References

124 M.L. Lanteri et al.
9.1.1
Auxins Control Root Development
Hormones are essential players for transmitting information and connecting
the whole plant. The action of hormones involves its perception, initiation
of both specific and nonspecific responses and, finally, the result
of a new steady state of growth and/or metabolic conditions. Among hormones,
auxin plays an important role during plant growth and developmental
processes; therefore, one of the essential aims of plant biologists is
to understand its action, regulation and target molecules. As a critical plant
hormone, auxin modulates diverse processes such as tropic responses to
light and gravity, root and shoot morphogenesis, organ patterning, vascular
development and growth in tissue culture (Davies 1995). Auxin is
known to influence cell division, cell expansion and cell differentiation
and to have a profound influence on root morphology. The control of root
length, the enhancement of lateral root (LR) formation and root hair density,
and the induction of adventitious root development are widely known
auxin-induced processes. Mutants that overproduce auxins tend to have
abundant lateral and adventitious roots (Boerjan et al. 1995; King et al.
1995); conversely, mutants deficient in auxin responses are often characterized
by long primary roots and few LRs (Estelle and Somerville 1987;
Hobbie and Estelle 1995).
Although many tissues can synthesize auxin (Ljung et al. 2001), it is
mainly produced in the shoot apical meristem. It was postulated that the
rate and direction of auxin movement through the cells and finally through
the whole plant is the key fate in auxin-mediated responses (Leyser 1998).
Auxin transport is complex and highly regulated, involving many identified
proteins (Morris 2000). One of the most characteristic features of auxin is
its polar cell-to-cell transport (Jones 1998). Both acropetal (Wilkins and
Scott 1968) and basipetal (Davies and Mitchell 1972) transport occurs in
roots. Opposing directions of auxin transport in roots is achieved by spatial
separation. Chemical and genetic approaches have revealed that transport
of auxin to distant sites is clearly required for normal development
(Friml 2003). For example, indole acetic acid (IAA, the main active auxin
in plants) transport is necessary for proper LR development (Reed et al.
1998; Bhalerao et al. 2002; Casimiro et al. 2001). Recent research suggests
that polar transport of auxin is accomplished via vesicular secretion linked
to endocytotic and recycling processes (Baluska et al. 2003). This would
imply that in addition to the hormone-like and morphogen-like properties,
auxin could also have a neurotransmitter-like behavior (Baluska et al. 2003,
2004). IAA biosynthesis, metabolism and transport are key features to orchestrate
plant development. Moreover, mechanisms downstream of auxin
transport must necessarily transduce the auxin message and be relevant