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Outline:

-Hyperaccumulation is an acute example of internal tolerance to toxic ions.

-Hyperaccumulating plants can withstand foliar concentrations of up to 1% of their shoot dry weight of different trace elements including arsenic, cadmium, nickel, zinc, and selenium.

-10 mg per gram dry weight.

-Hyperaccumulation is a comparatively uncommon plant response to potentially toxic ions that necessitates heritable genetic modifications that boost the expression of ion transporters involved in ion absorption and vacuolar compartmentalization.

-For metal hyperaccumulation in shoot tissues, long-distance transport of chelated ions from roots to shoots is also important. During this transport process, both the iron chelator nicotianamine and the free amino acid histidine have been implicated in metal chelation. Some ligands for ion chelation, such as phytochelatins, are also synthesized by plants. Metal Transporters Outline:

-An important trait of hyperaccumulating plant species is enhanced translocation of the absorbed metal to the shoot.

-Metal toxicity is tolerated by plant species that are native to metalliferous soils. Exclusion, in which plants resist undue metal uptake and transport, and absorption and sequestration, in which plants pick up vast quantities of metal and pass it to the shoot, where it is accumulated, are the two basic methods for metal tolerance.

-Hyperacctinttilators are plants that have both the second technique and the ability to absorb more than 100 times higher metal concentrations than typical organisms.

-T. caerulescens is found mostly in Zn/Pb-rich soils, as well as serpentines and non-mineralized soils. It was discovered to be a Zn hyperaccumulator. Because of its ability to extract vast quantities of heavy metals from soils.

-When grown on mildly polluted soils, a closely related species, Thlaspi ochroleucum, is a heavy metal-tolerant plant, but it accumulates much less Zn in the shoots than T. caerulescens.

-Thus, Thlaspi ochroleucum is a non-hyperaccumulator and of the same family T. caerulescens is a hyperaccumulator.

-The transfer of Zn from roots to shoots varied significantly between these two species. T. caerulescens had much higher shoot/root Zn concentration levels than T. ochroleucum, which always had higher Zn concentrations in the roots. When Zn was withheld, the amount of Zn previously accumulated in the roots in T.caerulescens decreased even more than in T. ochroleucum, with a concomitantly greater rise in the amount of Zn in the shoots. The decreases in Zn in roots may be mostly due to transport to shoots, since the volume of Zn in shoots increased during the same time span.

-A heavy metal transporter cDNA: mediate high-affinity Zn2+ uptake as well as low-affinity Cd2+ uptake.

-This transporter is expressed at very high levels in roots and shoots of the hyperaccumulator.

-According to (Pence et., al. 1999), an overexpression of a Zn transporter gene, ZNT1, in root and shoot tissue is an essential component of the Zn hyperaccumulation trait in T. caerulescens.

-This increased gene expression has been shown to be the basis for increased Zn21 uptake from the soil in T. caerulescens roots, and it is possible that the same process underpins the enhanced Zn21 uptake into leaf cells. Regulation outline:

-Transporters are good candidates for regulation since intracellular levels of heavy metals must be closely regulated.

-There are currently no examples of how they could be controlled in higher plants, although it might happen at the transcriptional level (control on initiation speeds, mRNA stability, differential mRNA splicing) or at the post-translational level (control on mRNA stability, differential mRNA splicing) (targeting, stability).

-Extracellular metal concentrations control the transcription of several metal transporters in other species through transcription factor proteins.