity in reconstituted assays and to decrease the C-4 PEPCase content and phosphorylation state in LiCl treated plants. Short-term (15 min) LiCl treatment increased IP3 levels, PPCK gene expression, and PEPCase-k accumulation. Extending the treatment (1 h) markedly decreased IP3 and PPCK gene expression, while PEPCase-k activity was kept high. The cytosolic protein synthesis inhibitor cycloheximide (CHX), which blocked the light-dependent up-regulation of the kinase in control plants, was found not to be active on this process in preilluminated, LiCl-treated leaves. This suggested that the salt causes the kinase turnover to be altered, presumably by decreasing degradation of the corresponding polypeptide. Taken together, these results establish PEPCase-k and PEPCase phosphorylation as lithium targets in higher plants and that this salt can provide a means to investigate further the organization and functioning of the cascade controlling the activity of both enzymes.