Selective mineral transport barriers at Cuscuta-host infection sites

The uptake of inorganic nutrients by rootless parasitic plants, which depend on host connections for all nutrient supplies, is largely uncharted. Using X-ray fluorescence spectroscopy (XRF), we analyzed the element composition of macro- and micronutrients at infection sites of the parasitic angiosperm Cuscuta reflexa growing on hosts of the genus Pelargonium. Imaging methods combining XRF with 2-D or 3-D (confocal) microscopy show that most of the measured elements are present at similar concentrations in the parasite compared to the host. However, calcium and strontium levels drop pronouncedly at the host/parasite interface, and manganese appears to accumulate in the host tissue surrounding the interface. Chlorine is present in the haustorium at similar levels as in the host tissue but is decreased in the stem of the parasite. Thus, our observations indicate a restricted uptake of calcium, strontium, manganese and chlorine by the parasite. Xylem-mobile dyes, which can probe for xylem connectivity between host and parasite, provided evidence for an interspecies xylem flow, which in theory would be expected to carry all of the elements indiscriminately. We thus conclude that inorganic nutrient uptake by the parasite Cuscuta is regulated by specific selective barriers whose existence has evaded detection until now.