Mining for the metals inside plants

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  • Published: Jun 1, 2016
  • Author: Ryan De Vooght-Johnson
  • Channels: HPLC
thumbnail image: Mining for the metals inside plants

Metals in moderation

Among the essential list of plant nutrients, which includes water, sunlight and oxygen, you may be surprised to find metals. Metals such as copper, iron, zinc and nickel are all essential for plant survival. They regulate photosynthesis, through which plants make their food, and are essential for maintaining enzyme activity inside cells.

Among the essential list of plant nutrients, which includes water, sunlight and oxygen, you may be surprised to find metals. Metals such as copper, iron, zinc and nickel are all essential for plant survival. They regulate photosynthesis, through which plants make their food, and are essential for maintaining enzyme activity inside cells.

Although an excess of certain metals can lead to severe toxicity, some so-called ‘hyperaccumulator’ plants can absorb huge amounts of metals without suffering toxic effects. In fact, over 400 different plant species are known to accumulate large amounts of metals in their shoots, which is thought to protect them from predators and pathogens.

Knowing the different metals circulating around plants is an important aspect of plant science – but also a challenging one. This is because metals are generally associated with organic molecules, present in low concentrations, and occur in a wide range of (often unstable) chemical forms.

Previously proposed techniques include liquid chromatography-mass spectrometry (LC-MS), X-ray analysis and computational prediction, but none has been able to surmount all these challenges to obtain a complete and accurate picture of the metal complexes inside a plant.

Recognising the lack of an analytical method for the large-scale detection, identification and determination of metal complexes in plants, a team of researchers from two French institutes came together to develop one.

Revisiting an old technique

In a paper published in New Phytologist, they present the method. The procedure is based on direct injection of samples using hydrophilic interaction LC (HILIC)—a type of LC that uses soluble stationary phases with reversed-phase type eluents—coupled with MS. Not the first time this combination has been applied, the technique was used in 2006 to identify metal complexes in Alpine Penny-cress, a plant found across Scandinavia.

Ten years on, the researchers revisited the technique, focusing on improving the separation of different metal species. They achieved this by using two methods of detection: inductively coupled plasma (ICP) MS and high-resolution electrospray ionisation (ESI) MS. “Our methodology is based on the parallel coupling of the HILIC column with elemental (ICP) and molecular (ESI) MS, which we developed to produce a large inventory of various types of metal complexes in plant fluids,” summarises Professor Laurent Ouerdane of the University of Pau and Pays de l'Adour.

After validating their method, they tested it on the common pea plant. They focused on the xylem—which moves water from the roots to the shoots and leaves of the plant—and the embryo sac liquid—which surrounds the egg cell. Both are important for the movement of metals.

Testing: easy peasy

They identified over 10 metals essential for plant growth, including iron, copper, zinc, manganese, cobalt and magnesium. The metals were in complex with various ligands, including nicotianamine (pervasive in higher plants), citrate and malate (key metabolic intermediates), and various amino acids (the building blocks of proteins—including histidine, glutamine, aspartic acid, asparagine and phenylalanine). Overall, they identified 50 different metal complexes, several of which were identified for the first time.

As well as shedding new light on the metal complexes found in the pea plant, this study advances understanding in the plant sciences more generally. For example, it confirmed that nicotianamine is important for metal homeostasis and possibly also transport.

Beyond the plant world, the approach could be used to detect metal complexes in other biological samples. “It could be a significant asset for future biochemical and genetic studies into metal transport/homeostasis in living organisms,” adds Professor Ouerdane. Indeed, a plethora of reactions in biology involve metal ions—almost half of all enzymes are thought to require metals.

Related Links

New Phytol., 2016. Flis et al., Inventory of metal complexes circulating in plant fluids: a reliable method based on HPLC coupled with dual elemental and high-resolution molecular mass spectrometric detection.

Heavy metals toxicity in plants

Which metals are essential for plant health?

Nature Education Knowledge 2010, 3(10):57. Boyd, R. Elemental Defenses of Plants by Metals.

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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