PNL VOLUME 18

1986 COMMENTARY

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]).

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.