PNL VOLUME 18
1986 COMMENTARY
67
GENERAL CONSIDERATIONS FOR THE USE OF PEA GENOTYPES FOR MODELS OF PLANT
MORPHOGENESIS
Ingensiep, H. W. Institute of Genetics, University of Bonn
Federal Republic of Germany
Genotypes of Pisum with clear morphological deviations from the normal
form are well known and described, especially those concerning the morphol-
ogy of the whole shoot, leaves, and flowers (e.g. by Gottschalk, Lamprecht,
Marx). These pea genotypes have been considered primarily from an evolu-
tionary point of view (1), but their significance for ontogenetic theories
and models has less frequently been discussed - this mainly because of a
lack of molecular genetic information and precise models for the underlying
mechanisms. However, in recent years useful models for developmental
processes have been elaborated (e.g. by Meinhardt and Gierer), which allow
computer simulations and an application of their basic principles to plant
developmental processes (2). The time has now come to attempt to interpret
different morphological genotypes in terms of these theories. In the
following questions I offer a rough guide and some suggestions for the use
of pea genotypes in such models.
1. Which organs of pea plants seem to be best suited for an inter-
pretation in terms of these models? I think that the leaf is very
appropriate, because it is not as complex as the whole shoot or flower and
differences in leaf morphology are easily observed, which is not the case
with roots. An advantage is that the leaves of Pisum are arranged in a
relatively simple manner along the shoot axis (distichous with a divergence
of 180 ) and show distinct morphological differences depending on their
position (trifid bracts, first true leaf, and adult leaves). Moreover, the
main parts of the normal leaf are easy to distinguish (stipules, leaflets,
tendrils) and show pair configuration in a polar, orderly manner along the
leaf axis.
2. Which principles are useful for the interpretation of the normal
morphogenesis of pea leaves? A specific model already has been proposed
for distichous phyllotaxis (2). It is more difficult to find the framework
for phyllomorphogenesis itself. A first approach for the differentiation
along the leaf axis during the embryonic and vegetative phase seems to be
the model for mutual activation of different determined cell types and of
positional information using a graded "morphogen" gradient within the leaf
primordia.
3. Which leaf mutants of Pisum might be interesting candidates to
test these models? Mutants with striking differences in the foliar con-
figuration are clear choices. Among these are the mutants afila (af)
(transformation of leaflets into tendrils), acacia (tl) (transformation of
tendrils in leaflets), tendri 1 led acacia (tac) as an intermediate form
between tl and the normal form, and the mutant unlfoliata (uni) having a
single leaflet at each node instead of paired leaflets on normal plants.
Another, more complex, case is presented by the mutant cochleata (coch )
with modified stipules which sometimes may resemble whole leaves (Lit. in
[1]).
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PNL VOLUME 18 1986 COMMENTARY
4. Which principles could be used to explain such leaf mutants of
Pisum? There is no satisfactory answer at the moment, but it can be as-
sumed that a change in the regulative part of the genetic program is
responsible for some of these morphological phenomena. If a model of gene
regulation like that of Britten and Davidson is applicable to plants, we
could expect sequence changes in regulatory elements like "integrator"
genes or at the site of "receptor" genes, or possibly at the top of the
hierarchy at the site of "sensor" genes or on the lowest level of
"producer" genes. This could lead to an increase or decrease of special
regulatory gene products ("morphogens"), which allow the establishment of
positional information for cells. The increase or decrease of such gene
products may lead to an abnormal interpretation of the positional Informa-
tion and consequently to the initiation of abnormal morphogenesis. For
instance, in the case of afila a defect in the regulatory element for the
initiation of leaflets on chromosome 1 could lead to an activation of the
genes for tendril formation on chromosome 7. A cause could be a higher
amount of "inhibitor" or a lower amount of "activator" for leaflet
formation. Similar considerations are possible for the acacia phenotype as
a consequence of an alteration in regulating elements in genes for normal
tendril formation on chromosome 7.
5. What role do recombinants of leaf mutants play for models of
morphogenesis? If the parental mutants, e.g. afila and acacia, are inter-
preted in terms of a model, it should be possible to predict the
morphological behavior of the foliar configuration of their recombinant
afila/acacia. This is at the moment impossible - the morphological con-
figuration is too complex - but this would be a very good test for the
underlying models developed for the single mutants and the normal form.
Another interesting point Is the use of recessive homozygotes (--),
dominant homozygotes (++) , and heterozygotes (+-) of presumed regulatory
mutations. Their morphological state might be correlated with normal (++)
or reduced (+—) amounts, or an absence (—) of a specific morphogen. If a
certain threshold in the amount of such a regulating substance is crossed,
we might expect a change in the morphogenetic process.
All these considerations give only a first hint for the use of pea
genotypes in ontogenetic models. Perhaps these models also afford an
Insight into evolutionary mechanisms of the leaf formation in legumes, for
instance into the theory of a reduction line of leaflets observable within
the legume family. However, many ideas, observations, and experiments are
still necessary before the current models i an be used to understand such
morphological phenomena.
1. Gottschalk, W. Die Bedeutung der Genmutationen fur die Evolution de
Pflanzen. Fischer, Stuttgart, 1971.
2. Meinhardt, H. Models of biological pattern formation. Academic
Press, London/New York, 1982.