Enhancing tonoplast Cd/H antiport activity increases Cd, Zn, and Mn tolerance, and impacts root/shoot Cd partitioning in Nicotiana tabacum L.

Abstract

Sequestration mechanisms that prevent high concentrations of free metal ions from persisting in metabolically active compartments of cells are thought to be central in tolerance of plants to high levels of divalent cation metals. Expression of AtCAX2 or AtCAX4, which encode divalent cation/proton antiporters, in Nicotiana tabacum cv. KY14 results in enhanced Cd- and Zn-selective transport into root tonoplast vesicles, and enhanced Cd accumulation in roots of plants exposed to moderate, 0.02 muM Cd in solution culture (Korenkov et al. in Planta 225:403-411, 2007). Here we investigated effects of expressing AtCAX2 and AtCAX4 in the same lines on tolerance to growth with near-incipient toxicity levels of Cd, Zn and Mn. Less growth inhibition (higher tolerance) to all three metals was observed in 35S::AtCAX2 and FS3::AtCAX4 expressing plants. Consistent with the tolerance observed for Cd was the finding that while root tonoplast vesicle proton pump activities of control and FS3AtCAX4 expressing plants grown in 3 muM Cd were similarly reduced, and vesicle proton leak was enhanced, root tonoplast vesicle antiporter activity of these plants remained elevated above that in controls. We suggest that CAX antiporters, unlike tonoplast proton pump and membrane integrity, are not negatively impacted by high Cd, and that supplementation of tonoplast with AtCAX compensates somewhat for reduced tonoplast proton pump and proton leak, and thereby results in sufficient vacuolar Cd sequestration to provide higher tolerance. Results are consistent with the view that CAX2 and CAX4 antiporters of tonoplast play a role in tolerance to high, toxic levels of Cd, Zn, and Mn in tobacco.

CAX‐ing a wide net: Cation/H+ transporters in metal remediation and abiotic stress signalling

“…Root‐specific expression of CAX transporters mitigates Cd 2+ content in tobacco leaves (Korenkov et al . 2007, 2009). Future work may also utilise CAX from microalgae such as Chlamydomonas to remediate Cd 2+ ‐polluted freshwater sites (Pittman et al .…”

Influence of Soil Chemistry and Plant Physiology in the Phytoremediation of Cu, Mn, and Zn

“…The P-type ATPases AtECA1 (localized at the endoplasmic reticulum) and AtECA3 (expressed in the Golgi complex) participate in the influx of Mn (Li et al, 2008;Mills et al, 2008). The cation/H exchanger (CAX) transporters AtCAX2 and AtCAX4, which were originally identified as Ca transporters, also have the ability to transport Mn into the vacuole (Connorton et al, 2012;Korenkov et al, 2007). Likewise, vacuolar Mn/H antiporter activity in the Arabidopsis cax2 knockout mutant is significantly reduced compared with wild-type, although it is not completely absent, suggesting the presence of additional vacuolar transporters (e.g., AtCAX5) that contribute to Mn transport (Edmond et al, 2009).…”

“…The Golgi-localized AtECA3 was introduced into tobacco, resulting in better growth of the plants and enhanced tolerance of high Mn and Zn concentrations (Barabasz et al, 2011). Further, the expression of AtCAX2 or AtCAX4 in Nicotiana tabacum results in enhanced tolerance of Mn (Korenkov et al, 2007). The chelation capacity of PCs and MTs to detoxify metals in plant can also be improved through plant engineering (Machado-Estrada et al, 2012;Vurro et al, 2011).…”

Functional analysis of metal tolerance proteins isolated from Zn/Cd hyperaccumulating ecotype and non-hyperaccumulating ecotype of Sedum alfredii Hance

“…HMA4 genes from Arabidopsis thaliana, Arabidopsis halleri and Thlaspi caerulescens are responsible for Zn 2+ and Cd 2+ root-to-shoot translocation [4][5][6]. AtHMA3, OsHMA3 and AtCAXs transporters have been suggested to transport Cd 2+ into vacuoles; the ABC-type transporters have been found to mediate Cd 2+ and As 3+ tolerance by transporting PC-Cd and PC-As into vacuoles; and the MTP1 gene has been found to be responsible for Zn 2+ or Ni + sequestration into vacuoles [7][8][9][10][11][12]. Metal tolerance protein (MTP1) is a member of cation diffusion facilitator (CDF) family and all members of the CDF family possess six putative transmembrane domains [13,14].…”