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  • Author or Editor: Meeghan Pither-Joyce x
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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-phosphosulfate (APS) reductase, γ-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. Steady-state transcript levels of APS reductase and high-affinity sulfate transporter in roots were significantly higher (2- to 3-fold) 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.

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Bulb onion (Allium cepa L.) is a globally significant crop, but the structure of genetic variation within and among populations is poorly understood. We broadly surveyed genetic variation in a cultivated onion germplasm using simple sequence repeat (SSR) markers and sequenced regions flanking expressed sequence tag (EST)-SSRs to develop single-nucleotide polymorphism (SNP) markers. Samples from 89 inbred and open-pollinated (OP) bulb onion populations of wide geographical adaptation and four related Allium L. accessions were genotyped with 56 EST-SSR and four genomic SSR markers. Multivariate analysis of genetic distances among populations resolved long-day, short-day, and Indian populations. EST-SSR markers frequently revealed two major alleles at high frequency in OP populations. The median proportion of single-locus polymorphic loci was 0.70 in OP and landrace populations compared with 0.43 in inbred lines. Resequencing of 24 marker amplicons revealed additional SNPs in 17 (68%) and five SNP assays were developed from these, suggesting that resequencing of EST markers can readily provide SNP markers for purity testing of inbreds and other applications in Allium genetics.

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