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

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

doi:10.3389/fmolb.2021.763795

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.

Highlights

Eukaryotic cells are highly organized into different compartments, such as the mitochondria, the endoplasmic reticulum, peroxisomes, and the Golgi apparatus
Based on current knowledge concerning the SAL1–phosphoadenosine 5′phosphate (PAP) pathway and taking all phenotypes reported for sal1 mutants into consideration, we hypothesized that PAP can act in different compartments, where PAP signaling can potentially execute different functions (Phua et al, 2018)
These intriguing scenarios prompted us to design experiments addressing the role of SAL1 in different cell compartments by expressing it in chloroplasts, the mitochondria, the nucleus, and the cytosol

Summary

Introduction

Eukaryotic cells are highly organized into different compartments, such as the mitochondria, the endoplasmic reticulum, peroxisomes, and the Golgi apparatus. Plastids and the mitochondria produce retrograde signals that modulate nuclear gene expression and organellar biogenesis or optimize their performance (Chi et al, 2015). Many signals in organellar retrograde pathways have been identified, including chlorophyll intermediates, ROS, and other metabolites (Chan et al, 2016; Ishiga et al, 2017; Pesaresi and Kim, 2019). The retrograde signaling molecule PAP (3′phosphoadenosine 5′-phosphate) is generated by the sulfate assimilation pathway and degraded by SAL1 into AMP and inorganic phosphate. Transcriptome analysis has shown that SAL1 regulates an overlapping set of genes with 3′ exoribonucleases (XRNs), suggesting that they function in a common signaling pathway (Estavillo et al, 2011)

Methods

Results

Conclusion