Factors involved in the rise of phosphoenolpyruvate carboxylase-kinase activity caused by salinity in sorghum leaves

Salinity increases phosphoenolpyruvate carboxylase kinase (PEPCase-k) activity in sorghum leaves. This work has been focused on the mechanisms responsible for this phenomenon. The light-triggered expression of SbPPCK1 gene, accountable for the photosynthetic C-4-PEPCase-k, is controlled by a complex signal transduction chain involving phospholipases C and D (PLC and PLD). These two phospholipase-derived signalling pathways were functional in salinized plants. Pharmacological agents that act on PLC (U-73122, neomycin) or PLD (n-butanol) derived signals, blocked the expression of SbPPCK1, but had little effect on PEPCase-k activity. This discrepancy was further noticed when SbPPCK1-3 gene expression and PEPCase-k activity were studied in parallel. At 172 mM, the main effect of NaCl was to decrease the rate of PEPCase-k protein turnover. Meanwhile, 258 mM NaCl significantly increased both SbPPCK1 and SbPPCK2 gene expression and/or mRNA stability. The combination of these factors contributed to maintain a high PEPCase-k activity in salinity. LiCl increased calcium-dependent protein kinase (CDPK) activity in illuminated sorghum leaves while it decreased the rate of PEPCase-k degradation. The latter effect was restrained by W7, an inhibitor of CDPK activity. Recombinant PEPCase-k protein was phosphorylated in vitro by PKA. A conserved phosphorylation motif, which can be recognized by PKA and by plant CDPKs, is present in the three PEPCase-ks proteins. Thus, it is possible that a phosphorylation event could be controlling (increasing) the stability of PEPCase-k in salinity. These results propose a new mechanism of regulation of PEPCase-k levels, and highlight the relevance of the preservation of key metabolic elements during the bulk degradation of proteins, which is commonly associated to stress.