title: Genotypic variation in sulphur assimilation and metabolism of onion (Allium cepa L.). II: Characterisation of ATP sulphurylase activity.
abstract: To investigate the regulation of sulphur (S)-assimilation in onion further at the biochemical level, the pungent cultivar W202A and the milder cultivar Texas Grano 438 PVP (TG) have been grown in S-sufficient (S(+); 4meqS(-1)) or S-deficient (S(-); 0.1meqS(-1)) growth conditions, and tissues excised at the seedling stage (pre-bulbing; ca. 10-weeks-old) and at the mature stage (bulbing; ca. 16-weeks-old). S-supply negatively influenced adenosine-5'-phosphosulphate (APS) reductase (APR) enzyme activity in both cultivars at bulbing only, and a higher abundance of APR was observed in both cultivars at bulbing in response to low S-supply. In contrast, S-supply significantly influenced ATP sulphurylase (ATPS) activity in leaf tissues of W202A only, and only at bulbing, while an increase in abundance in response to high S-supply was observed for both cultivars at bulbing. To investigate the regulation of the ATPS enzyme activity and accumulation further, activity was shown to decrease significantly in roots at bulbing in the S-deficient treatment in both cultivars, a difference that was only supported by western analyses in W202A. Phylogenetic analysis revealed that AcATPS1 groups in a broad monocot clade with the closest sequences identified in Sorghum bicolour, Zea mays and Oryza sativa, but with some support for a divergence of AcATPS1. Detection of ATPS in leaf extracts after two dimensional gel electrophoresis (2-DE) revealed that the protein may undergo post-translational modification with a differential pattern of ATPS accumulation detected in both cultivars over the developmental progression from the seedling to the bulbing stage. Treatment of leaf extracts of W202A to dephosphorylate proteins resulted in the loss of immuno-recognised ATPS spots after 2-DE separation, although enzyme activity was not influenced. These results are discussed in terms of the tiers of control that operate at the biochemical level in the reductive S-assimilation pathway in a S-accumulating species particularly during the high-S-demanding bulbing stage.
authors: Institute of Molecular BioSciences, Massey University, Private Bag 11222, Palmerston North, New Zealand.
authors: Ludivine
authors: L
authors: Thomas
authors: Institute of Molecular BioSciences, Massey University, Private Bag 11222, Palmerston North, New Zealand.
authors: Susanna
authors: S
authors: Leung
authors: Institute of Molecular BioSciences, Massey University, Private Bag 11222, Palmerston North, New Zealand.
authors: Mathew
authors: M
authors: Cumming
authors: New Zealand Institute for Plant and Food Research Ltd., Private Bag 4704, Christchurch, New Zealand.
authors: Martin
authors: M
authors: Shaw
authors: Institute of Molecular BioSciences, Massey University, Private Bag 11222, Palmerston North, New Zealand.
authors: Nick
authors: N
authors: Albert
authors: New Zealand Institute for Plant and Food Research Ltd., Private Bag 4704, Christchurch, New Zealand.
authors: John
authors: J
authors: McCallum
authors: Institute of Molecular BioSciences, Massey University, Private Bag 11222, Palmerston North, New Zealand.
authors: Michael T
authors: MT
authors: McManus
mesh_terms: D002736:Chloroplasts / Q000201:enzymology; D005838:Genotype; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D019697:Onions / Q000201:enzymology* / Q000235:genetics; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000378:metabolism; D010802:Phylogeny; D011499:Protein Processing, Post-Translational; D013430:Sulfate Adenylyltransferase / Q000235:genetics* / Q000378:metabolism; D013455:Sulfur / Q000378:metabolism*
chemical_list: D007527:Isoenzymes; D013455:Sulfur; D050862:Oxidoreductases Acting on Sulfur Group Donors; C019618:adenylylsulfate reductase; D013430:Sulfate Adenylyltransferase