Charles Darwin and the Plant Root Apex: Closing a Gap in Living Systems Theory as Applied to Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 42

the auxin flow into and out of the conducting cells (Friml and Palme 2002).
The columns of cells which comprise cylindrical plant organs (such as root
andshoot)arepolarisedsothatauxininputoccursatthebasalendofthe
cellsandauxinoutputoccursattheapicalend.
Some of the auxin which is transported acropetally towards the tip avoids
destruction within the quiescent centre and escapes into the root cap.
Once in the cap, the auxin becomes linked with the graviresponse system
(Ottenschläger et al. 2003) and perhaps with other environment-sensing
systems also. Observations from Arabidopsis thaliana show that cells of
the root cap meristem and the central statenchyme, upon receiving an
appropriate gravitational stimulus, can relocate their PIN IAA-exporter
protein, AtPIN3 (Friml et al. 2002). Together with AUX1 (Swarup et al. 2001),
the gravitationally reoriented PIN3 enables an asymmetric translocation of
auxin to take place back along the root. The basipetal movement is under the
regulation of PIN2 (Müller et al. 1998) and occurs through root epidermal
and outer cortical tissues (Rashotte et al. 2000; Ottenschläger et al. 2003).
It brings the auxin to the transition zone, where gravitropism is initiated.
The acropetal and basipetal auxin flows are carefully partitioned within
different tissues (Blancaflor and Masson 2003), the only route between one
auxin flow and the other usually being via the root cap tissue. However,
occasional crossover between the two auxin channels may account for
observations from which it has been concluded that, in addition to the
'strong' role of the cap in gravitropism, the root tip itself can play a 'weak'
role in this and other tropic responses (Wolverton et al. 2002).
3.3.3
The Muscular Root-Brain
Evidence is accumulating that the steps in the process of auxin transport
share features that characterise chemical synapses which are instrumental
in transmitting impulses along animal nerves (Lodish et al. 2000). Auxin
itself should be regarded as a plant neurotransmitter substance (Baluška et
al. 2003, 2004) and that its transport occurs via chemical synapses (Barlow
et al. 2004; Baluška et al. 2005). The rate of auxin transport is therefore also
therateofpropagationofaplantnervouschemicalimpulse.
Since auxin is transported acropetally into the root tip, and into the
root cap, in particular, whence it is redistributed and exported back along
the epidermal pathway, the portion of root distal to the transition zone,
including the root cap, can be considered to be like a 'brain'. It is here that
signals from the ambient root environment impinge, are suitably processed
or transduced, and are thence exported via a neuronal-type of efferent
channel to the 'muscular' proximal portion of the transition zone to effect