Auksin. Tropizmy and transport auksina

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The Bends caused by unilateral drawing auksina on a plant, remind bends of different bodies the plants arising under the influence of light or a gravity. Really, we know now that such bends (tropizmy) are connected with asymmetric distribution auksina in considered body. For example, if koleoptil oats to shine with light of low intensity on the one hand or light of various intensity from the different parties this body is usually bent aside brighter light (phototropism). Bending occurs because on the party turned to a light source, growth chokes with light a little, and on opposite - is accelerated.

If to remove a top odnostoronne shined koleoptilja and by means of described above the test for a bend to define the maintenance auksina in its both half in the shaded half auksina it will appear approximately twice more than in shined. However if to shade or shine entirely all top koleoptilja auksina in the shaded top will be no more, than in shined. From this physiologists have concluded that light causes body bending, influencing on lateralnoe distribution in it auksina. The difference in concentration auksina in turn causes unequal growth rate of body from its two parties, as leads to bend occurrence.

Similarly the bottom surface of the stalk which is in horizontal position (fig. 9.7), will contain after a while more auksina, than top. It will lead to the accelerated growth of the bottom party and finally to stalk bending upwards (geotropizm) (fig. 9.8). Growth of horizontally located root downwards is caused (on extreme a measure partially) by various sensitivity of a root to auksinu. In such root, as well as in a stalk being in horizontal position, auksin collects on the bottom party. However, as in a normal root already there is optimum or even a superoptimum concentration auksina, its further increase on the bottom party suppresses growth and by that causes root bending downwards. Later the conducted researches have shown that other hormones play more important role in occurrence tropizmov a root. For example, the assumption has been come out that abstsizovaja acid - the hormone ingibirujushchego actions - can collect also on the bottom party of a horizontal root and thus promote suppression there growth of cages. We will discuss this phenomenon more low.

Physiologists of plants do not know precisely, what mechanism with which help of a plant detect unilateral light and gravitational stimulus. The spectrum of action of phototropism shows that only dark blue light and a certain part of an ultraviolet cause formation of bends. As a photoreceptor the yellow pigment can participate in this process, possibly related to carotin or riboflavinu (fig. 9.9). Many other reactions in plants and mushrooms find out similar dependence on length of a wave. In some mushrooms the dark blue light absorbed by any enzyme, containing flavin, causes cytochrome restoration that it is possible to reveal with the help spektrofotometra. Now it is considered that this reaction occupies, possibly, central place in the mechanism responsible for phototropic perception at plants, and is connected in any yet absolutely clear image with the subsequent physiological response.

Especially dense grains of starch moving from one wall to another at an inclination of a cage, apparently, participate In the mechanism underlying reaction of a plant on a gravity (perception of a gravity) statolity, (fig. 9.10). By gravity or centrifugal force they settle down asymmetrically at cellular walls, signalling somehow about change of orientation of a stalk or a root. Besides, some other organelly in cages, such as endoplazmatichesky retikulum, rise to the top party of cages at an inclination of a top of a plant in this or that party. Thus, in perception of a gravity can be involved and other signals besides what are connected with moving starched statolitov. What was perceiving organella, the irritation turns to a chemical signal, obviously, thanks to affinity organelly to peripheral plazmalemme.

The Perception as directional light, and a gravity occurs in a tip of a root and a stalk top. In a root a sensitive zone is chehlik (fig. 9.11), rich with starched grains whereas in a stalk the irritation is perceived by a top including the youngest leaves. However the zone in which response on causing tropizm razdrazhitel develops, is on some distance (an order of several millimetres or centimetres) from the sensitive cages located in a tip of a root or a top of a stalk. So, auksin should migrate from a top of a stalk to a zone of growth lying under it. This process at normal temperature occurs approximately with a speed of 1 sm/ch by moving auksina on cages parenhimy koleoptilja or on serdtsevinnoj parenhime and to the youngest not differentiated vascular cages of a stalk. However such transport is unusual in the respect that it is carried out only in one net-direction, namely from a stalk top (fig. 9.12).

Therefore if to put the block containing auksin an agar to morphological apeksu a stalk it will lead to fast transition auksina in a stalk fabric, whence it can be collected a way ekstragirovanija or diffusions in bazalnyj the block. However if the same block to put to the basis of the same slice of a fabric auksin will not move from the basis to apeksu and further in retseptornyj the block. Unilateral transport auksina in stalks and koleoptiljah is called as polar transport. The physiological basis of polar transport auksina up to the end is not found out yet, but its mechanism, possibly, includes some from listed below stages (fig. 9.13). Auksin in the form of not ionised molecule IUK diffundiruet from the bottom end of one cage in the top part of other cage lying under it. As cytoplasm has almost neutral reaction, and the cellular wall - subacidic, IUK outside of a cage is less ionised, than in it. Hence, there is a diffusion gradient thanks to which not ionised IUK can get into a cage. Present at a cage ionised IUK it is insoluble in lipidah and consequently cannot diffundirovat outside whereas not ionised IUK continues to arrive in a cage. As a result of IUK collects in a cage. The assumption is come out that IUK can leave a cage by means of the specific fibers-carriers located in a cellular membrane on the bottom end of a cage. They are capable to grasp being in a cage ionised IUK and to transport it outside. As concentration ionised

IUK in a cage above, than outside, fibers-carriers operate in essence as turnstiles through which ionised IUK leaves a cage, and then takes a great interest in the next cage. These hypothetical carriers auksina are located only in the bottom part of a cage and consequently auksin can move only in one direction - from apeksa to the basis. Thus, it is supposed that polarity is caused by structural features of a cage at molecular level.

Polar transport demands also active metabolic energy which is necessary to support at least in cytoplasm higher rn, than outside. It can be carried out thanks to active pumping out of ions N + from a cage with use ATR as an energy source.

We should sum up now the discussion spent here tropizmov though in our understanding of these phenomena there are more many blanks. We do not know, how receptors odnostoronne a directional light or gravitational irritation influence on lateralnoe distribution auksina; the truth, non-uniform distribution auksina on two parties stimulirovannogo a stalk or a root arises, apparently, mainly owing to its cross-section migration. For example, symmetrically put mechennaja 14С IUK collects in the shaded longitudinal half fototropicheski stimulirovannogo body and the bottom half geotropicheski stimulirovannogo body.

One of possible explanations of such effect consists that bazipetalnyj transport (transport to the morphological basis) ingibiruetsja on stimulirovannoj to the party. As a result there is a continuous diffusion of substance from the shined half in shaded at phototropism of a stalk and from the top half in bottom at it geotropizme.

Slightly other situation is observed in a root. Root chehlik possesses sensitivity to a gravity. Having removed it from a root tip, it is possible to show that it is also a source powerful ingibitora of growth which at asymmetric distribution causes formation of bends. Experiences on pieces of the roots taken behind a zone of growth, testify that moving IUK to such pieces occurs in a direction to a root tip, instead of from it. In other words, the polar transport which is carried out only in one direction - from a top of a stalk to a tip of a root here takes place. Most part IUK which is found out in roots, is actually formed in a stalk top, and then transported downwards on a stalk in a root tip. It collects also in those places where new lateral roots develop, and promotes there to cell fission. Roots nevertheless develop very much a small amount auksina, most likely, in root chehlike. This auksin can migrate on distance of 2-3 mm (or a bit more) back in a zone of growth of a root. Conducted a root it is bent, auksin from a tip comes over to the bottom party of this zone where, probably, ingibiruet growth and by that leads to root bending downwards. Other component ingibitora root chehlika, possibly, is abstsizovaja acid (see gl. 10). Recently conducted researches show that in root tropizmah she plays more important role, than auksin.

At synthetic auksinov, used for regulation of growth and development of plants, ability to moving is connected with their activity. As a rule, the synthetic connections effectively stimulating growth, possess as well ability to polar transport. It means that some part of a molecule auksina, responsible for its action, answers and for its linkage with specific sites of transport fiber. When we measure supervised auksinom stalk growth, we always find that growth rate correlates not with the general maintenance auksina, and with that its quantity which is capable to diffusion and is allocated from a stalk if it to cut off and place a cut on the agar block. It testifies that all auksin in a cage makes stimulating impact on growth. Possibly, growth is supervised auksinom, containing in a certain part of a cage, for example in cytoplasm. This auksin also is accessible to transportation. However the significant amount auksina can be concentrated in other parts of a cage, such as vakuol that does its inaccessible to transportation and incapable to influence growth nevertheless if we ekstragiruem a fabric solvents and we will measure in an extract quantity auksina it will be thus considered also motionless auksin that will lead to a wrong estimation of the valid localisation of various forms auksina in studied cages. This phenomenon kompartmentatsii is of great importance for all physiology: often localisation of this or that connection or enzyme is more important, than its general maintenance. Such rule almost is for certain applicable not only to auksinu, but also to all other hormones.

The Quantity active auksina in any part of a plant depends on several factors: from synthesis level auksina in a top of a stalk, a share of a transported hormone, it kompartmentatsii in cages and, at last, from quantity auksina, podvergshegosja to disintegration or metabolizirovavshegosja other ways. It is known that auksin under the influence of enzyme IuK-oksidazy turns to an inactive product - 3-metilenoksindol (fig. 9.3 see). Some fabrics in a plant, especially in a root, extremely actively destroy auksin this way. Different reactions konjugatsii auksina with other molecules, such, as asparaginovaja acid and inozit take place also. The quantity of these products usually increases only when on a fabric put extremely high doses IUK. Therefore it is supposed that formed connections are spare substances or products detoksikatsii.

In seeds grain where practically is absent free IUK, at processing of a fabric by the diluted alkali there is a significant amount of this connection. In fabric IUK contains and the form glikozidov, i.e. In a combination to derivatives of sugars. At germination these substances are transported to a top koleoptilja where IUK is liberated and becomes accessible as already it was told above, for bazipetalnogo polar transport.