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extend horizontally from the primary root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
and serve to anchor the plant securely into the soil. This branching of roots also contributes to water uptake, and facilitates the extraction of nutrients required for the growth and development of the plant.
Many different factors are involved in the formation of lateral roots. Regulation of root formation is tightly controlled by plant hormones such as auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...
, and by the precise control of aspects of the cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
. Such control can be particularly useful: increased auxin levels, which help to promote lateral root development
Morphogenesis , is the biological process that causes an organism to develop its shape...
, occur when young leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
primordia form and are able to synthesise the hormone. This allows coordination of root development with leaf development, enabling a balance between carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
to be established.
Early Morphological Changes
The following description is for early events in lateral root formation of the model organism Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
, where lateral roots typically form when the plant is between seven and nine days old.
- Stage I: The first morphologically identifiable stage is the asymmetric division of two cells of the pericycle
The pericycle is a cylinder of parenchyma cells that lies just inside the endodermis and is the outer most part of the stele of plants.Although it is composed of non-vascular parenchyma cells, it is still considered part of the vascular cylinder because it arises from the procambium as do the...
, termed pericycle founder cells, which are adjacent to the protoxylem poles and from which the lateral roots are derived entirely. These cells then undergo further division, causing radial expansion.
- Stage II: The small, central cells then divide periclinally (parallel to the surface of the plant body) in a series of transverse, asymmetric divisions such that the young primordium
A primordium , in embryology, is defined as an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells...
becomes visible as a projection made up of an inner layer and an outer layer.
- Stages III and IV: At the third stage, the outer layer of cells divide so that the primordium is now made of three layers. The fourth stage is then characterised by the inner layer undergoing a similar division, such that four cell layers are visible.
- Stages V to VIII: Expansion and further division of these four layers eventually results in the emergence of the young lateral root from the parent tissue (the overlying tissue of the primary root) at stage eight.