A Cytosolic Arabidopsis Homologueof Bacterial GppA/PPX Retains Polyphosphate and pppGpp PhosphataseActivities and Regulates Photosynthetic Acclimation

Here, we performed biochemical and genetic characterization of a GppA/PPX-family phosphohydrolase from Arabidopsis thaliana (AtGPPA/PPX). Sixteen splice variants of AtGPPA/PPX are deposited in public databases and can be classified into two types based on differences in their N-terminal sequences. Green fluorescent protein fusions with either N-terminus indicated that neither possesses a transit peptide, suggesting that all AtGPPA/PPX splice variants localize to the cytosol. Biochemical analysis of the long splice variant of AtGPPA/PPX demonstrated phosphatase activity toward guanosine 5′-triphosphate 3′-diphosphate (pppGpp) and long-chain polyphosphate in vitro, although its substrate affinity and catalytic efficiency were lower than those ofEscherichia coli PPX. Heterologous expression of AtGPPA/PPX in anEscherichia coligppA mutant reduced intracellular pppGpp levels relative to the mutant control, indicating that AtGPPA/PPX possesses pppGpp dephosphorylation activity in vivo. Loss-of-function mutants of AtGPPA/PPX exhibited no overt growth defects under either standard or nitrogen-starvation conditions. However, chlorophyll fluorescence imaging revealed delayed and attenuated nonphotochemical quenching after actinic-light illumination without changes in the PSII quantum yield, suggesting faster electron transport and reduced need for thermal dissipation. Together, these results suggest that the cytosol-localized AtGPPA/PPX functions as a phosphatase with unknown physiological substrate(s) and plays a key role in photosynthetic acclimation to fluctuating light conditions.