Others are drawn to it because of the sense of dominance they think the drink provides. Principally to ensure military dominance, the conquerors made many main roads, mostly centering in London. But a raging storm, the sense of the absolute dominance of nature and the littleness of man, always exalted her. There was no hesitation, no stoppage in their service; under the dominance of mind these passive forces became active auxiliaries. Moran was in a dangerous frame of mind, and past the dominance of his employer.
New Word List Word List. Save This Word! According to the auxin transport canalization model, auxin export from a bud into a stem is a necessary condition for bud outgrowth. The stem is saturated with auxin from the shoot apex; buds cannot export auxin into the stem, and thus become dormant syn. The second, diversion, theory of the s explains the phenomenon of apical dominance as auxin from the shoot apex prevents growth promoting factors synthesized in roots and cotyledons from entering into buds and stimulate bud outgrowth.
The diversion theory is similar to the current second messenger model, which proposes apically derived auxin inhibits bud outgrowth by regulating the level or activities of other plant hormones Beveridge et al. Consistent with this auxin inhibits the biosynthesis of cytokinin in the stem that stimulates bud outgrowth and promotes the biosynthesis of strigolactone in the roots that inhibits bud outgrowth Beveridge et al.
More than eight decades of research on the role of auxin in apical dominance has led to the formulation of the two current theories: auxin transport canalization and second messenger. However, the modes of action of auxin in apical dominance are not yet completely understood. For example, bud outgrowth in response to decapitation occurs prior to any change in the level of auxin in the decapitated stem Morris et al. In addition, although inhibition of bud outgrowth by applying auxin to the stump of decapitated plants is dependent on strigolactones, auxin inhibits bud outgrowth in excised stems of strigolactone-deficient mutants Young et al.
Mutant analysis of some cytokinin-biosynthetic and response genes in Arabidopsis also questions the essential function of cytokinin in bud outgrowth resulting from decapitation Muller et al. Application of auxin to decapitated stump of Arabidopsis and bean Phaseolus vulgaris does not fully restore apical dominance indicating factors other than auxin inhibit bud outgrowth in intact plants Cline, Therefore, the auxin transport canalization and second messenger models cannot entirely explain the phenomenon of apical dominance for detailed discussion, see Domagalska and Leyser, The third, indirect, theory proposed in the s explains apical dominance as auxin promotes the growth of the stem below the shoot apex, and the growth of the stem indirectly inhibits bud outgrowth Snow, This third theory of apical dominance has been completely overlooked.
Furthermore, although shoot branching is regulated by environmental and hormonal signals, the focus of research on shoot branching during the past years was on apical dominance in eudicots such as pea because of the ease of application of hormonal treatments directly to the bud and measuring responses without damaging the plant Beveridge et al. With the advent of modern physiological and genetic tools in recent decades, characterization of shoot branching mutants in diverse species has been useful in identifying genes and intrinsic and environmental factors regulating dormancy and outgrowth of axillary buds Leyser, ; Kebrom et al.
The link between an increase in plant height and a reduction in shoot branching, and vice versa, in hormonal or light signaling mutants in diverse species Beveridge, ; Stirnberg et al. However, recent results demonstrating a link between stem internode elongation and inhibition of bud outgrowth Kebrom et al. It is well established that apically derived auxin promotes stem elongation and inhibits shoot branching, and highly branched mutants of auxin biosynthesis, transport or signaling pathways in diverse species are dwarfed.
For example, plant height is strongly reduced and shoot branching increased in the auxin resistant 1 axr1 mutant of Arabidopsis Lincoln et al. Reduced expression of the tomato auxin signaling gene SlIAA15 concomitantly reduces plant height and increases shoot branching Deng et al. Furthermore, auxin-overproducing yucca plants are elongated and display increased apical dominance. Transgenic reduction in the level of auxin suppressed the elongated primary shoot and increased apical dominance phenotype of yucca mutants Zhao et al.
Furthermore, shade signals promote shoot elongation and inhibit shoot branching Franklin and Whitelam, The elongation growth response to shade is mediated through an increase in the biosynthesis, transport and signaling of auxin in leaves Tao et al. It is also important to highlight that while apical dominance is often observed in eudicots such as pea it is not always apparent in grass species such as wheat Figure 2.
Plants grow through production of successive phytomers. Each phytomer has a leaf, a node, an internode and an axillary bud. In eudicots such as pea that display strong apical dominance, stem internodes elongate during the vegetative phase but buds do not grow Figure 2A.
When the growing shoot apex is decapitated, bud outgrowth begins immediately. A method known as shoot inversion in Ipomoea nil morning glory promotes bud outgrowth by suppressing the elongation of the stem section below the shoot tip Hosokawa et al. In the grasses, stem internode elongation is suppressed during the vegetative phase and the shoot apex, enclosed by the sheath and young leaves, remains close to the base of the plant near the soil surface.
During this phase tillers basal branches are formed and the tillering phase normally overlaps with the vegetative phase Figure 2B. Once the shoot apex transitions to flowering phase, internodes begin to elongate and the tillering phase ends McMaster, A more direct link between the inhibition of bud outgrowth by a growing stem has been identified in the tiller inhibition tin mutant of wheat Kebrom et al.
In tin , early cessation of tillering is associated with precocious internode elongation. In summary, there is ample evidence that supports the indirect theory that a growing stem inhibits bud outgrowth. Stem growth and shoot branching in eudicots and monocots.
A In eudicots such as pea, stem internodes elongate during the vegetative phase and shoot branching is inhibited. However, pea plants develop branches indicated by blue arrows from lower nodes adiacent to shortened internodes. B In monocots such as wheat, internodes do not elongate during the vegetative phase and tillers basal branches, indicated by blue arrows develop from the shortened internodes at the base of the shoot. C When grown at high density enriched with shade signals from neighbor plants, internodes of monocots such as sorghum elongate and bud outgrowth is suppressed.
When the density is reduced internode elongation is suppressed and branches indicated by blue arrows develop. The indirect theory of apical dominance proposed by Snow and others in the s did not detail how auxin induced stem growth might inhibit bud outgrowth. Recent discoveries on the role of sugars in shoot branching in wheat, pea, sorghum, Arabidopsis, chrysanthemum, Rosa species, grapevine, and poplar provide new insights into the indirect theory of apical dominance Kebrom et al.
The inhibition of bud outgrowth in the tin mutant wheat is associated with precocious stem internode elongation and reduced sugar level in the buds Kebrom et al. In pea, the sugar level in a dormant bud increases when the bud is stimulated to grow by decapitation that removes a growing shoot tip, which is a strong sink for sugars Mason et al.
In addition, dormant buds in intact pea plants grow when directly fed with sucrose providing conclusive evidence for the significance of sugars for bud outgrowth Mason et al. Bud dormancy in the phytochrome B mutant sorghum phyB-1 is associated with an increase in plant height and up-regulation of genes marker for sucrose deprivation in the buds Kebrom and Mullet, Therefore, inhibition of bud outgrowth in the tin mutant wheat, pea and phyB-1 sorghum is associated with enhanced growth of the main shoot and reduced sugar level in the dormant buds.
Defoliation experiments in sorghum demonstrate that a small reduction in photosynthetic leaf area inhibits bud outgrowth while a more sever defoliation inhibits the growth of other sink organs including newly formed leaves in the main shoot Kebrom and Mullet, In the presence of strong sink organs such as a growing stem and limited sugar production in the main shoot, buds may become dormant.
Therefore, the indirect theory of apical dominance can now be further elucidated as auxin-induced stem growth indirectly inhibits buds by depriving sugars necessary for their growth. Apical dominance refers to the inhibition of bud outgrowth by the shoot apex. The dormancy versus outgrowth fates of axillary buds, and thus shoot branching is also controlled by other intrinsic and environmental factors besides auxin that act within or outside the bud Leyser, ; Janssen et al.
A significant increase or decrease in plant height is commonly noticed in shoot branching mutants when the site of action of a gene is outside the bud. For example, strigolactones are synthesized primarily in the root, and almost all highly branched strigolactone biosynthesis mutants in diverse species are dwarf Beveridge, ; Stirnberg et al.
The reduction in plant height in strigolactone mutants is not due to enhanced lateral branching de Saint Germain et al. Since strigolactones promote internode elongation de Saint Germain et al.
In contrast, plant height and branching can be uncoupled when the site of action of the gene is in the bud. For example, the loss of function teosinte branched1 tb1 mutant of maize branch profusely while the height of the main shoot is not significantly different from the wild type Guan et al. Mutation in the tb1 ortholog brc1 gene in Arabidopsis is non-pleiotropic and specifically increases shoot branching Aguilar-Martinez et al. Furthermore, although cytokinins promote bud outgrowth when applied directly to the bud, buds in cytokinin deficient Arabidopsis plants grow in response to decapitation Muller et al.
Therefore, it appears that factors that control shoot branching by acting outside the bud override those that act within the bud and induce or inhibit bud outgrowth. As yet there is no known signal from the main shoot that is transmitted to the bud and controls its activity. However, sugar supply from the main shoot to the bud would be indispensable for bud outgrowth; the sucrose might also serve as a signaling molecule promoting bud outgrowth Rabot et al.
Since an increase in plant height in response to environmental and intrinsic factors in diverse species is associated with a reduction in shoot branching, and dwarfism is associated with enhanced shoot branching, it is likely that shoot branching is determined mainly by source—sink status of the main shoot.
The plant source—sink relationship is a very complex process that depends on many factors including photosynthetic leaf area and efficiency, size and position of competing sinks, plant hormone dynamics and growth stage of the plant, and availability of nutrients such as nitrogen, light, and water Lemoine et al. For example, a small reduction in photosynthetic leaf area due to disease or herbivory could result in the inhibition of bud outgrowth in particular during the early stage of plant growth and development Kebrom and Mullet, It is also possible that plants with relatively small photosynthetic leaf area at early stages of development such as Arabidopsis may not be able to develop branches during the vegetative stage.
In sorghum, stem internodes are formed during the vegetative phase and elongate in response to high planting density or shade signals Kebrom et al. As shown in Figure 2C , the length of internodes in a sorghum plant increased and reduced by alternating high and low plant density, respectively, and branches developed from buds adjacent to shortened internodes.
In pea that displays strong apical dominance branches can still develop from buds in the lower nodes Boyer et al. In maize, the length of internodes is negatively correlated to the number and size of ears that develop from axillary buds Xu et al. Therefore, the size of internodes adjacent to the buds determines the sink strength of the internodes for sucrose utilization and storage, and indirectly regulates availability of sugars to the buds.
However, a plant may grow taller and develop more branches when it synthesizes photoassimlates in excess. A concomitant reduction in plant height and shoot branching could also occur under poor growing condition. For example, Arabidopsis plants grown in low nitrogen are shorter and developed fewer branches than those grown at higher nitrogen de Jong et al. In addition, mutations that reduce the overall growth of a plant might reduce both plant height and shoot branching.
In fact, some of the plants reported as shoot branching mutants could be defective in the growth and development of the main shoot. For example, the reduced tillering tin wheat mutant is defective in the timing of development of internodes Kebrom et al. Therefore, it appears that the tremendous variation in the number of branches and their position observed within and between species of annual plants could be in part due to variations in source—sink status of the main or parent shoots indirectly affecting the dormancy versus outgrowth fates of axillary buds.
It is well established that during apical dominance auxin from the shoot apex inhibits bud outgrowth indirectly without entering into buds. The two current theories of apical dominance, auxin transport canalization, and second messenger, describe processes in the main shoot in response to auxin from the shoot apex, including an increase in the level of strigolactones and a decrease in the level of cytokinins, leading to enhanced stem growth and formation of vascular tissues.
Thus, decapitating pruning a shoot will cause it to branch! Plants, like animals, produce hormones to regulate plant activities, including growth. They need these hormones to respond well to their environment and to sustain growth, development, and dispersal.
Plant biologists recognize five major groups of plant hormones: auxins, gibberellins, ethylene, cytokinins, and abscisic acid. Find out in this guide the importance of each hormone in the life of a plant Read More. Plants produce hormones to regulate their growth. Auxins, for instance, influence plant growth. Know the role of auxin in plant growth and development in this tutorial Skip to content Main Navigation Search.
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