The maintenance of a multicellular organism or the exchange of genetic information between unicellular organisms requires numerous interactions of the involved cells. It is distinguished between neighbouring relations and long-distance effects. A cell has to send a signal in order to exert an influence and the cell to be influenced has to be able to recognize it. It needs a receiver. The terms effector (or elicitor) and receptor are more common than signal or receiver.
The signals of plant cells are either molecules or physical forces like the pressure inhibiting the neighbouring cell's growth that one cell may exert on another. Signal molecules may belong to the most different classes. It may be ions or simple metabolites that one cell supplies for another. It may be small, specific effectors like the phytohormones discussed in detail later on. It may finally be membrane- or cell wall-bound macromolecules participating in the cohesion and the control of interactions between neighbouring cells.
Plants have - in contrast to animals - no efficient system for the distribution of macromolecules and cells. No macromolecular long-distance effectors of plants are known, although the distribution of viruses within plant tissues shows that plant vessels may be used for the distribution of macromolecular complexes.
Plant cells within tissues occupy permanent positions. The positions are the result of their orthogenetic development. We have already discussed the information contained in a cell's specific position within a tissue. In the plant kingdom exist only few situations in which cells make contact with other cells and elicit specific reactions by active or passive movements. Among these situations are the sexual processes as well as interactions between host and parasite. Parasitically living flowering plants withdraw nutriments from their autotrophic hosts. They penetrate either root or shoot of the host with their haustoria until they have contacted the assimilate-conducting tissues. The interactions between host and parasite are species-specific and are dependent on an exact sequence of growth and differentiation steps.
The recognition signals that initiate the contact between host and parasite are secondary compounds like strigol that was isolated from cotton roots (R. KOLLMANN, I. DÖRR, 1987)
Sexuality is a highly specialized process since only genomes of the same kind can co-operate and fuse to a functioning zygote. In the course of evolution have all kingdoms of organisms developed security mechanisms that prevent the union of incompatible genomes while supporting that of compatible ones. Such mechanisms require several subsequent steps:
Attraction:The egg cell of unicellular and multicellular algae produces a low-molecular effector (a gamone) that is secreted into the surrounding media. The gamone is recognized by species-specific gametes and induces their movement in the direction of its highest concentration. The disadvantage of the process is the low range and the excess material needed. It works only if large quantities of male and female gametes are present. The hit rate is still pretty low.
Recognition and Adhesion. After the encounter do the two cells check whether they are compatible. Only compatible cells adhere to each other. Especially membrane- or cell wall-bound macromolecules participate in this process. The hydroxyproline-rich glycoprotein at the flagellar surface of Chlamydomonas, for example, causes the agglutination of cells at flagella. The pollen-stigma interaction of flowering plants belongs to this group of processes, too. Here again is the participation of glycoproteins decisive. From the adhesion and recognition processes of animal cells do we know that this class of molecules has an outstanding role in the formation of all kinds of intercellular contacts. Both the protein structure and the pattern of the sugar residues guarantee specificity. Lectins or lectin-like molecules take part in the bond between carbohydrate part and protein or act as bridges between carbohydrate residues of the same kind of neighbouring cells.
Specific interactions of this type occur, too, between plants and their parasites or symbionts. Details about the mutual relation between plants and bacteria, plants and fungi as well as plants and viruses are introduced elsewhere. In all cases studies till now it was shown that effector- and receptor molecules, localized in the cell wall are of eminent importance
Penetration of a Tissue by a Foreign Cell. This step is best illustrated by the fertilization of angiosperms. A pollen at the stigma's surface begins to germinate. It develops a pollen tube that grows into the stigma and through the style's tissue and clears its way to the egg cell. The enzymatic weakening of the contacts between the cells of the transfusion tissue permits penetration. Both pollen and stigma or style, respectively, supply the enzymes. The tuning of these activities, so that the pollen tube finds its way guided by the anatomy of the style works trouble-free only with pollen of the same species. We will meet the growth of one tissue through another tissue again when discussing host-parasite and host-symbiont relations.
The Fusion of Two Cells. A plant cell can neither be penetrated by another cell nor fuse with it as long as it is surrounded by a wall. Fusion occurs only, if the wall is missing or if it is locally perforated. At this stage becomes the sequentially orchestrated effect of several enzymes necessary for the union of the two cells. The cytosceletons restructures so that intracellular components are distributed anew.
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