C3 origins of the C4 pathway regulatory enzyme, PPDK-RP

Abstract

A current view of C4 and CAM evolution holds that all of the constituent enzymes of the C4 pathway pre-existed in C3 plants, albeit functioning in non-photosynthetic capacities. Problematic in this view is the C3 origin of presumably dedicated C4 pathway regulatory enzymes, such as PPDK regulatory protein (RP). RP is an unusual, bifunctional Ser/Thr-kinase/phosphatase that mediates light/dark regulation of C4/CAM PPDK activity via reversible phosphorylation of an active-site Thr. Because of its unique substrate requirements and localization in the stroma of C4 mesophyll cells, RP appears to be specific to C4 PPDK regulation. However, in this presentation we show that an RP-like activity exists in chloroplasts of C3 leaves. Specifically, immunoblot analysis of phospho- and dephospho- PPDK from illuminated and dark adapted C3 leaves (rice, F. pringlei, V. faba, spinach) revealed that PPDK phosphorylation/dephosphorylation is regulated in a light/dark-dependent manner. Further, the kinetics of the reversible activation process are similar to C4 plants, with light activation occurring rapidly (£ 15 min) and dark deactivation more slowly (³ 1 h). In vitro experiments with isolated intact spinach chloroplasts show the same light/dark modulation of PPDK phosphorylation state occurs, with light-induced dephosphorylation of phospho-PPDK being Pi dependent, inhibited by DCMU, but insensitive to MV. Hence, as with C4 RP, adenylates and stromal pools of Pi likely regulate the opposing bifunctional activities of the C3-like RP activity. Thus, evolution of RP into its C4/CAM role may have been no more problematic than for other C4/CAM pathway enzymes, as it apparently pre-exists in chloroplasts of C3 plants.