Blooming in Amherst: heavy metal relatives
I picked some Thlaspi arvense on the way in this morning. Shown here is one of T. arvense's distinctive fruits, which make it easy to identify. It's a weed, originally from Eurasia, but now all over the US. It's in the same family (Brassicaceae) as the model plant Arabidopsis thaliana, which in turn is in the same family as cabbage, broccoli, kale, and Brussels sprouts. The Brassicaceae family is very large (~3,700 species), and there are many weeks' worth of interesting stories to tell about them, but today I will stick to one: heavy metal hyperaccumulation.
Some plants can take up extremely high levels of heavy metals that would ordinarily be toxic to other plants. This trait, called heavy metal hyperaccumulation, has evolved multiple times in the Brassicaceae. Heavy metal hyperaccumulation might be useful to us, because we might be able to use plants that accumulate heavy metals to clean up contaminated areas (a strategy termed phytoremediation). T. arvense is not a hyperaccumulator, but one of its close-ish relatives (a second cousin?) is a zinc and cadmium hyperaccumulator: Noccaea caeruleans (formerl Thlaspi caeruleans).
We understand Zn/Cd hyperaccumulation best in Arabidopsis halleri. Scientists have identified a molecular heavy metal pump (HMA4) that is really important for heavy metal tolerance. This molecular pump sits in the plant cell membrane, and pumps metal into the plant cells, where it is sequestered from the soil and detoxified. Arabidopsis thaliana also has this pump, but the version found in A. halleri's is expressed at very high levels in plant cells. This high expression level is partly due to changes in the regulatory regions of the gene endocing the pump, and partly due to multiple copies of the gene in the A. halleri genome. Similar duplications of the pump gene were found in Noccaea caeruleans, implying a similar mechanism.