BIOLOGY Volume 2 - A Guide to General Biology - 2004

16. PLANT COORDINATION AND REGULATION

16.2. Plant Growth Substances

16.2.7. Cytokinins

Discovery of Cytokinins

During the 1940s and 1950s, considerable effort was devoted to improving plant tissue culture techniques. Such cultures make it possible to study individual developmental processes independently of influences from other PARTS OF THE Organism, as well as to observe the effects of various substances on Cells. Keeping cells alive proved relatively straightforward, but stimulating their division presented certain challenges. In 1954–1956, Skoog, working in the USA, discovered that coconut milk contained an unidentified factor that stimulated Cell Division in tobacco pith cultures. Coconut milk is liquid endosperm (the nutritive tissue of a seed), and the first indications that it contained growth-promoting substances came as early as the 1940s, when it was investigated as a potential source of embryo-growth-stimulating agents. The search for similar factors revealed their presence in aged samples of herring sperm DNA, whereas fresh preparations lacked the required activity. However, autoclaving (heating under pressure) fresh herring DNA imparted these properties to it. It was determined that this active ingredient was structurally similar to adenine, a nitrogenous base found in DNA (Chap. 3). It was named kinetin. Skoog himself used the term kinins for substances that regulate cell division. Later, the term cytokinins came into use; this was partly related to METABOLISM/2.html">THE CONCEPT OF cytokinesis (meaning cell division) and partly because the term kinin had already been used in animal physiology with an entirely different meaning (kinin being a type of Blood polypeptide). The first naturally occurring cytokinin with a determined Structure was isolated in 1963 from immature maize grains (Zea mays) and was therefore named zeatin. As shown in Fig. 16.22, like kinetin, it is a derivative of adenine.

Class="center">

Fig. 16.22. Structural formulas of kinetin, zeatin, and adenine.

Synthesis and Distribution of Cytokinins

Cytokinins are most abundant in zones of active cell division, particularly in fruits and seeds, where they are essential for embryo development. Evidence suggests that in mature plants they are often synthesized in the roots and then transported via the xylem in the Transpiration stream to the shoots. Their "re-export" from leaves via the phloem is also possible.

Action of Cytokinins

By definition, cytokinins stimulate cell division. However, they only do so in the presence of Auxins. Gibberellins may also be required, for instance, in the cambium. The interaction of cytokinins with other growth substances will be discussed in Section 16.3.2.

One of the most fascinating properties of cytokinins is their ability to delay normal leaf senescence. When a leaf is detached from a plant, it typically ages rapidly, losing its green color (chlorophyll), Proteins, and Nucleic Acids (RNA and DNA). However, if a drop of kinetin is applied to a specific area of a leaf, a green island of active tissue will persist in that spot amidst the surrounding dying Tissues. It has been shown that nutrients are redistributed to this area from other parts of the leaf (Fig. 16.23).

Fig. 16.23. Effect of kinetin on amino acid translocation in tobacco leaves. A drop of radioactive amino acid was applied to a leaf, and after a period of time sufficient for translocation, the leaf was placed on photographic film. On the resulting autoradiographs, radioactive areas appear black.

16.19. Examine Fig. 16.23 and answer the following questions.

a) What is the difference in The Fate of The amino acid applied to young versus old leaves?

b) How can this difference be explained?

c) How does kinetin affect the distribution of the radioactive amino acid in old leaves?

A similar, though less pronounced, effect is observed even when kinetin is applied to senescing leaves of an intact plant. Studies have shown that the concentration of natural cytokinins decreases in Aging leaves. Consequently, the program of natural senescence may involve The transport of cytokinins from older leaves to younger ones via the phloem.

Cytokinins are also involved in numerous processes related to Plant GROWTH AND DEVELOPMENT (Table 16.4 and Section 16.3).

MECHANISM OF ACTION of Cytokinins

The structural similarity between cytokinins and adenine (a nitrogenous base found in DNA and RNA) suggests that these phytohormones may play a vital role in NUCLEIC ACID METABOLISM. It has been found that certain unusual (minor) bases isolated from Transfer RNA molecules exhibit cytokinin activity, implying that these bases may be involved in tRNA synthesis. Nevertheless, even this possibility does not fully explain their mechanism of action as growth substances, and further research in this area is required.

Practical Application of Cytokinins

Cytokinins prolong the shelf life of fresh leafy vegetables, such as lettuce and cabbage (by delaying senescence), as well as cut flowers. Additionally, they can be used to break seed dormancy in certain plants.