Knock-Down of the Phosphoserine Phosphatase Gene Effects Rather N- Than S-Metabolism in Arabidopsis thaliana.

Sladjana Samuilov,Samantha Flachbart,Tabea Mettler-Altmann,Heinz Rennenberg,Stanislav Kopriva,Andreas P M Weber,Dominik Brilhaus,Nadine Rademacher,Leila Arab

doi:10.3389/fpls.2018.01830

Abstract

The aim of present study was to elucidate the significance of the phosphorylated pathway of Ser production for Cys biosynthesis in leaves at day and night and upon cadmium (Cd) exposure. For this purpose, Arabidopsis wildtype plants as control and its psp mutant knocked-down in phosphoserine phosphatase (PSP) were used to test if (i) photorespiratory Ser is the dominant precursor of Cys synthesis in autotrophic tissue in the light, (ii) the phosphorylated pathway of Ser production can take over Ser biosynthesis in leaves at night, and (iii) Cd exposure stimulates Cys and glutathione (GSH) biosynthesis and effects the crosstalk of S and N metabolism, irrespective of the Ser source. Glycine (Gly) and Ser contents were not affected by reduction of the psp transcript level confirming that the photorespiratory pathway is the main route of Ser synthesis. The reduction of the PSP transcript level in the mutant did not affect day/night regulation of sulfur fluxes while day/night fluctuation of sulfur metabolite amounts were no longer observed, presumably due to slower turnover of sulfur metabolites in the mutant. Enhanced contents of non-protein thiols in both genotypes and of GSH only in the psp mutant were observed upon Cd treatment. Mutation of the phosphorylated pathway of Ser biosynthesis caused an accumulation of alanine, aspartate, lysine and a decrease of branched-chain amino acids. Knock-down of the PSP gene induced additional defense mechanisms against Cd toxicity that differ from those of WT plants.

Highlights

Cysteine (Cys) constitutes the initial product of sulfur assimilation that is used for protein biosynthesis and all anabolic pathways that require reduced sulfur
Principal component analysis (PCA) showed a clear separation between metabolite amounts of wild type (WT) plants and the two mutants at day and night, as well as transcript amounts at day and night, except for a slight overlap between WT and psp-18 line at transcript level at day (Supplementary Figure S2)
Transcripts encoding all isoforms of glycine decarboxylase multi-enzyme complex (GDC) and SHMT genes of the photorespiratory pathway were lower compared to the WT at day and night, with significant differences observed for the psp-17 mutant line (Figure 1)

Summary

Introduction

Cysteine (Cys) constitutes the initial product of sulfur assimilation that is used for protein biosynthesis and all anabolic pathways that require reduced sulfur. It is an important precursor of all thiol containing organic sulfur compounds such as glutathione (GSH) (Noctor et al, 2012), methionine (Met) (Hesse et al, 2004; Wirtz and Droux, 2005), and glucosinolates (GLS) (Falk et al, 2007). GSH plays an important role in scavenging reactive oxygen species (ROS) in the ascorbate-glutathione cycle (Noctor et al, 2012), in the detoxification of xenobiotics by conjugation via GSH-S transferases (Gullner et al, 2001), and for the sequestration of heavy metals as metabolic precursor of phytochelatins that function as heavy metal chelators (Herbette et al, 2006; Lima et al, 2006). Glucosinolates are secondary sulfur-rich, cys-containing metabolites characteristic for Brassicaceae (Huseby et al, 2013) that protect plants against herbivores and pathogens (Halkier and Gershenzon, 2006)

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