Tuning of sulfur flow and sulfur seed metabolism in oilseed rape under sulfate-limited conditions

The response of oilseed rape to sulfur (S) restriction usually consists of increasing the components of S utilization efficiency (absorption, assimilation, and remobilization) to provide S to seeds. However, source-sink relationships and S management in developing seeds under sulfate restriction are poorly understood. To address this, impacts of sulfate restrictions applied at 'visible bud' or 'start of pod filling' stages were studied in two genotypes, Aviso and Capitol. The two cultivars have similar seed yield, but Capitol has higher seed weight and lower number of seeds per plant under non-limited conditions. S flow at the whole-plant level (using [34S]sulfate labelling) and S metabolism changes [S-compounds, and ATP sulfurylase and adenosine 5'-phosphosulfate reductase (APR) activities] were studied during seed development. Seed yield, protein quality, and accumulation of S metabolites were affected by sulfate restriction less and later in Aviso than in Capitol. This was related to higher S uptake and stronger remobilization of S from vegetative organs to seeds during early seed development in response to sulfate restriction. A higher seed APR activity was observed for Capitol in response to sulfate limitation, suggesting that APR is not limiting for sulfate assimilation and that seed S metabolism is principally devoted to S-amino acids and protein synthesis.