Metallothioneins (MTs) are low molecular weight, cysteine-rich metal chelators with an ability to bind heavy metal ions. MTs are able to bind a variety of metal ions by the formation of mercaptide bonds between numerous Cys residues (present in the proteins) and the metal,
and thus contribute to metal detoxification by buffering cytosolic metal concentration [1
]. MTs typically contain two metal-binding, cysteine-rich domains that give these metalloproteins a dumbbell conformation. They are widely distributed in animals, plants, fungi as well as cyanobacteria. Based on sequence similarities and their phylogenetic relationships, MTs have been broadly classified into three types [2
]. Class I MTs are widespread in vertebrates and have 20 conserved cysteine residues giving them the dumbbell conformation. Class II MTs do not have this strict arrangement of cysteine residues and are widespread in plants, fungi and invertebrates. On the other hand, phytochelatins, which are enzymatically synthesized metal binding peptides, are described as Class III MTs.
Plant MTs identified so far contain two cysteine-rich domains and a large spacer region (30–50 a.a. residues, devoid of cysteine) [5
]. These MTs have only a few histidines, while their Cys content varies between 10 and 17 residues. On the other hand, the number of aromatic amino acids in plant MTs varies from none to several. Based on the distribution of cysteine residues, number of aromatic amino acids as well as length of the spacer region, plant MTs are further classified into four types, type 1 through 4 [1
]. Analysis of various EST database shows that MTs are amongst the highly abundant transcripts in plants [8
]. Recent studies have established important roles for plant MTs in fruit development, root development and suberization besides heavy metal tolerance [9
]. Furthermore, the role of plant MTs in abiotic stresses such as oxidative, dehydration, senescence as well as hormonal alterations have also been shown [10
]. The antioxidant function of MTs is attributed to the presence of a large number of cysteine residues, which besides metal binding are also capable of ROS scavenging [6
]. Recently, a type 1 MT from mustard i.e. LSC54 has been reported to be induced by ROS production [15
]. Further, LSC54 has also been documented to be related to ROS imbalance during leaf senescence. Similarly, in rice, several type 1 and type 2 MTs have been found to play a direct role in antioxidation [16
Abiotic stresses, a major factor in reducing plant productivity, are proving to be an increasing threat to agriculture. Development of genetically modified abiotic stress tolerant varieties may provide a solution to this problem [17
]. Recently, we have characterized the molecular response of rice seedlings towards salinity stress based on their subtractive transcriptome profiling [18
]. One of the key members of this response, OsMT1e-P
– a type 1 MT isolated from a salinity tolerant genotype i.e. O. sativa
cv. Pokkali has been reported to be induced strongly in response to salinity stress. In the present communication, we provide detailed investigations on OsMT1e-P.
In essence, we have found that OsMT1e-P
is transcriptionally induced in response to salinity stress. Ectopic expression of OsMT1e-P in transgenic tobacco (under the control of 35 S constitutive promoter) provides stress tolerance against salinity, drought, cold, heat and heavy metals (Cu2+
). OsMT1e-P over expressing plants accumulate lower amount of ROS (H2
) under salinity stress. Further, we also show that at least five members of type 1 MT are tightly clustered on the distal arm of chromosome XII which show co-regulation in response to various abiotic stress conditions. Based on these results, we propose that OsMT1e-P
may serve as an important “candidate gene” for raising ‘multi-stress tolerant’ crops.