Interview with Dr. Liz Rylott on Bioremediation

Interview with Dr. Liz Rylott – Centre for Novel Agricultural Products (CNAP) – Department of Biology – University of York

Liz is a world leader in the development of GM and synthetic biology technologies for the phytoremediation of organic pollutants, especially explosives, such as TNT and RDX, and inorganic contaminants.

When did you first develop an interest in plant sciences?

“I have always been interested in plants, from before school. I grew up in a farming community so was exposed to plants from a young age. 

“At school you were encouraged to continue in whichever area you were good at, and I excelled in biology, and plant biology particularly, but didn’t know where that would end up.  Growing up in Lincolnshire, careers advice from school was to go into horticulture, growing vegetables or flowers in a glasshouse. That was the limit of their ambition.

“I went to the University of Manchester and took a degree in applied plant science. It was such a specific course, I couldn’t believe that I could find something that sounded so exactly like what I wanted to do. The course had a year in industry, and I spent this time at AstraZeneca, working on some of the first GM field trials.”

What is phytoremediation? What type of pollutants can be targeted?

“Phytoremediation is the use of plants to remediate environmental pollutants. This doesn’t necessarily mean that you are using the plants to take up the contaminants. It might be viaphytostabilization to remove or reduce exposure, purely through planting. It is seen as a relatively low cost, aesthetically pleasing alternative, and helps to restore the natural ecology in these areas. This can often lead to a natural attenuation process [the reduction in toxicity or mass of a contaminant without human intervention].”

“Both organic and inorganic pollutants can be targeted. Organic pollutants would include PCPs and dioxins. We work specifically on explosive compounds, such as 2, 4, 6 – trinitrotoluene [TNT] and RDX. These have been exemplar chemicals in the phytoremediation of organic pollutants. We have been fortunate in finding bacterial enzymes that are able to completely mineralise RDX, which is the holy grail of an organic phytoremediation project. But PCPs, for example, are more challenging, as they comprise a group of halogenated compounds, that bind tightly to the soil, requiring a number of different enzymes to break them down.”

“Inorganic pollutants include cadmium, arsenic, mercury, some of which are extremely toxic. These obviously cannot be mineralised; otherwise I would be very rich if we could! 

“You have two options, if you take the material up into the plant, you now have a plant that is full of cadmium; the price of cadmium is relatively low and it is toxic. You would have now made environmental waste that would have to be dealt with in landfill, which we try to avoid. So with inorganics, we often utilise phytostabilization to try and lock the pollutants down, or you can develop plants that will compartmentalise the metals away from the parts that people are going to eat.

“There are a lot of people in Asia, for example, who are eating rice contaminated with arsenic, and if you can reduce the amount of arsenic that is going into the grain, it’s not perfect, but it is something.”

Is there a way to recycle these heavy metals or rare metals?

“People have used chelating agents, such as EDTA, to loosen metals from the medium of the soil, in the hope that the plants will take them up. But in practice this can be environmentally extremely damaging, and is now looked upon as an incorrect approach. Plants have a very discrete area around their root that they will take metals up from. However, [with the use of chelating agents] these metals are indiscriminately released, so the plants do not reach a lot of them. The metals are then washed downstream, and could kill all of the fish, so this approach is no longer used. 

“A more sophisticated approach is to engineer the plants or microbes in areas around the roots, to exude compounds that would release the metals [from the soil] for uptake.

“With respect to nickel and rare metals such as, palladium, platinum or gold, we have been doing quite a lot of work in that area. We know that plants will take up some of the platinum group metals and gold, and they deposit those as discrete nanoparticles, with some catalytic activity. This can make [phytoremediation] more financially viable, as the material can be used in planta, rather than smelting the dried plant biomass  into an ingot, as this is too energy intensive to yield a profit.”

How easy is it to move from the model Arabidopsis thalianasystem to a more applicable/ real world system?

Arabidopsis thaliana is the model plant system for molecular biology studies. We can look at the genes very readily, it is a very small plant, we can grow it very quickly in large numbers and we know the full [genetic] sequence. We can also get mutants in the genes very readily too, and introduce genes from anywhere else. It is perfect for proof-of-concept, but absolutely imperfect for actually mining. It has poor tolerance for the environments that you would be growing these plants in: the heavy metals themselves, additional contamination, and a hostile place with poor soil.”

“Willow, miscanthus and poplar are good candidates. These are all used as biomass crops already, so we have the existing agricultural infrastructure in place.  For example, there have been tractors designed to harvest coppiced willow into bundles, straight to the power station. This makes using these species a more viable option.”

Do you have a recent example where bioremediation was applied successfully?

“There is a fern, Pteris vittat that hyperaccumulates arsenic, discovered in China. Hyperaccumulator plants are plant species that accumulate very high concentrations of one or more heavy metals from the soil.  They have evolved this ability to flourish in environmental niches where there are naturally occurring very high levels of cadmium, zinc or arsenic, for example. People have used this to clean up localised areas of metal contamination. China is investing massively into phytoremediation. Partly because, historically, they have produced quite a lot of contamination, and now they have realised they need the soil back to grow crops for people. The development and use of this Pteris fern is one of those technologies.” 

“The technology that we focus our research on is using GM, for organic explosives. We have taken bacteria that have evolved the ability to detoxify or degrade these compounds, and we have found the genes that encode for the enzymes that are responsible, and transform these into the plants, which has worked fantastically. The plants themselves haven’t had time to evolve any mechanisms of their own to do this. We have carried out field trials in the States [USA], last summer, and these provided very promising results. We can take these organic pollutants, remove them from the environment, mineralise [the organic pollutant] completely, and the plants even grow better because of the nitrogen from the explosives. There is no other viable alternative to cleaning up such large areas of the U.S.”

How do you deliver these phytoremediation plants to war torn countries, with high levels of explosive contaminated soil?

“Our technology has been designed for active training ranges where you need to contain the explosives to stop them leaching out, and remediate the site. They can keep [the site] active, and continue to contaminate it, but contain it. When you are looking at areas where there has been contamination from conflict, it may take a long time before it becomes a safe place to enter, and by then historical records may have been lost, and contamination isn’t very widely documented. It isn’t often the first thing on peoples list for recovering their country, with their economy, healthcare and infrastructure, taking priority, but one day, we hope to have the technologies ready for this.”

Could you give an outlook for the future of bioremediation?

“Hopefully we are still here on this planet for thousands more years, and when we think how far we have come with GM technology since the 90’s, so only 30 years further on now, there is a massive amount that we can achieve. I am very excited to wonder where we will be even in 50 years time. We can engineer transporters. These are the proteins on the cell membranes that can take up metals. At the moment there are hundreds of them encoded in plants, and quite a lot of them are fairly promiscuous, and will take up a range of different metals. Understanding the chemistry of how they bind the different metals, can we work out specificity there, so that we can take up a particular metal and partition that away somewhere so that it is not interfering with the plants own metabolism, because these metals can be quite toxic to the plant. So you cannot get the plant to take up cadmium because it dies, but we could make up a specific transporter that would take up the cadmium, then partition it to a particular compartment in the plant…

“Maybe we could even have artificial organelles that become depots for the metal that you want to take up. This is moving along into the future, but why not? One day there may well be someone working on that. There is massive promise in this area.”