Functional genomics to discover genes responsible for root architecture and heavy metal tolerance in Populus

Abstract

Anthropogenic activities have led to widespread of heavy metal contaminants such cadmium and arsenic. When left untreated, they pose risk to both human and ecosystem health as well as further reduce arable lands. Phytoremediation, which is the use of plants and their associated microorganisms to clean up such contaminants, is environmentally friendly and cost effective. Fast-growing trees such as Populus sp. are good candidates for phytoremediation because of their tolerance to heavy metals, high biomass and their distribution across much of the northern hemisphere. However, the molecular mechanisms underlying poplar’s phytoremediation are poorly understood. Although Populus is a model tree species, with a sequenced genome and many genetic and genomic resources, the identification of genes for important tree traits is still slower than in other model plants such as Arabidopsis. This study uses a functional genomics approach to identify genes related to phytoremediation by taking advantage of a large collection of activation tagged poplars (Populus tremula x P. alba hybrid 717-1B4) created by Dr. Sharon Regan’s Lab. After screening over 1700 independent transgenic lines for characteristics that could affect the phytoremediation, seven mutants had altered root biomass whereas 15 mutants had altered response to heavy metals. Of the seven root phenotypes identified, two previously studied mutants, called rippled leaf and adventitious root were further investigated. RT-qPCR analysis showed an up-regulation of CYCLIND1;2 and E3 ubiquitin-protein ligase XBAT32/33 in the roots of rippled leaf and adventitious root mutants respectively. The upregulation of CYCLIND1;2 is suspected to increase the root biomass through accelerated cell cycle division. XBAT32/33 on the other hand is suspected to promote the production of lateral roots through the regulation of ethylene biosynthesis. Altogether, this study provides a starting point in the quest to discover key genes responsible for phytoremediation and could lead ultimately to the development of biomarkers for selection of superior trees from natural population for clean-up purposes.