Further Growth Regulators

Oligosaccharines

Oligosaccharines are naturally occurring hydrocarbons with regulating effects. They have been discovered during the last few years in the group of P. ALBERSHEIM (Complex Carbohydrate Research Center, Athens, Georgia).The more they were studied the more functions were detected. Oligosaccharines influence growth and differentiation of cells, and they participate in the defence against fungi and bacteria. They are usually low molecular breakdown products of the cell wall whose amount is up to 2% of the whole cell wall material in monocots and dicots. Oligosaccharines are structurally astonishingly diverse. Roughly estimated are more than 100 enzymes required for their production. Beside the sugars common in the hemicellulose fraction like galactose, rhamnose, xylose and arabinose occur numerous new sugars with seven or eight C-atoms and partially unusual substitutions (additional –COOH groups). The sugar residues of the poly- or oligosaccharides of plant cell walls contain no nitrogen derivatives while half of all animal extracellular sugars have them (the sugars contain mostly amides).

A total of 65 different monosaccharides interconnected by more than 20 different types of links have been identified by now. Such an extensive spectrum of compounds was until now only known from proteins and nucleic acids. It looks as if the oligosaccharines have an importance for the plants that can be compared to that of peptide hormones in animals.

The release of oligosaccharines can be stimulated by auxines though fungi infections or damage of the plant cells have the same result. It was observed that oligosaccharines released after fungi infection induced the production of an antibiotic thus protecting the plant from further spreading of the fungi mycelium. Oligosaccharines are, too, able to kill neighbouring cells. They do thereby destroy the precondition of spreading for fungi, micro-organisms or viruses (hypersensitivity). Their effect is not species-specific. Oligosaccharines isolated from maple cells effect corn cells, too, (and vice versa). The addition of oligosaccharines to tissue cultures induces morphogenesis (shoot and root tissue). A nonasaccharide inhibits the elongation induced by auxin. It seems to have a regulating function for auxin (a feed-back modulator).

A Second Messenger: Calcium Ions

Hardly any other ion is as necessary for the regulation of cellular processes like the development of polarity, secretion, growth, division, and gene expression like calcium. Its cytosolic concentration is, compared to that of surrounding compartments like mitochondria, vacuoles or the endoplasmatic reticulum (including the cell wall), extraordinarily low. This indicated that the cells spend a lot of energy (ATP) for getting surplus calcium out of the cell. The uptake of calcium is stimulated by numerous factors, for example by auxine, cytokinin, and gibberellin, and by phytochrome or gravitation. All these observations point at a second messenger function. Calcium seems to be a member of a chain of cause and effect in which many signals are perceived and cleared independent of each other. The consequence of an increased calcium uptake by the cell may again initiate a number of other processes. Growth processes and bud development, for example, are correlated with a rise of the intracellular calcium concentration. Bud development does not take place when calcium uptake is inhibited. A lack of cytokinin or auxin has the same effect. It has been shown that abscisic acid acts as an antagonist of the second messenger activity of calcium by binding to the calcium channel on the outside of the membrane thus inhibiting the transport of calcium (OWEN, 1988).

In pollen tubes exists a gradient of membrane-bound calcium from tip to base. It is maintained by a continuous inflow of calcium at the end of the plasma tube. Calcium controls polar growth, vesicular transport along actin filaments, as well as membrane fusions (between vesicle membranes and the plasmalemma, for example; H. D. REISS (Universität Heidelberg, 1987). Intracellular calcium is usually bound to a protein called calmodulin. The calcium-calmodulin complex acts together with a further component (RE, response element, also a protein) as a protein kinase catalyzing the phosphorylation of numerous proteins that themselves control independent but partially parallel developmental processes, differentiation, and movements within the cell.

A. J. TREWAVAS (University of Edinburgh, 1987) isolated a membrane-bound calcium-dependent protein kinase. This enzyme is capable of autophosphorylation thus inactivating itself. It has a switch function and phosphorylates, dependent on its state of activity, also other proteins. Thereby are cellular processes activated or, when the enzyme is in a phosphorylated state, inhibited.