**8. Expression pattern under influence of various factors**

#### **8.1 Metal ions**

As homologs of animal MTs, it would be reasonable to assume that the MTs in plants are related to metal metabolism and detoxification of excess metal ions. Indeed, the expression of both Ec proteins and several class I MT genes vary under metal treatment. The wali l (type 2) in wheat was originally isolated as an aluminum-induced gene. Its expression is also increased by treatment with other toxic metal ions like Cd, Fe, Zn, Cu, Ga, In and La.

In rice, the transcript level of RgMT (type 1) was elevated by 250 μM CuCl2, in suspension-cultured cells [27, 28], while 100 μM CuSO4 increased ricMT (type 4) transcripts in both shoots and roots of seedlings. This expression-enhancing effect of Cu2+ are been reported in the type 2 MT genes from *Arabidapsis* and tobacco, and in type 3 MT from *Arabidopsis*. In contrast, copper ion are shown to have a suppressive effect on type 2 MT in soybean and *Vicia faba*; in the copper-tolerant plant *Mimulus guttatus*, transcription of the type l MT was inhibited by 5 μM CuS04, as well as by 5 μM CdSO4 and 15 μM ZnSO4. In addition, the mRNA Ievel of ricMT in the root of rice seedlings was reduced by 100 μM aqueous solutions of ZnSO4, CdCl2, FeCl3, Pb (Ac)2 and AlC13, respectively.

#### **8.2 Senescence**

Coupe *et al.* [29] isolated the type 2 MT gene JET12 from abscission zones of the ethylene-treated leaves of *Sambucus nigra* L. Northern analysis reported that JET12 gene was expressed only in senescent leaves, and in the ethylene-treated abscission zone.

Similarly, the type 3 MT LSC54 in rape (*Brassica napus*) was expressed during leaf senescence, and as senescence progressed, the mRNA level of LSC54 increased. Yu *et al*. [28] showed that the expression of rgMT (type 1) increased as young leaves progressed to senescence. These findings suggest that these MT genes may play a key role in senescence-associated metabolism of plants.

#### **8.3 Other stresses**

Considerable effort are made to clarify the relationships between plant MTs and environmental factors other than metal ions. The expression of type 2 MT seems to *Metallothioneins: Diverse Protein Family to Bind Metallic Ions DOI: http://dx.doi.org/10.5772/intechopen.97658*

be easily affected by stresses- For example; the MT in tobacco was strongly induced by tobacco mosaic virus and wounding [30] and expression of MT in *Vicia faba* was reduced when plants were treated with salt, cold, salicylic acid or UV-light. Furthermore, transcripts of OsMT-2 in rice can be induced by sucrose sta, heat shock and ABA and the expression of wali l in wheat was increased by low-nutrient conditions and wounding.

Regarding type I MT, the mRNA level of rgMT in rice increased under treatment with heat shock, a toxicant (DMSO) and starvation, and decreased in presence of abscisic acid.
