The effects of stress and endogenous security mechanisms

In biology today, the basic question concerning alkaloids is connected with the relation between their internal and external roles. It appears that the external role is only secondary, and the endogenous use of alkaloids as genetically coded is the primary function. Höft et al.344 have studied the sources of alkaloid formation and changes in Tabernaemontana pachysiphon plants. In this research the endogenous factors were leaf age, plant age, leaf position in the crown and teratological leaf dwarf growth on leaf alkaloid contents. Environmental factors were soil and other climatic factors controlled in a greenhouse in the case of young plants. In the case of the old trees, environmental factors were measured in natural habitat. Höft et al.344 clearly documented that higher leaf alkaloid content is thought to result from higher nitrogen and cation availability.

The relationship between nutrients in the soil and changes in alkaloid amount occurring in plants is one of the most important topics in alkaloid biology and furthermore in plant physiology and biochemistry. Alkaloid content in plants, for example in tobacco (Nicotiana) or lupine (Lupinus), may increase with treatments high in nitrogen. There are, however, many exceptions to this. Amounts of indole, purine and steroid alkaloids in plants do not change rapidly in response to such treatments. It does seem that alkaloid content is generally related to nitrogen levels available to plants. Two basic factors seem to influence this relation: (1) the biosynthetic nature of alkaloids themselves, and (2) the balance of nitrogen and other nutrients in the soil. The alkaloid biosynthetic pathway is important in this sense that during synthesis the nitrogen existing in the precursor can be liberated, or additional nitrogen may bind. Some precursors are richer in nitrogen than alkaloids, for example in the case of morphine, nicotine, hyoscyamine and so on. In the case of gramine or caffeine the amount of nitrogen is the same as in their precursors. In alkaloids such as tomatidine or coniine the amount of nitrogen is higher than in their precursors. This is the reason why some alkaloids are more sensitive to nitrogen availability than others. Moreover, the balance of nitrogen in the soil seems to be very important. High or low concentrations of nitrogen in soil seem to influence alkaloid content in the plant despite the biosynthetic nature of alkaloids (Figure 80). In both mentioned cases, the plant suffers from nutritional stress and the production of alkaloids seems to increase. Nutritional stress seems to be the reason for this. It is affected by absences and a high demand for nitrogen during metabolism. Plant stress in this sense can be determined as a force which strengthens alkaloid production for both continuing storage in vacuoles and for continuing their departure from vacuoles for the metabolic regulation of stress. It is necessary to mention that this topic has not been yet the object of larger specialized laboratory studies. Therefore, this explanation remains a strong hypothesis to be investigated in future studies. It is, however, known that nitrate uptake promotes alkaloid accumulation and is preferred over ammonium uptake. Soil acidity and temporary drought stress are also known to block

Figure 80. Mechanism of regulation of alkaloid content in plants. Abbreviations: RS - regulation system; OH - overhigh level; OL - overlow level: LGF - life growing factors. Observe that this regulation system is coded in genes. Life growing factors (light, water, CO2, nutrients including nitrogen, temperature, etc.) influence on stress, which is also dependent on this system.

Figure 80. Mechanism of regulation of alkaloid content in plants. Abbreviations: RS - regulation system; OH - overhigh level; OL - overlow level: LGF - life growing factors. Observe that this regulation system is coded in genes. Life growing factors (light, water, CO2, nutrients including nitrogen, temperature, etc.) influence on stress, which is also dependent on this system.

nitrification and may thus contribute to low leaf alkaloid accumulation345. The above-mentioned experiments by Höft et al.344 proved that in Tabernaemontana pachysiphon, the differences in alkaloid levels due to endogenous factors such as leaf age or dwarf growth were much more pronounced than any other difference caused by environmental factors. The influence of age or tetratological leaf growth differed depending on the alkaloid. Apparicine content was enhanced in very young leaves and equally high contents in dwarf-leaves of old trees344. The different positions of leaves may have been caused either by small differences in leaf age or by a plant's internal nutrient and water fluxes. According to Höft et al.344 differences in alkaloid levels according to tree age were rather marginal. Although their research does not directly answer the question of the internal and external roles of alkaloids, it does show the factors influencing alkaloid production. These factors also indirectly mean that alkaloids become more needed in a plant when the factors influencing their production are present. When summarizing and generalizing empirical results, it can be stated that stress and stressful situations in plants induce alkaloid production and their needs in regulatory processes of metabolism. The high content of alkaloids in old leaves suggests a metabolism regulation function similar to growth hormones, although it is also known that plant hormones such as cytokinins were found to stimulate the alkaloid synthesis346. Moreover, this also suggests that the basic biological function of alkaloids is endogenous. Many present research results suggest just this. For example, research by Henriques et al.347 has shown that alkaloid production and accumulation in plants of Psychotria leiocarpa (Rubiaceae) increased with plant age and light exposure. The alkaloids in this case are needed for physiological and metabolic regulation by a plant. Another good example of alkaloid production and accumulation and its function can be observed in the case of ft-carboline alkaloids in humans. These alkaloids occur in mammals211. As neurotransmitters they play a regulative role in various metabolic processes. The natural concentration of harman, an endogenous inhibitor of monoamine oxidase sub-type A with a high affinity in brain and peripheral organs, in rat brain is reported to be less than 0.5 ng/g tissue. Norharman induces pro-conflict behaviour in limbic-hypothalamic structures and alterations of motor activity211. This also proved that endogenous activity seems to be a basic function of some alkaloids. Alkaloids are therefore some kind of natural endogenous medicines needed for ordering metabolic processes by inhibiting or accelerating other active molecules. In this sense the external role, especially in growing environments and species interaction, seems to be secondary.

Nowadays, knowledge of alkaloid biological function is based on empirical results. The most important biological function in plants involves the chemical and biological protection of cells. They protect plant bodies from physical stresses like ultraviolet light and heat580. Other biological functions are protection against pathogens and herbivores, protection of generative reproduction, an acute source of nitrogen, nitrogen storage and the stimulation of growth and adaptation to the local environment.

2. Bioactivity

The general characteristics of alkaloids are their chemical flexibility in regards to structure, and as a consequence of this, the biological activity. Individual alkaloids do not play only one role. The same alkaloid in different cell conditions is able to change its structure and thereby its biological activity. This ability makes the alkaloids a special group of secondary compounds.

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