The parasitic plant Cuscuta sets up barriers to some minerals when infecting a host

If the parasitic plant dodder taps its host with a straw, then it's a specialised straw than can filter out some nutrients that the plant isn't using.

Rootless parasitic plants depend on their hosts for all their nutrients, but how do they collect them? Frank Förste and colleagues taking a close look at the connection between Cuscuta reflexa (Giant dodder) and Pelargonium. Their work, published in Physiologia Plantarum, shows not all inorganic nutrients are taken equally from the host plant.

Dodder. Image: Canva.

The team used X-ray fluorescence spectroscopy to look closely at the interface between Cuscuta and Pelargonium. Combining this with confocal microscopy showed that macronutrients, things like nitrogen, potassium or phosphorus, were present at similar concentrations to the host plant. The results were different for some micronutrients, in particular, calcium, manganese and chlorine. “This suggested that selective exclusion mechanisms of minerals might exist at the host-parasite border,” say the authors in their paper, “calling for a spatially resolved analysis of the host-parasite interfaces at the infection sites themselves.”

One reason for the drop in some nutrients might be because Cuscuta reflexa is leafless. Chlorine, the authors note, is found in the endophytic tissues of Cuscuta, the bits of the parasite living inside the host plant, but not the parts outside. Often chlorine is used to balance potassium and calcium in regulating cell turgor. Changing the water content of a cell makes it more or less flexible, which could open or close stomata to aid water flow through the plant. Having few stomata means Cuscuta has relatively little use for chlorine, except in the haustoria tapping the host plant.

The limited photosynthesis might also explain the low levels of manganese, say the authors. Manganese is used to split water in photosynthesis, so Cuscuta does not need a lot. Having an excess of manganese is a problem for a plant, say the authors. “Whether Cuscuta restricts the uptake of Mn as part of a control mechanism to avoid Mn phytotoxicity symptoms that are known from crops (Fernando and Lynch 2015), is currently unknown.”

Calcium is more of a puzzle. It is necessary for cell walls, yet it appears to be lacking in Cuscuta, compared to the host.

If Cuscuta only tapped into the host’s phloem, that could explain why manganese and calcium were depleted. They travel only in the xylem. But Förste and colleagues note there is a xylem connection too. “Xylem-mobile dyes, probing 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.”

It would appear that while Cuscuta dependent on its host for its nutrients, it’s not indiscriminate about what it takes.