Introduction:

                                         Metals
play a vital role for the development of plants. Other than the essential
nutrients plants required a trace amount of metal ions for their proper growth
and in performing various function which occurs in the cell. Metals are
essential for plant growth. Plants required a large number of metals as
fundamental micronutrients for the normal growth and development of plants.
These metals are also involved in various oxidation and reduction reactions,
development, signal transduction which is in turn essential to many cellular
functions of plants. Iron, copper, zinc, and
some other transition metals are essential nutrients for plants (Frausto da Silva and Williams, 2001; Clemens et al., 2002; Morrissey and Guerinot, 2009; Pilon, 2011). Metal elements should be
transport from the soil into the root, after that distributed throughout the
plants crossing both the cellular and organelles layer. Trace amount of metal
particles are present in soil. Plants must utilize the efficient mechanism to
aggregate these particles.  Deficiency of
the metals results in the low nutrient crop productivity and decrease the
stress tolerance (Akkermans et al., 2016)

It
is considered that 33% of the proteins of a typical cell are metalloprotein (Finkelstein, 2009).
Inside a cell metals ions are transport by diffusion and various ion channels
with the support  of the different
techniques like transporter identification by comparing the sequence, Yeast
Mutant Complementation and various gene activities assistance in  recognizing the various genes families and
transporters which are involved in metal transport such as  Zip Family, Cu regulatory transporters. Metals transporters have been examined traditionally
in great depth as models for understanding plant membrane transport.

Iron
is part of heam protein such as ferritin, it acts as a cofactor for enzymes and
involved in many biochemical process such as photosynthesis and respiration.
Although iron is an abundant metal but present in limited foam due to its
oxidized foam, Fe (III) is insoluble in neutral or basic PH. Iron deficient
plants use different strategies to take up iron. For example IRTI
transporter   play an important role in
uptake of iron. It releases a reducing chelating agent that binds to the iron
and helps in uptake of iron.  Members of
the Arabidopsis NRAMP family have been shown to be involved in iron homeostasis
(Curie et al., 2000; Thomine et al., 2000).

Zinc
is required by the plants in response to the demand of plants (Nouet, C., Motte, P., & Hanikenne, M. 2011).  Zn is involved in the various cellular process
and the essential part of proteins which provide stability to the membrane (Maret, W., & Li, Y. 2009).Zn is a non redox
element but play an essential role in proteins .Plants use different approaches
to overcome the deficiency of Zn. Zinc is taken up the plants in the foam of
divalent ions .Once Zinc is take up by the plant it is neither oxidize and
redox. The behavior of zinc in the cell depend upon the existence of a divalent
cations. Recent studies have shown that different gene families of transporters
are involved in the transport of divalent cation. For example ZIP family
produce a Protein ZIP1 and ZIP2 that expressed in roots and transport the zinc
from the soil into the plant

Manganese
is essential for all organism but required in small amount. Manganese also
plays a virtual role in photosynthesis process and in the detoxification of
free radicals of oxygen. Plants use various proteins helps in the transport of
manganese from root into the plant. NRAMP family produce a gene that are
involved in the transport of manganese to the plant .NRAMP gene encode a metal
transporter which are involved in the transportation of the Manganese from the
roots to the plants. They are a monomeric
protein having 11 Tran’s membrane domains (Ehrnstorfer et al., 2014)

Cu
is essential for the development of plant. Cu is involved in many physiological
process that occur in plants such as photosynthesis, respiration. It act as a
cofactor for many proteins and enzymes such as,
laccase, plastocyanin and polyphenol oxidase at cellular level, in the
signaling (C. M., & Pilon, M. (2009). Cu is essential for plants but when
present in excess amount it can cause serious damage to plants. Deficiency of a
Cu effects the physiological process in plants. Many transporters are reported
which are involve in the transport of Cu but most important is COPT transporter
(Curr Opin Chem Biol. 2002).

Therefore,
human diet in numerous ranges of the world does exclude the basic metal
supplement necessities, causing from minor immunological reforms to death (Akkermans et al., 2016). Efforts
have been done in all over the world to improve the efficiency of plants in
uptake of metals (López-Rayo et al., 2015).
Developing the crop varieties in which uptake of metals have been improved Eggert and von Wiren, 2013)

In
this review, I will briefly describe the different metal transporters, genes
and their families which are involve in transportation of metals to various
part of the plants from the soil. How we can overcome the deficiency of the
micronutrients in the world. 

Metal Transporter families in
Plants:

ZIP Family:

 ZIP family takes it names from the
first identified ZRT, IRT like protein which are present in Arabidopsis .It is
expressed in root in response to deficiency of the iron. ZIP family is involved
in the transportation of metals such as Cu, Mn, Cd, Zn (Mizuno et
al., 2005). ZIP protein are predicted to have eight Tran membrane
domain in which carbonyl group and amino group are present at outside of the
membrane (Antala et
al., 2015). The amino sequence chain of the ZIP protein is extend
from 309 to 476 (Saier Jr, M. H. 1997). The difference in the length is due to
the variable region. The cytoplasmic membrane is only variable between the 3
and 4 domain which contain different binding regions for metals .These region
allow for the binding of metal transporters. At the movement 25 members of ZIP
family have been recognized. These members were divide into two subfamily on
the basis of the amino sequence similarity of their genome. Subfamily I
includes 15 qualities in plants (11 from Arabidopsis, two from tomato, one from
pea and one from rice), two yeast classes (ZRT1 and ZRT2), and a class from the
protozoan Trypanosoma brucei. Subfamily II incorporates 8 qualities in the
nematode Caenorhabditis elegans, one class in Drosophila and two classes of
human.

The first member
of the ZIP gene family is Fe (II) which is involved in the transport of iron
from soil into the plant. Of the 16 ZIP transporters in Arabidopsis, few have
been functionally characterized (Maser et al.,
2001; Rampey et
al., 2013).AtZIP1, AtZIP2, ATZIP, AtZIP4, AtZIP5. AtZIP1 are mostly
expressed in the root .It transport the Mn from vacuole into the cytoplasm to
allow the transport of Mn to the shoot. AtZIP2 transport the Mn into the Ariel
tissues from the plant, they cannot supply directly from root. Deficiency of Zn
on developed soils is a general issue so making that can keep up development
and yield under the of deficiency in soil .Zn deficiency without the contribution
of fertilizers. Advance is being made in breeding projects with the execution
of genetic markers, while hereditary designing can possibly increment yield and
dietary quality under anxiety conditions of plants (Sadeghzadeh et al. 2015).
To overcome the Zn deficiency in plants and by improving the metal
transporters, overall development of a plant would be increased.. 

IRT1 family:

Iron
is an essential element for the proper growth and development of plants.  It is 
involved  in the in many process
occur  in plants such as photosynthesis,
respiration and act as a cofactor for many enzymes (Alberdi, M. (2010).Iron is present in abundant foam but the
problem is that it existing in insoluble oxidized foam Fe (III), which is
insoluble in neutral or acidic PH. Many iron deficient plants use different
strategies to take up iron.IRT1 family has high affinity for the transport of
iron. IRTI is the first gene encoding a Fe (II) transporters to be cloned from
plants.

. IRT1 is the significant transporter in control of
high-partiality metal take-up under iron defeiency.IRT1
produce a gene Fe (II) transporter, which converts the Fe (III) oxidized foam
of iron into Fe (II). Then it is transport by the into plants by two systems
depend upon the binding affinity of the substrate (Connolly, 2014)
.The transport system which has a low binding affinity for the receptor require
the Fe (II) encoded by the FET3 gene. The transport system which has the high
binding affinity for the substrate require an Fe (III) transporter’s encoded by
FET4 in this transport system In this system first insoluble oxidizes Fe (II)
is converted into Fe (III) after binding to the transporters. But the grasses
use different mechanism for the uptake of iron. Grasses released an oxidized
compound phytosiderophor into the soil that’s bind to ions and then transport
it (Maiti, R. K., & Satya, P. (2014).).
IRT2 show homology with the IRT1 and involve in the transportation of iron.

NRAMP family:

                              NRAMPs
family  transport a range of
fundamental  and insignificant metals,
they work in numerous natural process, For example, metal homeostasis,
photosynthesis, uptake of protein, digestion systems and natural anxiety
reactions (Ihnatowicz et al. 2014). NRAMP (natural
resistance-associated macrophage protein) were identified in various plants. NRAMP
gene was first identified in mice involve in resistance against intracellular
pathogens (Nevo and Nelson, 2006).NRAMP
genes play an crucial  role in the
transport of various metal ions.NRAMP genes are expressed under the deficiency
iron and phosphorous  metal ions .

  Many transporter’s
families are involved in the transportation of Mn, but NRAMP family also
involved in the transportation of Cu, Mn and Al (Nevo and Nelson, 2006; Xia et al., 2010). NRAMP protein
predicted to contain 10 trans- membrane domains with a consensus sequence
between the 8 and 9 domain which is linked with iron transporters. Functional characterization of AtNRAMP1, AtNRAMP3, and
AtNRAMP4 was first reported in 2000 (Thomine et al., 2000 . AtNRAMP1 is involved
in Mn transport in the root due to the up regulation of AtNRAMP1. AtMRAMP1 is
also over-expressed in Fe defiency.AtNRAMP1 genes are mostly involve in the
transport of Mn into the Cotyledon. (Donner et al., 2012).

AtMRAMP3
and AtNRAMP4 are expressed in the root and also involved in the transportation
of the Mn.  The Sequence analysis of both
AtNRAMP3 and AtNRAMP4 proteins demonstrate that they are restricted to the
vacuolar film, subsequently they are believed to be critical for metal
remobilization from the vacuole, in
planta.AtNRAMP2, AtNRAMP5, AtNRAMP6 are
 not analyzed, they are involved in the
transport of Mn.

YSL Family:

                         YSL family names comes
after Maize YS1 (ZmYS1), which amassed in the
deficit of Fe. YSL transporters belongs
to a group of oligopeptide transporters (OPTs), which can transport amino acids
and amino acid derivative compounds.YSL 
in non-grasses  use NA for the
metal transport in intracellular and intercellular .(Bashir et al., 2011). YSL
transporters are mostly found in the Plant, Bacteria and Animals. YSL
transporters studied in yeast shows that they are involved in the transport of
Mn, Cd, and Cu as well as iron. In rice, there are 18 putative YSL genes (Koike et al., 2004; Conte et al., 2013).OSYSL2
and OSYSL6 are involved in the transport of Mn. Electrophysiological and various studies suggested
that the OsYSL2 is associated with horizontal development of Mn2+-NA complexes
by means of the phloem and stacking into creating seeds.

OSYSL6 are expressed in the roots and shoots of cell. OSYSL6
is involved in refinement of Mn when present in excess amount (Sasaki et al., 2011).
Rice OsYSL9 is a unique transporter for Fe (II)-
nicotianamine ,that is responsible for the transport of iron inside
particularly from endosperm to the seed in development of embryo.OSYSL9 are
expressed in the root but not in the leaves in response to iron deficiency.
OSYS15 is an important for the transport of iron into the rice. It is expressed
in roots mostly in response to iron deficiency. OsYSL16 is involved in
Fe transport in rice organs via vascular bundles (Kakei et al. 2012; Lee
et al. 2012) .OsYSL16 also involve in the
transport of Cu with the help of phloem tissues( Zeng et al. 2012) . In rice OsYSL2, OsYSL9, OsYSL15 and OsYSL16. OsYSL9
indicated 82.9% (68.1%), 83.2% (68.5%) and 85.6% (72.1%) sequence similarity to
OsYSL2, OsYSL15 and OsYSL16 (Aoyama et al. 2009; Curie et al.
2009; Yordem et al. 2011)

COPT Family:

The copper (Cu) transporter (COPT/Ctr) quality family has a
critical role in the maintenance of Cu homeostasis in numerous species of the
plants. COPT transporters helps in uptake of Cu from soil into the pollens.  (Sancenón et al., 2004).They
have found in plants name as COPT transporter’s
(Sancenón et al., 2003), and in fungi known
as Ctr (Kim et al., 2013 .

Many
type of transporter’s are reported which are involve in the uptake of Cu .The
most important is COPT transporter. The expression of COPT transporters rely on
the level of Cu required by the plant. COPT transporter have three
trans-membrane regions (Ctr1. J Biol Chem. 2002)
.In Rice seven COPT transporters genes have been identified COPT1 to COT7
(Plant Cell. 2010, 22 ).COPT1 and COPT2 are mostly play an critical in the
transport of the Cu under the deficiency of Cu to the pollens and embryos. COPT3 and COPT5 
transporters having just a single Met-and His-rich box, which
demonstrate an transitional level of 
complementation and Cu transport .COPT4 are expressed in the root and
lack the met box, having essential motifs vital for the transport of Cu
(Sancenón et al., 2004)

. All COPT members have conserved functions. Proteins encoded
by the COPT transporters in which only COPTs, COPT2, COPT3, and COPT4 substantially
cooperated with COPT6  and COPT7 act
alone for the transport of Cu in the tissues .However,   COPT gene are involve in the Cu uptake are
transcriptionally impacted by insufficiency of iron, manganese, or zinc.
 (Yuan et al; licensee BioMed Central Ltd., 2011).  Under the deficiency of Cu, Cu transporters
activated which helps in the inflow of Cu through (COPTs)
which are persuaded by SPL7 transcription factor   (Park, H. J et al., 2016). 

ABC Transporters Family:

  (ATP-binding
cassette) ABC transporters proteins are capable to exchange
compounds across various natural layer in most cases against existing
electrochemical gradients, using energy released from ATP hydrolysis (Wilkens et al., 2015). ABC
transporters play a critical role for the growth of plants, development of
seed, germination, and secondary growth of plant and formation of protective
layers (Martinoi et al 2017). Plants have to face certain toxic materials such as toxins produce by
the pathogen, minerals within the soil. 
Since plants are insufficient in their ability
to avoid toxins. Plants developed an alternate systems to detoxify these
toxins S (Kang et al., 2011).ABC transporters mainly focus on protecting
the plants from pathogens by the transport of xenobiotic (Yim
S et al., 2016). Terrestrial plants required the
improvement of particular cell types such as cutin-covered epidermal cells
which inhibit the water loss, inflexible vascular bundles that oppose
gravitational pull and especially negative weight created in xylem created in
xylem during the transpiration process.

These particular cells required the transport of the
phytochromes which interact with the biosynthesis and differentiation. ABC
transporters have the ability to transport the phytochromes (Hwang
J-U et al 2016). The genes of ABC transporters
undergo multiplication and functional divergence that enable it to transport the
compounds virtual for the adaption in dry land (Jang et al 2016)

CDF/MTP Family:

In plants, the CDFs are better  known as MTPs because of their widespread
role in the detoxification of heavy metals (Ricachenevsky et al., 2013).CDF
family is present in all kingdom of life (Kolaj-Robin et al., 2015). They are bind with
the divalent metals such as Fe2+, Zn2+, or Mn2+,
attached with a H+antiport  (MacDiarmid et al., 2002; Rubio-Sanz et al., 2013; Gupta et al., 2014; Raimunda a Elso-Berberián, 2014). One gene of the CDF is involved in the transport of the Zn
(Guerinot  et al 2014).

Plants CDF family are categorized into three functional group
on the basis of their sequence similarity Zn-CDFs, Fe/Zn-CDFs, and Mn-CDFs (Gustin et al., 2011)
.CDF,s have six trans membrane domain and the eukaryotic CDF transporters
contain only rich histidine residues (Kolaj-Robin et al., 2015). Mn CDF,S contain the
conserved sequence DXXXD (where X = any amino acid) between 2 and 4
which is not present Zn-CDF, Fe-CDF,s (Montanini et al., 2007).Single change in an amino
acid sequence can alter the function of the metal transporters MTP,s, For
example a point mutation in AtMTP1  AtMTP1
(Podar et al., 2012) and
OsMTP1 (Menguer et al., 2013)
allow the transport of Mn which is not observed in the normal protein.

In rice there are five Mn –CDF are recognized as OsMTP8, and
their subfaimly OsMTP8.1, OsMTP9, OsMTP11 and OsMTP11.1.OsMTP8.1 mark off in
the cDNA of the rice that tolerate the Mn in yeast (Chen et al., 2013).
They are expressed in shoot in response to the excess of Mn.

 

Role of Metal
Transporters in Improving Food Security and Micronutrients:

Deficiency of essential micronutrients is a hot burning issue
in developing countries such as Zn, Fe, that lead to the improper  growth, and development. The population of
the world increase day by day and expected to be 10 million in 2050 (United
Nations, 2013) .Food security is a major challenge in the 21st
century due to the limited avaibility of the water recourses , fertilize land
and the declining in the micronutrients in plants ( Blindauer et al., 2010). One important feature that
effects both plant growth as well as the nutritional value of field crops are
micronutrients (Schmid et al., 2010). Low levels
of accessible mineral supplements in soil, improper administration of the
minerals, with the absence of the plants genotypes having lack of tolerance to
deficiency of nutrient are the major cause of food security and the deficiency
of essential micro nutrients in peoples. The fact that the 60% cultivated soil
have the problem of nutrients deficiency.

 Therefore,
human diet in numerous ranges of the world does exclude the basic metal
supplement necessities, causing from minor immunological reforms to death (Akkermans et al., 2016).
Deficiency of the nutrients occurs in all over the world (Hodson & Donner, 2013).The
FAO (2011).
Considers that almost a billion people are under-nourished, most in sub-Saharan
Africa (239?million) and Asia (578?million). The yield is low in these areas
due to their soil and deficiency of micronutrients such as Zn, Mn, Fe, these
areas required the improvement of nutrition uptake and the fertilizers. (Gregory et
al., 2014).It is estimated that 25-30% agriculture of developing and under
developing land have small availability of Fe, Zn, and Cu (Broadley et al., 2007;
White & Greenwood., 2013).

Many chronic disease that occur in societies due to under
nourishment related to the essential micronutrients .But this is masked by
improving the soil and the micronutrients uptake system (Graham et al., 2012).The deficiency of nutrient cannot only achieved only by the
fertilizers because 20-80% of nutrients present in fertilizers are lost in the
environment due to the complex chemistry of soil that reduce the nutrients (Sebilo et al., 2013).

There is no doubt that the production
of fertilizers with the improvement of the nutrients uptake system could reduce
the loss of nutrient and  use of
fertilizers (Withers et al.,2014 ) .In the 21 Century, plant nutrition
research become a major area in meeting the food production with the increase
in micronutrient. Developing the crop
varieties, in which uptake of metals have been improved (Eggert and von Wiren., 2013)

 Researchers could
solve this problem by producing the crops or plants genotype with improvement
of metal uptake potential or improvement in metal transporters mentioned above.
By producing such crops or plants genotype, in which the metal uptake potential
is improved, so that the micronutrients in the food is increase.

 Conclusion
and Future Prospects:

Metal
transporters play a virtual role in the transport of metals from soil to
different part of plant for the proper growth and development of plants. Expression
of metal transporters varies in tissues .deficiency of  micronutrients in people is a global problem
in all over the world .In the few past decades, we gain much information about
metal transporters families and which transporters helps in the homeostasis of
metal ion. It is necessary for ourselves to understand the transport system of
metal ions at plants level, with  the use
of such knowledge generate such plant that are able to survive in suboptimal soil conditions which would increase the crop production and
mineral content in plants. For example if we breed plants with high potential
metal uptake ,we can generate plants in which the availability of
micronutrients uptake  is improved in
plants ( Guerinot et at., 2014) .In
future with the help of knowledge of metal transport system in plants , we can
overcome the problem the micronutrients deficiency in all over the world .