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REVIEW
Micronutrients and their diverse role in agricultural crops:
advances and future prospective
Durgesh Kumar Tripathi
1
•Shweta Singh
2
•Swati Singh
2
•Sanjay Mishra
2
•
D. K. Chauhan
2
•N. K. Dubey
1
Received: 3 October 2014 / Revised: 16 February 2015 / Accepted: 23 May 2015 / Published online: 2 July 2015
Franciszek Go
´rski Institute of Plant Physiology, Polish Academy of Sciences, Krako
´w 2015
Abstract In plant sciences, the prodigious significance of
micronutrient is unavoidable since plant relies primarily on
micronutrient as it has profound influence on array of plant
activities. Although micronutrients are abundantly present
in the soil but plants usually acquire them in relatively
trace amounts; hence, regarded as tracer element. B, Cu,
Fe, Mn, Zn are such micronutrients required in minute
amounts by plants but inexorably play an eminent role in
plant growth and development. Plant metabolism, nutrient
regulation, reproductive growth, chlorophyll synthesis,
production of carbohydrates, fruit and seed development,
etc., are such effective functions performed by micronu-
trients. These tracer elements when present at adequate
level, elevate the healthy growth in plant physiological,
biochemical and metabolic characteristics while their
deficiency promotes abnormal growth in plants. Prevalence
of micronutrient deficiency has become more common in
recent years and the rate of their reduction has further been
increased by the perpetual demands of modern crop culti-
vars, high soil erosion, etc. On the basis of present existing
condition, it is not difficult to conclude that, the regular
increment of micronutrient deficiency will be mostly
responsible for the remarkable degradation in substantiality
of agricultural crops somewhere in near future and so that
this issue has now been the subject of intensified research
among the breeder, ingenuities and expertise of science.
These micronutrients can also be proven toxic when pre-
sent at accelerated concentrations and such toxicity level
endangers the plant growth. Taking this into consideration,
the current review unfolds the phenomenal participation of
micronutrients in plant sciences and gives a brief overview
of the current understanding of main features concerning
several micronutrient acquisitions in agricultural crop
plants.
Keywords Micronutrients Agriculture Deficiency
Toxicity Food security
Introduction
In the last few decades being widely cited, the term food
security has raised the major concern worldwide, among
the scientists, researchers, agronomists and policy makers.
Owing to anticipated changes in climatic conditions and
the perpetually looming anthropogenic activities, recent
years have witnessed much focus on food security and now
the countries from all over the world are continually paying
much more attention towards the security of food and thus
working under the common mission of global food security
(FAOUNS 2000; Devereux and Maxwell 2001; CFS 2005;
Clay 2002; Fresco 2009; Floros et al. 2010). Further, in the
wake of accumulated evidence, it becomes increasingly
important to broach here that food security is not only
meant for securing the quantity of food which was con-
sumed by global population but is also concerned with
quality and variety of food (Maxwell 1996; Hamm and
Bellows 2003; Faye et al. 2011; Chappell et al. 2013).
Communicated by A. K. Kononowicz.
&Durgesh Kumar Tripathi
dktripathiau@gmail.com
&D. K. Chauhan
dkchauhanau@yahoo.com
1
Center of Advanced Study in Botany, Banaras Hindu
University, Varanasi 221 005, India
2
D. D. Pant Interdisciplinary Research Laboratory,
Department of Botany, University of Allahabad,
Allahabad 211 002, India
123
Acta Physiol Plant (2015) 37:139
DOI 10.1007/s11738-015-1870-3
Nutritional insecurity, due to alarming micronutrient defi-
ciencies has substantially contributed to the global burden
of disease resulting in devastating health consequences as
the people face extreme hunger, which later results into life
threatening illness popularly known malnutrition or
undernutrition (Flores and Gillespie 2001; Pinstrup-An-
dersen and Rosegrant 2001; Barrett 2002; Thompson et al.
2012). Some recent reports on micronutrient status clearly
suggested that every year more than 10 million people die
due to the vulnerable effects of micronutrients while
micronutrient deficiency also reduces the immunological
capacity of plant and animals that enable them to resist
against several chronic diseases (Flores and Gillespie 2001;
Pinstrup-Andersen and Rosegrant 2001; Barrett 2002;
Thompson et al. 2012). Conventionally, from last many
years, nutrition safety issue has been taken as the most
significant realm of health professionals but still more
focus is required to meet the food security challenges
(Parnell et al. 2001; Floros et al. 2010; Chappell and
LaValle 2011). It is reported that agri-food chain has
always played a vital role to provide the sufficient nutrients
either to plants or to human beings (Soil Association 2000;
Thrupp 2000; Heaton 2001; Williams 2002; Maathuis and
Diatloff 2013); therefore, agri-food interaction is also
another venture for the food scientists. With agriculture
being the prime source of nutrition in India, several gov-
ernment policies in this respect has been introduced to
draw full attention towards higher agricultural yield and
also to understand the food security goals, including
nutrition security and its importance in plant and human
life (Ha
¨nsch and Mendel 2009; White and Brown 2010;
Marschner 2012).
From the above discussion, an integrated solution for
securing sufficient and healthy food for all could prefer-
entially be drawn as a sustainable opportunity to reduce
hidden hunger because micronutrient plays an important
role in plant growth, their development and more specifi-
cally it also involved in multitudinous metabolic processes
(Singh et al. 2011; Marschner 2012; Tripathi et al. 2012a,
b,c;2014a,b; Kumar et al. 2014a,b). Mineral nutrients
protect the plants from various hurdles and maximally
perform irreplaceable role during the entire life cycle of
plants (Singh et al. 2011; Tripathi et al. 2012a,b,c; Kumar
et al. 2015; Liang et al. 2013; Tripathi et al. 2014a,b).
Minerals nutrients that are found in the soil, water, air and
plants were classified previously by imminent plant sci-
entists on the basis of their utility and requirement
(Marschner 2012; Sperotto et al. 2014). They discriminate
it in two different classes, i.e., macronutrients and
micronutrients, further micronutrients are those nutrients
which are required by plant in lesser amount whilst
macronutrients is needed in relatively larger amount to
complete the life cycle of plants (Tripathi et al. 2011,
2012a,b,c,2014a,b;2015; Chauhan et al. 2011; Singh
et al. 2011; Marschner 2012).
Owing to its increased necessity in intensive cropping
system for attaining higher yield productivity, in recent
years, micronutrients gained profound significance (Dell
et al. 2006; Liu et al. 2015). Moreover, regarding the
importance and the role of micronutrients in crop produc-
tion, promising changes have been found in modern sce-
nario of agriculture that also deals with the level of
micronutrients in the main staple food crops as well as diets
of humans and animals.
Micronutrients i.e. boron (B), copper (Cu), chlorine (Cl),
iron (Fe), zinc (Zn), manganese (Mn), molybdenum (Mo)
are regarded as essential plant nutrients taken up and
consumed by the plants in relatively lesser amount. These
micronutrients play an eminent role in plant growth,
development and plant metabolism. However, their defi-
ciencies may induce several diseases in plants and later,
can reduce the quality as well as quantity of food. Thus
investigation related to the role of micronutrients in plants
has resulted in breathtaking curiosity and is a matter of a
huge significance among the researchers. It has been well
documented that micronutrients play multifarious roles and
their adequate supply increases the growth and yield of
plants, thereby protecting the plants from adverse effects of
various biotic and abiotic stresses (Fig. 1). Therefore, in
this current review, we have summarized the overall ben-
efits and significance of some micronutrients in plants.
Boron (B)
Soil basically serves as the unique/prime source of trace
elements for vascular plants (Fig. 2). Among them, boron
has been recognized as one of the most essential, ubiqui-
tously distributed microelements of group III of the long
form of periodic table. Boron show marked difference from
the other members of its group because it is a highly
electronegative element and exhibits intermediary proper-
ties between metals and non-metals (Bolan
˜os et al. 2004;
Herrera-Rodrı
´guez et al. 2010). Though its average con-
centration inside the soil solution is 10 ppm, but the most
preferable range for which plants encounter neither defi-
ciency nor toxicity is very low i.e., 0.3–1 ppm (Lee and
Aronoff 1967; Shelp 1993; Blevins and Lukaszewski
1998). Though boron has ubiquitous distribution in nature,
the important role of boron in making proper growth and
productivity is still not well known. Plant acquires boron
(B) basically in the form of undissociated/uncharged boric
acid (H
3
BO) that tends to form borate ester by reacting
with apiose residues of two rhamnogalacturonan II (RGII)
molecules and the resulting RGII borate dimers further
show the cross-linking with pectins of the cell wall thereby
139 Page 2 of 14 Acta Physiol Plant (2015) 37:139
123
initiating the formation of three-dimentional pectic net-
work and thus, maintaining structural integrity of the cell
wall (Kobayashi et al. 1996; Camacho-Cristo
´bal et al.
2008; Beato et al. 2011).
Firstly, Warington (1923) recognized boron (B), as the
essential microelement and also established its importance
in improving the optimal growth of plant cell. In the
beginning of the twentieth century, the necessity of boron
was first considered and as the time passed boron has been
declared as the critically essential micronutrient by a
number of investigators. It has also been well documented
that smaller concentration of boron could facilitate the
proper growth and development of the higher plant and
their deficiency would lead to the impairment of metabolic
and physiological processes (Nable et al. 1997; Blevins and
Lukaszewski 1998; Bolan
˜os et al. 2004;Reid2007;
Camacho-Cristo
´bal et al. 2008).
In addition to playing incredible role in the biosynthesis
of cell wall and lignifications, boron is also involved sig-
nificantly in a variety of physiological and biological pro-
cesses such as tissue differentiation, vegetative growth,
phenolic metabolism, and membrane integrity, etc.
(Table 1). Apart from this, the bioavailability of boron is
also necessary for nitrogen fixation and nitrate assimilation
(Camacho-Cristo
´bal and Gonza
´lez-Fontes 1999; Matas
et al. 2009; Reguera et al. 2010; Beato et al. 2010), for
oxidative stress (Pfeffer et al. 1998; Kobayashi et al. 2004)
and root development (Dugger 1973; Martı
´n-Rejano et al.
2011). However, its presence beyond the permissible limit
hampers plant growth and crop productivity worldwide
(Stangoulis and Reid 2002; Reid et al. 2004). Boron is an
essential micronutrient required by the plant for proper
growth and development (Miwa et al. 2007) and its defi-
ciency would impose several inimical effects on plants,
such as yellowing of the leaf tips, chlorosis and necrotic
spot, stunted growth, and inhibition of root and shoot
length, greenish-grey spots on fruits (Table 1) (Nable et al.
1997). Therefore, from the above studies and recent find-
ings it can be well said that boron is the sole element and it
has been proved beneficial for the proper growth and
development of plant only if it is absorbed and accumu-
lated in required amount (Table 1).
Copper (Cu)
The magnificent role of copper (Cu) in the plant world makes
it a unique element. Being a transition element, it occupies
place in 11th group of periodic table with atomic number of
29 and atomic mass of 63.5. It is considered to be a very
Fig. 1 Response of
micronutrients in different
abiotic and biotic stresses
Acta Physiol Plant (2015) 37:139 Page 3 of 14 139
123
important element for every life being present on the earth
due to its several characteristics. Plant growth is highly
dependent on the availability of Cu as it plays pivotal role in
regulating multiple biochemical reactions in plants (Table 2).
Arnon and Stout (1939) declared Cu as an important nutrient
for plants in their experiments with tomato.
Being the stable cofactor of various enzymes and pro-
teins, Cu plays an indispensable role in regulating several
metabolic and physiological processes of plants (Rehm and
Schmitt 2002). Cu actively takes part in many physiolog-
ical processes in plants as it is present in the form of oxi-
dized Cu (II) and reduced Cu (I) states in histidine and
cysteine or methionine, respectively (Yruela 2005; Grata
˜o
et al. 2005; Pilon et al. 2006; Burkhead et al. 2009a,b). It
participates in oxidation-reduction reaction as an electron
carrier in chloroplasts and mitochondria as well as in
oxidative stress response (Raven et al. 1999; Yruela 2005;
Grata
˜o et al. 2005; Pilon et al. 2006; Kra
¨mer and Clemens
2006; Puig et al. 2007; Yruela 2009).
The other effective functions of Cu in the plant world at
cellular level can be enlisted as follows:
•Significantly contributes in cell wall metabolism and
signal pathway of transcription,
•Cu actively participates in oxidative phosphorylation
and iron mobilization,
•Cu plays important role in the biogenesis of molybde-
num cofactor and protein trafficking machinery.
As mentioned earlier, Cu acts as a cofactor of enzymes
and plays significant role in respiration, photosynthesis,
lignifications, phenol metabolism, protein synthesis, regu-
lation of auxins, etc. (Table 2). Some of these Cu-enzymes
are cytochrome oxidase, Cu/Zn-superoxide dismutase (Cu/
ZnSOD), laccase, ascorbate oxidase, amino oxidase,
polyphenol oxidase and plastocyanin (Yruela 2005; Ravet
et al. 2011; Rout et al. 2013). Plastocyanin, being the most
abundant copper protein promotes electron transport in the
thylakoid lumen of chloroplasts (Yruela 2005; Abdel-
Fig. 2 Adequate level of micronutrients in plants and their diverse response under deficiency and toxicity (Epstein and Bloom 2005; Marschner
2012)
139 Page 4 of 14 Acta Physiol Plant (2015) 37:139
123
Table 1 Integrated response of agricultural crops towards the bioavailability, deficiency and toxicity of certain essential micronutrients influencing its probable sustainability
Microelement boron (B) in
variable concentration
Symbol and the form of
absorption by plants
Concentration in plant Response of agricultural crops towards variable
concentration of boron in soil solution
References
Boron (in adequate
concentration)
(B) H
3
BO
3
(Boric acid) 0.3–1 ppm
3–100 lgg
-1
dry weight
Boron (B) plays an indispensable role in
Biosynthesis of cell wall and lignifications
Tissue differentiation
Phenolic metabolism
Vegetative and reproductive growth
Membrane integrity
Nitrogen fixation and nitrate assimilation
Reguera et al. (2010), Cristo
´bal et al. (1999),
Matas et al. (2009), Beato et al. (2010)
Boron-deficiency Less than 0.3 ppm and
0.14 mg kg
-1
and
Boron deficiency in soil results in
Stunted growth
Inhibition of cell expansion
Cracking or rotting of fruits
Wilted or curled leaves
Water soaked petiole
Silva and Uchida (2000), Dear and Weir (2004)
Boron-toxicity Above 0.3–1 ppm and
3–100 lgg
-1
dry weight
Higher concentration of boron in plants would
lead to
Yellowing of the leaf tips and distorted shoot
growth
Chlorotic and necrotic patches in the margin/
older leaves spots on fruits
Nable et al. (1997), Stangoulis and Reid (2002),
Reid et al. (2004), Nable et al. (1997)
Acta Physiol Plant (2015) 37:139 Page 5 of 14 139
123
Table 2 Cumulative response of agricultural crops towards variable concentration of certain essential micronutrients
Presence of
Micronutrients
Symbol and the form of absorption by
plants and concentration
Concentration in
plant
Response of agricultural crops towards variable concentration
of Zinc and Copper in soil solution
References
Adequate amount
of Zinc
(Zn) and Zn
2?
15–20 mg Zn kg
-1
DW
Regulates the biological membranes
Zinc finger regulates transcription directly through effects on DNA/RNA binding
Regulation of chromatin structure
RNA metabolism and protein–protein interactions
Antioxidative defence enzymes
Sinclair and Kra
¨mer (2012), Rengel
(2004), Ha
¨nsch and Mendel
(2009), Broadley et al. (2007)
Deficiency of Zn Below 15 mg Zn
kg
-1
DW
Impaired stem elongation in tomato
Root apex necrosis (‘dieback’)
Interveinal chlorosis (‘mottled leaf’)
Development of reddish-brown or bronze ‘bronzing’
Internode shortening (‘rosetting’)
Epinasty, inward curling of leaf lamina
‘Goblet’ leaves) and reductions in leaf size (little leaf)
Tsonev et al. (2012), Alloway
(2004a,b), Disante et al. (2010),
Skoog (1940)
Toxicity level of
Zn
Above 20 mg Zn
kg
-1
DW
Reduced yields and stunted growth
Leafy vegetable crops are sensitive to Zn toxicity
Soybean and rice has been recognized as Zn sensitivity
crops in which Zn toxicity instigate genetic variation
Phototoxic concentrations of Zn
Increase lipoxygenase activity
Lipid peroxidation
Enhancing antioxidative activity in plants
Hafeez et al. (2013), Boawn and
Rasmussen (1971); Chaney,
(1993); Weckx and Clijsters
(1997)
Adequate amount
of copper
(Cu) and Cu
1?
,Cu
2?
6lgg
-1
DW Cell wall metabolism
Electron transport in chloroplast, mitochondria etc.
Oxidative phosphorylation and iron mobilization
Biogenesis of molybdenum cofactor
Nitrogen assimilation, abscisic acid (ABA) biosynthesis
Raven et al. (1999), Yruela (2005),
Grata
˜o et al. (2005), Pilon et al.
(2006), Kra
¨mer and Clemens
(2006), Puig et al. (2007), Yruela
et al. (2009)
Deficiency of Cu Below 5 lgg
-1
DW
Improper growth rate and distortion or whitening (chlorosis) of young leaves
Decrease in cell wall formation lignification in several tissues and curling of leaf
margins
Damages apical meristem, fruit formation, pollen development, the fruit and seed
production, wood production
Inhibits embryo development, seed viability and plant development
Marschner (1995), Epstein and
Bloom (2005), Ruiter (1969),
Ku
¨pper et al. (2003), Yruela
(2005), Burkhead et al. (2009a,b)
Toxicity of Cu Above 20 lgg
-1
DW or higher
Chlorosis and necrosis, stunting, and inhibition of root and shoot growth
Inhibit enzyme activity and protein function, which later produces highly toxic
hydroxyl radicals leading to oxidative damage of plant cell
Grata
˜o et al. (2005), Vinit-Dunand
et al. (2002), Ku
¨pper et al.
(2003), Yruela et al. (2009)
139 Page 6 of 14 Acta Physiol Plant (2015) 37:139
123
Ghany and Pilon 2008). Apart from this, some copper
proteins that are localized in cytoplasm, stroma of
chloroplast, peroxisomes and other plant organelles act as
the most effective scavenger of reactive oxygen species
(Yamasaki et al. 2008; Montes et al. 2014). Furthermore,
Rehm and Schmitt (2002) suggested the unique role of Cu
in optimal seed production and chlorophyll formation
which is necessary for optimal enzyme activity, whereas
Bernal et al. (2006) explained the prominent role in thy-
lakoid grana stacking. Additionally, Cu plays important
part in nitrogen assimilation, abscisic acid (ABA) biosyn-
thesis. It was also confirmed by Kuper et al. (2004) that Cu
ion provisionally occupies the site for Mo lodging in the
bound molybdopterin substrate (Burkhead et al. 2009a,b).
Copper (Cu) levels in plants have been reported as
2–50 lgg
-1
DW (ppm) considering 6 lgg
-1
to be suit-
able for shoots (Epstein and Bloom 2005). Cu level below
5lgg
-1
DW in vegetative tissues shows its deficiency
while above 20 lgg
-1
DW, toxicity level of Cu has also
been noticed (Marschner 1995; Burkhead et al. 2009a,b).
Below 5 lgg
-1
of Cu level, several effects of deficiency
can be noticed such as improper growth rate, distortion or
whitening (chlorosis) of young leaves, decrease in cell wall
formation and lignification in several tissues. It causes
curling of leaf margins, damages apical meristem and
badly hampers fruit formation, pollen development, the
fruit and seed production, wood production and inhibits
embryo development, seed viability and plant development
(Ruiter 1969; Marschner 1995;Ku
¨pper et al. 2003; Epstein
and Bloom 2005; Yruela 2005; Burkhead et al. 2009a,b).
Copper (Cu) level above 20 lgg
-1
DW or higher is
proven to be toxic for plant growth and development as it
causes inhibition of root and shoot growth, stunting,
chlorosis and necrosis (Yruela 2005; Hossain et al. 2012).
By binding to sulfhydryl groups in proteins, they inhibit
enzyme activity and protein function which later produces
highly toxic hydroxyl radicals leading to oxidative damage
of plant cell (Yruela 2005; Hossain et al. 2012). Hence
from the above it could be well documented that for the
healthy growth and development of plants, proper acqui-
sition, assimilation and regulation of Cu should be main-
tained in different cells and organelles (Yruela 2005). This
can only be possible by detailed investigation and
advanced study of application of copper in plants to
explore the still hidden facts (Table 2).
Iron (Fe)
Besides silicon, oxygen, and aluminium, iron (Fe) is typi-
cally regarded as the fourth most abundant and virtually
essential microelement on the earth crust. It belongs to the
4th period and group VIII of the long form of periodic table
with atomic number 26 and atomic weight of 55.845 which
intervene a variety of cellular processes (Puig et al. 2005).
Iron is an indispensable microelement and required by
plant in a tracer amount for their optimal growth and
productivity (Curie and Briat 2003). Gris (1843) firstly
recognized it as an essential micronutrient for plant growth
and also establishes its relative significance in eliminating
the adverse effect of chlorosis in plants (Table 3). Because
of low soil solubility its bioavailability to plants cell in
inorganic form is limited (Chatterjee et al. 2006). Fe-ac-
quisition in plant occurs by two efficient strategies called
Strategy I and Strategy II that operates in different phylo-
genic groups (Romheld and Marschner 1986; Romheld
1987; Ma and Nomoto 1996; Curie and Briat 2003; Sch-
midt 2003;Ma2005; Donnini et al. 2010). Although vas-
cular plant requires relatively a lower concentration of
micronutrient for their optimum development, but its
slighter deficiency (moderate decrease in micronutrient
content) and toxicity (modest increase in micronutrient
content) would lead to the impairment of several physio-
logical and metabolic processes and thus, poses great
constraint to overall crop productivity (Robinson et al.
1999; Schmidt 2003;Ma2005; Donnini et al. 2010).
Iron is found to be localized inside the different cel-
lular compartments such as chloroplasts, mitochondria,
and vacuoles (Jeong and Guerinot 2009; Adamski et al.
2012;Viganietal.2013). It also acts as a redox cofactor
in a variety of plant cellular metabolism (Puig et al.
2005). Iron (Fe) is an unavoidable and one of the most
prominent constituent of a number of proteins and
enzymes that plays important roles in key metabolic
processes, including cellular respiration, oxygen trans-
port, lipid metabolism, the tricarboxylic acid (TCA)
cycle, gene regulation, synthesis of metabolic intermedi-
ates, and DNA biosynthesis as well as making it essential
for photosynthesis and chlorophyll biosynthesis (Jeong
and Connolly 2009; Adamski et al. 2012). To organize the
range of physiological and metabolic function in a con-
vectional manner and to minimize nutritional disorder
both the abundance and deficiency of micronutrients
should be maintained properly. Therefore, to conquer the
ill effects imposed by iron deficiency, plants are equipped
with an efficient and promising tolerance strategies that
make their survival possible in such stressful conditions
and such adapted mechanism of plants facilitates the
controlled uptake of iron, the process termed as iron
homeostasis (Marschner et al. 1986; Jeong and Guerinot
2009; Ramirez et al. 2009;Wangetal.2012). However,
homeostasis of this metal is essential for plant growth and
development, because in several studies it has been
demonstrated that it seems to be harmful when present in
both excessive and limiting amounts (Sharma 2007;
Adamskietal.2012;Wangetal.2012). The main and one
Acta Physiol Plant (2015) 37:139 Page 7 of 14 139
123
of the most visually appeared characteristics features of
iron deficiency is chlorosis in young leaves, which is
caused by decreased chlorophyll biosynthesis (Sharma
2007;Wangetal.2012). Several reports have shown that,
those structural component that permit iron to act as an
efficient catalyst and cofactor in cellular redox reactions,
also have tendency to make it a potent toxin on similar
structural chemical properties, when it is up taken by
plants in excess of cellular needs, and as a consequence
lead to overproduction of toxic oxygen radicals (Olaleye
et al. 2009; Gill and Tuteja 2010;Sharmaetal.2012).
Moreover, elevated iron concentrations lead to enhanced
oxidative stress and the excessive production of highly
reactive and toxic oxygen species (ROS) (Robello et al.
2007;Sharmaetal.2012).
ROS are extremely destructive in nature because they
seriously pose a threat to a variety of cellular components,
including lipids, proteins, carbohydrates, and nucleic acids,
thus due to their destructive activity leading to express
diverse morphological, biochemical, and physiological
alterations (Fang et al. 2001; Gill and Tuteja 2010; Sharma
et al. 2012). In the case of excess iron, one of the ways to
limit progressive oxidative damage is to stop the uncon-
trolled oxidation caused by antioxidant enzymes. Super-
oxide dismutase (SOD) plays a protective role against the
damaging effects of ROS that requires Fe, Mn, Cu, and Zn
as metal cofactors and is present in various cellular com-
partments, and is involved in the detoxification of O
2
-
to
H
2
O
2
and O
2
(Sinha and Saxena 2006; Sharma et al. 2012).
In addition to SOD, CAT and peroxidases have been
revealed to participate in this protective mechanism (Costa
et al. 2005; Sharma et al. 2012). Accelerated concentration
of iron affects the uptake and accumulation of other min-
eral nutrients such as calcium (Ca), magnesium (Mg),
potassium (K), phosphorus (P), and of iron itself (Zhang
et al. 1999). Hence from the above mentioned fact and
recent findings, it can be well said that the role of iron in
plant metabolism is indispensible and hence efforts should
be made to abolish iron (Fe) stress (deficiency or toxicity)
for avoiding several nutritional disorders.
Manganese (Mn)
Manganese (Mn), after iron (Fe) is being recognized as one
of the most essential and ubiquitously distributed
microelement on the earth crust belonging to the group VII
of the extended form of periodic table (Kluwer et al. 2010).
In nature, it occurs in a variety of oxidation states 0, II, III,
IV, VI but the most preferable range in biological system is
II, III and IV (Hebbern et al. 2009). Bioavailability of
manganese in nature is basically affected by the redox
condition and pH level of the soil (Marschner 1995;
Table 3 List of certain essential micronutrients whose variable distribution in nature directly or indirectly influences the sustainability of agricultural crops
Microelement (in
variable
composition)
Symbol and the
form of availability
to plants
Concentration
in plants
Probable response of agricultural crops References
Manganese (in
appropriate
composition)
(Mn) and Mn
2?
,Mn
3?
10–100 lgg
-1
Manganese plays a pivotal role in biosynthesis of ATP acyl lipids, proteins and fatty acids. Besides, it
also participates in RuBP carboxylase reactions, oxidation-reduction process of photosynthesis,
photolysis of water at PSII of photosynthesis. Bioactivation of enzymes
Pfeffer et al. (1986), Ness and
Woolhouse (1980), Houtz et al.
(1988)
Manganese
deficiency
Less than
10–100 lgg
-1
Deficiency symptoms: Interveinal chlorosis, in young tissue, appearance of greenish-grey specks at the
lower base of monocots, development of brown necrotic spots on the cotyledons of legume plants,
premature leaf fall, white–grey spots of leaf and delayed maturity
Schulte and Kelling (1999)
Manganese toxicity Above
10–100 lgg
-1
Toxicity symptoms: Higher concentration of Mn interferes with absorption and utilization of other
mineral elements; it also affects the energy metabolism, decreases photosynthetic rates and also causes
oxidative stress
Demirevska-Kepova et al. (2004),
Clark (1982), Fecht-Christoffers
et al. (2003)
Iron (in adequate
concentration)
(Fe) and Fe–S–Fe,
heme, nonheme,
Fe
2?
,Fe
3?
50–150 lgg
-1
Iron plays a pivotal role in biosynthesis of chlorophyll, bioactivation of certain enzymes Being a part of
protein ferodoxin, Fe is required in sulphate and nitrate reduction, also associated with protein
metabolism
Schulte and Kelling (1999),
Chatterjee et al. (2006)
Iron (Fe) deficiency Less than
50–100 lgg
-1
Deficiency symptoms: Interveinal chlorosis in young leaves, caused by decreased chlorophyll synthesis,
retarded/stunted growth and reduced activity of hill reaction
Goos et al. (2004), Nikolic and
Kastori (2000)
Iron (Fe) toxicity Above
50–100 lgg
-1
Toxicity symptoms: Growth inhibition, reduced chlorophyll synthesis, inhibition of photosynthesis Chatterjee et al. (2006)
139 Page 8 of 14 Acta Physiol Plant (2015) 37:139
123
Porter et al. 2004; Millaleo et al. 2010a,b). Though its
concentration inside the soil solution is relatively large but
plant acquires only a small fraction of it for their optimum
growth and development (Kluwer et al. 2010).
Manganese plays a pivotal role not only in variety of
metabolic process but is also involved directly or indirectly
in stress tolerant mechanism of higher plants by serving as
the cofactor of various antioxidative enzymes (Burnell
1988). Furthermore, the role of manganese (Mn) in pho-
tosynthesis is indispensable as it participate in photolysis of
water at photosystem II that provides electrons needed for
the onset of electron transport system (Millaleo et al.
2010a,b).
Kenneth (2012) significantly demonstrated the efficient
role of manganese during oxygen evolution step of pho-
tosynthesis. Further it has well documented that manganese
is significantly involved in the biosynthesis of ATP, acyl
lipids, proteins and fatty acids besides, it also participate
in RuBP carboxylase reactions (Ness and Woolhouse
1980; Pfeffer et al. 1986; Houtz et al. 1988; Millaleo et al.
2010a,b).
Being the structural constituent of photosynthetic pro-
teins and enzyme it is predominantly involved in the
bioactivation of approximately 35 crucial enzymes of
plants, such as: Mn-SOD, Mn-CAT, phosphoenol pyruvate
carboxy kinase and pyruvate carboxylase (Burnell 1988;
Ducic and Polle 2005). Biogenesis of chlorophyll, amino
acid (tyrosine) and several other secondary metabolites
such as flavenoids and liginin are another eminent role of
manganese in plants (Lidon et al. 2004). Millaleo et al.
(2010a,b) signifies the profitable role of manganese and
also ascribed their distribution, accumulation and resistant
mechanism inside the plant when its concentration inside
the soil solution exceeds the permissible limit, it becomes
severely toxic to plant cell, thereby limiting its growth and
yield productivity worldwide.
Excessive-limed soil or the soil rich in organic matter
(above 6.0 %) with high pH level (more than 6.5) are
primarily responsible for the prevalence of manganese
deficiency (Reichman 2002; Demirevska-Kepova et al.
2004; Millaleo et al. 2010a,b). But the uptake and accu-
mulation of manganese inside the plant shows adverse
relation with the available iron content of soil (Demir-
evska-Kepova et al. 2004; Millaleo et al. 2010a,2010b). As
a trace element it’s over expression led the generation of
highly reactive oxygen species (ROS) particularly OH
,
that
ultimately triggers oxidative injuries inside the plant
(Demirevska-Kepova et al. 2004; Millaleo et al. 2010a,b).
Likewise, the appearance of interveinal chlorosis, brown
necrotic spots, premature leaf fall, white and gray spots of
leaf and delayed maturity is another characteristic deficiency
symptom of Mn resulting from its limited supply inside the
plant. Therefore, its proper optimization inside the plant is
necessary to further lessen the exogenous demand of fertil-
izers. Although the potentially significant physiological and
biological role of manganese is well elucidated from the
above studies however, further investigation is still needed
with the insight of improving gross productivity and main-
taining the human/plant nutrient status on a global scale.
Zinc (Zn)
The essentiality of zinc (Zn) as a micronutrient in plant is
phenomenal, bearing atomic number 30, Zn is another
transition element and observed as the 23rd most copious
element on earth with five stable isotopes (Broadley et al.
2007). Zn
2?
has distinct characteristics of Lewis acid and
also considered to be the redox-stable due to completely
filled d-shell orbitals by electron unlike in Fe
2?
and Cu
2?
(Barak and Helmke 1993; Auld 2001; Broadley et al. 2007;
Sinclair and Kra
¨mer 2012; Hafeez et al. 2013). Broadley
et al. (2007) mentioned that idea of importance of Zn in
plants was originated for the first time when its significance
was shown by Maze
´(1915) in maize and barley and dwarf
sunflower by Sommer and Lipman (1926). Interestingly,
Zn plays eminent role by being a structural constituent or
regulatory co-factor for different enzymes and proteins. At
organism level, the significant role of ‘zinc finger’ as a
structural motif is worth mentioning as it regulates tran-
scription (Klug 1999; Englbrecht et al. 2004; Broadley
et al. 2007). The optimal crop growth is generally main-
tained by intake of Zn in its divalent form. Henceforth,
performing several important functions in different plants
which can be enumerated as:
•Regulation of carbonic anhydrase for fixation to
carbohydrates in plants (Carbon dioxide ?reactive
bicarbonate species).
•Promotion of the metabolism of carbohydrate, protein,
auxin, pollen formation (Marschner 1995) etc.
•Governs biological membranes and performs defence
mechanism against harmful pathogens.
•Presence of Zn in SOD and CAT as a cofactor, protects
plant from oxidative stress.
•The fundamental attribute of Zn is being the component
of all the six enzyme classes’ ?oxidoreductases,
transferases, hydrolases, lyases, isomerases, ligases.
Additionally, Zn being participatory in the structure of
Rubisco, activates several biochemical reactions in the
photosynthetic metabolism (Brown et al. 1993; Alloway
2004a,b; Tsonko and Lidon 2012). In the thylakoid
lamellae, Zn inhibits the production of high toxic hydroxyl
radicals in Haber–Weiss reactions due to its high affinity
with cysteine and histidine (Cakmak 2000; Alloway 2004a,b;
Brennan 2005; Disante et al. 2010; Tsonko and Lidon 2012).
Acta Physiol Plant (2015) 37:139 Page 9 of 14 139
123
Furthermore, Alscher et al. (1997) and later Cakmak
(2000) confirmed the savior role of Zn against oxidative
stress by being involved in multiple antioxidative enzymes
such as ABX and glutathione reductase. The availability of
water to plant has also been noticed to be affected by Zn
(Barcelo
´and Poschenrieder 1990; Kasim 2007; Disante
et al. 2010; Tsonko and Lidon 2012). Pahlsson (1989) and
Coleman (1992) suggested formation of complexes of Zn
with DNA and RNA. The tryptophan synthesis and active
role in signal transduction are also some of the valuable
functions of Zn reported so far (Brown et al. 1993; Alloway
2004a,b; Lin et al. 2005;Ha
¨nsch and Mendel 2009).
Nonetheless, prominent participation of Zn in regulation of
membranes by combining with phospholipids and sul-
phydryl groups of membrane proteins is also equally
important to be known.
The sufficient Zn concentration required for proper
growth of plant is estimated to be 15–20 mg Zn kg
-1
DW
as mentioned by Marschner (1995). Deficiency of Zn has
been reported below this level in several research works.
For instance, Skoog (1940) observed the disturbances in
stem elongation in tomato.
Several other symptoms and responses of plants towards
Zn deficiency are as follows:
•Necrosis at root apex and inward curling of leaf lamina,
•Mottled leaf due to inter veinal chlorosis,
•Bronzing and internodes shortening as well as size
reductions in leaf.
While on exposure of leaf with elevated level of Zn i.e.
above 0.2 mg g
-1
dry matter, multiple abnormal func-
tioning in plant can be observed. This toxicity level gives
rise to deterioration of leaf tissue and at the same time
decline the productivity of plant by making their growth
stagnant. Sensitivity towards toxic Zn concentration has
also been noticed in Soya bean and Rice. Boawn and
Rasmussen (1971) and Chaney (1993) took chance to show
the effect of Zn toxicity in leafy vegetable crops as they
tend to accumulate high concentration of Zn noticed in
spinach and beet. Therefore, it can be interpreted that,
although Zn is toxic at excess level, it is an indispensable
component of thousands of proteins in plants. Hence,
adequate supply of Zn is one of the prime-most demands
for plants growth and development which can be reached
by detail research work on understanding the concept of
application, acquisition and assimilation of Zinc in plants.
Conclusion and future outlook
Micronutrients, though required in relatively tracer amount
(at \100 mg kg
-1
dry weight) by plants, play a virtually
significant role in a variety of cellular and metabolic
processes such as, gene regulation, hormone perception,
energy metabolism and signal transductions etc. Based on
their precise requirement in higher plants, boron (B),
chloride (Cl), copper (Cu), iron (Fe), manganese (Mn),
molybdenum (Mo), nickel (Ni), and zinc (Zn) are typically
classified under essential micronutrients. Each organism
requires an adequate supply of these micro elements which
subsequently requires a complete metal homeostasis net-
working including their uptake and accumulation inside the
plant, mobilization, storage and intracellular trafficking etc.
(Ha
¨nsch and Mendel 2009). Insufficient supply or low
phyto-availability of these elements would result in limited
crop productivity worldwide. Therefore, plants require an
requisite and constant supply of these micronutrients
throughout their entire growth phase for optimal produc-
tivity. But, the over expanding human population and
mindless exploitation of natural repositories makes it dif-
ficult for the plants to ensure their adequate supply for
future reference and therefore, causing impulsive challenge
for the expertise of science. Recently, it has been well
elucidated that approximately 2/3 of the world’s population
is being suffering from the risk of nutrient-deficiency
(White and Broadley 2009; Stein 2010). However, the
marked deficiency of mineral nutrient could be reduced by
the judicious exogenous supply of mineral fertilizers or by
the cultivation genotypically modified crops (GM crops)
with higher metal concentrations. In addition, crop hus-
bandry, breeding or genetic manipulation could also be
recognized as one of the most efficient, recent and reliable
technique of improving mineral status of soil (White and
Broadley 2009). For sure, these above mentioned approa-
ches can definitely create new horizon in the field of
micronutrient application in plant and crop sciences, but in
order to achieve greater successful results, more advanced
and scientific research works on this deep topic are nec-
essarily required.
Author contribution statement DKT, SS, SS, DKC, SM
and NKD designed the manuscript, DKT, SS, SS and DKC
wrote the manuscript.
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