PNL Volume 14
Ingensiep, H. W. Institute of Genetics, University of Bonn
Federal Republic of Germany
Many of the morphological differences among mutants or recombinants
may result from the action of genes that control plant growth regulators
such as auxins. Experiments show that exogeneously applied auxins are
able to induce several morphological changes in plant form and size. In
such experiments different morphogenetic effects often are caused by
different auxins applied under equimolar conditions, and for Pisum
sativum these different effects could be correlated with differences in
metabolic availability and translocation of the applied auxins (1).
These studies led us to propose a simple test system to screen for
mutants which affect auxin metabolism, using intact pea seedlings.
Experimental background: Our previous experiments showed that nor-
mal pea seedlings exhibit a characteristic morphogenetic effect when
auxins 2,4-D, IAA, and NAA were applied via the root system (1).
Seedlings treated with 2,4-D were strongly inhibited in root and shoot
development, while IAA- and NAA-treated seedlings were not. This could
be correlated with higher metabolic availability of the IAA and NAA
molecules by different mechanisms: IAA is mainly decarboxylated or
conjugated to aspartic acid; NAA is mainly conjugated; and 2,4-D is
neither decarboxylated nor conjugated to any extent. In the case of IAA
and NAA, these mechanisms prevent the morphogenetic alteration of the
seedling by inactivating many of the free molecules. Hence, these
auxins are not able to reach the vascular tissue in the root in high
amounts and are not translocated into the shoot to cause morphogenetic
aberrations there. On the other hand, 2,4-D does reach the vascular
tissue and is actively translocated in lethal amounts into the shoot.
PNL Volume 14
This experimental background was used for the following two-step
screening system for auxin-sensitive pea mutants. It is supposed that
the roots act as selective "filters" for the auxins and the shoots as
visible indicators of auxin action in the plants. If the shoot of a
given plant differs morphologically from the initial line, this may be a
hint that the plant exhibits genetically controlled changes in auxin me-
tabolism. For instance, if IAA-treated pea seedlings show strong
inhibition compared to the control, this may be due to the lower inac-
tivation capacity of this mutant. On the other hand, plants may occur
that show normal shoot development after being treated with 2,4-D. This
could be due to metabolic changes affecting the free molecule to greater
extent and resulting in resistance. Between these extreme reactions
other seedlings may occur which show auxin-sensitivity. These mor-
phological deviations could be observed in the first step, where the
initial line and mutants are treated with the three auxins as described.
In a second step, suspicious plants could be analyzed by physiological
methods to characterize quantitative changes in auxin metabolism, for
instance by chromatographical analysis of labelled auxins as described
(1). It must, of course, be recognized that other factors may be in-
volved in the observed changes.
Nevertheless this assay could be of help in detecting auxin-
sensitive pea mutants in those cases where auxin-dependent variations
are suspected. The advantage is that the procedure is simple, inexpen-
sive, and rapid (possible effects are detected within two weeks with a
few seeds). The more involved in vitro culture methods could be applied
after prescreening intact pea seedlings with this system.
1. Ingensiep, H. W. , ejt_ ai- 1981. PNL 13:21-23.