How Can Plants Choose the Most Promising Organs? - Communication in Plants: Neuronal Aspects of Plant Life - page 53

4 How Can Plants Choose

the Most Promising Organs?

T. S achs

Abstract Branch death is an important component of the processes that generate tree form.
Many branches are formed and the plant appears to “choose” the most promising alter-
natives for survival and further development. The purpose of this chapter is to consider
how this choice could occur in the absence of a central decision center. Experiments were
carried out on a model system of pea seedlings with two shoots. In most plants only one
of these shoots continued to grow. We studied the conditions that influence the choice of
the “winning” branch. These included removing entire branches or leaves at different ages
and modifying the environment of the entire plant or of individual shoots. The evidence
from experiments and from comparative observations of many trees supports the following
hypothesis. All components of a shoot are sources of auxin and possibly of other signals.
The level of auxin synthesis depends on the immediate environment and the developmental
stage of individual leaves. The responses to auxin include the orientation of vascular differ-
entiation towards organs that are its strongest source. This oriented auxin response results
in competition between alternative organs. This is a prominent example of “developmen-
tal selection,” an alternative to developmental programs and prepatterns, which also has
various other roles in the generation of biological form.

4.1
Introduction: Developmental Selection
of Branch Configurations

Plants develop their complicated forms by reiterating developmental pro-
cesses, forming repeated structural metamers (Halle et al. 1978; Barlow
1989). This can be readily verified by following the development of the
young tips of leading branches. Reiteration is also necessary on theoret-
ical grounds, for there is no other way in which the information coded
in a genome could suffice to determine the detailed form of a large tree.
Yetreiterationcannotbethewholestory.Evenasuperficialobservation
of most trees suggests that while all buds are potential branches, most of
them do not develop further. Those that do become branches develop at
variedratesthatdependontheirimmediateenvironmentandthepresence
of neighboring branches, not only on a reiterated program. Further, the
majority of the branches that start growing in a young canopy are shed
within a few years (Sachs and Novoplansky 1995). Dead branches are read-
ily found below large trees, especially following a storm. The shedding of
branches is also the origin of bare trunks, which commonly develop from