The following points highlight the four main levels of action of controls. The levels are: 1. Genetic Level 2. Biochemical Level 3. Cellular Level 4. Organizational Level.
1. Genetic Level:
The concept of totipotency is widely accepted. Given proper nutrients, growth hormones and congenital environments, single cells can be made to divide, enlarge and differentiate into tissue, organs and a plant. Evidently every cell has information for all the developmental processes in the life of an organism.
Another control type could be when cytokinins may form ribosides structurally similar to the ribosides of RNA. These have been located in tRNA. Recent evidences have indicated that activity of cytokinins was not directly related to their presence in RNA. Possibly presence of cytokinins in RNA may be involved indirectly in the regulation of enzyme synthesis or function.
2. Biochemical Level:
The role of gibberellins in the stimulation of α-amylase activity is widely accepted. The stimulation of a-amylase synthesis in germinating cereal seeds by gibberellins has been extensively investigated from different angles.
This hormone acts to release a pre-existing but a repressed operon and makes it active (Fig. 19-6). Accordingly, α-amylase is synthesized. This enzyme brings about the hydrolysis of starch during seed germination. In due course of time hopefully effects of other hormones on biochemical actions will be discovered.
3. Cellular Level:
At the cellular level array of control mechanisms exist. Several of them are the direct result of the regulation of some biochemical systems. In the following we shall discuss possible effects at three levels: cell division, cell enlargement and cell differentiation.
The meristematic cells are small having thin cell walls, intercellular spaces and are full of protoplasm with a prominent nucleus. The meristematic cells grow as follows:
The amount of protoplasm gradually increases, resulting in an increase in the cell size. Subsequently, the nucleus divides and so does the cytoplasm. The newly formed daughter cells are separated from each other by new cell walls. Cells divide repeatedly in this way. The region of cell division is a centre of great metabolic activity being rich in carbohydrates and proteins.
Cellulose and pectic compounds, etc. are built up from carbohydrates. Protoplasmic proteins are formed from amino acids and other related compounds. Such substances are translocated to the meristems from the tissues where they are synthesized.
Some are also synthesized in the meristems themselves. Some of the metabolites are oxidized in respiration and the latter provides energy consumed during the anabolic processes.
The new cells imbibe water through cell wall and the protoplasm and thus swell up. The meristematic cells receive essential mineral elements either in inorganic or organic form. The translocation of water and solutes beyond the vascular elements occurs by the cell to cell movement. Cell division requires the presence of numerous growth regulating substances, enzymes, co-enzymes, vitamins, etc. in a cell.
The increase in the size of the cell involves an increase in the cell wall.
Increase in the thickness of cell walls occurs as a result of the following physiological manipulations:
(a) Due to the enlargement of a cell when additional cell wall material is added and the quantity of the cell wall material is proportionately greater than the increase in the quantity of protoplasm. Hence the amount of carbohydrate and proteins assimilated during the cell enlargement is greater than during the cell division;
(b) As the cell increases in volume, water enters the enlarging cell vacuoles osmotically. As a result the cytoplasm is reduced to a thin parietal layer abutting on the cell wall and is held there by the turgor pressure of the cell sap;
(c) The respiratory activity is high;
(d) The rapid translocation of water and mineral salts takes place by cell movement and respiration provides the energy expanded on it; and
(e) The increase in the area of the cell walls occurs in the presence of the auxins. In general two views regarding the mechanism of cell enlargement are accepted.
First, there is reversible or irreversible stretching of the cell sap which is followed by an increase in the substances of the cell wall either by the intercalation of the additional molecules in the wall (intussusception) or by the deposition of the additional molecules on the cell wall layers already present (apposition) or by a combination of both these processes.
Second, the first step in cell enlargement is intercalation of additional molecules to cause additional protoplasm increase accompanying the cell walls. The increase in the cell volume is by the entrance of water into the cell. During the period of elongation, cells have a low turgor pressure and also high water potential.
During the cell differentiation, enlarged cells are now modified into the primary tissues of the mature organ, e.g., protective, storage, conducting and mechanical. Cells of various tissues differ in size and structure. Accordingly they elongate along the growth axis. The walls in some cells become pitted or thickened as in tracheids and vessels.
The structural differentiation is accompanied by chemical differentiation. Walls of the pith cells, phloem and cortex retain their original cellulose pectic compounds. On the contrary xylem duct walls as well as sclerenchyma become lignified.
Walls of the cork cells are impregnated with suberin. Cells which become suberized or lignified have their contents disappear. The respiratory activity of fully differentiated cells is less than the dividing cells.
4. Organizational Level:
Organization takes place as a consequence of polarized cell division and cell specialisation. The reaction at the individual cell level controls shape, size and form. The hormones balance plays an important role in cell and tissue organization. Here it will suffice to mention that auxin is translocated from the tip to the base of the plant but reverse does not happen.
This polarity of transport exists even if a plant tissue is cut and placed up-side down. Thus, in a stem cutting roots are always formed on the physiological lower side whereas shoots are produced on the physiological upper end. Recently several instances of acropetal auxin transport have also been brought out.
As regards other plant growth substances they do not undergo polar translocation. Cytokinins move slowly whereas gibberellins move rapidly. The movement of phytohormones needs utilization of metabolic energy which is derived from respiration. Hormones also participate in morphological processes when applied exogenously.
They alter several developmental events. The role of auxin in inducing rooting is well known. In brief all morphogenetic and organizational patterns in plants are affected by different growth regulators. The level and the method of coordination of these plant growth regulators is still conjunctural and much remains to be understood.