Zinc (Zn) is an essential micronutrient but in excess is highly toxic to plants. Plants regulate Zn homeostasis and withstand excess Zn through various pathways; these pathways are generally tightly regulated by a specific set of genes. However, the transcription factors involved in excess Zn tolerance have yet to be identified. Here, we characterized a Populus ussuriensis heat shock transcription factor A4a (PuHSFA4a) that acts as a positive regulator of excess Zn tolerance in P. ussuriensis. We used overexpression (PuHSFA4a-OE) and chimeric dominant repressor (PuHSFA4a-SRDX) lines to identify the targets of PuHSFA4a. PuHSFA4a transcription is specifically induced in roots by high Zn. Overexpression of PuHSFA4a conferred excess Zn tolerance and a dominant repressor version of PuHSFA4a increased excess Zn sensitivity in P. ussuriensis by regulating the antioxidant system in roots. PuHSFA4a coordinately activates genes related to abiotic stress responses and root development and directly binds to the promoter regions of glutathione-s-transferase U17 (PuGSTU17) and phospholipase A₂ (PuPLA₂). PuGSTU17 overexpression significantly increased GST activity and reduced reactive oxygen species levels in roots while PuGSTU17-RNA interference lines exhibited the opposite phenotype. Furthermore, PuPLA₂ overexpression promoted root growth under high Zn stress. Taken together, we provide evidence that PuHSFA4a coordinately activates the antioxidant system and root development-related genes and directly targets PuGSTU17 and PuPLA, thereby promoting excess Zn tolerance in P. ussuriensis roots.

The DNA-binding One Zinc Finger (Dof) genes are ubiquitous in many plant species and are especial transcription regulators that participate in plant growth, development and various procedures, including biotic and abiotic stress reactions. In this study, we identified 41 PtrDof members from Populus trichocarpa genomes and classified them into four groups. The conserved motifs and gene structures of some PtrDof genes belonging to the same subgroup were almost the same. The 41 PtrDof genes were dispersed on 18 of the 19 Populus chromosomes. Many key stress- or phytohormone-related cis-elements were discovered in the PtrDof gene promoter regions. Consequently, we undertook expression profiling of the PtrDof genes in leaves and roots in response to osmotic stress and abscisic acid. A total of seven genes (PtrDof14, 16, 25, 27, 28, 37 and 39) in the Populus Dof gene family were consistently upregulated at point in all time in the leaves and roots under osmotic and abscisic acid (ABA) stress. We observed that 12 PtrDof genes could be targeted by 15 miRNAs. Moreover, we mapped the cleavage site in PtrDof30 using the 5’RLM-RACE. The results showed that PtrDofs may have a role in resistance to abiotic stress in Populus trichocarpa.

Salix matsudana Koidz. is a fast growing tree species. It has a high cadmium (Cd) tolerance capacity, making it potentially suitable for phytoremediation. Presently, transcriptomic and physiological Cd response mechanisms are poorly understood. Transcriptomic analysis in early response to high (50 μM) Cd levels was investigated in leaf and root of Cd-resistant S. matsudana Koidz.. Analysis of the response profiles demonstrate the existence of a complex transcriptional network in the root and leaf when exposed to Cd. The main response in the root involved up-regulation of genes associated with defence response via callose deposition in the cell wall and cell wall thickening. In the leaf, transcripts related to biotic stress signalling and secondary metabolism were activated. Additionally, many lignin and brassinosteroids synthesis pathway genes were induced mainly in the leaf, indicating that gene response to Cd was tissue-specific. The Cd transcriptome results were consistent with observed physiological changes. The sub-localization, transcriptional network, and physiological regulation demonstrate the tissue-specific manner of Cd response, and provide a novel insight into in early response of tree species to Cd exposure.