Plants: nature’s sponge?

We rightly celebrate the tremendous range of compounds that plants make by dint of their own biosynthetic abilities, as exemplified by the great variety of so-called secondary compounds they contain. However, to that innate capability must also be added a largely unappreciated capacity of plants to absorb ready-made organic compounds from the environment. That has been dramatically demonstrated by Dirk Selmar et al. using Mentha × piperita (peppermint) and nicotine (“a potent alkaloid found in the Solanaceae and a stimulant drug … It constitutes approximately 0.6–3.0% of the dry weight of tobacco”).

Although the peppermint plants contained some nicotine naturally, they also exhibited the ability to absorb it, both from nicotine-contaminated soil and from cigarette smoke added to the atmosphere surrounding the plant. Inside the plant it is likely that the nicotine is moved within the transpiration stream. However, the study also indicated that levels of the exogenously derived nicotine decreased in the plant with time, indicating that it was being metabolised by the plant*. Thus, peppermint not only has the capacity to remove nicotine from terrestrial and aerial (and aquatic …?) environments, it can also break it down (hopefully to less-harmful compounds). Which must be something akin to the holy grail of phytoremediation, wherein plants are used to clean up the environment by absorbing and accumulating within their tissues harmful chemicals, but usually without metabolising them.

Whether peppermint – or other plant species – could also extract the remainder of the cocktail of carcinogens, etc. present in cigarette smoke and thereby ‘purify’ the air and reduce the chances of non-smoking humans breathing in such stuff (passive smoking) would be an interesting and suitable subject for further tests.

However, as if the ability of plants to sequester ‘natural’ chemicals from the environment wasn’t unusual enough, Yeonjong Koo et al. show that arabidopsis has the capacity to take up nanoparticles from the soil. Nanoparticles (‘particles between 1 and 100 nanometers [sic.] in size’) are big business nowadays and exploited in the emerging discipline of nanotechnology, the ‘application of extremely small things’(!) that is used across all the other science fields, such as chemistry, biology, physics, materials science and engineering. As a new field of human endeavour – and one that is also human-created – there are understandable concerns about how safe this technology is and whether it may have health or environmental implications.

Study of the effects of nanoparticles on living systems is therefore needed, and hence the study by the Rice University (Houston, Texas, USA) team on quantum dots (QDs) – nanocrystals of semiconductor materials that are small enough to exhibit quantum mechanical properties. Using a range of QDs they demonstrated that these particles could not only be taken up by arabidopsis roots and leaf petioles from hydroponic growing medium, but they could also be transferred to Trichoplusia ni (strictly speaking the caterpillar – larval stage – of the cabbage looper moth) that fed on the plant material.

Although the degree to which the QDs were stable and accumulated in the plant varied with the QD coating used, this does demonstrate that QD transfer from environment to plant to herbivore can occur. Should we be concerned? Well, considering that the QDs used were cadmium-containing entities, and given that cadmium is a rather unpleasant heavy metal, I think the answer is: yes. However, the full implications of this work probably remain to be … err … quantified.

* Whether such metabolisation of exogenously sourced organic carbon sources could also have nutritional benefit for the plants is an intriguing notion, which threatens their true autotrophic nature; maybe plants are opportunistic heterotrophs? Discuss …