The very act of tolerating some forms of soil pollution may give trees an advantage in the natural world, says University of Montreal plant biologists. Their findings were published this week in BMC Plant Biology. High chemical tolerant plants can be used to rehabilitate¹ land contaminated with heavy metals or petroleum² by-products - some 30,000 such sites exist in Canada and 342,000 sites in Europe - through a process termed phytoremediation. The research team compared the molecular³ response of willow⁴ trees growing in contaminated or non-contaminated soil and found that several plant genes⁵ were expressed differently between both treatments. "The most fascinating result, however, comes from the fact that genetic⁶ information (RNA) from other organisms, such as fungi⁷, bacteria and insects were also found to be expressed differentially in plant tissues. Notably⁸, 99% of RNA from spider mites⁹, a common plant pest, was in higher abundance in trees growing without contamination," explained Nicolas Brereton, co-first author of the study. "This suggests that trees growing in contaminated soils might have reacted in a way that makes them less prone¹⁰ to herbivore attacks by priming their defense¹¹ machinery¹²."

 

Decontamination of polluted sites, often many hectares in scale, is costly¹³ and in itself can have a high environmental impact. "Phytoremediation plants must have a very high tolerance¹⁴ to pollution as well as high biomass yields. This second trait brings an additional value stream to the process of phytoremediation, outside of the direct benefit of rejuvenating¹⁵ land" Brereton said. Short rotation¹⁶ coppice willow are some of the highest yielding trees, having the ability to produce very large amounts of wood in temperate¹⁷ regions in a very short time and requiring low nitrogen fertilization. "By producing high yields we can use the produced biomass, for example wood, for processes such as lignocellulosic bioenergy production. We term the integration¹⁸ of these two complementary benefits added-value cultivation¹⁹." 

 

The genetic information exchange the researchers identified is in step with a new field in biology which has rapidly expanded since the advent²⁰ of modern next-generation genetic sequencing technology: the systems biology approach relating to the "metaorganism." The researchers, directed by Michel Labrecque, Frederic Pitre and Simon Joly, look at all the interacting organisms as a single, dynamic biological entity²¹ in order to understand natural complexity²². "One of the major discoveries we've been exploring is that when you extract genetic information from any plant tissue, such as RNA, you always also find genetic information from fungi, bacteria and even animals, such as insects and arachnids. In this case, the tree's defense against contamination, which is an abiotic stress, improves resistance to spider mites, a biotic stress," said Emmanuel Gonzalez, co-first author of the study. "The important point here is that genes have been switched on across multiple interacting organisms. This is what we call meta-transcriptomics, meta referring to metaorganism and transcriptome to the activation²³ of genes. The ability to get such a comprehensive snapshot of genetics is very new. While cross-tolerance is known to occur in trees, it has yet to be documented in a phytoremediation context and certainly not using this cutting-edge next-generation sequencing technology."

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