Biochemistry of Flower Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 75

tolerance to endogenous ethylene. Much more work has been done using the mutated dom-
inant ethylene-resistance gene etr1-1 from Arabidopsis . Early experiments with this gene
were done in Petunia (Wilkinson et al., 1997; Clark et al., 1999; Gubrium et al., 2000) and
continued by Celvenger et al. (2004). In these very thorough experiments, a CaMV35S::
etr1-1 construct was used, resulting in constitutive expression of the etr1-1 gene. The re-
sults showed that etr1-1 from Arabidopsis conferred ethylene insensitivity to the plants,
but also that constitutive expression of the gene gave some additional effects that, though
expected, are unacceptable from a grower's point of view. The effect of the transforma-
tion proved to be dependent on the genetic background and on the temperature the plants
were kept.
Typical effects observed in these plants were poor root development of cuttings, lower
seed weight, less efficient seed germination, less efficient rooting and delayed growth of
seedlings, slightly faster flowering, enhanced ethylene production in pollinated flowers,
delayed flower abscission, delayed fruit ripening, and delayed flower senescence.
Delayed flower senescence was seen both in the presence and absence of ethylene,
and both in pollinated and nonpollinated flowers. From a grower's point of view, the most
interesting of these effects is the delayed senescence and abscission of flowers; the poor
rooting ability of cuttings is the most unacceptable. Most of these effects can be explained
by the role ethylene plays at the different developmental stages. The enhanced ethylene
production in flowers probably is due to the uncoupling of the feedback from the plant's
reaction to pollination.
Shaw et al. (2002) have achieved ethylene-insensitive Petunia using a mutated ers
homolog from Brassica oleracea (boers). The results from this experiment resemble those
mentioned above, except for a higher mortality, due to a higher susceptibility to fungal
diseases. This is not a surprising result, as ethylene is known to play a role in defense against
pathogens. Also, these plants showed prolonged flower longevity. The ornamental Nemesia
strumosa has also been transformed for enhanced flower longevity by altered ethylene
perception controlled by a constitutive promoter (Cui et al., 2004). In this case the plants
were transformed with a mutated etr1-homolog from melon. The gene in question contained
a mutation resembling that of etr1-1 from Arabidopsis . Wilkinson et al. (1997) suggested the
use of tissue-specific promoters in order to avoid the unintended side effects due to the use
of the constitutive CaMV35S promoter. Bovy et al. (1999) followed this recommendation
when they used a flower-specific promoter from Petunia (fbp1) in their attempt to transform
carnation with etr1-1. The resulting plants showed strong insensitivity to ethylene without
the unwanted side effects of earlier experiments, though not all agronomical aspects were
investigated.
The gene construct used in the latter experiment has also been used in the transforma-
tion of Campanulacarpatica . A very effective method for transformation and regeneration
of this species has been developed (Sriskandarajah et al., 2001; Frello et al., 2002), and a
number of transformed plants have been obtained and transferred to soil (Sriskandarajah
et al., 2004). Another line of experiments have been carried out in Petunia by Chang et al.
(2003). They used a construct (Psag12-IPT) containing a promoter from a senescence-
associated gene coupled to a cytokinin biosynthetic gene from Agrobacterium tumefa-
ciens to control the cytokinin production of senescing flowers. Several simultaneous effects
were seen: the cytokinin level was enhanced, and this was accompanied by a delay in
ethylene production and by enhanced ethylene tolerance of the flowers of the transgenic