Sulfur deprivation and genotype affect gene expression and metabolism of onion roots

Genetic and environmental factors affect onion (Allium cepa L.) pungency but the molecular basis for this variation is not understood. To initiate molecular analysis of onion sulfur metabolism we isolated cDNAs from onion associated with sulfur assimilation and compared gene expression and sulfur metabolism of mild and pungent onion cultivars. We isolated cDNAs encoding homologues of 5'adenosine-phosphosuffate (APS) reductase, gamma-glutamylcysteine synthetase and serine acetyl transferase using a homology-based RT-PCR approach. Homologues of high-affinity sulfate transporters and sulfite reductase were isolated from an onion root differential cDNA library enriched for genes up-regulated by 48 hours sulfur deprivation. The influence of genotype and sulfur nutrition on root expression of selected genes was measured in an experiment in which a low pungency onion cultivar ('Houston Grano') and a high pungency cultivar ('Canterbury Longkeeper') were grown hydroponically in low (0.1 meq.L-1) or high (4.0 meq.L-1) sulfate medium and harvested before bulbing. 'Canterbury Longkeeper' contained higher concentrations of (+)-S-methyl-L-cysteine sulfoxide in leaf and root than 'Houston Grano' but cultivars did not differ in leaf trans-(+)-S-(1-propenyl)-L-cysteine sulfoxide concentrations. 'Houston Grano' accumulated significantly higher concentrations of total N, nitrate, and basic amino acids in leaves and roots, suggesting these cultivars differ markedly in maintenance of S/N homeostasis. Steadystate transcript levels of APS reductase and high-affinity sulfate transporter in roots were significantly higher (2- to 3-old) at low sulfate. By contrast, steady state levels of ATP sulfurylase transcript were significantly higher at high sulfate levels and in 'Canterbury Longkeeper'. We conclude that differences in regulation of the sulfur assimilation pathway may underlie genetic differences in pungency.