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Root primordia (brown spots) as seen on the butt of a freshly cut pineapple crown intended for vegetative reproduction. Root primordia.JPG
Root primordia (brown spots) as seen on the butt of a freshly cut pineapple crown intended for vegetative reproduction.

A primordium ( /prˈmɔːrdiəm/ ; plural: primordia; synonym: anlage) in embryology, is an organ or tissue in its earliest recognizable stage of development. [1] 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.


Although it is a frequently used term in plant biology, the word is used in describing the biology of all multicellular organisms (for example: a tooth primordium in animals, a leaf primordium in plants or a sporophore primordium in fungi. [2] )

Primordium development in plants

Two primordia
New primordium forming
Generative spiral.png
Generative spiral
Leaf migration.png
Leaf migration

Plants produce both leaf and flower primordia cells at the shoot apical meristem (SAM). Primordium development in plants is critical to the proper positioning and development of plant organs and cells. The process of primordium development is intricately regulated by a set of genes that affect the positioning, growth and differentiation of the primordium. Genes including STM (shoot meristemless) and CUC (cup-shaped cotyledon) are involved in defining the borders of the newly formed primordium. [3]

The plant hormone auxin has also been implicated in this process, with the new primordium being initiated at the placenta, where the auxin concentration is highest. [3] There is still much to understand about the genes involved in primordium development.

Leaf primordia are groups of cells that will form into new leaves. These new leaves form near the top of the shoot and resemble knobby outgrowths or inverted cones. [4] Flower primordia are the little buds we see at the end of stems, from which flowers will develop. Flower primordia start off as a crease or indentation and later form into a bulge. This bulging is caused by slower and less anisotropic, or directionally dependent, growth.

Primordium Initiation

Primordia initiation is the precursor for the start of a primordium, and typically confers new growth (either flowers or leaves) in plants once fully mature. In pines, the leaf primordia develop into buds, which eventually elongate into shoots, then stems, then branches. [5] Though primordia are typically only found in new flower and leaf growth, root primordia in plants can also be found, but are typically referred to as lateral root primordium or adventitious roots. The process of lateral root primordium initiation has been studied in Arabidopsis thaliana , though the process in other angiosperms is still under analysis. [6] [7] Primordia are initiated by local cell division and enlargement on the shoot apical meristem. [8] At least in wheat plants, leaf primordium initiation rates increase with increasing ambient temperature, and the leaf number of some varieties decrease with increasing daylength. [9]

Auxin's Role in Primordial Development

Auxin is a group of plant hormones, or phytohormones, that plays a key role in almost all areas of the growth and development plants. [10] Auxin concentrations affect mitosis, cell expansion, as well as cell differentiation. [11] There is a lot of current research being conducted to explain the role that it assists in the process of plant primordium. It is believed to control these processes by binding to a specific receptor on plant cells and influences gene expression. [10] It affects transcription factors that control the upregulation or downregulation of auxin genes that relate to growth. [12]  This has led researchers to believe that auxin accumulation as well as decreases in auxin levels might control different phases of primordium development. [13] Auxin concentration gradients are necessary to initiate and continue primordial growth.  Higher concentrations allow them to bind to cells and results in downstream effects that lead to primordial growth. [14] Auxins have a large impact on plant primordium development because of their effect on gene regulation.

Root Primordium

Lateral roots are one of the most important tissues in a plant's anatomical structure. They provide physical support and uptake water and nutrients for growth. Before the emergence of lateral roots in the morphogenetic process, a new lateral root primordium which consists of primordial cells is formed. Localized cell divisions in the Pericycle give rise to the lateral root primordia. This pattern of growth gives rise to a bundle of tissue. The subsequent accumulation of cell division and enlargement in this bundle of tissue gives rise to a new structure known as the root primordium. [15] The root primordium emerges as a new lateral rootlet by creating its own root cap and apex. Both genetic and physiological studies point to the importance of Auxin in the LR initiation and primordium development in the LR formation process, but cytokinin negatively regulates the growth of the LR. [16] However, it is not fully understood the full mechanisms of how these different hormones affect the transport, signaling, or biosynthesis of the others. The PUCHI gene (specifically an Auxin regulated AP2/EREBP gene), plays a vital role in coordinating the organization/pattern of cell division during lateral root primordium (LRP) development, in Arabidopsis thaliana. PUCHI expression is regulated via Auxin concentration, and because of this, exogenous Auxin is required to increase the transcription of PUCHI genes. [17] This allows us to infer that the PUCHI gene must be downstream to Auxin signaling. One method used to test the theory that PUCHI is responsible for LRP development, was by using Arabidopsis Thaliana accession col as the wild type (WT) strain, and isolating the PUCHI-1-mutant from the T-DNA insertion. The function of the PUCHI gene was demonstrated by using the PUCHI-1 mutant (using Arabidopsis Thaliana as the model plant), which if backcrossed three times to Arabidopsis Thaliana accession col (WT), it was demonstrated to affect lateral root and flower primordium development by stunting LR growth. [17] One of the many theories out there, is that Auxin promotes downstream PUCHI expression via a cascade signaling effect, by triggering ARF and Aux/IAA protein functions. PUCHI genes act as a transcriptional regulator of lateral root primordium development by controlling its cell division during this stage.

Leaf Primordium

Early events in leaf development fall into three main processes:

1.       Initiation of the leaf primordium

2.       Establishment of dorsoventrally (abaxial-adaxial polarity) which is established with bulging of the primordia

3.       Development of a marginal meristem [18]

Lateral organ and leaf development initiation is dependent upon the structure of the shoot apical meristem (SAM). [18] In the center of the SAM, there is a central zone of many indeterminate, undifferentiated cells where cell division is infrequent. [18] Cells divide more frequently in the peripheral zones flanking the SAM and are incorporated into leaf primordia, also referred to as founder cells for leaves. Cells are recruited from the flanks of the shoot apical meristem which initiates the development of leaf primordia. [19]

Signals propagated in the epidermis initiate primordia growth in directions away from the cotyledons (in dicotyledonous plants) in simple patterns, known as phyllotaxis. [20] Phyllotaxis are the arrangement of leaves on an axis or stem and can either be arranged in a spiral or whorl pattern moving out radially by continually dividing cells at their central edges. [20] Phyllotactic patterns determine plant architecture and the positions of where new leaves will develop can be easily predicted by observing the locations of existing leaf primordia. [21]

The key instructive signal for phyllotactic pattern formation is Auxin. [22]   Leaf primordia are specified as auxin maxima in a flanking region of the SAM following the rules of phyllotaxy. Phyllotactic spiral patterns, as observed in Arabidopsis, have an unequal auxin distribution between left and right sides, resulting in asymmetrical growth of leaf laminas. [18]  For example, in clockwise phyllotactic spiral patterns, the left side will grow more than the right side and vice versa for counterclockwise phyllotactic spiral patterns. Leaf initiation requires high intracellular auxin concentration and is generated by directional auxin transport through the SAM. [22] Once in the meristem, developing organ primordia act as a sink, absorbing and depleting auxin from the surrounding tissue. [22] The accumulation of auxin in the developing organ primordia induces the formation of new leaf primordium. The SAM continues to produce leaf primordia regularly on its flanks throughout the vegetative phase. [19]

See also

Related Research Articles

Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of stem cells in the adult organism.

<i>Arabidopsis thaliana</i> Model plant species in the family Brassicaceae

Arabidopsis thaliana, the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. A. thaliana is considered a weed; it is found along the shoulders of roads and in disturbed land.

Root Part of a plant

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 most often lie 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.

Apical dominance

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.

Meristem 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.

Phyllotaxis Arrangement of leaves on the stem of a plant

In botany, phyllotaxis or phyllotaxy is the arrangement of leaves on a plant stem. Phyllotactic spirals form a distinctive class of patterns in nature.

Auxin plant hormone

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 (1904-1997) 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.


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. Folke Skoog discovered their effects using coconut milk in the 1940s at the University of Wisconsin–Madison.

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.

Plant embryogenesis is a process that occurs after the fertilization of an ovule to produce a fully developed plant embryo. This is a pertinent stage in the plant life cycle that is followed by dormancy and germination. The zygote produced after fertilization must undergo various cellular divisions and differentiations to become a mature embryo. An end stage embryo has five major components including the shoot apical meristem, hypocotyl, root meristem, root cap, and cotyledons. Unlike animal embryogenesis, plant embryogenesis results in an immature form of the plant, lacking most structures like leaves, stems, and reproductive structures. However, both plants and animals pass through a phylotypic stage that evolved independently and that causes a developmental constraint limiting morphological diversification.


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.

Florigen is the hypothesized hormone-like molecule responsible for controlling and/or triggering flowering in plants. Florigen is produced in the leaves, and acts in the shoot apical meristem of buds and growing tips. It is known to be graft-transmissible, and even functions between species. However, despite having been sought since the 1930s, the exact nature of florigen is still disputed.

ABC model of flower development Model for genetics of flower development

The ABC model of flower development is a scientific model of the process by which flowering plants produce a pattern of gene expression in meristems that leads to the appearance of an organ oriented towards sexual reproduction, a flower. There are three physiological developments that must occur in order for this to take place: firstly, the plant must pass from sexual immaturity into a sexually mature state ; secondly, the transformation of the apical meristem's function from a vegetative meristem into a floral meristem or inflorescence; and finally the growth of the flower's individual organs. The latter phase has been modelled using the ABC model, which aims to describe the biological basis of the process from the perspective of molecular and developmental genetics.

Lateral root

Lateral roots, emerging from the pericycle, extend horizontally from the primary root (radicle) and over time makeup the iconic branching pattern of root systems. They contribute to anchoring the plant securely into the soil, increasing water uptake, and facilitates the extraction of nutrients required for the growth and development of the plant. Lateral roots increase the surface area of a plant's root system and can be found in great abundance in several plant species. 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.

Evolutionary developmental biology (evo-devo) is the study of developmental programs and patterns from an evolutionary perspective. It seeks to understand the various influences shaping the form and nature of life on the planet. Evo-devo arose as a separate branch of science rather recently. An early sign of this occurred in 1999.

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.

Lateral shoot

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.

Lewis Jeffrey Feldman is a professor of plant biology at the University of California, Berkeley and is Associate Dean for Academic Affairs in the College of Natural Resources. He is in the Department of Plant and Microbial Biology. Feldman has taught at Berkeley since 1978. He received Berkeley's Distinguished Teaching Award in 1996. Feldman's research focuses on regulation of development in meristems/stem cells, root gravitropism, and redox regulation of plant development.

CLE peptides are a group of peptides found in plants that are involved with cell signaling. Production is controlled by the CLE genes. Upon binding to a CLE peptide receptor in another cell, a chain reaction of events occurs, which can lead to various physiological and developmental processes. This signaling pathway is conserved in diverse land plants.

A cytokinin signaling and response regulator protein is a plant protein that is involved in a two step cytokinin signaling and response regulation pathway.



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