ABA Transport and Distribution in Relation to Its Function in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 66

tissues as a result of passive diffusion (Bruggeman et al. 2001 ). Supposing that the
observed translocation of ABA from wild-type tissues to ABA-deficient tissues is
the result of passive ABA diffusion, one would expect ABA transport from high
to low ABA-contained tissues, such that the ABA transport among tissues should
be a universal and important phenomenon in regulating seed development and
dormancy.
Many plant species utilize fruit to disperse seeds, and seed development is
tightly coupled with the development of the whole fruit. For example, once a
seed is fully developed, the fruit is also fully ripened, therefore promoting the dis-
semination of the seeds. The tight relationship between seed and fruit develop-
ment implies that materials and messages are transferred between seeds and other
tissues of the fruit (i.e., the pericarp and receptacle). Phytohormones are central
regulators of both seed and fruit development and ripening. Ripening of the respi-
ration climatic fruits is determined by ethylene. Recently, ABA has been increas-
ingly reported to play important roles in development and ripening in many fruit
species, especially in the non-respiration climatic species (Giribaldi et al. 2010 ; Jia
et al. 2011 ). Deytieux et al. ( 2005 ) reported that, in grape berry ( V. vinifera L), a
non-climacteric fruit, ABA levels greatly increased in both whole berries and sepa-
rated pericarps when the berries changed color and, interestingly, the ABA level in
pulp rapidly declined during the fruit ripening and dramatically increased in the
skin tissue. This study suggests that a specific repartition of ABA occurs between
the different tissues of the berry. Knowledge of the origin and movement of the
ABA signal among fruit tissues is particularly important for understanding the
mechanisms underlying fruit development and ripening, but unfortunately, little is
known about these topics.
4.4 Long-Distance Transport in Relation to Plant Systemic
Signaling
Higher organisms not only rely on the communication of materials and signals
among cells, but also among organs. The communication between organs gener-
ally requires long-distance transport of a signal, and this enables higher organisms
to respond more rapidly and more positively to environmental stimuli. This is best
demonstrated in animals, where nerve-mediated signaling swift and purposeful
reactions, thus protecting the animal from possible harm. As in nerve-mediated
signaling in animals, long-distance signaling in plants is a prerequisite for plants
being capable of perceiving environmental stimuli. Recently, numerous studies
have suggested that ABA may act as a long-distance transporter signal to mediate
root-to-shoot communication under drought stress conditions.
Drought stress is a major factor affecting plant growth and development. As
more than 90 % of water is lost through stomatal transpiration, the regulation of
stomatal movement is particularly important for plants' adaptation to drought
stress. Leaf stomatal conductance was initially thought to be regulated by a