Auxin as an intercellular signal - Intercellular Communication in Plants - page 17

meristems, which enable a perpetuation of postembryonic growth. Meristems con-
tain populations of 'stem' cells, which, according to their position, differentiate into
different cell types. The Arabidopsis root meristem displays a highly regular and
predictable pattern of cell divisions and differentiation and is, therefore, an ideal
system for studies on mechanisms of meristem patterning. Manipulation of auxin
distribution as well as auxin signaling have demonstrated a role for auxin in reg-
ulating the root meristem activity (Kerk & Feldman, 1994; Ruegger et al ., 1997;
Sabatini et al ., 1999). Accumulation of auxin and auxin response was detected in the
columella initial and first columella layer cells (Sabatini et al ., 1999; Benkova et al .,
2003). Interestingly, the PIN4 auxin efflux regulator displays a polar localization
pointing toward the same area, suggesting a role of PIN4 in maintenance of this
auxin accumulation. In support of this, pin4 mutation or chemical inhibition of PAT
disrupts this auxin accumulation, which results in changes in cell fate specification
(Friml et al ., 2002b). These observations suggest that PIN4 mediates PAT through
the central root meristem tissues, thus actively maintaining an auxin gradient with
its maximum in the distal root tip (Plate 1.1M). The PIN4 function appears to be also
necessary for local auxin turnover, since pin4 mutant root tips display elevated auxin
levels and fail to canalize exogenously applied auxin properly (Friml et al ., 2002b).
Following the most plausible scenario, auxin from upper tissues is actively concen-
trated by PIN-dependent transport in the distal root tip, which serves as an 'auxin
sink'. There, part is probably rendered inactive by an as yet unknown mechanism and
other part is redistributed back by PIN2 action through the outer layers (Plate 1.1M).
1.5.5 Tropisms
Tropisms are growth responses to external stimuli such as light (phototropism) or
gravity (gravitropism), resulting from differential elongation rates on either side of a
plant organ. The Cholodny-Went hypothesis proposed that differential growth rates
result from the asymmetrical distribution of auxin, which subsequently promotes or
inhibits cell growth and elongation (Went, 1974). Indeed, differential auxin or auxin
response distributions were visualized in various plant organs including gravity
stimulated tobacco shoots (Li et al ., 1991), light and gravity stimulated Arabidopsis
hypocotyls (Friml et al ., 2002a) or developing peanut gynophores (Moctezuma,
1999). Because AEIs interfere with the asymmetric distribution of auxin as well as
with tropisms, PAT has been implicated as the process underlying asymmetric auxin
distribution (Lehman et al ., 1996; Friml et al ., 2002a) and the existence of a lateral
auxin transport in shoots has been proposed. This would facilitate the exchange of
auxin between the basipetal stream in vasculature and peripheral regions, where
control of elongation occurs (Fig. 1.2). The analysis of localization and function
of the auxin efflux regulator PIN3 provided molecular support for this concept
(Friml et al ., 2002a). The pin3 mutants are defective in hypocotyl phototropism and
gravitropism as well as root gravitropism, although these defects are rather subtle,
suggesting functional redundancy. In addition, PIN3 is predominantly localized at
the lateral side of shoot endodermis cells, where it is perfectly positioned to regulate
lateral auxin flow (Friml et al. , 2002a).