Wound callus

Mixed Cytokinins. The first cytokinin, kinetin [525-79-1] (3), was isolated from stale herring sperm (8) but, like so many biologically active natural products, it was later found in the vascular system of tobacco stems and leaves (9). Yeast also proved to have a very high titre of kinetin (see Yeasts) (8). The compound is very active in increasing cell division in tobacco wound callus tissue that has been cultured on White s agar medium supplemented with 2 mg/L of indole-3-acetic acid (IAA) [87-51 -4], The presence of IAA is mandatory to induce cell division in the presence of kinetin. 

Bark, wound callus, and specialized tissues such as the endodermis that controls entry into the root vascular system, have walls lined with suberin. Suberin, like cutin, is a... 

In his elegant study of hormonal control of the normal differentiation of phloem fibers (described below), Aloni (1976) pointed out that the fibers did not differentiate from the parenchymatous cells of the wound callus, even though sieve tubes and tracheary cells did. The differentiation of phloem fibers was always limited to the longitudinal vascular strands, never being found in the intervening parenchyma. In that sense, one could say that they do not regenerate however, their regeneration has been seldom studied. 

Crown galls are plant tumors that are induced by Agrobacterium tumefaciens. Their occurrence is in no way limited to roots they can occur on any part of the plant (Fig. 139). A precondition is a wound. In the first place it is necessary for the bacteria to penetrate into the plant, but, in addition, the wound leads to a conditioning of the plant cells. For if the bacteria are introduced without injury into the plant no tumor is formed. What this conditioning is, is unknown. At the site of the wound, a wound callus is first formed, as is usual after an injury. Later the tumor grows out of this callus. Wound callus and tumor can be readily distinguished from each other. This is because call divisions in cellus tissue are oriented periclinally, whereas in tumor tissue no orientation at all is to be found. 

Fig. 140. Evidence for the participation of DNA in the formation of crown galls. Both halves of the blade of a leaf of Bryophyllum daigremontianum are wounded and infected with Agrobacterium tumefaciens. The left half of the leaf is treated, in addition, with 5-fluorodeoxyuridine, a competitive inhibitor of thymidilate synthesis. On this side no tumors form, only a wound callus (modified from Bopp 1963).

Many other findings point in the same direction. We shall mention the experiments of Jacobs, which were carried out on another system at the beginning of the 1950s. Jacobs inflicted wounds on the internodes of Coleus and then investigated the development of xylem in the wound callus that formed. The xylem fibers that were newly formed in the callus then connect with those already present in the internodes. In these experiments it was also shown that the leaves above the site of the wound and, to a lesser extent, those below, exert a strong influence on the differentiation of xylem. Xylem formation is inhibited by removal of the leaves but recommences if lAA is applied to the cut surface of the leaf stalks after the leaves have been cut off. The role of lAA is further emphasized by a close parallel between the content of lAA which is transported from the leaves above the wound downward and from the leaves below the wound upward, and the number of xylem fibers induced in the callus (Fig. 211). In addition, these experiments also show that, in Coleus, the lAA transport can occur not only in a strictly polar downward... 

Fig. 211. Parallels between amount of auxin transported and number of xylem fibers bormed in the wound callus. Studied in shoot sections of Coleus (from Jacobs 1956).

Treating tree wounds. Wounds in tree bark, caused by equipment or animals, are an easy target for pests and diseases. Treat the wound by using a sharp knife to smooth the wound edges and shape the area into an elliptical form. This promotes the development of callus ti.s.sue and helps the wound close quickly. 

Electret materials are meanwhile used in a large number of modern high-tech applications including microphones, acoustic sensors, transducers, radiation and pollution dosimeters, power generators, filters, and many more. Additionally, electret technology is of great interest in the field of biomaterials, for instance in callus formation and wound healing [10, 11], When used in cellular or in multilayer sandwich structures, polymer electrets can exhibit piezoelectricity. Such materials are ferroelectrets, as they show typical features of ferroelectric materials such as piezo-and pyroelectricity [12-17],... 

Extracts from the root of Ko-shikon (Lithospermum erythrorhizon) have long been used in Japan as dyes and for their anti-bacterial activity in the treatment of wounds, and burns. The active principles, a series of anthraqmnone derivatives known as shikonins, accumulate in the root to a level of 2% (dry wt.). The total supply of Ko-shikon is imported from China and Korea and it takes the plants 5-7 years to grow to commercial size. Pure natural shikonin sells for about 4500/kg. Based on an earlier report showing shikonin production in root derived callus cultures... 

FIGURE 22.3 Estimation of the wound overgrowing (a) wound covered with transparent scotch-tape to avoid infection and withering (b) surface of callus occupies less than 5% of the wound (1 point) (c) surface of callus occupies 85.5-100% of the wound (9 points). 

Even during World War I, Lobmayer claimed that the Turkish soldiers suffered severely from scurvy and that in those who developed the disease both skin and flesh wounds and fractures healed poorly. In many cases the fractures showed not the slightest sign of the formation of a callus, even after several months. In confirmation of the fact that it was lack of the antiscorbutic substance which was directly responsible for this failure of fractures to unite, he quoted two cases who suffered from pseudoarthrosis of the humerus and who recovered rapidly and completely as soon as they were put on a diet which was rich in antiscorbutic material. 

The response strategy of the tree is twofold first, to close the wound with new callus tissue, thus re-establishing that unbroken layer of living tissue and, second, to develop a reaction layer in the uninjured sapwood tissue adjacent to the damaged cells. The rate of wound closure and the production of the reaction zone are dependent not only upon the size of the wound but also upon the growth rate or vigor of the host. 

We have already mentioned that wound tissue, a callus, first forms on the cut surface of explants from higher plants. Let us now consider an explant from the xylem. It also first forms a callus out of undifferentiated cells (Fig. 209). Then pockets of cambium develop in this callus. This cambium then differentiates, xylem elements being formed on the side towards the original xylem explant and phloem elements on the other side. Now let us do the same experiment with an explant from the phloem (Fig. 209). In this case the cambium forms phloem on the side oriented towards the phloem explant and xylem elements on the other. Evidently there exists a gradient of material from the explant to the callus which specifies and directs the differentiation of the vascular tissue. 

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