Iron in complex with the alleged phytosiderophore 8-hydroxyquinoline induces functional iron deficiency and non-autolytic programmed cell death in rapeseed plants

• Iron supply in the form of a Fe–HQ complex induced functional Fe deficiency.
• Fe–HQ supplied plants developed chlorosis.
• Fe–HQ-supplied plants developed non-autolytic PCD similar to Fe-deprived plants.
• Fe–HQ induced ROS and various enzyme activites resembled to Fe-deprived plants.

The alkaloid 8-hydroxyquinoline (HQ) was pointed out as potential phytosiderophore that is taken up unspecifically by iron acquisition strategy I and II plants. HQ in root exudate of the knapweed Centaurea diffusa even was considered as a factor contributing to the invasive success of this species. This study compares the iron supply efficiencies of the Fe–HQ complex and Fe–EDTA in hydroponic cultures of rapeseed (Brassica napus) as a plant model system to explore the proposed function. Iron (FeCl3) was supplied in 2 and 10 μM concentrations in a 1:1 ratio with the ligand (HQ). After 20 days, Fe content, lipid peroxidation, superoxide anion, hydrogen peroxide, and ascorbic acid concentrations, various enzyme activities associated with antioxidant defences and programmed cell death (PCD), and nuclear condensation were determined. Iron supply in the form of a Fe–HQ complex clearly was less efficient. These plants developed chlorosis and showed symptoms of non-autolytic PCD similar to those that had been subjected to Fe deprivation. Reactive oxygen species (ROS) concentration levels and enzyme activities (superoxide dismutase, catalase, alkaline protease, caspase-3-like and deoxyribonuclease) resembled more those observed in plants which suffered from Fe deprivation, than those of Fe-sufficient Fe–EDTA supplied plants. The results do not support the putative phytosiderophore function alleged to HQ in the studied concentration range (2–10 μM) but instead corroborate the one hypothesis explaining HQ toxicity: Fe in complex with HQ is not released efficiently enough from the complex to be available for metalloenzymes that require it as a co-factor.