Transcriptome Profiling of Black Poplar under Boron Toxicity Revealed Genetic Regulation of Boron Uptake, Transport and Detoxification

Poplars are known to be highly tolerant species to boron toxicity and accumulation. However, genes and molecular networks responsible in boron toxicity tolerance have not been investigated yet. Therefore, we performed a pot experiment with 20 black poplar clones collected from the vicinity of boron mines and polluted areas to investigate its potential role in phytoremediation and to select the most boron toxicity tolerant genotype. Trees were treated with irrigation water containing seven elevated boron concentrations from 0 to 160 ppm. Then a microarray based comparative transcriptome profiling was conducted to identify boron toxicity regulated genes responsible in defence responses of black poplar. The results of the study indicated that black poplar is quite suitable for phytoremediation of boron pollution. It could resist 15 ppm soil B content and < 1600 mg/kg boron accumulation in leaves which are highly toxic concentrations for almost all agricultural plants. Transcriptomics results of study revealed totally 1625 and 1419 altered probe sets under boron toxicity in leaf and root tissues, respectively. The highest induction were recorded for the probes sets annotated to tyrosine aminotransferase, ATP binding cassette transporters, glutathione S transferases and metallochaperone proteins. Strong up regulation of these genes attributed to internal excretion of boron into the cell vacuole and existence of detoxification processes in black poplar. Many candidate genes functional in signalling, gene regulation, antioxidation, boron uptake, transport and detoxification processes were also identified in the current study. This is the first transcriptomic study identifying boron toxicity regulated poplar genes and their potential role in boron toxicity tolerance. Total RNA used in microarray experiment was isolated from the leaves and roots of black poplar clone; N.92.237 which accumulated the highest amount of boron its tissues. Total RNA used in the microarray experiment was isolated from leaves and roots of three black poplar saplings grown in ~ 2 ppm (control) and ~ 15 ppm (toxic) soil B contents. RNA isolation was made according to Lithium chloride precipitation method described in Chang et al. (1993). These three isolated RNAs (biological replicates) for each tissue loaded onto three Affymetrix poplar Gene Chips (technical replicates). Totally, 12 GeneChips (2 tissues × 2 different B treatment × 3 biological replicates) were used for transcriptional analysis.