Molecular Plant Breeding, 2016, Vol.7, No.16, 1-7
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biochemistry, molecular biology and physiology has
been flourishing. The extracted knowledge of studies
on ethylene synthesis in plants and its role and impact
in plant physiology and development has
advantageous and beneficial to various agricultural
systems and the trend is advancing and expediting.
1 Biosynthetic Pathway of Ethylene
Ethylene biosynthetic pathway is entrenched in higher
plants (Bleecker and Kende, 2000). Biosynthetic and
signaling pathways of ethylene have been studying
extensively for last few decades. These pathways have
gained much attention in the area of plant hormone
physiology (Kende, 1993). Major breakthrough in this
subject came through the identification of
S-adenosyl-L-methionine
(S-AdoMet)
and
1-aminocyclopropane-1-carboxylic acid (ACC), as
ethylene precursors (Yang and Hoffman, 1984). ACC,
a non-protein cyclic compound is produced from
S-AdoMet in a reaction carried out by a specialized
enzyme ACC synthase (ACS) (Kende, 1993).
However, S-adenosyl-L-methionine is formed from
methionine via S-AdoMet synthetase (SAM
synthetase) enzyme at the cost of ATP (Ravanel et al.,
1998).
Methionine is not only the essential brick of protein
building while about 80% of cellular methionine is
also involved in the formation of S-AdoMet (prime
methyl donor in plants) (Kevin et al., 2002). In the
first committed step of ethylene biosynthetic pathway,
besides ACC,
another compound 5´-methyl-
thioadenosine (MAT) is also formed that is
subsequently converted into methionine via modified
methionine cycle (Bleecker and Kende, 2000). As
intermediate compound of ethylene synthesis pathway,
ACC is then being used as substrate of another
enzyme of this route, ACC oxidase (ACO). This
enzyme mediates the reaction of ethylene formation
from ACC that is the immediate precursor of ethylene
(Yang and Hoffman, 1984).
Modified methionine cycle is a type of salvage
pathway that keeps and save methylthio group from
every round and for another cycle of ethylene
production at the expense of one ATP molecule.
Thereby ethylene can be synthesized even at high rate,
when there is scarcity of methionine in a cell
(Bleecker, 2000 and Kevin et al., 2002). In addition to
ethylene, cyanide and CO2, are also the end products
of this biological route. As cyanide is toxic, thus cell
has to flush out this compound, as sake of its survival.
Therefore by the activity of β-cyanoalanine synthase
(CAS) that detoxifies cyanide and converts it into
β-cyanoalanine, a non-toxic compound, cell is
prevented to toxicity (Kevin et al., 2002) (Figure 1).
2 Regulation of Ethylene Biosynthesis
Related Genes
In plants, progress in ripening stage had proved to be
linked with the induction and upregulation of various
genes. Hence involvement of ethylene dependent and
ethylene independent gene regulations have been
observed using molecular and expression analyses of
ripening related genes by developing mutants and
transgenic plants (Picton et al., 1993 and Theologis et
al., 1993). Impact of positive as well as negative
feedback regulation on ethylene biosyntheses is also
well established (Barry et al., 2000). Genes encoding
ACC synthase and ACC oxidase belong to small
multigene families. The differential expression of
these enzymes is regulated through, hormonal,
environmental d developmental stimuli or signals
(Zarembinski and Theologis, 1994 and Barry et al.,
2000). A complex regulatory network comprising
environmental and developmental signals is existed
for regulating the expression of genes involved in
biosynthetic pathway of ethylene (Johnson and Ecker,
1998). Regulation of ACC synthase gene is based on
the expression of this gene in response to internal and
external cues. Various isoforms of ACS is present such
as in tomato, Le-ACS2, Le-ACS4 and Le-ACS6.
However their feedback regulation is different when
ethylene is synthesized during fruit ripening thereby
former two are positively regulated whereas last one is
negatively regulated by ethylene synthesis at this stage
(Nakatsuka et al., 1998). The only common feature of
these isoforms of ACS gene is their spatial and
temporal regulation by various endogenous as well as
exogenous stimuli or signals.
3 Ethylene Regulated Genes
The mechanism of ethylene-regulated genes has
unraveled by molecular characterization of the
promoter regions of ripening-related genes and
thereby an ethylene-responsive element containing 8
bp motifs (A (A/T) TTCAAA) has been identified
(Montgomery et al., 1993 and Itzhaki et al., 1994).
