In this article we will discuss about the Definition and Factors of Organogenesis.
Definition of Organogenesis:
From cells of tissue culture various organs, such as, roots, stems, leaves or flowers may be initiated. This is called organogenesis. Such organ development does not require any pre-existing–initials. These new organs are formed in two stages.
In the first phase (dedifferentiation) cells of the explant divide and form undifferentiated cells. In the second phase cell differentiation takes places. Organ primordia are formed from single cells or small groups of differentiated cells. These cells form small meristem with cells containing dense cytoplasm and large nuclei.
According to Torrey (’66) meristemoids (i.e., meristematic zones) occur near the tissue medium interface.
Root formation in culture is called rhizogenesis and shoot initiation is called caulogenesis.
Root formation on culture has been noted in several cases. In culture of carrot cells root formation was first observed by Nobecourt (’39b). Explants taken from any part of a plant may produce roots. Like roots shoot buds are also formed frequently. Leaves develop less frequently than roots and shoots.
Root formation on culture of Jerusalem artichoke is influenced by mineral salts, auxin, sugar, temperature and light.
Root formation stops after several subcultures. These may be due to (a) some substances required for root initiation may be exhausted, (b) culture tissue is incapable of rhizogenesis or (c) epigenetic changes of some genes may occur.
Few layers of epidermal or sub-epidermal cells from various plants under regulated condition on culture can produce organs. In Begonia rex explants from epidermal or sub-epidermal layers near the midveins of leaves can produce roots or shoots rapidly. Root initiation occurs in a medium supplemented with zeatin and NAA. Shoot initiation takes place in presence of zeatin but in absence of auxin.
In short term cultures organization of the new meristem bears a relationship with the original organization of the explant. In culture of tobacco stalk shoot primordium arises from the external phloem.
In culture of Convolvulus roots shoot primordium originates near the protoxylem. In culture of carrot cells root primordium arises in association with the protoxylem strands and when this is transferred to an agar medium, it forms a complete plant.
In long term cultures shoot and bud primordia develop exogenously. But in some cases, as in Convolvulus callus shoot primordia develop either exogenously or endogenously. Culture of explants from lower internodes produce vegetative buds, whereas explants from young upper parts or from inflorescence produce flower buds.
For flower formation high level of nitrogen, presence of cytokinin and various constituents of nucleic acid in the medium, are necessary. Presence of auxin, gibberellin and organic nitrogenous compounds have an inhibitory effect on flower formation.
Factors Influencing Organogenesis:
(1) Age of Culture:
A young culture frequently produces organs. As the culture becomes older, this capacity decreases and ultimately disappears. But there are few exceptions. The; culture of Amorphophallus retains its regeneration capacity indefinitely. Culture of carrot cells can produce roots for many years.
(2) Ploidy Level:
In culture there is instability of genome. Only a few weeks after isolation of a diploid callus various degrees of ploidy (mostly tetraploid) have been noticed in various plants (e.g. culture of medullary parenchyma of Nicotiana tabacum, Haplopapptis shoots, pollens of Ginkgo).
Only in a few cases the culture tissues maintain their normal diploid sets, as in culture of tubers of Helianthus tuberosus, leaves of Crepis capillaris and Medicago sativa.
In certain cases, as in culture of pea root callus with increase in ploidy there is decrease in organogenesis. But in some plants, under suitable conditions organs may develop with polyploid meristem. Loss of capacity to organogenesis is reversible in some cases. This may be due to conditions of culture and other non-genetical factors.
Plant hormones have some effect on organogenesis. Gautheret (’45) noted that auxin at proper concentration can induce root primordia formation in carrot explants. Skoog first said that organogenesis can be chemically controlled. Skoog and Miller (’57) observed in tobacco a high auxin cytokinin ratio favours root initiation and a low ratio favours shoot initiation.
Other scientists also observed that under regulated auxin, cytokinin ratio and carbohydrate supply formation of roots and floral or vegetative buds occur. In serveral dicots shoot formation occurs when the ratio between exogenous cytokinin and auxin is 100: 10. But a ratio of 10: 100 favours formation of root primordia.
In monocots shoot initiation occurs on a medium with high 2-4-D and kinetin ratio for four days and then transferred to a medium lacking hormone. In absence of auxin shoot initiation occurs in some cases. For bud initiation certain plants do not require an exogenous supply of cytokinin.
Endogenous gibberellin retards root and shoot initiation. In callus during shoot initiation starch accumulates. Gibberellin lowers the concentration of starch and thereby inhibits shoot initiation.
Endogenous ethylene retards organ initiation during early stages of culture, but in later stages it helps shoot initiation. This has been observed in culture of tobacco cotyledons and Lilium bulb tissue.
According to some scientists phenolic compounds in addition to auxin are more effective for root initiation than auxin alone. On culture of Helianthus tuberosus explants in addition to the auxin the presence of sugar, light, temperature etc. play a major role in root initiation.
(4) Phosphate Concentration:
Increase in phosphate concentration favours shoot formation and suppresses or weakens root initiation.
(5) Photoperiodism and Vernalization:
Flower formation on culture can be induced by photoperiodism in Plumbago indica and by vernalization in Cichorium intybus and Lunaria annua.
Plantlet Formation form Tomato Leaves:
From surface sterilised young leaves rectangular explants (6 mm x 8 mm) are cut out. Each explants is placed on a tube containing 15 c.c. of Murashige and Skoog’s medium, which is composed of mineral salts, thiamine (0.4 mg/l), myo-inositol (100 mg/l), sucrose (30,000 mg/l) and a growth regulator.
For root initiation 2 mg/l IAA and 2 mg/l kinetin are needed at 12 hours photoperiod and a temperature of 25°C. After initiation of root the callus is transferred to a medium containing 4 mg/l kinetin and 4 mg/l IAA. In this medium shoot “initiation occurs after four weeks. Culture is transferred to a medium without any hormone. Subculture can be done every 3-4 weeks.