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. Therefore, apically derived-auxin stimulates the growth of stem internodes in the main shoot and internode growth, which is a strong sink, inhibits buds indirectly by depriving sugars necessary for their growth Figure 3.
Intrinsic and environmental factors besides auxin that promote the growth and development of new sink organs including stem internodes and reproductive organs could also inhibit shoot branching indirectly by limiting sugars available for bud outgrowth. On the other hand, dwarfism in the absence of either auxin or strigolactones might stimulate shoot branching by making excess sugars available for growing buds. Therefore, shoot branching might be an unintended consequence of source—sink relationships and result from an overflow of sugars to axillary buds that cannot be utilized by the main shoot.
While bud outgrowth depends on sugar supply from the main shoot, subsequent growth of the developing branch depends on an ample supply of nutrients and water from the roots. Nutrients are also one of the major factors determining the source—sink status, and thus indirectly regulate shoot branching. A model for the inhibition of bud outgrowth by a growing stem.
A The growth of stem intemodes in auxin or strigolactone deficient mutant plants or plants grown at high light intensity or low planting density is suppressed. A short intemode is not strong sink for sucrose. Therefore, excess sucrose exported from photosynthetic leaves to the stem overflow into axillary buds and induces bud outgrowth.
B Intrinsic factors such as auxin and strigolactones and environmental factors such as shade promote stem intemode elongation in the main shoot. Elongated intemode, which is a strong sink, inhibits bud outgrowth indirectly by limiting sugar supply to axillary bud. The plant source—sink relations is regulated by intrinsic and environmental factors making shoot branching a complex trait that cannot be predicted easily without considering the growth and developmental status of the whole plant and prevailing environmental conditions.
Reappraisal of the source—sink status in shoot branching mutants and wild-types and systematic study of the effect of source—sink status of the main shoot on dormancy and outgrowth of axillary buds might advance our knowledge of the physiological basis of apical dominance and shoot branching in plants. Future studies should accurately determine the sink or source status of an organ being manipulated. For example, the cotyledons in pea contribute to seed germination.
The nutrient reserve and biomass of the cotyledons are exhausted within the first 10 days after sowing, during which the plant transitions from heterotrophic to autotrophic growth Hanley et al. Experiments involving cotyledon removal or defoliation of young newly formed or old non-photosynthetic leaves assuming that they are source of nutrients or photoassimilates might lead to incorrect conclusions. Besides their role in shoot branching, sugars are also important in many other aspects of plant growth and development including phase transitions from juvenile to adult and from vegetative to flowering Wahl et al.
Therefore, when investigating plant growth and development, sugar demand and supply should be taken into consideration. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author thanks Dr. Aguilar-Martinez, J. Plant Cell 19, — Albacete, A. Hormonal and metabolic regulation of source-sink relations under salinity and drought: from plant survival to crop yield stability.
Barbier, F. Sucrose is an early modulator of the key hormonal mechanisms controlling bud outgrowth in Rosa hybrida. Beveridge, C. Long-distance signalling and a mutational analysis of branching in pea. Plant Growth Regul. Pea has its tendrils in branching discoveries spanning a century from auxin to strigolactones. Plant Physiol.
Booker, J. Auxin acts in xylem-associated or medullary cells to mediate apical dominance. Plant Cell 15, — Boyer, F. Structure-activity relationship studies of strigolactone-related molecules for branching inhibition in garden pea: molecule design for shoot branching.
Cline, M. Exogenous auxin effects on lateral bud outgrowth in decapitated shoots. Auxin and strigolactone signaling are required for modulation of Arabidopsis shoot branching by nitrogen supply. Strigolactones stimulate internode elongation independently of gibberellins. Auxin-mediated plant architectural changes in response to shade and high temperature. Deng, W. The tomato SlIAA15 is involved in trichome formation and axillary shoot development.
New Phytol. Dierck, R. Response to strigolactone treatment in chrysanthemum axillary buds is influenced by auxin transport inhibition and sucrose availability. Acta Physiol. Domagalska, M. Signal integration in the control of shoot branching. Cell Biol. Ferguson, B. Roles for auxin, cytokinin, and strigolactone in regulating shoot branching.
Finlayson, S. Phytochrome regulation of branching in Arabidopsis. Franklin, K. Phytochromes and shade-avoidance responses in plants. Guan, J. Diverse roles of strigolactone signaling in maize architecture and the uncoupling of a branching-specific subnetwork. Hall, S. Correlative inhibition of lateral bud growth in Phaseolus vulgaris L.
Planta , — Hanley, M. Early plant growth: identifying the end point of the seedling phase. Hosokawa, Z. Apical dominance control in ipomoea-nil - the influence of the shoot apex, leaves and stem.
Ishikawa, S. Suppression of tiller bud activity in tillering dwarf mutants of rice. Plant Cell Physiol. Janssen, B. Regulation of axillary shoot development. Plant Biol. Kebrom, T. Tillering in the sugary1 sweet corn is maintained by overriding the teosinte branched1 repressive signal.
Plant Signal. Phytochrome B represses Teosinte Branched1 expression and induces sorghum axillary bud outgrowth in response to light signals.
Inhibition of tiller bud outgrowth in the tin mutant of wheat is associated with precocious internode development. Dynamics of gene expression during development and expansion of vegetative stem internodes of bioenergy sorghum.
Biofuels 10, Photosynthetic leaf area modulates tiller bud outgrowth in sorghum. Plant Cell Environ. Transcriptome profiling of tiller buds provides new insights into PhyB regulation of tillering and indeterminate growth in sorghum.
Grasses provide new insights into regulation of shoot branching. Trends Plant Sci. Lemoine, R. Source-to-sink transport of sugar and regulation by environmental factors. Plant Sci. Leyser, O. The control of shoot branching: an example of plant information processing.
Lincoln, C. Growth and development of the axr1 mutants of Arabidopsis. Plant Cell 2, — Mason, M. Sugar demand, not auxin, is the initial regulator of apical dominance.
McMaster, G. Phytomers, phyllochrons, phenology and temperate cereal development. Morris, D. Transport of exogenous auxin in two-branched dwarf pea seedlings Pisum sativum L.
Planta , 91— Morris, S. Auxin dynamics after decapitation are not correlated with the initial growth of axillary buds. Muller, D. Auxin, cytokinin and the control of shoot branching. Not all bacteria utilize the above route of nitrogen fixation. Many that live free in the soil, utilize other chemical pathways. Nitrogen uptake and conversion by various soil bacteria. Roots have extensions of the root epidemal cells known as root hairs. While root hairs greatly enhance the surface area hence absorbtion surface , the addition of symbiotic mycorrhizae fungi vastly increases the area of the root for absorbing water and minerals from the soil.
Role of the root hairs in increasing the surface area of roots to promote increased uptake of water and minerals from the soil. Animals have a circulatory system that transports fluids, chemicals, and nutrients around within the animal body. Some plants have an analogous system: the vascular system in vascular plants; trumpet hyphae in bryophytes. Root hairs are thin-walled extensions of the epidermal cells in roots. They provide increased surface area and thus more efficient absorption of water and minerals.
Water and dissolved mineral nutrients enter the plant via two routes. Water and selected solutes pass through only the cell membrane of the epidermis of the root hair and then through plasmodesmata on every cell until they reach the xylem: intracellular route apoplastic.
Water and solutes enter the cell wall of the root hair and pass between the wall and plasma membrane until the encounter the endodermis , a layer of cells that they must pass through to enter the xylem: extracellular route symplastic. The endodermis has a strip of water-proof material containing suberin known as the Casparian strip that forces water through the endodermal cell and in such a way regulates the amount of water getting to the xylem.
Only when water concentrations inside the endodermal cell fall below that of the cortex parenchyma cells does water flow into the endodermis and on into the xylem.
Xylem is the water transporting tissue in plants that is dead when it reaches functional maturity. Tracheids are long, tapered cells of xylem that have end plates on the cells that contain a great many crossbars. Tracheid walls are festooned with pits. Vessels , an improved form of tracheid, have no or very few obstructions crossbars on the top or bottom of the cell.
The functional diameter of vessels is greater than that of tracheids. Water is pulled up the xylem by the force of transpiration , water loss from leaves.
Mature corn plants can each transpire four gallons of water per week. Transpiration rates in arid-region plants can be even higher. Water molecules are hydrogen bonded to each other.
Water lost from the leaves causes diffusion of additional water molecules out of the leaf vein xylem, creating a tug on water molecules along the water columns within the xylem. This "tug" causes water molecules to rise up from the roots to eventually the leaves.
The loss of water from the root xylem allows additional water to pass throught the endodermis into the root xylem. Cohesion is the ability of molecules of the same kind to stick together. Water molecules are polar, having slight positive and negative sides, which causes their cohesion. Inside the xylem, water molecules are in a long chain extending from the roots to the leaves. Adhesion is the tendency of molecules of different kinds to stick together. Water sticks to the cellulose molecules in the walls of the xylem, counteracting the force of gravity and aiding the rise of water within the xylem.
Transpiration exerts a pull on the water column within the xylem. The lost water molecules are replaced by water from the xylem of the leaf veins, causing a tug on water in the xylem. Adhesion of water to the cell walls of the xylem facilitates movement of water upward within the xylem.
This combination of cohesive and adhesive forces is referred to as the Cohesion-Adhesion Theory. Guard cells are crescent-shaped cells of the epidermis that flank the stoma and regulate the size of the opening. Together, the guard cells and stoma comprise the stomatal apparatus.
The inner wall of the guard cell is thicker than the rest of the wall. When a guard cell takes up potassium ions, water moves into the cell, causing the cell to become turgid and swell, opening the stoma. When the potassium leaves the guard cell, the water also leaves, causing plasmolysis of the cells, and a closing of the stoma.
Plants make sugar by photosynthesis, usually in their leaves. Some of this sugar is directly used for the metabolism of the plant, some for the synthesis of proteins and lipids, some stored as starch.
Other parts of the plant also need energy but are not photosynthetic, such as the roots. Food must therefore be transported in from a source, an action accomplished by the phloem tissue. Phloem consists of several types of cells: sieve tube cells aka sieve elements , companion cells , and the vascular parenchyma. Sieve cells are tubular cells with endwalls known as sieve plates. Most lose their nuclei but remain alive, leaving an empty cell with a functioning plasma membrane. Companion cells load sugar into the sieve element sieve elements are connected into sieve tubes.
Fluids can move up or down within the phloem, and are translocated from one place to another. Sources are places where sugars are being produced. Sinks are places where sugar is being consumed or stored. Food moves through the phloem by a Pressure-Flow Mechanism.
Sugar moves by an energy-requiring step from a source usually leaves to a sink usually roots by osmotic pressure. The pressure causes the sap to flow toward an area of lower pressure, the sink. In the sink, the sugar is removed from the phloem by another energy-requiring step and usually converted into starch or metabolized.
One plant response to environmental stimulus involves plant parts moving toward or away from the stimulus, a movement known as a tropism. Nastic movements are plant movements independent of the direction of the stimulus. Charles Darwin and his son Francis studied the familiar reaction of plants growing toward light: phototropism. The Darwins discovered that the tips of the plant curved first, and that the curve extended gradually down the stem. By covering the tips with foil, they prevented the plant from curving.
They concluded that some factor was transmitted from the tip of the plant to the lower regions, causing the plant to bend.
We now know, from the experiments of Frits Went, that auxin, a plant hormone produced in the stem tip auxins promote cell elongation , moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant.
Geotropism is plant response to gravity. Roots of plants show positive geotropism, shoots show negative geotropism. Geotropism was once thought a result of gravity influencing auxin concentration.
Several new hypotheses are currently under investigation. Note the root grows down irregardless of the orientation of the seed. Thigmotropism is plant response to contact with a solid object. Tendrils of vines warp around objects, allowing the vine to grow upward. Note the tendril of this passion flower wrapping around the metal rod. Nastic movements, such as nyctinasty , result from several types of stimuli, including light and touch.
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