Light Mitigates Bismuth Toxicity While Sustaining Iron Homeostasis in Lepidium sativum Seedlings

Bismuth (Bi) is increasingly used as a substitute for lead (Pb) in several industrial applications, raising concerns about its potential environmental impact. However, the effects of Bi on early plant development and nutrient homeostasis remain poorly understood. In this study, toxicological and transcriptional responses were investigated in garden cress (Lepidium sativum L.) exposed in vitro to increasing Bi concentrations (0, 30, 60, 121, and 242 mg L−1) under dark and light conditions. In darkness, Bi progressively reduced the germination index and root growth. In contrast, under light conditions, low Bi concentrations stimulated seedling growth, whereas this effect decreased at higher doses. Gene expression analyses showed that Bi differentially affected key genes involved in iron (Fe) uptake and homeostasis, including LsIRT1 and LsFRO2, which displayed divergent expression patterns in dark- and light-grown seedlings. Micro-X-ray fluorescence (µ-XRF) analysis revealed distinct Fe and Bi accumulation profiles under dark and light conditions. Moreover, LsPCS1 expression, a marker of heavy metal detoxification responses, was strongly induced in the shoots of light-grown seedlings, where Bi accumulation was detected. Taken together, these results show that Bi inhibits early seedling development in darkness by impairing Fe uptake and homeostasis, whereas light promotes tolerance to Bi by enhancing these processes.