Deficiency in the Phosphorylated Pathway of Serine Biosynthesis perturbs sulfur assimilation in Arabidopsis

Although the plant Phosphorylated Pathway of L-serine Biosynthesis (PPSB) is essential for embryo and pollen development, and for root growth, its metabolic implications have not yet been investigated in depth. A transcriptomics analysis of PPSB-deficient mutants at nighttime, where PPSB activity is thought to be more important, pointed the sulfate assimilation process. As sulfate assimilation takes place mainly during the light period, we also investigated this process in PPSB-deficient lines in the daytime. Key genes in the sulfate starvation response, such as the Adenosine 5'phosphosulfate reductase (APR) genes along with sulfate transporters (SULTR), especially those involved in sulfate translocation in the plant, were induced in the PPSB-deficient lines. However, sulfate content did not lower in these lines, and the steady-state level of glutathione (GSH) increased in roots. This scenario suggests that PPSB deficiency perturbs the sulfate assimilation process between tissue/organs. Alteration of thiol distribution in leaves from different developmental stages, and between aerial parts and roots in plants with reduced PPSB activity provided confirmatory support for this idea. Flux analysis indicated that diminished PPSB activity caused an enhanced flux of 35S into thiol biosynthesis especially in roots. GSH turnover also accelerated in the PPSB-deficient lines, which supports the notion that not only biosynthesis, but also transport and allocation of thiols were perturbed in the PPSB mutants. Our results suggest that PPSB is required for sulfide assimilation in specific heterotrophic tissues and that lack of PPSB activity perturbs sulfur homeostasis between photosynthetic and non-photosynthetic tissues. Overall design: Aerial parts and roots of Arabidopsis wild type plants and mutants of the phosphoserine phosphatase were compared, in triplicate. The sequencing step was performed using a SOLID 5500XL equipment of 75 nucleotides using the Exact Call Chemistry.