Dissecting the Role of SAL1 in Metabolizing the Stress Signaling Molecule 3'-Phosphoadenosine 5'-Phosphate in Different Cell Compartments

Natallia Ashykhmina; Kai Xun Chan; Henning Frerigmann; Frank Van Breusegem; Stanislav Kopriva; Ulf-Ingo Flügge; Tamara Gigolashvili

doi:10.3389/fmolb.2021.763795

Dissecting the Role of SAL1 in Metabolizing the Stress Signaling Molecule 3'-Phosphoadenosine 5'-Phosphate in Different Cell Compartments

Front Mol Biosci. 2022 Jan 21:8:763795. doi: 10.3389/fmolb.2021.763795. eCollection 2021.

Authors

Natallia Ashykhmina¹, Kai Xun Chan^{2

3}, Henning Frerigmann⁴, Frank Van Breusegem^{2

3}, Stanislav Kopriva¹, Ulf-Ingo Flügge¹, Tamara Gigolashvili¹

Affiliations

¹ Institute for Plant Sciences, Cologne Biocenter, University of Cologne, Cologne, Germany.
² Department of Plant Biotechnology and Bioinformatics, Ghent University, Ghent, Belgium.
³ VIB Center for Plant Systems Biology, Ghent, Belgium.
⁴ Max Planck Institute for Plant Breeding Research, Cologne, Germany.

Abstract

Plants possess the most highly compartmentalized eukaryotic cells. To coordinate their intracellular functions, plastids and the mitochondria are dependent on the flow of information to and from the nuclei, known as retrograde and anterograde signals. One mobile retrograde signaling molecule is the monophosphate 3'-phosphoadenosine 5'-phosphate (PAP), which is mainly produced from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) in the cytosol and regulates the expression of a set of nuclear genes that modulate plant growth in response to biotic and abiotic stresses. The adenosine bisphosphate phosphatase enzyme SAL1 dephosphorylates PAP to AMP in plastids and the mitochondria, but can also rescue sal1 Arabidopsis phenotypes (PAP accumulation, leaf morphology, growth, etc.) when expressed in the cytosol and the nucleus. To understand better the roles of the SAL1 protein in chloroplasts, the mitochondria, nuclei, and the cytosol, we have attempted to complement the sal1 mutant by specifically cargoing the transgenic SAL1 protein to these four cell compartments. Overexpression of SAL1 protein targeted to the nucleus or the mitochondria alone, or co-targeted to chloroplasts and the mitochondria, complemented most aspects of the sal1 phenotypes. Notably, targeting SAL1 to chloroplasts or the cytosol did not effectively rescue the sal1 phenotypes as these transgenic lines accumulated very low levels of SAL1 protein despite overexpressing SAL1 mRNA, suggesting a possibly lower stability of the SAL1 protein in these compartments. The diverse transgenic SAL1 lines exhibited a range of PAP levels. The latter needs to reach certain thresholds in the cell for its impacts on different processes such as leaf growth, regulation of rosette morphology, sulfate homeostasis, and glucosinolate biosynthesis. Collectively, these findings provide an initial platform for further dissection of the role of the SAL1-PAP pathway in different cellular processes under stress conditions.

Keywords: 3′-phosphoadenosine 5′-phosphate (PAP); SAL1; chloroplasts; cytosol; metabolite signaling; mitochondria; nuclei; nucleotidase/phosphatase.