Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FLOWER SENESCENCE 57
ethylene-sensitive flowers, and a number of commercial products based on these chemicals
for treatment of carnation and orchid flowers have entered the market (Son et al.,
1994; Woltering and Harkema, 1994). However, blocking ethylene effects at the receptor
level is more effective as it will protect against both endogenous and exogenous ethylenes
(Serek and Reid, 1993). Several ethylene antagonists have been discovered during the last
30 years, and some of them have successfully been used by the floral industry to block
ethylene responses. Another approach to interfere with ethylene is the use of molecular
breeding techniques, in which ethylene responses are modified in transgenic plants. Central
in the synthesis of ethylene in plants are two enzymes: 1-aminocyclopropane-1-carboxylic
acid (ACC) synthase (ACS) and ACC oxidase (ACO), formerly known as ethylene-forming
enzyme (EFE) (Kende, 1993). The production of ethylene is autocatalytic in many plants,
meaning that the presence of ethylene in the atmosphere or in the plant tissue causes a
positive feedback, leading to a rise in the production of the hormone. The key protein responsible
for the reactions to ethylene is the ethylene receptor, which consists of a family
of membrane-embedded proteins that bind ethylene. In Arabidopsis, five members of this
family have been identified: ETR1, ETR2, ERS1, ERS2, and EIN4. The first receptor to
be identified was ETR1 (Chang et al., 1993), and homologs to this and some of the other
proteins have been found in a number of other plants. Later work by Chang and others
have elucidated the function of this receptor, as well as other proteins involved in the ethylene
response pathway. These results can be summarized as follows: the ethylene signaling
pathway is initiated by a family of membrane-embedded proteins (receptors) leading via a
signal transduction cascade to altered gene expression patterns. The ethylene receptors are
thought to exist as dimers that belong to the so-called two component histidine kinase receptor
family. The receptors are negative regulators of ethylene responses, which mean that
ethylene binding represses receptor signaling. When ethylene is not present, the receptor
activates a protein called CTR1. CTR1 is a serine/threonine kinase and is presumably part
of a mitogen-activated protein kinase signaling cascade, a highly conserved signaling route
known to regulate a variety of cellular processes in different organisms. In the absence of
ethylene, CTR1 is active and represses further signaling; when ethylene is present and binds
to the receptor, this binding presumably induces a conformational change resulting in inactivation
of CTR1, thereby releasing the repression on the signal transduction chain leading to
the activation of transcription factors and the genes responsible for ethylene effects (Chang,
2003). For any reason the receptor is not capable of binding ethylene, CTR1 will be active
and the signaling cascade will be repressed. ETR1 was first identified in Arabidopsis by its
dominant mutant, called etr1-1 that does not show the normal triple response of seedlings to
ethylene (Chang et al., 1993). It has been shown that this mutant protein does not bind ethylene
(Hall et al., 1999). The dominant nature of this mutant can be understood considering
that the different receptors feed into the same signaling cascade. The presence of one type
of actively signaling receptor and, hence, activated CTR1, apparently is sufficient to repress
the signaling cascade leading to gene expression and subsequent ethylene symptoms even
when other receptor types are inactivated. In addition, increased dosage of wild-type alleles
in triploid lines led to the partial recovery of ethylene sensitivity, indicating that dominant
ethylene insensitivity may involve either interactions between wild-type and mutant receptors
or competition between mutant and wild-type receptors for downstream effectors
(Hall et al., 1999). Through this knowledge, we can understand that the plant’s reaction to
ethylene can be prevented by (1) application of a chemical that blocks the receptor in such a