Plant development: Difference between revisions

Revision as of 14:09, 21 June 2025

Revision as of 14:29, 21 June 2025

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=== Direct organogenesis ===

Direct organogenesis is a method of plant tissue culture in which organs like roots and shoots develop directly from meristematic or non-meristematic cells, bypassing the callus formation stage. This process takes place through the activation of shoot and root apical meristems or axillary buds, influenced by internal or externally applied plant growth regulators. As a result, specific cell types differentiate to form plant structures that can grow into whole plants. This technique is commonly used for propagating various plant species, including vegetables, fruits, woody plants, and medicinal plants. Shoot tips and nodal segments are typically used as explants in this process. In some cases, adventitious structures arise from somatic tissues under specific conditions, allowing for the regeneration of shoots or roots in areas where they would not naturally develop. This approach is particularly effective in herbaceous species, and while adventitious regeneration can lead to a higher rate of shoot formation, axillary shoot proliferation remains the most widely used method in micropropagation due to its efficiency and practicality. The general sequence of organ development in this process follows the pattern: Primary Explant → Meristemoid → Organ Primordium.{{cn|date=June 2025}}

=== Indirect organogenesis ===

Indirect organogenesis is a developmental process in which plant cells undergo dedifferentiation, allowing them to revert from their specialized state and transition into a new developmental pathway. This process is characterized by an intermediate callus stage, where cells lose their original identity and become morphologically adaptable, serving as the foundation for organ formation. The progression of indirect organogenesis involves several key phases, beginning with dedifferentiation, which enables the cells to attain competence, followed by an induction stage that leads to a fully determined state. Once determination is achieved, the cells undergo morphological changes, ultimately giving rise to functional shoots or roots. This process follows a structured developmental sequence: Primary Explant → Callus → Meristemoid → Organ Primordium, ensuring the organized formation of plant organs.{{cn|date=June 2025}}

== Factors affecting organogenesis ==

=== Explant ===

The ability to regenerate plants successfully depends on selecting the right explant, which varies among species and plant varieties. In direct organogenesis, explants sourced from meristematic tissues, such as shoot tips, lateral buds, leaves, petioles, roots, and floral structures, are often preferred due to their ability to rapidly develop into new organs. These tissues have high survival rates, fast growth, and strong regenerative potential in vitro. Meristems, shoot tips, axillary buds, immature leaves, and embryos are particularly effective in promoting regeneration across a wide range of plant species. Additionally, mature plant parts, including leaves, stems, roots, petioles, and flower segments, can also serve as viable explants for organ formation under suitable conditions. Plant regeneration occurs through the formation of callus, an undifferentiated mass of cells that later gives rise to new organs. Callus formation can be induced from various explants, such as cotyledons, hypocotyls, stems, leaves, shoot apices, roots, inflorescences, and floral structures, when cultured under controlled conditions. Generally, explants containing actively dividing cells are more effective for callus initiation, as they have a higher capacity for cellular reprogramming. Immature tissues tend to be more adaptable for regeneration compared to mature tissues due to their increased developmental plasticity. The size and shape of the explant also influence the success of culture establishment, as larger or more structurally favorable explants may enhance the chances of survival and growth. Callus development is primarily triggered by wounding and the presence of plant hormones, which may be naturally present in the tissue or supplemented in the growth medium to stimulate cellular activity and organ formation.{{cn|date=June 2025}}

=== Culture medium, plant growth regulators, and gelling agent ===

Culture media compositions vary significantly in their mineral elements and vitamin content to accommodate diverse plant species requirements. Murashige and Skoog (MS) medium is distinguished by its high nitrogen content in ammonium form, a characteristic not found in other formulations. Sucrose typically serves as the primary carbohydrate source across various media types.{{cn|date=June 2025}}

The interaction between auxins and cytokinins in regulating organogenesis is well-established, though responses vary by species. Some plants, such as tobacco, can spontaneously form shoot buds without exogenous growth regulators, while others like ''Scurrula pulverulenta'', ''Lactuca sativa'', and ''Brassica juncea'' strictly require hormonal supplementation. In ''B. juncea'' cotyledon cultures, benzylaminopurine (BAP) alone induces shoot formation from petiole tissue, similar to radiata pine where cytokinin alone suffices for shoot induction.{{cn|date=June 2025}}

Research indicates that endogenous hormone concentrations, rather than exogenous application levels, ultimately determine organogenic differentiation. Among the various cytokinins (2iP, BAP, thidiazuron, kinetin, and zeatin) used for shoot induction, BAP has demonstrated superior efficacy and widespread application. Auxins similarly influence organogenic pathways, with 2,4-D commonly used for callus induction in cereals, though organogenesis typically requires transfer to media containing IAA or NAA or lacking 2,4-D entirely. The auxin-to-cytokinin ratio largely determines which organs develop.{{cn|date=June 2025}}

Gibberellic acid (GA3) contributes to cell elongation and meristemoid formation, while unconventional compounds like tri-iodobenzoic acid (TIBA), abscisic acid (ABA), kanamycin, and auxin inhibitors have proven effective for recalcitrant species. Natural additives like ginseng powder can enhance regeneration frequency in certain cultures. Since ethylene typically suppresses shoot differentiation, inhibitors of ethylene synthesis such as aminoethoxyvinylglycine (AVG) and silver nitrate (AgNO3) are often employed to promote organogenesis, with documented success in wheat, tobacco, and sunflower cultures.{{cn|date=June 2025}}

Agar is not an essential component of the culture medium, but quality and quantity of agar is an important factor that may determine a role in organogenesis. Commercially available agar may contain impurities. With a high concentration of agar, the nutrient medium becomes hard and does not allow the diffusion of nutrients to the growing tissue. It influences the organogenesis process by producing adventitious roots, unwanted callus at the base, or senescence of the foliage. The pH is another important factor that may affect organogenesis route. The pH of the culture medium is adjusted to between 5.6 and 5.8 before sterilization. Medium pH facilitates or inhibits nutrient availability in the medium; for example, ammonium uptake in vitro occurs at a stable pH of 5.5 (Thorpe et al., 2008).

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==== Season of the year ====

The timing of explant collection significantly impacts regenerative capacity in tissue culture systems, with seasonal variations playing a crucial role in organ formation success. This phenomenon is clearly demonstrated in ''Lilium speciosum'', where bulb scales exhibit differential regenerative responses based on collection season. Explants harvested during spring and autumn periods readily form bulblets in vitro, while those collected during summer or winter months fail to produce bulblets despite identical culture conditions.{{cn|date=June 2025}}

Similar seasonal dependency is observed in ''Chlorophytum borivillianum'', a medicinally valuable species that shows markedly enhanced in vitro tuber formation during monsoon seasons compared to other times of year. This seasonal variation in morphogenic potential likely reflects differences in the physiological state of the source plant, including endogenous hormone levels, carbohydrate reserves, and metabolic activity that fluctuate throughout the annual growth cycle.{{cn|date=June 2025}}

==== Oxygen gradient ====

Oxygen has a key role in tissue culture, which influences the organ formation. In some cultures, shoot bud formation takes place when the gradient of available oxygen inside the culture vessel is reduced, while induction of roots requires a high oxygen gradient.{{cn|date=June 2025}}

==== Light ====

Light conditions, including both intensity and spectral quality, function as significant morphogenic signals in plant tissue culture systems. Spectral composition research has revealed distinct wavelength-dependent responses, with blue light generally promoting shoot organogenesis while red light wavelengths typically favor root induction. Sequential photoperiod exposure—blue light followed by red light—has been documented to effectively stimulate specific organogenetic pathways in certain species.{{cn|date=June 2025}}

The regulatory effect of different wavelengths demonstrates how light quality can selectively control morphogenic outcomes. Artificial fluorescent lighting produces variable responses depending on the species, promoting root formation in some cultures while inhibiting it in others. Some species exhibit specialized light requirements, as observed in ''Pisum sativum'' (garden pea), where shoot bud initiation occurs optimally in darkness before exposure to light stimulates further development.{{cn|date=June 2025}}

For most tissue culture applications, standard lighting protocols typically recommend illumination of approximately 2,000-3,000 lux intensity with a 16-hour photoperiod. However, certain species demonstrate exceptional light intensity requirements, exemplified by ''Nicotiana tabacum'' (tobacco) callus cultures, which require substantially higher light intensities of 10,000-15,000 lux to induce shoot bud formation or somatic embryogenesis.{{cn|date=June 2025}}

==== Temperature ====

Temperature serves as a critical environmental factor in plant tissue culture systems, with optimal incubation temperatures varying significantly among species based on their natural habitat requirements. While 25°C represents the standard incubation temperature suitable for many plant species in vitro, species-specific temperature adaptations should be considered to maximize organogenic potential.{{cn|date=June 2025}}

Geophytic species from temperate regions typically require lower temperature regimes than the standard protocol. Notable examples include bulbous plants such as ''Galanthus'' (snowdrop) which exhibits optimal growth at approximately 15°C, while certain cultivars of ''Narcissus'' (daffodil) and ''Allium'' (ornamental onion) demonstrate enhanced regeneration efficiency at around 18°C.{{cn|date=June 2025}}

Conversely, species of tropical origin generally require elevated temperatures for optimal growth and organogenesis in culture. Date palm cultures thrive at 27°C, while ''Monstera deliciosa'' (Swiss cheese plant) exhibits peak regenerative performance at 30°C. These temperature requirements reflect evolutionary adaptations to the plants' native environmental conditions.{{cn|date=June 2025}}

==== Ploidy level ====

Variation in chromosome number, that is, aneuploidy, polyploidy, etc., in plant cell culture has been well documented in the past. Chromosome instability of the cells results in gradual decline of morphogenetic potentiality of the callus tissue. Therefore, to maintain organogenic potential of the callus tissue and the chromosome stability, it is suggested that the time and frequency of subculture should be regularly followed.{{cn|date=June 2025}}

==== Age of culture ====

Age of culture is often the key to successful organogenesis. A young culture/freshly subcultured material may produce organs more frequently than the aged ones. The probable reason for this is the reduction or loss of the organogenic potential in old cultures. However, in some plants, the plant regeneration capacity may retain indefinitely for many years{{cn|date=June 2025}}

== Developmental process ==

=== Dedifferentiation ===

The ability of cells to undergo organogenesis largely depends on the application of plant growth regulators (PGRs), which influence the developmental direction of the tissue. The balance between auxins and cytokinins plays a critical role in determining whether shoots or roots will form. A lower auxin-to-cytokinin ratio favors shoot regeneration, whereas a higher auxin concentration promotes root formation. For example, in ''Medicago sativa'' (alfalfa) cultures, an elevated level of kinetin combined with a low concentration of 2,4-D (a synthetic auxin) leads to shoot development, whereas increasing 2,4-D while reducing kinetin concentration encourages root formation. However, successful organogenesis is not solely dependent on PGR treatment. The physical size of the callus or developing tissue must reach a certain threshold to support proper organ formation, highlighting the importance of intercellular signaling in coordinating developmental processes.{{cn|date=June 2025}}

=== Induction ===

The induction phase in organogenesis represents the transition period between a tissue achieving competence and becoming fully determined to initiate primordia formation. During this stage, an integrated genetic pathway directs the developmental process before morphological differentiation occurs. Research suggests that certain chemical and physical factors can interfere with genetically programmed developmental pathways, altering morphogenic outcomes. In the case of ''Convolvulus arvensis'', these external influences were found to inhibit shoot formation, leading instead to callus development.{{cn|date=June 2025}}

The conclusion of the induction phase is marked by a cell or group of cells committing to either shoot or root formation. This determination is tested by transferring the tissue from a growth regulator-supplemented medium to a basal medium containing essential minerals, vitamins, and a carbon source but no plant growth regulators. At this stage, the tissue completes the induction process and becomes fully determined to its developmental fate.{{cn|date=June 2025}}

A key concept in this process is canalization, which refers to the ability of a developmental pathway to consistently produce a standard phenotype despite potential genetic or environmental variations. If explants are removed from a shoot-inducing medium before full canalization occurs, shoot formation is significantly reduced, and root development becomes the dominant outcome. This phenomenon highlights the morphogenic plasticity of plant tissues in vitro, demonstrating their ability to adjust to external conditions and developmental cues.{{cn|date=June 2025}}

=== Differentiation ===

During this phase, the process of morphological differentiation begins, leading to the formation and development of the nascent organ. The initiation of organogenesis is characterized by a distinct shift in polarity, followed by the establishment of radial symmetry and subsequent growth along the newly defined axis, ultimately forming the structural bulge that marks organ initiation.{{cn|date=June 2025}}

The sequential development of organogenesis can be observed in species such as ''Pinus oocarpa Schiede'', where shoot buds are regenerated directly from cotyledons through direct organogenesis. However, the specific developmental patterns may vary across different plant species grown in vitro. The progression of organ formation includes distinct morphological changes, beginning with alterations in surface texture, the emergence of meristemoids, and the expansion of the meristematic region either vertically or horizontally. This is followed by the protrusion of the meristematic region beyond the epidermal layer, the formation of a structured meristem with visible leaf primordia, and eventually, the full development of an adventitious bud.{{cn|date=June 2025}}

A notable characteristic of in vitro organogenic cultures is the simultaneous formation of multiple meristemoids on a single explant, with varying degrees of differentiation. Within the same explant, buds may exist in different developmental stages, ranging from early initiation to fully developed structures. Once the elongated shoots surpass a length of 1 cm, they are transferred to either in vitro or ex vitro rooting substrates, allowing for the completion of plantlet regeneration and the establishment of a fully formed plant.{{cn|date=June 2025}}

== Advantages and limitation ==

In the process of direct organogenesis, axillary shoots are generated directly from pre-existing meristems located at the shoot tips and nodes, offering a high rate of multiplication. One of the key advantages of this method is the low likelihood of mutations occurring in the organized shoot meristems, ensuring that the resulting plants maintain genetic consistency. This technique is particularly valuable for the production and conservation of economically and environmentally significant plants, as it allows for the efficient generation of multiple shoots from a single explant, maintaining uniformity across the propagated plants. Furthermore, all plants produced via direct organogenesis are true-to-type, meaning they are genetic clones of the original plant.{{cn|date=June 2025}}

However, there are some limitations to organogenesis. Somaclonal variation, which can result in unwanted genetic diversity, is a potential issue, particularly in the indirect organogenesis process. Additionally, this technique may not be suitable for recalcitrant plant species, which are those that do not respond well to in vitro culture or regeneration protocols. These limitations highlight the need for ongoing research and optimization of methods for different plant species to overcome these challenges in plant propagation and conservation.{{cn|date=June 2025}}

===Cell elongation===

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[[File:Growth of a Plant.svg|thumb|This image shows the development of a normal plant. It resembles the different growth processes for a leaf, a stem, etc. On top of the gradual growth of the plant, the image reveals the true meaning of phototropism and cell elongation, meaning the light energy from the sun is causing the growing plant to bend towards the light aka elongate.]]

Plant growth and development are mediated by specific [[plant hormone]]s and plant growth regulators (PGRs) (Ross et al. 1983).<ref name="ross">Ross, S.D.; Pharis, R.P.; Binder, W.D. 1983. Growth regulators and conifers: their physiology and potential uses in forestry. p. 35–78 ''in'' Nickell, L.G. (Ed.), Plant growth regulating chemicals. Vol. 2, CRC Press, Boca Raton FL.</ref> Endogenous hormone levels are influenced by plant age, cold hardiness, dormancy, and other metabolic conditions; photoperiod, drought, temperature, and other external environmental conditions; and exogenous sources of PGRs, e.g., externally applied and of rhizospheric origin.{{cn|date=June 2025}}

=== Morphological variation during growth ===

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[[File:Leaf samples from the giant ragweed.jpg|thumb|right |280px |Variation in leaves from the giant ragweed illustrating positional effects. The lobed leaves come from the base of the plant, while the unlobed leaves come from the top of the plant.]]

There is variation among the parts of a mature plant resulting from the relative position where the organ is produced. For example, along a new branch the leaves may vary in a consistent pattern along the branch. The form of leaves produced near the base of the branch differs from leaves produced at the tip of the plant, and this difference is consistent from branch to branch on a given plant and in a given species.{{cn|date=June 2025}}

The way in which new structures mature as they are produced may be affected by the point in the plants life when they begin to develop, as well as by the environment to which the structures are exposed. Temperature has a multiplicity of effects on plants depending on a variety of factors, including the size and condition of the plant and the temperature and duration of exposure. The smaller and more [[Succulent plant|succulent]] the plant, the greater the susceptibility to damage or death from temperatures that are too high or too low. Temperature affects the rate of biochemical and physiological processes, rates generally (within limits) increasing with temperature.{{cn|date=June 2025}}

Juvenility or [[heteroblasty]] is when the organs and tissues produced by a young plant, such as a [[seedling]], are often different from those that are produced by the same plant when it is older. For example, young trees will produce longer, leaner branches that grow upwards more than the branches they will produce as a fully grown tree. In addition, leaves produced during early growth tend to be larger, thinner, and more irregular than leaves on the adult plant. Specimens of juvenile plants may look so completely different from adult plants of the same species that egg-laying insects do not recognize the plant as food for their young. The transition from early to late growth forms is sometimes called [[vegetative phase change]].<ref>{{Cite journal | doi = 10.1086/314215 | issn = 1058-5893 | volume = 160 | issue = S6 | pages = 105–S111 | last = Jones | first = Cynthia S. | title = An Essay on Juvenility, Phase Change, and Heteroblasty in Seed Plants | journal = International Journal of Plant Sciences | date = 1999-11-01 | pmid = 10572025 | s2cid = 21757481}}</ref>

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===Adventitious structures===

Plant structures, including, roots, buds, and shoots, that develop in unusual locations are called ''adventitious''.{{cn|date=June 2025}}

Adventitious roots and buds usually develop near the existing vascular tissues so that they can connect to the [[xylem]] and [[phloem]]. However, the exact ___location varies greatly. In young stems, adventitious roots often form from [[Ground tissue#Parenchyma|parenchyma]] between the [[vascular bundle]]s. In stems with secondary growth, adventitious roots often originate in phloem parenchyma near the [[vascular cambium]]. In stem cuttings, adventitious roots sometimes also originate in the [[Callus (cell biology)|callus]] cells that form at the cut surface. Leaf cuttings of the ''[[Crassula]]'' form adventitious roots in the epidermis.<ref>McVeigh, I. 1938. Regeneration in ''Crassula multicava''. ''American Journal of Botany'' 25: 7-11. [https://www.jstor.org/stable/2436624]</ref>

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The ability of plant stems to form adventitious roots is utilised in commercial propagation by [[Cutting (plant)|cutting]]s. Understanding of the physiological mechanisms behind adventitious rooting has allowed some progress to be made in improving the rooting of cuttings by the application of synthetic auxins as rooting powders and by the use of selective basal wounding.<ref>{{cite journal | url=https://doi.org/10.1007%2Fs11627-999-0076-z | doi=10.1007/s11627-999-0076-z | title=Review the formation of adventitious roots: New concepts, new possibilities | year=1999 | last1=De Klerk | first1=Geert-Jan | last2=Van Der Krieken | first2=Wim | last3=De Jong | first3=Joke C. | journal=In Vitro Cellular & Developmental Biology - Plant | volume=35 | issue=3 | pages=189–199 | s2cid=44027145 | url-access=subscription }}</ref> Further progress can be made in future years by applying research into other regulatory mechanisms to commercial propagation and by the comparative analysis of molecular and ecophysiological control of adventitious rooting in 'hard to root' vs. 'easy to root' species.{{cn|date=June 2024}}

Adventitious roots and buds are very important when people propagate plants via cuttings, [[layering (horticulture)|layering]], [[plant tissue culture|tissue culture]]. [[Plant hormones]], termed [[auxin]]s, are often applied to stem, [[Shoot (botany)|shoot]] or [[leaf]] cuttings to promote adventitious root formation, e.g., [[African violet]] and [[sedum]] leaves and shoots of [[poinsettia]] and [[coleus]]. Propagation via root cuttings requires adventitious bud formation, e.g., in [[horseradish]] and [[apple]]. In layering, adventitious roots are formed on aerial stems before the stem section is removed to make a new plant. Large houseplants are often propagated by [[air layering]]. Adventitious roots and buds must develop in tissue culture propagation of plants.{{cn|date=June 2025}}

=====Modified forms=====

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An external [[stimulus (physiology)|stimulus]] is required in order to trigger the [[cellular differentiation|differentiation]] of the meristem into a flower meristem. This stimulus will activate [[mitosis|mitotic]] cell division in the meristem, particularly on its sides where new [[primordium|primordia]] are formed. This same stimulus will also cause the meristem to follow a [[biological development|developmental]] pattern that will lead to the growth of floral meristems as opposed to vegetative meristems. The main difference between these two types of meristem, apart from the obvious disparity between the objective organ, is the verticillate (or whorled) [[phyllotaxis]], that is, the absence of [[plant stem|stem]] elongation among the successive [[whorl (botany)|whorls]] or [[whorl (botany)|verticils]] of the primordium. These verticils follow an acropetal development, giving rise to [[sepal]]s, [[petal]]s, [[stamen]]s and [[carpel]]s. Another difference from vegetative axillary meristems is that the floral meristem is «determined», which means that, once differentiated, its cells will no longer [[cell cycle|divide]].<ref name="azcón-bieto">{{cite book | author = Azcón-Bieto | title = Fundamentos de fisiología vegetal | year = 2000 | publisher = McGraw-Hill/Interamericana de España, SAU | isbn = 84-486-0258-7 |display-authors=etal}}{{page needed |date=April 2013}}</ref>

The identity of the organs present in the four floral verticils is a consequence of the interaction of at least three types of [[gene products]], each with distinct functions. According to the ABC model, functions A and C are required in order to determine the identity of the verticils of the [[perianth]] and the reproductive verticils, respectively. These functions are exclusive and the absence of one of them means that the other will determine the identity of all the floral verticils. The B function allows the differentiation of petals from sepals in the secondary verticil, as well as the differentiation of the stamen from the carpel on the tertiary verticil.{{cn|date=June 2025}}

===Floral fragrance===