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Get the answer of: Are all cells in culture totipotent?
A callus mass contains thousands of cells. Theoretically, if the totipotency of all cells are expressed at a time, then it is expected that equal number of shoots or roots or embryoids will be regenerated from such totipotent cells. But in experiment, such results are not obtained.
The reasons behind such limited expression of totipotency are many and vary from plant to plant. It has been observed that variation of chromosome number in the cells of callus tissue is one of the main factors that causes the limited expression of totipotency. This variability may be attributed to either pre-existing variation in the somatic cells of the explant (genetic) or variation generated during tissue culture (epigenetic).
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Changes in chromosome number are aneuploid, polyploid etc. As a result, a mixoploid callus tissue is formed in the subsequent growth. But, very often, from these mixoploid callus cultures, organogenesis and/or embryogenesis occur mostly from diploid cells. Therefore, all cells of the callus tissue are not able to express their totipotency.
Observations on organogenesis or embryogenesis in callus culture have led to propose that an association of cells may be sometimes necessary to provide the appropriate environment for certain individuals to express their totipotency.
It is also proposed that endogenous hormone level of a cell and exogenously supplied hormone makes a threshold level which actually induces the totipotent ceil to express in culture. But the cells that comprise the callus tissue absorb the hormones and the nutrients forming a gradient. Therefore, the availability of hormones is not equal to all cells. As a result, this fact imposes a barrier to reach a threshold level of hormone equally in all cells for their expression of totipotency.
In culture, some cells are highly recalcitrant and in such cases totipotent cells do not respond to any morphogenetic stimuli. These cells cannot be easily differentiated. Besides these factors, heterogeneity in physical structure of the cells make a great difference in the degree of chemo differentiation of the cells that make an asynchronous situation for which all cells are not able to express their totipotency at a time.
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Therefore, it seems apparently that all cells are not totipotent. More or less homogenous thin tissue layer such as epidermal tissue and free unicellular pollen grain, free cells from suspension culture, protoplasts are more efficient system for the synchronous expression of totipotency.
