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The action of genes controlling apical dominance in Pisum sativum L.

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Beveridge, CA (1994) The action of genes controlling apical dominance in Pisum sativum L. PhD thesis, University of Tasmania.

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Abstract

Four non-allelic mutants with increased branching,
rmsl, rms2, rms3 and rms4, were utilised to investigate
the control of apical dominance in the garden pea, Pisum
sativum L. The role of indole-3 -acetic acid ( IAA) was
examined in detail by gas chromatography-mass spectrometry
(GC-MS), quantification of free IAA in shoot tissue of
various ages, and by the application of IAA and the auxin
transport inhibitor 2,3,5-triiodobenzoic acid. The
procedure for the purification and quantification of
endogenous IAA was refined to allow rapid routine analysis
of this substance. Wedge grafts of young seedlings were
performed to determine the site of action of the four
ramosus genes and to determine whether or not a grafttransmissible
substance is involved in their action. The
levels of c ytokinins zeatin riboside and dihydrozeatin
riboside were also quantified by GC-MS from the xylem sap
of rms2 and wild-type roots, after the development of a
suitable extraction and quantification technique.
Branching was inhibited in rmsl and rms2 scions when
grafted to wild-type stocks while rmsl and rms2 stocks
were not able to promote branching in wild-type scions.
The most plausible explanation of these results is that
the rmsl and rms2 mutations act to promote branching by
altering the level of a hormone-like substance which is
produced in the rootstock and shoot. Further grafting
studies indicated that the Rmsl gene acts prior to the
Rms2 gene in the biosynthetic pathway for the same
substance. It is unclear whether the rmsl and rms2
mutations act by increasing the level of a branching
promoter, or by decreasing the level of a branching inhibitor,
although the latter seems more likely.
Similar grafting studies indicated that the rms3
mutation acts predominantly in the shoot but also in the
rootstock and appears to alter the level of a grafttransmissible
substance other than that controlled by the
Rmsl and Rms2 genes. As above, the specific nature of the
substance controlled by the Rms3 gene has not been
elucidated. In contrast, the rms4 mutation does not
appear to control the level of a graft-transmissible
substance. IAA quantifications from mutant and wild-type shoot
tissue indicated that the increased branching in each of
the mutant shoots was not attributable to a reduced level
of endogenous IAA in comparison with the level in wildtype
plants.
Mutant rms4 plants contained normal levels of IAA in
comparison with wild-type plants. Branching in rms4
plants was not inhibited by IAA application. Furthermore,
rms4 rootstocks appeared over-responsive to the grafttransmissible
substance produced by rms2 scions since rms4
rootstocks exhibited a promotion of lateral growth at the
cotyledonary node when grafted to the rms2 mutant scions
compared with the rms4 rootstocks grafted to rms4 or wildtype
scions. It is therefore suggested that the Rms4 gene
influences the response to factors involved in the control
of branching.
The rmsl, rms2 and rms3 plants contained an elevated
lev.el of IAA in comparison with wild-type plants, prior
to, during, and after bud release. This accumulation of
IAA in rmsl, rms2 and rms3 plants was not simply due to
imminent or actual lateral bud growth or release as
firstly, it did not occur in comparable rms4 plant
portions, and secondly, was present in rms2 scions in
which branching was inhibited by grafting to wild-type
rootstocks. Possible explanations for the accumulation of
IAA in rmsl, rms2 and rms3 plants are discussed in view of
the conventional theory that IAA acts to inhibit lateral
branching.
In the case of the rms2 plants, the level of zeatin
riboside and dihydrozeatin riboside in the xylem sap of
the rootstock did not appear to be significantly different
from that in wild-type plants. Since these cytokinins may
be the most a bundant and physiologically important
cytokinins in the root xylem sap it appears unlikely that
the graft-transmissible substance controlled by the Rms2
gene in the rootstock is a cytokinin. The possibility
that a novel substance may be involved in the control of
branching in pea is suggested.
Studies on the effect of the flowering genes Sn, Dne
and Ppd on branching in pea supported the proposition that
these genes influence the pattern of branching along the
stem.

Item Type: Thesis (PhD)
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Date Deposited: 09 Dec 2013 02:46
Last Modified: 15 Sep 2017 01:06
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