Nuclei of the plant cells are the first loading site for arsenic
By: Seema Mishra*
Arsenic initially accumulates in the nuclei of plant cells. This has been revealed by an x-ray examination of the aquatic plant rigid hornwort (Ceratophyllum demersum). The distribution study of the toxic metalloid in plant leaves by Dr. Seema Mishra at the Deutsches Elektronen-synchrotron hamburg using the x-ray source PETRA III demonstrated that at low cellular concentration arsenic predominantly occupies the nucleus of the leaf cells. Even at comparatively low concentrations, arsenic floods the vacuole, which takes up most of the cell. Dr. Seema Mishra (now working at National Botanical Research Institute in Lucknow) made this discovery in the course of a project which she initiated during her Humboldt post-doctoral research on speciation and distribution of arsenic in plants. The experiment was set up in the lab of Prof. Kuepper in University of Konstanz, Germany. The findings have been reported in the “Journal of Experimental Botany”. The results could help researchers breed plants that absorb less arsenic.
Arsenic is highly toxic and poses a growing environmental and health problem all over the world. The concentration of arsenic in the soil is increasing as a result of human activities, and in many countries – especially on the Indian subcontinent – the concentration of arsenic in the groundwater has become a problem. Although arsenic is a naturally present in the environment, due to human intervention for example drilling of wells it mobilised gets from the underground rock, contaminating the drinking and irrigation water. In human beings, arsenic can cause cancer, necrosis, or acute renal and circulatory failure. The metalloid is also toxic to plants. It is taken up by the same transport mechanism as phosphorus, an element that is essential to plants, and even at levels far below the lethal concentration, it impairs plant growth and therefore reduces the yield of crops.
On top of this, human beings consume the plant produce and feed them to their livestock, in whose bodies arsenic accumulates and eventually ends up in human beings, through food chain. The project was aimed to find out exactly how arsenic poisoning occurs in plants at ecologically and physiologically relevant concentrations. The results showed that concentrations of just one micromole per litre are already relevant in terms of plant physiology while in arsenic contaminated areas the concentration over 30µM is not uncommon in irrigation water and soil solution. The initial findings were published in ‘Plant physiology in 2013’.
In this experiment the plants exposed to arsenic concentrations between one and five micromoles per litre were subjected to narrowly focused x-ray beam from PETRA III through leaves. PETRA III made it possible to look inside the individual cells of the plant for the first time and allowed to localise the arsenic more precisely within the cell. The plant first deposits the toxin in its outer layer, the epidermis. It was surprising to see that arsenic initially accumulates in the cell nuclei. Only when the concentration rises to five micromoles per litre, a level that the plants are unable to withstand for prolonged periods, arsenic floods the vacuole and thus ` more or less the entire cell. “This means that the capacity of the epidermis is exhausted and the plant can no longer get rid of the toxin, and that’s when it spreads to the mesophyll, where photosynthesis takes place.
The second part of the current study shows that arsenic damages the enzymes that are responsible for producing chlorophyll. Arsenic first inhibits photosynthesis, which is not due to the removal of chlorophyll but rather to the reduced production of the pigment.
At low concentrations (1µM), arsenic accumulates predominantly in the nuclei of epidermal cells while at high concentrations (5µM), it floods the whole cell (Mishra et al., Journal of Experimental Botany, doi:10.1093/jxb/erw238).
*DST SERB Scientist, CSIR-National Botanical Research Institute, Lucknow,
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