Lateral root

Last updated January 05, 2023

Lateral roots, emerging from the pericycle (meristematic tissue), extend horizontally from the primary root (radicle) and over time makeup the iconic branching pattern of root systems.^[1] They contribute to anchoring the plant securely into the soil, increasing water uptake, and facilitate the extraction of nutrients required for the growth and development of the plant.^[2] Lateral roots increase the surface area of a plant's root system and can be found in great abundance in several plant species.^[1] In some cases, lateral roots have been found to form symbiotic relationships with rhizobia (bacteria) and mycorrhizae (fungi) found in the soil, to further increase surface area and increase nutrient uptake.^[1]

Several factors are involved in the formation and development of lateral roots. Regulation of root formation is tightly controlled by plant hormones such as auxin, and by the precise control of aspects of the cell cycle.^[3] Such control can be particularly useful, as increased auxin levels help to promote lateral root development, in young leaf primordia. This allows coordination of root development with leaf development, enabling a balance between carbon and nitrogen metabolism to be established.

Morphology and Development

The general zones of the primary root (taproot) that gives rise to eventual lateral roots are presented below from top to bottom. The most mature and developed tissue is found near the top, while the newly dividing cells are found near the bottom.^[1]

Maturation Zone: Cells in this stage have developed differentiated characteristics and have completed maturation and elongation. The xylem system is seen to develop in this zone along with lateral root development.

Elongation Zone: Cells in this stage are rapidly elongating and parts of the phloem system (sieve tubes) start to develop. As you move up closer to the maturation zone, cell division and, elongation decrease.

Meristematic Zone: Right above the root cap and contains the "stem cells" of the plant. In this zone, cells are dividing quickly and there is little to no differentiation present.

Root Cap: Protective layer of cells that covers the meristematic tissue. The cells in this part of the root have been seen to play a critical role in gravitropic response and releasing secretions to mobilize nutrients.

The following description is for early events in lateral root formation of the model organism Arabidopsis thaliana:

Lateral root formation is initiated in pericycle (located between the endodermis and vascular tissue) of the root system, and begins with a process referred to as priming. In this stage, you have rhythmic bouts of gene expression and responses to auxin. If sufficient signaling is present, pre-branching sites are developed in basal portions of meristematic tissue that are stable in the presence of high auxin environments. These pre-branching sites go on to form the pericycle founder cells after they are stable and have high auxin accumulations. In some cases, the activation of auxin biosynthesis takes place in these founder cells to reach a stable threshold.^[2]

Stage I: The first morphologically identifiable stage is the asymmetric division of two cells of the pericycle, 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.^[4]
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 becomes visible as a projection made up of an inner layer and an outer layer.^[4]
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 characterized by the inner layer undergoing a similar division, such that four cell layers are visible.^[4]
Stages V to VIII: Expansion and further division of these four layers eventually result in the emergence of the young lateral root from the parent tissue (the overlying tissue of the primary root) at stage eight.^[4]

The number of lateral roots corresponds to the number of xylem bundles,^[4] and two lateral roots will never be found directly across from one another on the primary root.^[2]

Signaling

Signaling is important for the overall development and growth of a plant, including the production of lateral roots. Several hormones are used by plants to communicate, and the same molecule can have starkly different effects in varying parts of the plant.^[1] Auxin is a good example of this, as it generally stimulates growth in the upper part of a plant when in high concentrations, but in roots, inhibits the elongation and growth of the roots when found in high concentrations.^[1] Root growth is often stimulated by another hormone, called ethylene, which is prevented from being produced in the roots when auxin levels are high. Additionally, it was found that low levels of auxin are actually found to stimulate the growth and elongation of the root system, even without the presence of ethylene.^[1] Cytokinin, another plant hormone, has also been seen to play a role in maintaining and developing the meristematic tissue of the root, and can often have an antagonistic relationship with auxin in root development.^[1]

Auxin Signaling

In a research study of auxin transport in Arabidopsis thaliana, auxin was found to be a critical plant hormone in the formation of lateral roots. In Stage I of early morphological stages, the division of pairs of pericycle founder cells were found in groups of eight or 10, suggesting that before this initial morphological stage, transverse divisions must be conducted first to precede lateral root initiation.^[5]

A specific auxin transport inhibitor, N-1-naphthylphthalamic acid (NPA) causes indoleacetic acid (IAA) accumulation in the root apical meristem, while simultaneously decreasing IAA in radical tissue required for lateral root growth.^[5]

Numerous mutants associated with auxin indicated an effect on lateral root development:

alf4, which blocks the initiation of lateral root emergence.
alf3, which inhibits the development of plant organs shortly after later root emergence.

The results from these mutants indicate that IAA is required for lateral roots in various stages of development.^[5]

Also, researchers found a close relationship between the position of the first division of lateral root formation and the root tip.^[5] A cycB1:1::uidA selectable marker was used as a reporter for lateral root initiation and its early mitotic events.^[6] This marker was histochemically stained for beta-glucuronidase (GUS) in Arabidopsis thalia seedlings, which highlighted activity in the lateral root primordium and the transition zone between the hypocotyl and the root. Seedlings were harvested every day for a week and stained for GUS activity, then measured the primary root length as well as the distance to the root tip, the ratio between these two numbers being consistent. From this study, the following was concluded:

There is a defined distance from the initiation of the lateral root and leaf primordia to their apical meristems.
The tissues with zones of lateral root initiation are co-localized with the same root tissues that are involved in basipetal auxin transport.
Basipetal auxin transport is necessary for the localization of IAA to the zone of lateral root initiation.^[5]

PIN Transport Proteins

Auxin is responsible for generating concentration gradients to allow for proper plant development. As of 2020, one auxin transporter was identified as a means to flood the hormone into cells: AUXIN-RESISTANT1 (AUX1)/AUX1-LIKEs (LAXs). Also, two auxin transporters that allowed for the hormone to exit cells, PIN-FORMEDs (PINs) were established, as well as ATP-binding cassette Bs (ABCBs)/P-glycoproteins (PGPs).^[7] PIN proteins steer auxin to areas of necessity throughout the plant. These proteins present in the apical meristem of the plant direct auxin downward through the plant, a process independent of gravity.^[1] Once in the vicinity of the root, vascular cylinder cells shuttle auxin towards the center of the root cap. Lateral root cells then absorb the phytohormone through AUX1 permease.^[1] PIN proteins recirculate the auxin upwards to the plant shoots for direct access to the zone of elongation.^[1] Once utilized there, the proteins are then shuttled back to the lateral roots and their corresponding root caps. This entire process is known as the foundation model.^[1]

In Arabidopsis thaliana, PIN proteins are localized in cells based on the size of their loop that connects the intercellular matrix to the extracellular matrix. Shorter PIN proteins (PINs 1-4, 6, 7) are found intracellularly as well as nearest to the plasma membrane, whereas the longer proteins (PINs 5, 8) are found almost exclusively by the plasma membrane.^[7]

The protein PIN8 significantly influences the development of lateral roots in a plant.^[7] When a nonfunctional mutant of the protein, pin8, was inserted into a plasmid, the lateral roots of Arabidopsis thaliana had a decrease in root density.^[7] It was shown that this mutant had no lingering effects on the development of the primary root. When further investigated, it was discovered that the pin8 mutant was significant only as the lateral root was beginning to appear in the plant, suggesting that a function PIN8 protein is responsible for this action.^[7]

Related Research Articles

In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the surface of the soil, but roots can also be aerial or aerating, that is, growing up above the ground or especially above water.

In botany, apical dominance is the phenomenon whereby the main, central stem of the plant is dominant over other side stems; on a branch the main stem of the branch is further dominant over its own side twigs.

<span class="mw-page-title-main">Meristem</span> Type of plant tissue involved in cell proliferation

The meristem is a type of tissue found in plants. It consists of undifferentiated cells capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells continue to divide until a time when they get differentiated and then lose the ability to divide.

<span class="mw-page-title-main">Plant hormone</span> Chemical compounds that regulate plant growth and development

Plant hormone are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and through to reproductive development. Unlike in animals each plant cell is capable of producing hormones. Went and Thimann coined the term "phytohormone" and used it in the title of their 1937 book.

Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essential for plant body development. The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s. Kenneth V. Thimann became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA). Went and Thimann co-authored a book on plant hormones, Phytohormones, in 1937.

<span class="mw-page-title-main">Cytokinin</span> Class of plant hormones promoting cell division

Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence.

Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation form the internal organs of the organism.

Hydrotropism is a plant's growth response in which the direction of growth is determined by a stimulus or gradient in water concentration. A common example is a plant root growing in humid air bending toward a higher relative humidity level.

Gravitropism is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feature of all higher and many lower plants as well as other organisms. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems grow in the opposite direction. This behavior can be easily demonstrated with any potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing upwards. Herbaceous (non-woody) stems are capable of a degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside. The mechanism is based on the Cholodny–Went model which was proposed in 1927, and has since been modified. Although the model has been criticized and continues to be refined, it has largely stood the test of time.

Indole-3-butyric acid (1H-indole-3-butanoic acid, IBA) is a white to light-yellow crystalline solid, with the molecular formula C₁₂H₁₃NO₂. It melts at 125 °C in atmospheric pressure and decomposes before boiling. IBA is a plant hormone in the auxin family and is an ingredient in many commercial horticultural plant rooting products.

A primordium in embryology, is an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells. A primordium is the simplest set of cells capable of triggering growth of the would-be organ and the initial foundation from which an organ is able to grow. In flowering plants, a floral primordium gives rise to a flower.

Polar auxin transport is the regulated transport of the plant hormone auxin in plants. It is an active process, the hormone is transported in cell-to-cell manner and one of the main features of the transport is its asymmetry and directionality (polarity). The polar auxin transport functions to coordinate plant development; the following spatial auxin distribution underpins most of plant growth responses to its environment and plant growth and developmental changes in general. In other words, the flow and relative concentrations of auxin informs each plant cell where it is located and therefore what it should do or become.

Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant always has embryonic tissues. By contrast, an animal embryo will very early produce all of the body parts that it will ever have in its life. When the animal is born, it has all its body parts and from that point will only grow larger and more mature. However, both plants and animals pass through a phylotypic stage that evolved independently and that causes a developmental constraint limiting morphological diversification.

A lateral shoot, commonly known as a branch, is a part of a plant's shoot system that develops from axillary buds on the stem's surface, extending laterally from the plant's stem.

Phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light contain a hormone called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the furthest side from the light. Phototropism is one of the many plant tropisms or movements which respond to external stimuli. Growth towards a light source is called positive phototropism, while growth away from light is called negative phototropism. Negative phototropism is not to be confused with skototropism which is defined as the growth towards darkness, whereas negative phototropism can refer to either the growth away from a light source or towards the darkness. Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them. The combination of phototropism and gravitropism allow plants to grow in the correct direction.

Somatic embryogenesis is an artificial process in which a plant or embryo is derived from a single somatic cell. Somatic embryos are formed from plant cells that are not normally involved in the development of embryos, i.e. ordinary plant tissue. No endosperm or seed coat is formed around a somatic embryo.

<span class="mw-page-title-main">Cholodny–Went model</span> Botany model

In botany, the Cholodny–Went model, proposed in 1927, is an early model describing tropism in emerging shoots of monocotyledons, including the tendencies for the shoot to grow towards the light (phototropism) and the roots to grow downward (gravitropism). In both cases the directional growth is considered to be due to asymmetrical distribution of auxin, a plant growth hormone. Although the model has been criticized and continues to be refined, it has largely stood the test of time.

Feronia, also known as FER or protein Sirene, is a recognition receptor kinase found in plants. FER plays a significant part in the plant immune system as a receptor kinase which assists in immune signaling within plants, plant growth, and plant reproduction. FER is regulated by the Rapid Alkalinization Factor (RALF). FER regulates growth in normal environments but it is most beneficial in stressful environments as it helps to initiate immune signaling. FER can also play a role in reproduction in plants by participating in the communication between the female and male cells. FER is found in and can be studied in the organism Arabidopsis thaliana.

Phenotypic plasticity is the ability of an individual organism to alter its behavior, morphology and physiology in response to changes in environmental conditions. Root phenotypic plasticity enables plants to adapt to an array of biotic and abiotic constraints that limit plant productivity. Even though the exploitation of soil resources through root activity is energetically costly, natural selection favors plants that can direct root activity to exploit efficiently the heterogeneous distribution of soil resources.

Ethylene (CH^₂=CH^₂) is an unsaturated hydrocarbon gas (alkene) acting naturally as a plant hormone. It is the simplest alkene gas and is the first gas known to act as hormone. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, the abscission (or shedding) of leaves and, in aquatic and semi-aquatic species, promoting the 'escape' from submergence by means of rapid elongation of stems or leaves. This escape response is particularly important in rice farming. Commercial fruit-ripening rooms use "catalytic generators" to make ethylene gas from a liquid supply of ethanol. Typically, a gassing level of 500 to 2,000 ppm is used, for 24 to 48 hours. Care must be taken to control carbon dioxide levels in ripening rooms when gassing, as high temperature ripening (20 °C; 68 °F) has been seen to produce CO₂ levels of 10% in 24 hours.

References

1 2 3 4 5 6 7 8 9 10 11 12 Taiz, Lincoln (2018). Fundamentals of Plant Physiology. Cary, NC, USA: Oxford University Press. ISBN 9781605357904.
1 2 3 Santos Teixeira, J.A.; ten Tusscher, K.H. (February 2019). "The Systems Biology of Lateral Root Formation: Connecting the Dots". Molecular Plant. 12 (6): 784–803. doi: 10.1016/j.molp.2019.03.015 . ISSN 1674-2052. PMID 30953788.
↑ Malamy, Jocelyn E.; Benfey, Philip N. (October 1997). "Down and out in Arabidopsis: the formation of lateral roots". Trends in Plant Science. 2 (10): 390–396. doi:10.1016/s1360-1385(97)90054-6. ISSN 1360-1385.
1 2 3 4 5 Casimiro, Ilda; Beeckman, Tom; Graham, Neil; Bhalerao, Rishikesh; Zhang, Hanma; Casero, Pedro; Sandberg, Goran; Bennett, Malcolm J. (April 2003). "Dissecting Arabidopsis lateral root development". Trends in Plant Science. 8 (4): 165–171. doi:10.1016/s1360-1385(03)00051-7. ISSN 1360-1385. PMID 12711228.
1 2 3 4 5 Casimiro, Ilda; Marchant, Alan; Bhalerao, Rishikesh P.; Beeckman, Tom; Dhooge, Sandra; Swarup, Ranjan; Graham, Neil; Inzé, Dirk; Sandberg, Goran; Casero, Pedro J.; Bennett, Malcolm (2001-04-01). "Auxin Transport Promotes Arabidopsis Lateral Root Initiation". The Plant Cell. 13 (4): 843–852. doi:10.1105/tpc.13.4.843. ISSN 1040-4651. PMC 135543 . PMID 11283340.
↑ Ferreira, Paulo C. G.; Hemerly, Adriana S.; de Almeida Engler, Janice; Montagu, Marc Van; Engler, Gilbert; Inze, Dirk (December 1994). "Developmental Expression of the Arabidopsis Cyclin Gene cyc1At". The Plant Cell. 6 (12): 1763–1774. doi:10.2307/3869906. JSTOR 3869906. PMC 160560 . PMID 7866022.
1 2 3 4 5 Lee, Hyodong; Ganguly, Anindya; Lee, Richard Dongwook; Park, Minho; Cho, Hyung-Taeg (2020-01-31). "Intracellularly Localized PIN-FORMED8 Promotes Lateral Root Emergence in Arabidopsis". Frontiers in Plant Science. 10: 1808. doi: 10.3389/fpls.2019.01808 . ISSN 1664-462X. PMC 7005106 . PMID 32082353.

Péret, B., Rybel, B. D., Casimiro, I., Benková, E., Swarup, R., Laplaze, L., … Bennett, M. J. (2009). Arabidopsis lateral root development: an emerging story. Trends in Plant Science, 14(7), 399–408. doi: 10.1016/j.tplants.2009.05.002

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[:0-1] 1 2 3 4 5 6 7 8 9 10 11 12 Taiz, Lincoln (2018). Fundamentals of Plant Physiology. Cary, NC, USA: Oxford University Press. ISBN 9781605357904.

[:2-2] 1 2 3 Santos Teixeira, J.A.; ten Tusscher, K.H. (February 2019). "The Systems Biology of Lateral Root Formation: Connecting the Dots". Molecular Plant. 12 (6): 784–803. doi: 10.1016/j.molp.2019.03.015 . ISSN 1674-2052. PMID 30953788.

[3] Malamy, Jocelyn E.; Benfey, Philip N. (October 1997). "Down and out in Arabidopsis: the formation of lateral roots". Trends in Plant Science. 2 (10): 390–396. doi:10.1016/s1360-1385(97)90054-6. ISSN 1360-1385.

[:1-4] 1 2 3 4 5 Casimiro, Ilda; Beeckman, Tom; Graham, Neil; Bhalerao, Rishikesh; Zhang, Hanma; Casero, Pedro; Sandberg, Goran; Bennett, Malcolm J. (April 2003). "Dissecting Arabidopsis lateral root development". Trends in Plant Science. 8 (4): 165–171. doi:10.1016/s1360-1385(03)00051-7. ISSN 1360-1385. PMID 12711228.

[:4-5] 1 2 3 4 5 Casimiro, Ilda; Marchant, Alan; Bhalerao, Rishikesh P.; Beeckman, Tom; Dhooge, Sandra; Swarup, Ranjan; Graham, Neil; Inzé, Dirk; Sandberg, Goran; Casero, Pedro J.; Bennett, Malcolm (2001-04-01). "Auxin Transport Promotes Arabidopsis Lateral Root Initiation". The Plant Cell. 13 (4): 843–852. doi:10.1105/tpc.13.4.843. ISSN 1040-4651. PMC 135543 . PMID 11283340.

[6] Ferreira, Paulo C. G.; Hemerly, Adriana S.; de Almeida Engler, Janice; Montagu, Marc Van; Engler, Gilbert; Inze, Dirk (December 1994). "Developmental Expression of the Arabidopsis Cyclin Gene cyc1At". The Plant Cell. 6 (12): 1763–1774. doi:10.2307/3869906. JSTOR 3869906. PMC 160560 . PMID 7866022.

[:3-7] 1 2 3 4 5 Lee, Hyodong; Ganguly, Anindya; Lee, Richard Dongwook; Park, Minho; Cho, Hyung-Taeg (2020-01-31). "Intracellularly Localized PIN-FORMED8 Promotes Lateral Root Emergence in Arabidopsis". Frontiers in Plant Science. 10: 1808. doi: 10.3389/fpls.2019.01808 . ISSN 1664-462X. PMC 7005106 . PMID 32082353.

[1]

[2]

[3]

[4]

[5]

[6]

[7]