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The action of genes controlling apical dominance in Pisum sativum L.
thesisposted on 2023-05-26, 01:51 authored by Beveridge, CA
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.
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