The main objective in plant cultures is to regenerate a plant or plant organ from the callus culture. The regeneration of plant or plant organs only taken place by the expression of cellular totipotancy of the callus tissues. Scattered areas of actively dividing cells, known as meristematic centres, develop as a result of differentiation and their further activity results in the production of root and shoot primordia. These processes can be controlled by adjusting the cytokinins: auxin ratio in culture medium. The production of adventitious roots and shoots from cells of tissue is called organogenesis.
Definition of Organogenesis:
“The development of adventitious organs or primordia from undifferentiated cell mass in tissue culture by the process of differentiation is called organogenesis.
“The formation of roots, shoots or flower buds from the cells in culture in manner similar to adventitious root or shoot formation in cuttings is called organogenesis.
Type of organogenesis by which only adventitious shoot bud initiation take place in the callus tissue.
Type of organogenesis by which only adventitious root formation takes place in the callus tissues.
In some culture tissues, an error occurs in development programming for organogenesis and an anomalous structure is formed. Such anomalous organs like structures are known as Organoids. Although Organoids contain the dermal, vascular and ground tissues present in plant organs, they differ from true organ in that the Organoids are formed directly from the periphery of the callus tissue and not from organised mersitemoids.
Meristemoid is localized group of meristematic cells that arise in callus tissue and may give rise to shoots and or roots. They are also termed as nodules or growth centres.
In plant tissue culture, during growth and maturation of callus tissue or free cells in suspension culture, few dedifferentiated cells undergo cytoquiescece and cytosenescence and this twin phenomenon are mainly associated with redifferentaition of vascular tissues, particularly tracheary elements. The whole developmental process is termed as cytodifferetiation.
General Account of Organogenesis
In vitro organogenesis in the callus tissue derived from small piece of plant tissue, isolated cells, isolated protoplasts, microspores etc can be induced by transferring them to a suitable medium or a sequences of media that proliferation of shoot or root or both. The suitable medium is standarize by trial and error method. Organ formation generally follows cessation of unlimited proliferation. Individual cells or groups of cells of smaller dimensions may from small nests of tissue scattered throught the cells of smaller dimension may from small nests of tissue scattered throught the callus tissue, so called meristemoids which become transformed into cyclic nodules from which shoot bud or root primordia may differentiate.
In most calli, initiation of shoots buds may procede Rhizogenesis or vice versa or the induced shoot bud may grow as rootless shoot. Shoot bud formation may decrease with age and subculture of the callus tissue, but the capacity of the rooting may persist for longer period. In some calli, rooting occurs more often than other form of organogenesis. During organogenesis, if the roots are first formed, then it is very difficult to induce shoot bud formation from the same callus tissue.
But if the shoots are first formed, it may form root later on or may remain as rootless condition unless and until the shoots are transformed to another media or hormone less medium or condition that induce root formation. In certain cases, shoot and root formation may occur simultaneously. But the organic connection between two different organ primordia may or may not be established. Therefore, organic connection between soot and root primordia is essential for the regeneration of complete plantlet from the same culture. Shoot formation followed by rooting is the general characteristics of organogenesis.
The callus tissue may cases shows a high potential for organogenesis when initiated but gradually a decline sets in as subculture proceeds with eventual loss of organogenic response. The loss of potential for organogenesis may be due to either a genetic or physiological change induced by either prolonged cultural conditions or the composition of the culture media. The effects in callus tissue are reflected in changes of chromosome structures or number such as anuploidy, polyploidy, cryptic chromosomal rearrangement etc. It is generally observed that shoot bud formation take place from the diploid cells of callus tissue. At early stage of culture, the callus tissue exhibits maximum number of diploid cell.
According to physiological hypothesis, subculture often leads to loss of many endogenous factors or morphogens present at the critical stages of growth. Such factors present in the callus tissue at the initial stages may not be synthesized at all or synthesized only in insufficient quantities at later stages. as a result , callus tissue fails to exhibit the potential for organogenesis at later stages. as a result , callus tissue fails to exhibit the potential for organogenesis or embryogenesis. However, if these factors are supplemented to the medium during subculture, then restoration of organogenicpotential should be regained. Generally, high concentration of cytokinin brings about shoot bud initiation, whereas high levels of auxin favour rooting.
Certain phenolic compounds peroxidise, and accumulation of higher amount of starch before shoot induction, and synthesis of enzymes of EMP pathway and pentose phosphate pathway, are playing important role in organogenesis.