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Electronic Journal of Biotechnology
Universidad Católica de Valparaíso
ISSN: 0717-3458
Vol. 12, Num. 2, 2009
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Electronic Journal of Biotechnology, Vol. 12, No. 2, April 15, 2009
Review Article
AgNO3 - a potential regulator of ethylene activity and
plant growth modulator
Vinod Kumar1 , Giridhar Parvatam2, Gokare Aswathanarayana
Ravishankar*3
1Plant Cell Biotechnology
Department,
Central Food Technological
Research Institute,
Mysore-570 020,
Karnataka State, India
2Plant Cell Biotechnology
Department,
Central Food Technological
Research Institute,
Mysore-570 020,
Karnataka State, India
3Plant Cell Biotechnology
Department,
Central Food Technological
Research Institute,
Mysore-570 020,
Karnataka State, India
Tel: 91 821 2516501
Fax: 91 821 2517 233
E-mail: pcbt@cftri.res.in
*Corresponding author
Financial support: Department of Biotechnology, Government of India.
Received March 6, 2006
/ Accepted August 7,2008
Code Number: ej09015
Abstract
The aim of
this review is to critically analyze the role of silver nitrate (AgNO3)
in modulating plant growth and development. In recent years, basic studies on
ethylene regulation opened new vistas for applied research in the area of micro-propagation,
somatic embryogenesis, in vitro flowering, growth promotion, fruit
ripening, and sex expression. Silver nitrate has proved to be a very potent
inhibitor of ethylene action and is widely used in plant tissue culture. Few
properties of silver nitrate such as easy availability, solubility in water,
specificity and stability make it very useful for various applications in
exploiting plant growth regulation and morphogenesis in vivo and in
vitro. Silver ion mediated responses seem to be involved in polyamines,
ethylene- and calcium- mediated pathways, and play a crucial role in regulating
physiological process including morphogenesis. The molecular basis for
regulation of morphogenesis under the influence of silver nitrate is completely
lacking. This review compiles published reports of silver nitrate-mediated in
vitro and in vivo studies and focuses on fundamental and applied
aspects of plant growth modulation under the influence of silver nitrate.
Keywords: calcium, ethylene, morphogenesis, polyamines, silver nitrate, somatic
embryogenesis.
Abbreviations: |
ACC: 1-Amino-cyclopropane-1-carboxylic
acid
ADC: arginine decarboxylase
AgNO3: silver nitrate
AVG: aminoethoxyvinylglycine
BA: 6-benzylaminopurine
DFMA: α-DL-difluromethyl arginine
DFMO: α-DL-difluromethyl ornithine
IAA: indole acetic acid
NAA: α-naphthalene acetic acid
ODC: ornithine decarboxylase
PA: polyamine
Put: putrescine
SAM: S-adenosyl-L- methionine
STS: silver thiosulphate
Spd: spermidine
Spm: spermine |
In recent
years, advances in plant genetic engineering have opened new avenues for crop
improvement and various plants with novel agronomic traits have been produced.
The success of plant genetic engineering relies on several factors which
include an efficient tissue culture system, for regeneration of plants from
cultured cells and tissues (Pua et al. 1996). Shoot generation and rooting are
important in the realization of the potential of the cell and tissue culture
techniques for plant improvement (Purnhauser et al. 1987). Silver ions in the
form of nitrate, such as AgNO3, play a major role in influencing
somatic embryogenesis, shoot formation and efficient root formation which are
the prerequisites for successful genetic transformation (Bais et al. 1999; Bais et al. 2000a; Bais et al. 2000b; Bais et al.
2001a; Bais et al. 2001b; Bais et al. 2001c). Silver ions are also
employed in the form of silver thiosulphate in several tissue culture studies (Eapen
and George, 1997).
Ethylene
is recognized as a ubiquitous plant hormone (Lieberman,
1979; Yang,
1985), which influences growth
and development of plants (Abeles, 1973; Yang
and Hoffman, 1984; Mattoo and
Suttle, 1991). In vitro studies have indicated that ethylene
can affect callus growth, shoot regeneration and somatic embryogenesis in
vitro (Purnhauser et al. 1987; Songstad
et al. 1988; Roustan et al. 1989; Roustan
et al. 1990; Biddington, 1992; Pua
and Chi, 1993). Thus, by regulating the
production or action of ethylene, the growth and development of some
tissue cultures can be controlled to a certain extent (Beyer,
1976c; Davies, 1987; Purnhauser
et al. 1987; Songstad
et al. 1988; Chi and Pua, 1989; Bais
et al. 2000a; Giridhar et al. 2003).
AgNO3 has
been known to inhibit ethylene action (Beyer, 1976a)
and cobaltous ions are known to inhibit ethylene synthesis (Lau
and Yang, 1976) (Figure 1). Silver ion
is capable of specifically blocking the action of exogenously applied
ethylene in classical responses such as abscission, senescence
and growth retardation (Beyer, 1976c). These
observations led to its application in tissue culture. Addition
of AgNO3 to the culture media greatly improved the
regeneration of both dicot and monocot plant tissue cultures (Beyer,
1976c; Duncan et al. 1985; Davies, 1987; Purnhauser
et al. 1987; Songstad et al.
1988; Chi and
Pua, 1989; Veen
and Over Beek, 1989; Bais
et al. 2000a; Giridhar et al. 2003). The exact mechanism of AgNO3 action
on plants is unclear. However, few existing evidences suggest its
interference in ethylene perception mechanism (Beyer,
1976c). In recent years, AgNO3 has been employed in
tissue culture studies for inhibiting ethylene action because of
its water solubility and lack of phytotoxicity at effective concentrations
(Beyer, 1976a).
Ethylene
Ethylene
is a gaseous plant hormone involved in many aspects of plant life
cycle (Figure 1) such as seed
germination, root hair development, root nodulation, flower senescence,
abscission, and fruit ripening (Johnson and Ecker,
1998; Bleecker and Kende,
2000). Its biosynthesis (Wang et al. 2002)
is tightly regulated by internal signals and environmental stimuli
from biotic and abiotic stresses, such as pathogen attack, wounding,
hypoxia, ozone, chilling, or freezing (Wang et al.
2002). The role of ethylene in morphogenesis has been well documented
in an earlier review (Kumar et al. 1998a). Mutants
have also been identified that display a constitutive triple response
in the absence of ethylene (Kieber et al.
1993). This can be divided into subgroups based on whether or
not the constitutive triple response can be suppressed by inhibitors
of ethylene perception and biosynthesis, such as silver ions and
aminoethoxyvinyl glycine (AVG). Mutants that are unaffected by these
inhibitors are termed constitutive triple-response (ctr)
mutants, whereas mutants whose phenotype reverts to normal morphology
are termed ethylene-overproducer (eto) mutants, which are defective
in the regulation of hormone biosynthesis. To date, data is lacking
on the molecular basis for silver ion interaction with the mutants,
which are insensitive to ethylene.
Ethylene biosynthesis
To understand
the role of silver ions in regulating morphogenesis, it is important to know
the aspects of ethylene biosynthesis (Figure 1). The biochemistry of
ethylene biosynthesis has been a subject of intensive study in plant hormone
physiology (reviewed by Wang et al. 2002). In brief, the biosynthesis of
ethylene starts with conversion of the amino acid methionine to S-adenosyl-L-
methionine (SAM, also called Adomet) by the enzyme Met Adenosyltransferase. SAM
is subsequently converted to 1-aminocyclopropane-1-carboxylic-acid (ACC) by the
enzyme ACC synthase (ACS). The activity of ACS is the rate-limiting step in
ethylene synthesis. The final step requires oxygen and involves the action of
the enzyme ACC-oxidase (ACO), formerly known as the ethylene forming enzyme
(EFE) (Wang et al. 2002).
A major
breakthrough in the ethylene synthesis pathway was the establishment of
S-adenosylmethionine (S-AdoMet) and ACC as the precursors of ethylene (reviewed
in Yang and Hoffman, 1984; Kende, 1993). On the basis of this knowledge, the
enzymes that catalyze these reactions were characterized and purified. The
first successes in molecular cloning of the ACC (Sato and Theologis, 1989) and
ACO (Hamilton et al. 1991; Spanu et al. 1991) genes led to the demonstration of
these enzymes belonging to a multi-gene family and are regulated by a complex
network of developmental and environmental signals responding to both internal
and external stimuli (reviewed by Johnson and Ecker, 1998). In addition to
being an essential building block of protein synthesis, nearly 80% of cellular
methionine is converted to S-AdoMet by S-AdoMet synthetase (SAM synthetase) at
the expense of ATP utilization (Ravanel et al. 1998). S-AdoMet is the major
methyl donor in plants and is used as a substrate for many biochemical
pathways, including polyamines and ethylene biosynthesis (Ravanel et al. 1998).
Ethylene
signal perception
Ethylene is
perceived by a family of five membrane-localized receptors that are homologous
to bacterial histidine kinases involved in sensing environmental changes (Figure 2). Ethylene binding occurs at the N-terminal transmembrane domain of the
receptors, and a copper co-factor is required for the binding. The system
typically consists of a histidine kinase as the sensor that autophosphorylates
an internal histidine residue in response to environmental signals, and a
response regulator that activates the downstream components upon receiving a
phosphate from the histidine residue of the sensor on its aspartate residue (Wurgler-Murphy
and Saito, 1997). Five ethylene receptors exist in Arabiodpsis: ETR1,
ETR2, ERS1, ERS2, and EIN4 (Chang et al. 1993; Hua et al. 1995; Hua and
Meyerowitz, 1998; Sakai et al. 1998). Further characterization of ethylene
binding to ETR1 has revealed that it occurs at the hydrophobic pocket located
at the N- terminus of the receptors and requires a transition metal, copper, as
a co-factor (Figure 2) (Schaller and Bleecker, 1995; Rodriguez et al.
1999; Wang et al. 2002). Further findings indicated that
RAN1 is involved in the delivery of copper to the ethylene receptor and
that this copper-delivery pathway is required to create functional ethylene
receptors in plants (Figure 2) (Wang et al. 2002). Cu ions are also
known to form complexes with ethylene (Coates et al. 1968). But the studies of Beyer (1976c) revealed that the
effect of silver ions could be explained on the basis that silver ions
substitute for Cu ions, thereby interfering with ethylene action. This may be
due to the similarity in size, the same oxidation state, and the ability of
both Cu ion and Ag ion to form complexes with ethylene (Coates et al. 1968).
The possibility of the anti-ethylene property of silver was later well explored
in various plant systems. At present there are no concrete evidences to show
the involvement of silver ions with signaling networks which leads to down
regulation of physiological responses governed by ethylene. Therefore, focus on
the elucidation of molecular basis for diverse developmental process in plants
such as abscission, flowering, fruit ripening, morphogenesis and sex
expression, that are known to be regulated by silver ions, would be
interesting.
Possible
mechanisms of action of silver nitrate on ethylene action inhibition
Silver ions are capable
of generating ethylene insensitivity in plants (Zhao et al. 2002).
Ethylene-insensitive mutations (Hall et al. 1999) and silver ions are thought
to perturb the ethylene binding sites (Rodriguez et al. 1999). The ethylene
receptor, ETR1, contains one ethylene-binding site per homodimer and binding is
mediated by a single copper ion (Cu) present in the ethylene-binding site. The
replacement of the copper co-factor by silver also serves to lock the receptor
into a conformation such that it continuously represses ethylene responses
(Zhao et al. 2002).
There are
different views and experimental evidences on this subject. According to one
view, the ethylene action in plants is inhibited by week antagonists such as CO2 and strong antagonists like silver compounds. This is possibly due to oxidation
of ethylene by a metal-ion enzyme system (Abeles, 1973). In Arabidopsis,
insensitivity to ethylene is conferred by dominant mutation in receptors
(Bleecker et al. 1988). Another hypothesis is that AgNO3 inhibits
ethylene action by means of silver ions by reducing the receptor capacity to
bind ethylene (Yang, 1985), which would result in higher titers of ethylene in
the tissues, thus inhibiting the earlier steps of its own pathway. Miyazaki and
Yang (1987) reported the influence of putrescine and AgNO3 on the
competitive utilization of SAM. Bais et al. (2000b) also postulated that the
utilization of SAM by putrescine for its conversion to spermidine would
possibly result in a lower availability of SAM for ethylene biosynthesis (Figure 3). The introduction of ethylene antagonists into the culture media affects
the level of ACC, thereby affecting ethylene levels (Gong et al. 2005).
Polyamines
Other
important substances responsible for regulation of morphogenesis
are polyamines. The polyamines (PAs) are organic compounds having
two or more primary amino groups. Polyamines have been implicated
in several important cellular processes like cell division, morphogenesis,
protein synthesis, DNA replication, and plant response to abiotic
stress (Tabor and Tabor, 1984; Smith,
1985; Smith, 1993; Van Den
Broeck et al. 1994; Walden
et al. 1997; Kumar and Rajam, 2004). They bind
to DNA, and are essential for cell viability (Flink
and Pettijohn, 1975). Polyamines
are also known to be involved in DNA helix stabilization, stabilization
of loops in RNA molecules, membrane permeability, DNA replication,
cell division, gene expression, regulation of enzyme activities,
membrane stabilization, morphogenesis, fruit ripening etc. (Bais
and Ravishankar, 2002; Kumar and
Rajam, 2004). It has been postulated that polyamines and related
compounds are a type of growth regulator or secondary hormonal messenger
(Galston, 1983; Davies, 1987). PAs are found in plant cells at
significantly higher levels than plant hormones. There is evidence
that PAs are taken up by cell suspension cultures (Evans
and Malmberg, 1989). Interestingly, it seems that there is a
strong link between ethylene, polyamines, and calcium-mediated signaling.
This triangle is expected to be a potential target for silver ions.
This is because both ethylene and polyamines are metabolically related
(Figure 3)
and utilize the same precursor, SAM, for their synthesis (Evans
and Malmberg, 1989; Bais and Ravishankar, 2002).
It has also been suggested that polyamines and ethylene may regulate
each other’s synthesis. For instance, ethylene has been shown
to inhibit arginine decarboxylase and S-adenosyl methionine
decarboxylase activities in pea seedlings (Apelbaum
et al. 1985). These enzymes are necessary
for polyamine synthesis (Smith, 1985). It has
been proved beyond doubt that polyamines play crucial roles in plant
growth and development as well as basic biological process (reviewed
by Kumar and Rajam, 2004). Since polyamines have
been reported to promote embryogenesis (Feirer et al.
1984), the promotive
effect of ethylene inhibitors, such as AgNO3, on regeneration
was thought to be due to enhanced polyamine synthesis rather than
reduced ethylene production. Pua et al. (1996) clearly
described the synergistic effect of AgNO3 and putrescine
on shoot regeneration in Chinese radish. Miyazaki and
Yang (1987) reported the influence of putrescine and AgNO3 on
the competitive utilization of SAM. Bais et al. (2000b) postulated
that, utilization of SAM by putrescene for its conversion to spermidine
would possibly result in a lower availability of SAM for ethylene
biosynthesis (Figure 3). On the other hand, Pua
and Chi (1993) also
reported the same stimulatory effect of AgNO3 feeding
on ethylene production and its contribution to increased titers of
polyamines in mustard. Polyamines also regulate the growth and secondary
metabolism (Bais et al. 1999; Bais et al. 2001b; Bais and
Ravishankar, 2002). Reports on somatic embryogenesis in carrot
(Roustan et al. 1990; Nissen, 1994) indicate that the potent ethylene
action inhibitor, AgNO3,
causes the increase of ADC activity, which in turn increases the
levels of endogenous polyamines in carrot embryogenic cultures.
Involvement
of calcium in polyamine-mediated response
Polyamines
are associated with Ca2+ ions in signaling events (Majewska-Sawka
et al. 1998). They supported the hypothesis of transportation
of spermidine/spermine within protoplasts through a carrier-mediated
mechanism (Antognoni et al. 1994, Majewska-Sawka
et al. 1997). Majewska-Sawka et al.
(1998) found that spermidine/spermine may result in change in
distribution of Ca2+ ions. It is reasonable to conclude
that Ca2+ ions may be involved in the
mechanism of polyamine action in plant cells (Bush,
1995). This aspect is very
relevant here because, apart from ethylene regulation, silver nitrate
is known to regulate the polyamine pool in plant systems.
Application
of silver nitrate in plant tissue culture
So far we have
discussed the possible mechanisms of regulation of morphogenesis by silver
nitrate. Interestingly, a large number of reports are accumulating on the
utility of silver nitrate in tissue culture and other applications, with
significant contributions towards the development of plant biotechnology and
transgenic research. The following section deals with a brief compilation of
published research pertaining to the effect of silver nitrate in plant
morphogenesis (Table 1).
Table
1. Composition of medium in transformation experiment. |
Plant |
Response |
Reference |
Albizzia julibrissin |
In vitro shoot formation |
Sankhla et al. 1995 |
Andrographis paniculata |
Somatic embryogenesis |
Martin, 2004 |
Apple |
Higher efficiency of regeneration and transformation |
Seong et al. 2005 |
Arachis hypogea |
Regeneration |
Pestana, 1999 |
Arachis hypogea |
Multiple shoot formation |
Ozudogru et al. 2005 |
Albizzia procera |
Plant regeneration |
Kumar et al. 1998b |
Bactris gasipaes |
Enhance embryogenic competence |
Steinmacher
et al. 2007 |
Brassica campastris |
Shoot regeneration |
Palmer, 1992 |
Brassica juncea |
Microspore embryogenesis |
Prem et al. 2005 |
Capsicum annuum |
Shoot development and plant regeneration |
Hyde and Phillips, 1996 |
Cicer arietinum |
Somatic embryo induction |
Patil et al. 1999 |
Cichorium intybus |
Shoot length, shoot number, flowering |
Bais
et al. 2001a; Bais et al. 2001b; Bais et al.
2001c |
Coffea arabica |
Shoot growth
Somatic embryogenesis
Direct somatic embryogenesis |
Ganesh
and Sreenath, 1996
Giridhar et al. 2003
Giridhar et al. 2004 |
Coffea canephora |
Shoot growth
Somatic embryogenesis
Direct somatic embryogenesis |
Giridhar et al. 2003
Giridhar
et al. 2004; Kumar et al. 2007
Fuentes et al. 2000
|
Cucumis sativus |
Shoot regeneration
Sex expression |
Mohiuddin et al. 1997
Atsmon and Tabbak, 1979 |
Daucus carota |
Somatic embryogenesis |
Nissen et al. 1994 |
Decalepis hamiltonii |
In vitro root formation and shoot formation |
Bais
et al. 2000a; Bais et al. 2000b; Reddy et al.
2001 |
Egyptian maize |
Enhanced the formation of embryogenic type II callus |
El-Itriby et al. 2003 |
Eleusine coracana |
Plant regeration |
Kothari-Chajer et al. 2008 |
Glycine max |
Shoot formation on hypocotyls |
Wang and Xu, 2008 |
Gossypium sp. |
Leaf abscission
Enhance multiple shoot production from hypocotyl
segments |
Beyer,
1976a
Divya et al. 2008 |
Helianthus annuus |
Regeneration and shoot organogenesis |
Chraibi et al. 1991 |
Hordium vulgare |
Somatic embryogenesis
Plant regeneration |
Castillo
et al. 1998
Jha et al. 2007 |
Ipomoea batatas |
Shoot
regeneration |
Gong
et al. 2005 |
Manihot esculenta |
Shoot
organogenesis |
Zang et al. 2001 |
Morus Alba |
Modification of sex expression |
Thomas, 2004 |
Nicotiana plumbaginifolia |
Shoot regeneration |
Purnahauser et al. 1987 |
Oryza sativa |
Androgenesis |
Lentini et al. 1995 |
Paspalum scrobiculatum |
Plant regenerarion |
Kothari-Chajer et al. 2008 |
Passion fruit |
Regeneration |
Reis et al. 2003 |
Penisittum glaucum |
Plant regeneration |
Oldach et al. 2001 |
Penisittum glaucum |
Regeneration from zygotic embryos |
O'Kennedy et al. 2004 |
Pennisettum americanum |
Plant regeneration |
Plus et al. 1993 |
Phaseolus vulgaris |
Shoot development |
Cruz de Carvalho et al. 2000 |
Phoenix dactylifera |
Somatic embryogenesis |
Al-Khayri
and Al-Bahrany, 2001; Al-Khayri and
Al-Bahrany, 2004 |
Picea glauca |
Somatic embryogenesis |
Kong and Yeung, 1994 |
Picea glauca |
Maturation of somatic embryos |
El Meskaoui et al. 2000 |
Pisum sativum |
Shoot growth |
Beyer, 1975 |
Punica granatum |
Adventitious shoot regeneration. |
Naik and Chand, 2003 |
Quassia amara |
somatic embryogenesis |
Martin and Madassery, 2005 |
Raphanus sativus |
Shoot regeneration |
Pua et al. 1996 |
Rubus sp |
Inhibits callous formation during shoot
multiplication |
Tsao and Reed, 2002 |
Sorghum bicolor |
Plant regeneration |
Oldach et al. 2001 |
Stenotaphrum secundatum |
Embryogenic callus shoot regeneration. |
Fei et al. 2000 |
Tagetes erecta |
Plant growth flowering and seed viability |
|
Triticum aestivum |
Plant regeneration from callus
Improved embryogenic callus frequency |
Yu et al. 2008
Wu et al. 2006 |
Vanilla planifolia |
Shoot growth and in vitro root formation |
Giridhar et al. 2001 |
Vicia faba |
Promoted root formation |
Khalafalla and Hattori, 2000 |
Vigna ungiculata |
In vitro regeneration |
Brar
et al. 1999 |
Zea mays |
Regenerable
callus
Embryogenic
calli from embryo scutellum |
Songstad
et al. 1988
Valdez-Ortiz,
2007 |
Somatic
embryogenesis
Theoretically,
each living plant cell is capable of forming somatic embryos. Somatic
embryos are formed from vegetative plant cells. Applications of
this process include: clonal propagation of genetically uniform
plant material, elimination of viruses, provision of source tissue
for genetic transformation, generation of whole plants from single
cells called protoplasts, and development of synthetic seed technology.
Plant growth regulators in the tissue culture medium can be manipulated
to induce callus formation and subsequently changed to induce embryos
from the callus or directly from intact tissues. The ratio of different
plant growth regulators required to induce callus or embryo formation
varies with the type of plant. The use of silver nitrate
improved somatic embryogenesis in several plant species such as buffalograss
(Fei
et al. 2000), Coffea sp. (Fuentes et al.
2000; Giridhar
et al. 2004),
carrot (Nissen, 1994), white spruce (Kong
and Yeung, 1994), Triticum durum (Fernandez
et al. 1999), and Zea mays (Vain Hort and
Flament, 1989; Vain
Hort et al. 1989; Songstad et al. 1991).
Multiple
shoot induction and shoot regeneration
Silver
nitrate is known to promote multiple shoot formation in different
plants. In vitro shoot formation was improved by incorporating
silver nitrate in the culture medium. Ganesh and
Sreenath (1996) reported in vitro sprouting of apical
buds of Coffea under the influence of AgNO3. The
addition of N6-benzyladenine with AgNO3 greatly enhanced
the rate of sprouting. At low concentration, AgNO3 was
found to cause delayed senescence resulting in improved growth of
the proliferated shoots in Coffea canephora (Fuentes
et al. 2000). AgNO3 enhanced in vitro shoot
growth of C. arabica and C. canephora (Giridhar
et al. 2003). Shoot
regeneration of Chinese radish Cv Red coat was improved when
cultured in media supplemented with 2030 µM AgNO3 (Pua
et al. 1996). Brassica sp. are poorly responsive to tissue
culture manipulations (Narasimhulu
and Chopra, 1988). B. campestris produces high levels
of ethylene causing abnormal growth and development of the plant
in tissue culture conditions (Lentini et al. 1988),
and also inhibits de novo shoot
regeneration in vitro (Chi et al. 1990; Chi
et al. 1991; Palmer, 1992; Pua and Chi, 1993). The cotyledons and hypocotyls
of 7 cultivars belonging to B.
campestris spp. chinensis, spp. pekinensis and spp. parachinensis
exhibited improved shoot regeneration on culture media supplemented
with growth regulators and AgNO3.
The effects of the ethylene precursor,
1-aminocyclopropane-1-carboxylic acid (ACC), and two inhibitors, silver
thiosulfate and aminoethoxyvinylglycine (AVG), were tested in yellow passionfruit (Passiflora
edulis) axillary buds cultured in vitro (Reis et al. 2003). The
organogenesis was assessed by the number of buds per explant, mean leaf area
per explant, and shoot length. ACC-supplemented medium significantly inhibited
all evaluated responses. When ethylene action and biosynthesis were inhibited,
a significant enhancement of buds and leaf area was observed. The results
suggest beneficial effects of silver nitrate on in vitro development of
axillary buds.
Inhibition of ethylene action by
AgNO3 stimulated regeneration of shoots from cotyledon explants of Helianthus
annus (Chraibi et al. 1991). In many plants, the regeneration potential of
cultured cells and tissues decreases with increased cycles of subcultures
(Ogura and Shimada, 1978; Vasil, 1987). This phenomenon is evident in Pennisetum
americanum (Pearl millet) and Plus et al. (1993) effectively addressed this
issue by incorporating AgNO3 in the culture medium to restore the
regeneration potential. AgNO3 enhanced shoot regeneration frequency
in silk tree (Albizzia julibrissin) and Nicotiana plumbaginifolia.
The
work done in our laboratory has shown that exogenous feeding of
putrescine and silver nitrate influenced morphogenesis in chicory
(Chichorium intybus) shoot
cultures (Bais et al. 2000b). Putrescine and AgNO3 induced
shoot multiplication and in vitro flowering. The chicory plants,
which flower biennially, could be forced to flower experimentally
for studies on in
vitro pollination and seed development (Bais et
al. 2000b).
Silver
nitrate was found to be beneficial in the regeneration and clonal
propagation of several economically important plants (Table
1) such as peanut (Pestana et al. 1999),
cowpea (Brar et
al. 1999), Brassica sp. (Eapen and George,
1997; Pua et al. 1999), Capscicum sp.
(Hyde and Philips, 1996, Kumar et
al. 2003a), watermelon (Lim
and Song, 1993), Coffea canephora (Kumar
et al. 2003b), Cucumber (Mohiuddin et al. 1997),
Pomegranate (Naik and Chand, 2003), White marrygold
(Misra and Datta, 2001),
Cassava (Zang et al. 2001), Petunia (Gavinlertvatana
et al. 1980), etc.
In vitro rooting
Decalepis
hamiltonii Wight, Arn (swallow root), belonging to
Asclepiadaceae, is a monogeneric climbing shrub, native of the Deccan peninsula
and forest areas of Western Ghats in India. It is used as a culinary spice due
to its aromatic roots. In vitro root formation is a major issue in the
tissue culture of this plant. Effects of AgNO3 on in vitro root formation of Decalepis hamiltonii were studied. Addition of 40 µM AgNO3 resulted in root
initiation and elongation (Bais et al. 2000a; Reddy
et al. 2001).
Vanilla
is an important spice crop of commercial value. The effect of AgNO3 on
rooting and shooting was elucidated in Vanilla
planifolia (Giridhar et al. 2001). Maximum
number of shoots and highest shoot length was obtained on medium
containing 20 µM AgNO3. AgNO3 not only
induced shoot multiplication but also influenced rooting of vanilla
explants. The plantlets obtained on medium containing 40 µM
AgNO3 exhibited 100% survival. Silver nitrate also
induced rooting and flowering in vitro on the rare, rhoeophytic
woody medicinal plant, Rotula aquatica Lour. Dipping of the
basal end of shoots in NAA (2.69 µM) and silver nitrate (11.7 µM)
solution improved rooting efficiency (Sunandakumari
et al. 2004).
Modification
of sex expression
The inhibition
of ethylene action by silver nitrate was employed to suppress the development
of female flowers and induce male flowers (Beyer, 1976c; Atsmon and Tabbak,
1979; Takahashi and Jaffe, 1984). Mulberry (Morus alba L.) is a dioecious
plant and the male and female flowers are seen in separate plants. Bisexual
flowers never occur under natural conditions (Thomas, 2004). By treating the
nodal cuttings with silver nitrate, bisexual flowers could be induced in female
plants. The histological analysis of these bisexual flowers showed both ovule
and anther in the same flower (Thomas, 2004). Bisexual flowers were also
induced in cucumber by silver nitrate treatment (Stankovic and Prodanovic,
2002). Ethylene and auxin promote the formation of female flowers whereas
gibberellins promote the formation of male flowers (Mohan Ram and Jaiswal,
1970; Saito and Takahashi, 1986). The enhancement of femaleness by auxin
possibly occurs through the induction of ethylene biosynthesis (Takahashi and
Jaffe, 1984; Trebitsh et al. 1987). Ethylene evolution is highly correlated
with sex expression in plants and dioecious plants produce more ethylene than
monoecious ones (Rudich et al. 1972; Trebitsh et al. 1987). In view of all
these evidences, silver nitrate may possibly be a potent candidate compound to
regulate the sex expression in plants.
Fruit
ripening
Ethylene plays
a crucial role in initiating and accelerating the ripening-related process.
Treatment of tomato with silver ions has been shown to inhibit ethylene action
and fruit ripening (Hobson et al. 1984). Furthermore, if silver ions were
applied at stages of ripeness well after the breaker stage, ripening can be
arrested (Tucker and Brady, 1987). The growth
regulator 1-methylcyclopropane (1-MCP), like silver ions, is an extremely
effective antagonist for plants or harvested plant products (Serek et al.
1995a; Serek et al. 1995b; Serek et al. 1995c; Sisler et al. 1996).
Leaf
abscission
Ethylene
that stimulated leaf abscission in cotton is blocked by the silver
ion (Beyer,
1976c). Without AgNO3, all the leaves had abscised
on the 7th day in ethylene. Plants treated with increasing
concentrations of AgNO3 and placed in ethylene showed
progressively less leaf abscission. Treatment with 25 mg/l of AgNO3 reduced
the time required to reach 100% leaf abscission by 2 days. Silver
nitrate treatment also reduced the growth retarding effects of ethylene.
Other similar experiments with mature cotton plants have demonstrated
a similar ability of AgNO3 to prevent young fruit and
flower abscission (Beyer, 1976c).
Concluding Remarks
In this review, an
attempt has been made to discuss the role of ethylene, polyamines, and silver
ions as potent regulators of morphogenesis in plants. The interplay of
polyamines, ethylene, and calcium signaling is also discussed. The influence of
exogenously applied silver ions in the form of AgNO3 in plant tissue
culture media significantly regulates the ethylene activity in most of the
plant systems. We have clearly brought out the major physiological effects of
AgNO3 in plant systems viz direct or indirect organogenesis,
somatic embryogenesis, in vitro rooting of micro shoots, induction of
flowering, early flowering, sex expression, and control of leaf abscission.
However, there is a gap in information on the molecular mechanisms of
interaction between silver ions and the ethylene receptors. Further research on
the regulation of morphogenesis through the use of metal ions like silver would
throw light on an array of functions of these relatively simple molecules that
play a marvelous role in influencing growth, development, and adaptation of
plants to the environment. This opens new dimensions
in understanding plant morphogenesis. Hence, it is necessary to elucidate the
physiological mechanisms at the gene regulation level to find out the actual
role of silver ions in signaling and to see how they influence
regulation of ethylene action in plants.
Acknowledgments
VK is grateful to the CSIR, New Delhi for the award
of Research Fellowship. Authors thank Mr. Rithesh Narayanpur for his technical
assistance in preparation of the manuscript.
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