Cytosolic pyruvate,orthophosphate dikinase functions in nitrogen remobilization during leaf senescence and limits individual seed growth and nitrogen content

Abstract

SUMMARYThe protein content of seeds determines their nutritive value, downstream processing properties and market value. Up to 95% of seed protein is derived from amino acids that are exported to the seed after degradation of existing protein in leaves, but the pathways responsible for this nitrogen metabolism are poorly defined. The enzyme pyruvate,orthophosphate dikinase (PPDK) interconverts pyruvate and phosphoenolpyruvate, and is found in both plastids and the cytosol in plants. PPDK plays a cardinal role in C 4 photosynthesis, but its role in the leaves of C 3 species has remained unclear. We demonstrate that both the cytosolic and chloroplastic isoforms of PPDK are up-regulated in naturally senescing leaves. Cytosolic PPDK accumulates preferentially in the veins, while chloroplastic PPDK also accumulates in mesophyll cells. Analysis of microarrays and labelling patterns after feeding 13 C-labelled pyruvate indicated that PPDK functions in a pathway that generates the transport amino acid glutamine, which is then loaded into the phloem. In Arabidopsis thaliana, overexpression of PPDK during senescence can significantly accelerate nitrogen remobilization from leaves, and thereby increase rosette growth rate and the weight and nitrogen content of seeds. This indicates an important role for cytosolic PPDK in the leaves of C 3 plants, and allows us to propose a metabolic pathway that is responsible for production of transport amino acids during natural leaf senescence. Given that increased seed size and nitrogen content are desirable agronomic traits, and that efficient remobilization of nitrogen within the plant reduces the demand for fertiliser applications, PPDK and the pathway in which it operates are targets for crop improvement.

Changes in Transcript Abundance in Chlamydomonas reinhardtii following Nitrogen Deprivation Predict Diversion of Metabolism

“…It has also been associated with suppressed phosphoenolpyruvate carboxykinase activity (Osterås et al, 1997) and salt stress (Fisslthaler et al, 1995). Recently, this enzyme has been shown to play an important role in N remobilization (Taylor et al, 2010). Likewise, the transcript abundances for enzymes of the pentose phosphate cycle predicted to be localized in the cyto-sol, Glc-6-P 1-dehydrogenase and phosphogluconate dehydrogenase (decarboxylating), were increased under those conditions.…”

Transcriptomes of eight Arabidopsis thaliana accessions reveal core conserved, genotype- and organ-specific responses to flooding stress

“…This transcript encodes the cytosol-localized form of PPDK (Parsley and Hibberd, 2006) that is suggested to be important during amino acid remobilization and senescence (Taylor et al, 2010) and for gluconeogenesis (Eastmond et al, 2015). Interestingly, several other genes involved in gluconeogenesis were quantitatively up-regulated or down-regulated in response to the compound and darkness treatments (Supplemental Fig.…”

Nitrogen use efficiency in rapeseed. A review

“…During flowering, the vegetative plant segments switch from being sink organs to source organs, with associated modifications in the N fluxes from older to younger tissues and to the reproductive organs (Le Deunff and Malagoli 2014). Labeling studies in Arabidopsis thaliana (Taylor et al 2010) and B. napus (Rossato et al 2001;Malagoli et al 2005a) showed that the N accumulated in the seeds at harvest originates mainly from the degradation of proteins in vegetative plant segments. phase sequentially, multiple overlapping source-sink situations are possible in rapeseed.…”