Other Hormonal Signals during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 218

unlike the case regarding ethylene in
climacteric fruits, which has been studied
more intensively (see Grierson, Chapter 10,
and Kumar and Sharma, Chapter 11, this
volume). More work is needed to better
defi ne the roles and relative importance of
these 'other' hormones. Particular areas of
interest include detailed knowledge of
hormone perception and signalling, the
resulting transcriptional changes and the
resultant changes in the protein profi le and
hence metabolism. Although we have
discussed some examples where cross-talk
between hormones seems to be occurring,
there is relatively little direct evidence for
this in fruits, and much is inferred from
what we know from vegetative tissues. It
should be remembered that effects caused
by hormone application, for example the
delay or enhancement of ripening, do not
necessarily confi rm the action of a hor-
mone during 'normal' fruit development.
The use of inhibitors and accurate
measurement of endogenous hormone
levels add weight to these arguments, but
such studies are lacking for a number of
hormones in a range of fruits.
While the role of ethylene has been
established in many fruits, its role in others
is still uncertain, and it seems likely that
there will be a spectrum of response with
the classic defi nitions of climacteric and
non-climacteric at either end. Indeed, as
our knowledge of the action of hormones
other than ethylene during ripening
increases, there may be a number of
different models required to explain the
control of ripening in a broad range of fruit
species. Much of the focus is on those
hormones that promote ripening, but it is
understood that there are some that appear

to delay ripening. This inhibition seems to
occur in both climacteric and non-
climacteric fruits, and, as the release of this
inhibition is required for ripening, it could
be argued that the true control over
ripening initiation, but not the conduct of
the ripening process itself, is vested in
those hormones whose levels are low at the
initiation of ripening, such as auxins. The
diffi culty in gaining direct evidence of a
role for these hormones in inhibiting
ripening is that they are essential for fruit
development and so downregulating their
levels to test function is diffi cult. This is
especially challenging as more than one
hormone may be involved in inhibition.
Powerful new techniques, such as deep
RNA sequencing and microarray analysis,
are well suited for studying the hormonal
control of fruit development, as hormonal
control often involves changes on a large
scale with the transcriptional response
being rapid and wide-reaching. However, it
is clear that detailed functional analysis is
still essential in determining the role of the
hormone in plant development.
The number of hormones that may
infl uence ripening and the complexity of
the interactions between them make it
diffi cult to be categorical about their
individual roles. In many cases, more than
one hormone may simply be moderating
the effects of others through subtle in-
fl uences on synthesis, transport, perception
and signalling. This intricate network may
be required to provide fl exibility in
controlling the complexities of ripening
where the fruit must adapt to change and,
in the case of fl eshy fruit, store large
amounts of precious carbon/energy while
interacting with the developing seed.

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