text: Several gene families are involved in the processes of hyperaccumulation including upregulation of absorption and sequestration of heavy metal metals. These hyperaccumulation genes (HA genes) are found in over 450 plant species, including the model organisms Arabidopsis and Brassicaceae. The expression of such genes is used to determine whether a species is capable of hyperaccumulation.  Expression of HA genes provides the plant with capacity to uptake and sequester metals such as As, Co, Fe, Cu, Cd, Pb, Hg, Se, Mn, Zn, Mo and Ni in 100–1000x the concentration found in sister species or populations.
  The ability to hyperaccumulate is determined by two major factors: environmental exposure and the expression of ZIP gene family.
  Although experiments have shown that the hyperaccumulation is partially dependent on environmental exposure (i.e. only plants exposed to metal are observed with high concentrations of that metal), hyperaccumulation is ultimately dependent on the presence and upregulation of genes involved with that process. It has been shown that hyperaccumulation capacities can be inherited in Thlaspi caerulescens (Brassicaceae) and others. As there is a wide variety among hyperaccumulating species that span across different plant families, it is likely that HA genes were eco typically selected for. 
  In most hyperaccumulating plants, the main mechanism for metal transport are the proteins coded by genes in the ZIP family, however other families such as the HMA, MATE, YSL and MTP families have also been observed to be involved. The ZIP gene family is a novel, plant-specific gene family that encodes Cd, Mn, Fe and Zn transporters. The ZIP family plays a role in supplying Zn to metalloproteins.
  In one study on Arabidopsis, it was found that the metallophyte Arabidopsis halleri expressed a member of the ZIP family that was not expressed in a non-metallophyte sister species. This gene was an iron regulated transporter (IRT-protein) that encoded several primary transporters involved with cellular uptake of cations above the concentration gradient. When this gene was transformed into yeast, hyperaccumulation was observed. This suggests that overexpression of ZIP family genes that encode cation transporters is a characteristic genetic feature of hyperaccumulation. 
  Another gene family that has been observed ubiquitously in hyperaccumulators are the ZTP and ZNT families. A study on T. caerulescens identified the ZTP family as a plant specific family with high sequence similarity to other zinc transporter4. Both the ZTP and ZNT families, like the ZIP family, are zinc transporters. It has been observed in hyperaccumulating species, that these genes, specifically ZNT1 and ZNT2 alleles are chronically overexpressed.
  While the precise mechanism by which these genes facilitate hyperaccumulation is unknown, expression patterns strongly correlate with individual hyperaccumulation capacity and metal exposure, implying that these gene families play a regulatory role. Because the presence and expression of zinc transporter gene families are highly prevalent in hyperaccumulators, the ability to accumulate a diverse range of heavy metals is most likely due to the zinc transporters inability to discriminate against specific metal ions. The response of the plants to hyperaccumulation of any metal also supports this theory as it has been observed that AhHMHA3 is expressed in hyperaccumulating individuals. AhHMHA3 has been identified to be expressed in response to and aid of Zn detoxification. In another study, using metallophytic and non-metallophytic Arabidopsis' populations, back crosses indicated pleiotropy between Cd and Zn tolerances.  This response suggests that plants are unable to detect specific metals, and that hyperaccumulation is likely a result of an overexpressed Zn transportation system.
  The overall effect of these expression patterns has been hypothesized to assist in plant defense systems. In one hypothesis, "the elemental defense hypothesis", provided by Poschenrieder, it is suggested that the expression of these genes assist in antiherbivory or pathogen defenses by making tissues toxic to organisms attempting to feed on that plant. Another hypothesis, "the joint hypothesis", provided by Boyd, suggests that expression of these genes assists in systemic defense.