Abstract
Populus alba ‘Villafranca’ clone is well-known for its tolerance to cadmium (Cd). To determine the mechanisms of Cd tolerance of this species, wild-type (wt) plants were compared with transgenic plants over-expressing an aquaporin (aqua1, GenBank GQ918138). Plants were maintained in hydroponic conditions with Hoagland’s solution and treated with 10 µM of Cd, renewed every 5 d. The transcription levels of heavy metal transporter genes (PaHMA2, PaNRAMP1.3, PaNRAMP2, PaNRAMP3.1, PaNRAMP3.2, PaABCC9, and PaABCC13) were analyzed at 1, 7, and 60 d of treatment. Cd application did not induce visible toxicity symptoms in wt and aqua1 plants even after 2 months of treatment confirming the high tolerance of this poplar species to Cd. Most of the analyzed genes showed in wt plants a quick response in transcription at 1 d of treatment and an adaptation at 60 d. On the contrary, a lower transcriptional response was observed in aqua1 plants in concomitance with a higher Cd concentration in medial leaves. Moreover, PaHMA2 showed at 1 d an opposite trend within organs since it was up-regulated in root and stem of wt plants and in leaves of aqua1 plants. In summary, aqua1 overexpression in poplar improved Cd translocation suggesting a lower Cd sensitivity of aqua1 plants. This different response might be due to a different transcription of PaNRAMP3 genes that were more transcribed in wt line because of the importance of this gene in Cd compartmentalization.
Highlights
Received: 1 October 2020 Accepted: 25 December 2020 Published: 29 December 2020Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.Cadmium (Cd) is a toxic element that can affect mineral homeostasis, photosynthesis, water balance, and nutrient uptake in plants [1,2,3]
The physiological modifications and the dynamics of Cd accumulation and distribution within plant tissues as well as the function of the main heavy metal transporters involved in Cd uptake, translocation, and detoxification are well-known in poplar plants [4,5,6,7,8,9]
Several putative metal transporter genes are involved in these processes, such as transporters belonging to: (i) Zinc/Ironregulated transporter-like Proteins (ZIPs), (ii) Natural Resistance and Macrophage Proteins (NRAMPs), and (iii) plasma membrane Heavy Metal proton-ATPases (HMAs) [10,11]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. After the mobilization from soil, Cd uptake within plant tissues is determined by plasma membrane transporters located in root cells. Members of NRAMP family are considered general metal ion transporters due to their ability to transport several ions, including Cd [12,13] These transporters play a crucial role in the response, translocation, and accumulation of Cd [14,15,16]. A study found that overexpression of the ABCG36 gene of Populus tomentosa is effective in enhancing Cd tolerance in Arabidopsis plants, indicating the function of this transporter as a heavy metal extrusion pump [29]. Ariani et al [42] tested the heavy metal tolerant species P. alba ‘Villafranca’ clone overexpressing the aqua gene under Zn excess. The aim was to understand if overexpression of the aqua gene in poplar: (i) enhanced short- and long-term Cd transport and accumulation; (ii) interfered with the gene transcription of metal transporters related to Cd uptake, translocation, and accumulation
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