Auxin as an intercellular signal - Intercellular Communication in Plants - page 4

acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA). Also, some naturally occurring
flavonoid compounds show similar effects as AEIs and have been considered to
be natural regulators of PAT (Jacobs & Rubery, 1988). Despite very limited data
on the molecular mechanism of AEI function, they facilitated establishment of the
central role of auxin efflux in plant development. When applied to plants, AEIs
interfere with a number of processes such as axis establishment during embryoge-
nesis (Hadfi et al ., 1998), lateral root and aerial organ initiation (Reinhardt et al. ,
2000; Casimiro et al ., 2001), root meristem patterning (Kerk & Feldman, 1994;
Ruegger et al ., 1997), vascular patterning (Mattson et al. , 1999), hypocotyl and
root elongation in light (Jensen et al ., 1998), apical hook formation (Lehman et al .,
1996) and most prominently tropic responses (Marchant et al ., 1999). Owing to the
lack of similar inhibitors of auxin influx, the characterization of the role of auxin
influx in plant development was slowed down. Nevertheless, recently, compounds
such as 1-naphthoxyacetic acid (1-NOA) and 3-chloro-4-hydroxyphenylacetic acid
(CHPAA) were shown to specifically inhibit auxin uptake in tobacco culture cells
(Imhoff et al ., 2000) and were used to demonstrate a role of auxin influx in root grav-
itropism (Parry et al ., 2001). The manipulation of PAT using inhibitors established
PATasthe major, developmentally important route for auxin translocation.
1.2.2 Chemiosmotic model
The wealth of physiological findings on PAT such as its cell-to-cell character, satura-
bility, energy and protein synthesis dependence, together with the known chemical
nature of auxin, led to the formulation of the chemiosmotic hypothesis in the middle
of the 1970s (Rubery & Sheldrake, 1974; Raven, 1975). The chemiosmotic hypoth-
esis provides a coherent model for the polar transport of auxin through cell files and
postulates the existence of auxin-specific carrier proteins (Fig. 1.3). In the relatively
acidic environment of the cell wall (pH around 5.5), part of IAA exists in its pro-
tonated form (IAAH). This noncharged, lipophilic molecule passes easily through
the plasma membrane by diffusion. In the more basic cytoplasm (pH around 7), the
majority of IAAH dissociates and hence the resulting polar IAA − anion is 'trapped'
in the cell because of its poor membrane permeability and can leave the cell only
by the activity of specific efflux carriers. The unidirectionality of auxin flow was
explained by postulating asymmetrically distributed auxin efflux carriers within the
cells. Thus the polar auxin export at the single cell level would multiply within a
file of cells and result in the directed translocation of auxin throughout plant tissues.
In addition, auxin influx was proposed to participate in PAT (Goldsmith, 1977) and
later physiologically demonstrated (Benning, 1986). This classical model was rein-
forced after identification and characterization of candidate proteins for auxin influx
(AUX1/LAX family) and efflux (PIN family) (Bennett et al. , 1996; Galweiler et al. ,
1998; Luschnig et al. , 1998). Numerous circumstantial evidence demonstrate their
role in auxin transport (Friml & Palme, 2002) and most strikingly both AUX1 and
PIN proteins have been shown to be asymmetrically localized in auxin transport
competent cells in accordance with the known directions of auxin flux (Galweiler