On the functional roles of the xylulose 5-phosphate/phosphate translocator and related transporters in primary and secondary metabolism of Arabidopsis thaliana

• Applicant: Dr. Rainer E. Häusler
• Subject Area: Plant Physiology
• Term: from 2015 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 267440908

Phosphate translocators (PTs) of the inner envelope membrane of plastids mediate the strict counter-exchange of phosphorylated metabolites and inorganic phosphate between the stroma and the cytosol. The four members of the PT family differ in their substrate spectra, their temporal and tissue-specific expression profiles, and thereby in their functional roles. The triose phosphate (TP)/PT (TPT) is the main exporter of assimilated carbon from chloroplasts in the light. In contrast, the glucose 6-phosphate (Glc6P)/PT (GPT) provides carbon skeletons to heterotrophic plastids, and the phosphoenolpyruvate (PEP)/PT (PPT) delivers PEP to both green and non-green plastids to support e.g., the shikimate pathway. The role of the xylulose 5-phosphate Xu5P/PT (XPT), which is ubiquitously expressed in Arabidopsis, is less clear. We could show that the XPT mediates the retrieval of Xu5P as endproduct of the extraplastidial oxidative pentose phosphate pathway (OPPP) to the stroma. The reason for a missing phenotype of both allelic mutants, xpt-1 and xpt-2, shall be investigated in the first subproject of this proposal. Mutant and wild-type plants will be subjected to conditions that increase the flux in extraplastidial OPPP. This should lead to an accumulation of Xu5P in the cytosol and a tailback of metabolites involved. Double mutants with a simultaneous defect in XPT and TPT show a clear growth retardation, combined with a massive accumulation of maltose (deriving from starch breakdown) and pentose phosphates as well as diminished photosynthesis rates. In the first funding period we could show that triple mutants with a defect in TPT, XPT, and starch synthesis are not viable. When compensational starch breakdown is not possible anymore, the TP transport activity of the XPT thus supports assimilate export from the chloroplasts. The second subproject is aiming at clarifying mechanisms responsible for the phenotype of the tpt/xpt double mutants. A particular focus will be on the consequences the deprivation of cytosolic TP bring along in leaves. As main strategies analyses of the transcriptome, metabolome, and metabolic fluxes will be applied with plants expressing or repressing transporters or other functions by an EtOH-inducible promoter system. By this transient expression system changes in the measuring parameters can be captured in a time dependent way upon induction. The XPT can also transport PEP beside of TPs. We could demonstrate that triple mutant lacking both PPTs and the XPT are capable of a 40% PEP transport compared to the wild type. In the third subproject we will analyse to which extent both GPTs, which possess substantial PEP transport capacities, are responsible for this effect.

• DFG Programme: Research Grants