Agriculture Reference
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4.2 Cellular Compartmentalization and Redistribution
Since ABA is a weak acid, it may exist in undissociated or protonated forms and
in a dissociated anion form. The protonated form permeates freely across mem-
branes, whereas the dissociated anion does not. Consequently, the amount of ABA
distributed in different compartments is largely determined by pH; the greater the
difference in pH between compartments, the greater the amount of ABA accumu-
lates in the more alkaline compartment (Hartung 1983 ; Baier and Hartung 1988;
Hartung and Slovik 1991 ). Based on related parameters, such as membrane con-
ductance, anion permeability, pKa value, and so on, Cowan and Railton ( 1986 )
developed a mathematic model of ABA distribution, which showed that 4.4,
68.4, 17.2, and 10 % of the total cell ABA occurred in apoplasts, chloroplasts,
the cytoplasm, and vacuoles, respectively. Based on a mathematical model devel-
oped using morphological and physiological data experimentally determined in
Valerlanella locusta , Slovik and Hartung ( 1992 ) described the pattern of ABA
distribution in mesophyll cells, epidermal cells, guard cells, phloem, and the apo-
plasm. According to their analysis (Slovik and Hartung 1992 ), the highest concen-
tration of ABA occurs in guard cells (i.e., 426.4, 467.6, and 195.7 nM of ABA in
the cytosol, chloroplast, and vacuole of guard cells, respectively). By contrast, the
ABA content in mesophyll and epidermal cells was much lower; the ABA content
in the cytosol and vacuole was only 9 and 2.8 nM for mesophyll cells and 55.1 and
5.6 nM for epidermal cells, respectively. Regardless of the cell type, chloroplasts
always had the greatest ABA content.
Immunolocalization techniques using anti-ABA serum or monoclonal anti-
bodies allow ABA to be monitored directly at the cellular and subcellular lev-
els. Using rabbit anti-ABA serum, Sotta et al. ( 1985 ) developed an indirect
immunohistochemical technique involving staining of the peroxidase-antiper-
oxidase (PAP) complex to indicate ABA localization in the buds and leaves of
Chenopodium plants. The staining patterns showed an apicobasal gradient of
ABA. Dense staining appeared in buds and vascular tissues. Phloem cells of the
main axis and chloroplasts of the leaves were specifically labeled. In older buds,
ABA was mainly distributed in quiescent meristematic cells. Using a combination
of ABA monoclonal antibody and 568 goat anti-mouse IgG antibodies, Schraut
et al. ( 2004 ) visualized ABA localization and transport in maize roots using confo-
cal laser scanning microscopy and showed that ABA is mainly concentrated in the
root cap and meristematic root tip and that the exodermis may act as a barrier to
control the lateral transport of ABA in maize roots. Similarly, immunolocalization
of ABA in roots and needles of radiate pine (Pinus radiate ) showed that ABA is
mainly located in the exodermis of roots and guard cells of needless (Diego et al.
2013 ).
ABA can be observed at the subcellular and ultrastructural levels using immu-
nogold electron microscopy localization. Sossountzov et al. ( 1986 ) used this tech-
nique to show that ABA is mainly localized to two sites in the axillary-bud-bearing
nodes of Chenopodium polyspermum L , i.e., in the plastids of cortical cells and
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