Control Systems in the Living World - Building the Most Complex Structure on Earth - page 47

result by using different genes or transcriptional programs, but in the process, they
crosstalk and influence each other ( Nemhauser et al., 2006 ).
Ample evidence has been accumulated on the control and regulation of gene
expression and the role of epigenetic mechanisms, such as DNA methylation, histone
acetylation/deacetylation, and chromatine remodeling. We have a detailed picture of
signal transduction pathways in plants. However, this is only a partial picture; it elu-
cidates the control at the cellular level. Knowledge of gene expression is of no help
in understanding how tall a plant will grow, how long it can live, how often it will
flower, how big its fruit will grow, how it will react to tissue injuries, etc. These are
emergent plant phenomena that can neither be predicted by its cells or genome.
We need to know what determines the development of the plant multicellular struc-
ture, as well as what monitors its state, detects deviations from the norm, and sends
instructions for restoring it for long periods of time, sometimes thousands of years.
The examples of plant control mechanisms described briefly and in simplified
form above unquestionably show the progress in the field of molecular mechanisms
of control at the plant cell and supracellular levels. The evidence at hand has shed
light on some important aspects of the control mechanisms in plants, but they are far
from anything that could be considered an actual control system in plants. All the evi-
dence on hormonal regulation presented in this brief review has an essential flaw: hor-
monal pathways described so far take the regulation of first element of the pathway to
be self-explanatory, as if the timing and switching on/off of the hormone synthesis do
not require explanation. But the mechanism of its regulation cannot be excluded from
the general scheme of the control system. Further work to piece together all the avail-
able facets into a comprehensive control mechanism that is comparable to the control
system as we know it in animals will be necessary to fill the present gaps.
Searching for the Controller of the Control System in Plants
We have seen that the highly ordered structure of the cell in unicellular organisms
is determined by a control system consisting of genetic and epigenetic mechanisms.
Both genetic control by the genome and epigenetic control by the cytoskeleton,
described earlier, are cell level controls and do not extend beyond the individual
cells. Hence, they are not relevant to the multicellular structure of animals and
plants. The development and maintenance of that structure requires memorization
of that structure and special biological devices to direct its construction. Expressed
more simply, this implies regulation in space and time of the arrangement of indi-
vidual cells of different types in strictly determined patterns that allow the system to
develop into an organism of its species and perform species-specific functions at the
tissue, organ, and organismic levels. The question that we face in the case of plants is
not whether a control system exists, but where it is. In this regard, there is no ques-
tion that the situation for plants is different from the one in metazoans. The picture
of the control system in plants, as can be seen from the few examples of hormonal
control of growth of various organs, with a number of controls at the level of tissues
and organs, is still fragmentary.