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 41

exist elsewhere, such as the channels whereby information passes into and
out of the root tip. As indicated, a major sensor of information is located in
and around the root cap. This is the afferent limit of the root-brain, just as,
for example, the eye is the afferent, information-gathering outpost of the
brain of a vertebrate animal. Decisions may also be made in this portion
of the plant brain as to which tropic response to follow (Takahashi et al.
2003). These so-called decisions might result from conflicts between the
respective perception systems.
3.3.2
Clues from the Polarity of Auxin Flow
Besides their sensitivity to external stimuli of various kinds, the perception
of which in many cases resides in the root cap, root tips are important sinks
for trophic substances which drive their growth. In fact, the evolutionary
development of roots themselves may have been a response to a need of the
shoots of early photosynthetic plants to rid themselves of excess sucrose
by sinking it into new carbonaceous structures which could conveniently
be placed underground where their mass would help stabilise the upward-
growing shoot. With the development of leaves came not only additional
carbohydrate but also the synthesis of auxin (indole acetic acid, IAA),
a phytohormone (Sztein et al. 2000). This substance, with its ability to
stimulate cell division, may also have played an important role in the
evolution of the root. The chi-chi on the trunks and branches of old Ginkgo
biloba trees (Fujii 1895) may, for example, be extant prototypical root
organs.
In present-day plants, auxin is transported into the root, and it is here
that surplus auxin is catabolised. The growing root tip is thus a sink not
only for surplus carbohydrates, but for surplus auxin also. This directed
auxin pathway may be instrumental in organising the regular pattern of
transverse divisions along the meristematic cell files (Barlow et al. 2004).
In the chi-chi of Ginkgo , by contrast, cell divisions in their tips are oriented
irregularly (Kurczy´nska and Barlow 1999), possibly as the result of excessive
retention of auxin which is only slowly broken down within this organ.
The site within the root where auxin catabolism occurs is the quiescent
centre, a small group of a few hundred cells sandwiched between root
cap and root proper. The catabolic agent is auxin oxidase, an enzyme
whose activity is up-regulated by the incoming IAA (Kerk et al. 2000). The
pathway for auxin movement into the root tip is via the xylem parenchyma
of the primary tissue. If the plant is secondarily thickened, the auxin moves
through the vascular cambium (Schrader et al. 2003). The auxin transport
process is mediated by PIN and AUX proteins which, respectively, assist