Auksin. As operates auksin

you are: Auksin

As we saw, auksin can render set of various influences on plants, regulating such processes, as division, a stretching and a direction of development of cages during their differentiation. It causes variety of physical, chemical and physiological changes in cages, and some of them occur extremely quickly. for example, speed of movement of cytoplasm in pieces koleoptilja oats increases in some minutes after drawing by them auksina. Though this fast reaction can be early display of action auksina, is difficult connect it directly with a stretching of cages which at some instant should cause essential changes in a cellular wall. It is improbable that one and only ' dominating reaction ' with participation auksina is responsible for all these effects. Therefore physiologists of plants investigated some different processes. We now have quite comprehensible representation about how auksin regulates a stretching of cages. We gradually approach to a mechanism solution with which help auksin supervises other aspects of development.

Unlike cages of animals protolayers of vegetative cages are concluded in semifixed ' wooden ' a box-.kletochnuju a wall. Irrespective of the fact which changes occur in a box, the cage cannot increase in the sizes if its walls are not capable to be stretched. The similar situation can be observed and at animals: for example, at the representative of arthropods - a lobster at whom all body is concluded in the rigid external skeleton consisting mainly from chitin. The animal can grow only when the old rigid external skeleton is dumped in the course of a moult, and the new layer lying under it remains during any time soft and extensible. Cages of plants cannot dump the cellular walls just as lobsters dump the external skeletons. For them a unique way to increase in volume is to modify an existing cellular wall, to make its more extensible. It has been shown that auksin plays this process an essential role.

Growth of vegetative cages two factors operate: an extensibility of cellular walls and the tourist's mountain pressure of cellular contents operating on a cellular wall. We can imagine this situation by analogy with not completely nadutym a balloon, in which volume increase to proportionally internal pressure and in inverse proportion to the resistance rendered by a wall of a sphere. The sphere will increase in sizes if we raise internal pressure inflating in it of air. It is similar to increase in tourist's mountain pressure in a cage. On the other hand, if we could make a wall of a sphere of more extensible, influencing it chemicals softening rubber the sphere also would increase in sizes, but without increase of initial internal pressure. Through 1-2 ch after drawing auksina or koleoptil their cages increase by a stalk in sizes much more, than corresponding cages in a control variant without auksina. Such increase occurs basically at length, instead of width. It is caused by a spiral arrangement cellulose mikrofibrill in a cellular wall that promotes more likely to lengthening, than radial growth of a cage, and also that auksin increases an extensibility of a cellular wall. we can measure it, if at first we will boil a piece of a stalk or koleoptilja to kill him and to eliminate thereby tourist's mountain pressure in cages, and then, having clamped both ends of a piece, we will define the force necessary for its stretching. This force is connected with an extensibility of a wall inverse relationship. The wall extensibility can be spread out to two components: a plastic extensibility (irreversible deformation) and elasticheskuju (reversible deformation). If before to kill segments of a stalk of a plant, to process them auksinom it will appear that for their irreversible stretching to a certain limit it is required less efforts, than in a check experiment without application auksina. Such induced auksinom the increase in a plastic extensibility of a cellular wall is observed only in live cages. Auksin does not render any influence on an extensibility at its drawing directly on cellular walls in pieces of a dead stalk or koleoptilja.

How it is possible to explain loosening action auksina on cellular walls? Whether it can be caused, for example, increase in a cage of quantity of enzyme tselljulazy, destroying a cellular wall? Really, activity of this enzyme sharply increases in some hours after drawing auksina, and, hence, enzyme could destroy some cellulose mikrofibrilly, having raised thereby a wall extensibility. Though in itself such idea is probable, activity tselljulazy, apparently, increases only after auksin has already started to influence growth. Clearly that any comprehensible theory of action auksina should include process which begins to stimulirovannogo auksinom growth rate increases.

Precisely to define, since what moment growth rate after drawing auksina starts to increase, scientists use the device represented on fig. 9.16.

By means of this device it is possible to measure time during which there are any changes of length of a piece of a stalk to within several seconds. Through 10-15 mines after addition auksina it is not marked yet growth rate increases. Then growth is quickly accelerated and reaches the highest speed within the next 15 minutes Thus, the time interval from 10 to 15 mines after drawing auksina is required to cause changes in a metabolism, leading as a result to growth acceleration. These changes should reach the maximum approximately in 30 minutes Therefore any changes which are not found out within an hour or a bit later, cannot concern initial action auksina on indutsirovanie growth. However they could be a part of mechanisms which should join later for maintenance induced auksinom growth.

What change occurs for such short period, what it can be connected with induced auksinom a stretching of cages? The answer, possibly, is concluded in the phenomenon which for the first time has been found out in the thirties, but not attracted attention of the majority of scientists. Then it has been opened repeatedly and described in the early seventies. It has been established that the sour environment (low rn) promotes a stretching of cages. If pieces of a stalk or koleoptilja to place in the device represented on fig. 9.16, having shipped them in a sour solution their growth will amplify. If for improvement of penetration of acid from pieces to remove epidermis optimum strengthening of growth and an extensibility of cellular walls will be observed at rn nearby 5. If dead pieces to stretch mechanically, effort, which is necessary for putting to reach certain degree of a stretching, will be less in that case when pieces are shipped in a solution with rn 3,5, instead of with rn 7,0. Thus, influence of acid on a stretching does not demand presence metabolicheski active cages. The stretching of cages is a direct consequence of action of ions N + on wall components. That they make similar impact on growth testifies to communication between acid and auksinom. They differ with that, first, acid operates on walls, and auksin - on cages and, secondly, acid influence only short time, and influence auksina - at least lasts some hours. Between acid and auksinom, probably, there is a simple dependence. If under influence auksina occurs podkislenie cellular walls it can cause their the subsequent razryhlenie. This mechanism explains only initial stimulation of growth as the materials necessary for the further stretching of cages, are quickly settled. That growth could proceed, auksin should show one more action which would provide synthesis of a new wall. This last process is absent, when the wall stretching is induced only by one acid.

It is known that in cellular membranes and in membranes various organell cages are available metabolic pumps for various substances, such, as mineral ions, sucrose, hormones and hydrogen ions (see gl. 7). According to one widespread theory, auksin stimulates work localised in plazmalemme the pump which is pumping over ions N + from cytoplasm in a cellular wall. Such pumps represent the fibers receiving energy for the work as a result of splitting ATR to ADP and inorganic phosphate. Resintez ATR depends, of course, on a metabolism. This theory assumes that the pump which is pumping over ions N +, functions only in the event that with it is connected IUK, operating as effektor (activator) (fig. 9.17). Such communication, apparently, is reversible. It is formed and breaks up depending on concentration IUK in surrounding cytoplasm. This model basically has proved to be true, when has been shown that pieces koleoptilja with removed epidermisom at their premise in a solution acidify it only after addition IUK on Wednesday. Thus, despite the fact that what the theory activated auksinom proton (N +) has not resolved the pump of all problems and it it is necessary to add with some details, it Will be co-ordinated with the majority of the data and represents a comprehensible explanation of action auksina on a stretching of cages.

But we yet have not finished. Why podkislenie promotes a stretching of cellular walls? To answer this question, we should return to structure of a cellular wall of a plant (fig. 2.31). The wall consists of the long molecules of cellulose connected among themselves polisaharidnymi by bridges, formed by a mix of sugars, such, as galaktoza, arabiioza and ksiloza. The Polisaharidnye bridges making a part of a wall, usually name gemitselljulozoj. The long cellulose molecules connected with each other by these cross-section communications, cause rigidity of a cellular wall. At an irreversible stretching of a cellular wall that occurs usually in the course of growth, cellulose chains should slide rather each other, and it can be reached only in the event that cross-section communications between cellulose chains will be broken off. When the wall stretching comes to the end, cross-section communications are formed again, fixing cellulose chains in their new position (fig. 9.18). As a result the sizes of a cage increase. That acid could promote increase in an extensibility of a wall, it, obviously, should break cross-section links between cellulose molecules. The assumption has been come out that in a cellular wall there is the rn-dependent enzyme destroying such communications. This enzyme does not operate at rn 6,0-7,0, but becomes highly active at rn nearby 5,0. When under influence auksina, present in a cage, ions N + are pumped up in a cellular wall, enzyme is activated and destroys cross-section communications; cellulose chains are released and have an opportunity to slide one concerning another. Such sliding is carried out under the influence of tourist's mountain pressure of cellular contents that causes a stretching of a wall and increase in the sizes of a cage. We yet do not know the nature of this hypothetical enzyme to which existence that some chemical compounds interfering usually action of enzymes, are capable ingibirovat the stretching of walls of the isolated cages induced by acid testifies.

In absence auksina the growth induced by acid lasts only short period, and at its presence proceeds within many hours. What itself represents this long-term process activated auksinom? If to add ingibitor fiber synthesis (such, as tsiklogeksimid) to stimulirovannym auksinom to stalk pieces they stop growth approximately through 15 mines after a stop of synthesis of fiber. If ingibitor it is added long before drawing auksina last nevertheless stimulates growth, but only within the same 15/minutes From here we can conclude that the initial stimulation of growth caused by pumping out of ions N + from cytoplasm under influence auksina, does not depend on fiber synthesis,

Whereas further induced auksinom growth requires fiber synthesis. One of necessary fibers is, apparently, the fiber which are carrying out function of the pump as at presence tsiklogeksimida induced auksinom podkislenie the external environment stops. Hence, at presence auksina this fiber-pump should have short enough time of a life and constantly be synthesised. For growth continuation are necessary as well other processes. To keep a cage wall in the stretched condition and to make its rigid, it is necessary to reconstruct polisaharidnye cross-section communications. To prevent gradual utonchenie walls (it is known that its thickness during all period of growth of a cage remains approximately identical), it should be synthesised more cellulose and polisaharidov, forming cross-section communications. So, it is remarkable that auksin as it has been established, raises activity of the enzymes responsible for synthesis of cellulose. These enzymes are localised in a cellular membrane and in vials diktiosom which, as it is known, participate in synthesis of components of a wall.

Constant synthesis of new fibers is necessary For continuous division, a stretching and differentiation of cages under influence auksina. In growth of a cage of the squirrel are used for construction of additional components of cytoplasm of a cage, and at differentiation the new enzymes participating in processes, characteristic only for the differentiated condition of a cage are required. It has been shown that auksin causes increase in speed of synthesis of fiber by synthesis increase mrnk in a kernel. Together with other hormones and regulators auksin, possibly, changes also type formed mrnk, changing thereby type of enzymes in a cage. It is as a result synthesised more enzymes specific concerning growth and differentiation of cages. Besides, it is synthesised more ribosomnoj RNK to satisfy the raised requirement for it, arising at fiber synthesis. We yet do not know, what enzymes synthesised under influence auksina, induce long growth of cages and their differentiation. Such problems hardly give in to the decision, however gradually we will learn more and more about the details connected with growth and differentiation of cages, and eventually we can is better imagine, where it is necessary to search for answers to unresolved questions.