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    • br Introduction Cadmium is a major component of

    2023-01-04


    Introduction Cadmium is a major component of heavy metal pollution. Phytoremediation is considered to be the more effective method for recovering contaminated soil [[1], [2], [3], [4]] via cadmium absorption, transport, and accumulation. Several cadmium-associated proteins have previously been well characterized in plants, including phytochelatins (PCS) [5,6], zinc-regulated transporter(ZRT)/iron-regulated transporter (IRT1)related proteins (ZIPs) [7], natural resistance associated macrophage proteins (NRAMPs) [8], yellow stripe-like (YSL) proteins [9], cation exchanger proteins(CAX) [10], cation dexamethasone acetate facilitator transporters (CDFs) [11], and heavy metal ATPases (HMAs) [12,13]. HMA, a P1B-type of ATPase protein found in plant species, has been widely studied because of its well-known role in root-to-shoot cadmium translocation and efflux into vacuoles [[14], [15]]. Several HMA genes have been identified and functionally analyzed such as CrHMA1–3 in Chlamydomonas reinhardtii L. [16], OsHMA1–9 in Oryza sativa L. [14], HvHMA1–10 in Hordeivulgaris [17], AtHMA1–8 in Arabidopsis thaliana L. [18], AhHMA3–4 in Arabidopsis haller L. [[19], [20]], and TcHMA4 in Thlaspi caerulescens L. [21]. Populus is a model species of woody plants that has been recognized as having great potential for phytoremediation, especially for cadmium and zinc, because of its high biomass, rapid growth, wide planting, and strong adaptability. The cadmium content of poplar roots can reach 9962mg/kg (in 50μM cadmium for 3w) [22], while514.08mg/kg (in 100μM cadmium for 4weeks) in the leaves [23]. Unfortunately, little is known about the movement and storage of cadmium in poplar. The purpose of this study was, therefore, to identify an HMA gene from Populus and evaluate its role in cadmium transport and its potential in phytoremediation via gene transformation of tobacco plants. This would help us understand the mechanisms underlying phytoremediation of cadmium-contaminated soil in woody plants, and would provide a reliable gene resource for the phytoremediation-related molecular breeding program.
    Materials and methods
    Results
    Discussion HMA, a P1B-type member of the ATPase family in plants, has been widely studied because of its important role in phytoremediation [34,35]. Based on the sequence variation and the ability to bind to different kinds of heavy metal ions, HMA genes were majorly divided into two categories of Cu2+/Ag2+ (called Cu2+-ATPase) and Zn2+/Co2+/Cd2+/Pb2+ (called Zn2+-ATPase), respectively [[36], [37]]. Sequence alignment here clearly showed PtoHMA5 asa typical Cu2+-ATPase. ButPtoHMA5 expression could be induced by Cd2+, especially in stem tissues. PtoHMA5could also advance Cd2+ transport from roots to leaves and increase the transfer coefficient by 16.01%–43.25%. Obviously, PtoHMA5 had the same function as other Zn2+- ATPase members, even though it belongs to the Cu2+-ATPase group. In fact, some studies had previously confirmed the extensive roles of HMA proteins. For example, OsHMA9, a Cu2+-ATPase, could be induced by various heavy metal ions ofCu2+, Zn2+, and Cd2+ [17].AtHMA1, a Zn2+-ATPase, was also recognized as being associated with dynamic equilibration of Cu2+ in plants. The Cu2+ content of an AtHMA1 mutant was reduced by 10% when compared with the WT control [38]. Besides, each HMA protein might work on various heavy metal ions, likely with differing efficiency. Right here, over-expression of PtoHMA5 in tobacco plants could enhance Cd2+ transport by 33.51% on average, while the cadmium content in leaves increased by 25.04%. Similarly, a previous report showed that OsHMA2 mutant decreased the cadmium content by 20–30% in leaves when compared with WT plants in treatment of 50 um Cd2+ [14], but the cadmium content in the stems of this mutant decreased by 60%–80% when grown in 5μg/L CdCl2 [39].Also, overexpression of AtHMA3 increased cadmium accumulation by approximately 2–3-fold when compared with WT plants. Therefore, identification of HMA orthologs in different species (especially cadmium hyperaccumulators) and evaluation of their function, would be necessary for phytoremediation breeding.