EPIGENETIC EXPRESSION AND CELL DETERMINATION

Plants cells have an amazing ability to respond to a signal, which almost invariably is a plant growth substance.  When the cells can respond, they can change from a previously determined developmental pathway.  For example, a leaf cell can change so that it will begin to divide and a root or shoot will form.

            In tissue culture and plant propagation, the ratio of cytokinin : auxin is important (Chapter 6).  When cytokinin is in excess, adventitious shoots will form, when auxin is in excess, adventitious roots or somatic embryos will form, and when there are moderate to high levels of both, callus will develop.  This is the normal wound healing process in plants.  Many plant species have the remarkable ability to replace wounded or lost organs.  Following wounding, the endogenous hormone levels change and callus formation and organogenesis are often the result.  Similarly, with some species, because of high auxin levels associated with sexual fertilization in the seed, adjacent somatic cells change and produce embryos through the process of apomixis.  In plant propagation and plant tissue culture, we exploit this healing process and the apomictic ability of cells.

            Different cells within a tissue will not all respond the same way to a change in the cytokinin : auxin ratio.  Understanding what distinguishes certain cells from others in their ability to respond to the plant growth substance is important.  As our knowledge of the molecular biology of cells increases we may understand why there are these cellular differences.  With such knowledge, we may gain the ability to regenerate recalcitrant genotypes, such as adult forms of many plants.

Christianson (1987) and Christianson and Warnick (1983, 1988) stated that there are competent cells in a plant or explant (see Chapter 10).  These are the cells that are capable  of recognizing the plant growth regulator signal  and changing, or becoming determined.  When the plant growth regulator signal is removed, the determined cells will continue to respond.  They will dedifferentiate (become meristematic again), and their daughter cells will differentiate into the new shoot, root, or somatic embryo.

Not all cells in a tissue  are competent.  Exactly what makes one cell competent to perceive a plant growth regulator signal and its adjacent cell not to be competent is not well understood.  It is known that applied plant growth regulators elicit specific mRNA molecules (Christianson and Warnick, 1988).  This is evidence of specific genes being expressed in response to the exogenous plant growth substances.  However, this specific gene expression is more clearly related to changes in determined cells than to what makes cells competent.

A problem with trying to understand what makes a cell competent is that it is difficult to ascertain that a specific cell is competent until after it responds to the plant growth substances and becomes determined.  Once determined, the cell changes and its gene expression can be studied.  However, it is difficult to know which cell is competent and which one is not before the cell becomes determined.

            It is likely that epigenetic expression is the reason that a cell is competent.  Perhaps there is a gene or genes expressed for the receptor molecule of the elicitor plant growth regulator, which would allow that competent cell to recognize the plant growth substance.  However, there are other possibilities that could lead to competence, such as a expression of an important gene in the signal transduction pathway in a competent cell.

The predetermination of cells and tissues can often be changed in culture by cell differentiation and cell division, leading to the direct formation of organogenic or embryogenic meristems or callus. But if the tissue is not competent to undergo these changes, or the growth regulators in the medium are not suitable, previously predicated developmental pathways may not be interrupted.  Some aspects of determination may nevertheless survive in newly-established cultures and can occasionally persist for many callus transfers.