Auxin as an intercellular signal - Intercellular Communication in Plants - page 1

1 uxin as an intercellular signal
Jirı Friml and Justyna Wisniewska

1.1 Introduction

Auxin has a prominent position among the classical plant hormones since it medi-
ates multiple aspects of plant growth and development. At the cellular level, auxin
is required for cell division as well as for elongation. Moreover, it plays a role in cell
fate acquisition and hence in multiple patterning processes. At the whole plant level,
auxin acts as a correlative signal between cells, tissues and organs. Thus, macro-
scopically, auxin mediates a surprising variety of processes such as cell divisions
in the cambium and in tissue culture, vascular tissue differentiation, embryo devel-
opment, organ initiation and growth, tropic responses as well as apical dominance
(Davies, 1995).
The discovery of auxin dates back to Charles and Francis Darwin's experiments
on the phototropism of canary grass coleoptiles, which indicated the existence of a
transported signal (Darwin & Darwin, 1881). They demonstrated the transmission
of some 'influence' from the place of light perception at the tip to the other tis-
sues where differential growth (i.e. elongation) was induced. Later, Boysen-Jensen
(1913) was able to pass this 'influence' through gelatine, and thus demonstrated the
chemical nature of this growth-promoting substance, which was therefore termed
auxin (Gk. auxein -toincrease, to grow). Despite the multitude of auxin effects,
its chemical nature is rather simple. The most abundant naturally occurring auxin is
indole-3-acetic acid (IAA); other auxins are indole-3-butyric acid (IBA) (Fig. 1.1)
and 4-chloroindole-3-acetic acid (4-Cl-IAA). In agricultural applications, synthetic
auxins with higher metabolic stability, such as 1-naphthylacetic acid (1-NAA) and
2,4-dichlorophenoxyacetic acid (2,4-D), are commonly used (Fig. 1.1). Besides the
physiologically active, free forms, conjugates to sugars, amino acids or to small
peptides have been isolated (Cohen & Bandurski, 1982). Several lines of evidence
support the hypothesis of regulated degradation of free IAA (Tam et al ., 1995). Con-
sequently, the momentary concentration of auxin in target cells depends on the net
outcome of the metabolic processes such as biosynthesis, degradation, conjugation
and, most importantly, auxin transport. Auxin is unique among plant hormones in be-
ing directionally transported from the source organs in young, apical regions (Ljung
et al ., 2001) to target cells in most plant tissues (Davies, 1995). Almost a century
of physiological as well as genetic studies revealed that regulated auxin distribution
between cells underlies most of the known auxin effects and thus highlighted the
central role of intercellular auxin transport in plant biology.