Control Systems in the Living World - Building the Most Complex Structure on Earth - page 46

Figure 1.27 Bud activation and auxin transport. One model for the mechanism by which auxin
regulates bud outgrowth relies on establishment of auxin export out of the bud as a prerequisite
to allow bud outgrowth. An initial auxin flow towards an auxin sink promotes auxin transport
canalization along this path. The polarization and upregulation of auxin transport feeds back to
promote auxin flow further. The canalization of auxin transport into cell files connecting the auxin
source (bud) to the auxin sink (main stem) allows sustained auxin export out of the bud. This
means that auxin export out of the bud can be prevented if the main stem is a weak auxin sink.
This can be achieved by increasing the amount of auxin transported in the polar auxin transport
stream (PATS) in the main stem. Therefore, if cytokinin (CK) in the primary apex promotes auxin
synthesis and/or PATS, this would inhibit bud outgrowth. On the other hand, if CK promotes auxin
synthesis and/or auxin flow in the bud, it would promote bud outgrowth ( Müller and Leyser, 2011 ).
Symbols? indicate possible but unconfirmed involvement of various substances and processes.
Other important plant hormones are abscisic acid (a stress hormone), ethylene,
jasmonates, brassinosteroids, etc.
Searching for the Plant Control System
Biologists are still puzzled that a relatively small number of hormones, many of
them functionally redundant, enable plants to accomplish, during embryogenesis and
in later life, many phenotypic outcomes and give rise to the observed diversity of
forms in the plant kingdom.
This may be related to a number of genetic and epigenetic factors. In plants, hor-
monal signaling pathways are connected between them, thus giving rise to pathway
networks where the combined activity of different pathways can lead to amplifica-
tion, attenuation, or fine tuning of phenotypic outcomes ( Kuppusamy et al., 2009 ).
Different hormonal pathways may be involved in producing the same phenotypic