Auxin as an intercellular signal - Intercellular Communication in Plants - page 21

perception and signaling, the elucidated complex transcriptional network of acti-
vating and repressing ARFs modulated by interacting AUX/IAA inhibitors provide
sufficient diversity to elicit the various developmental programs, which are known
to be mediated by auxin.

1.5.7 Auxin as morphogen
The term morphogen was introduced as a theoretical term in mathematical models
of self-organizing systems. The concept of positional information proposes
concentration gradients of morphogens, which instruct cells within a homogeneous
field about their position, thus leading to their commitment to different cell types
(Wolpert, 1998). In contrast to plants, in animals, several molecules with morphogen
properties have been identified, such as Decapentaplegic (DPP) or Wingless (WG)
in Drosophila imaginal discs (reviewed in Teleman et al ., 2001). With increasing
knowledge on these 'classical morphogens', the meaning of the term morphogen
has shifted. The general description defines a morphogen as a substance which
forms a concentration gradient and which is involved in patterning (Wolpert,
1998). More stringent definitions provide conditions, which 'real' morphogens
must meet; i.e., they must show a stable concentration gradient and show direct
and concentration-dependent instruction to responding cells (Teleman et al. , 2001;
Entchev & Gonzalez-Gaitan, 2002). Auxin comes close to meeting some of these
criteria. Auxin is certainly a compound linked to patterning in multiple develop-
mental contexts. In addition, by several independent approaches, differences in
auxin concentration between neighboring cells have been detected, and these can
be described as concentration gradients (Uggla et al. , 1998; Sabatini et al. , 1999;
Casimiro et al. , 2001; Benkova et al. , 2003; Friml et al ., 2003). This gradient is ac-
tively maintained and controlled by a PIN-dependent auxin transport. Interestingly,
also in the animal field, the classical morphogens such as WG or DDP have been
shown to move actively through a field of cells (Entchev & Gonzalez-Gaitan, 2002).
The question remains whether such an auxin gradient is instructive for patterning.
There is clear correlation: genetic or chemical interference with the gradient also
results in patterning defects - cells are not properly specified, often display mixed
fates and are in wrong positions (Sabatini et al. , 1999; Friml et al ., 2002a, 2003).
Nonetheless, a direct and concentration-dependent auxin effect on the responding
cells has not been demonstrated based on our current limited knowledge on
auxin-gradient perception and downstream signaling. Whatever the outcome will
be of rather academic discussions on whether auxin is a morphogen or not, the
crucial role of auxin and its graded distribution in many plant patterning processes
is now firmly established.

1.6 Conclusions

More than a century of physiological work, as well as recent contributions from
molecular genetics, has clearly shown that auxin is the most prominent intercellular
signal in plants, and that it regulates a wide variety of developmental processes.