IRKUTSK, RUSSIA
Siberian Institute of Plant Physiology and Biochemistry
Abscisic acid in maize roots at low positive temperature --Akimova, GP, Sokolova, MG, Nechayeva, LV, Luzova, GB Identification of abscisic acid (ABA) functioning mechanisms has made considerable progress in the last few years. However, there are still quite a few unclear points, including that relating to ABA participation in the maintenance of plant organism homeostasis in unfavorable environmental conditions. Abscisic acid has been shown to accumulate in the plants in the conditions of osmotic stress, soil drought and supercooling. ABA's role in different plant bodies may vary: in leaves its antitranspiration effect prevails, in roots ABA activity is associated with an influence on ion adsorption and cell permeability for water. Pilet (Plant Physiol. 83:33-88, 1987) demonstrated that ABA amount goes up as the root age increases. The inhibitor is localized in the root apex and affects growth processes.

The present work was aimed at the identification of the impact of a temperature of 10 C on abscisic acid content in the growth zones of maize seedlings in connection with the change of growth speed with lowered temperature.

Roots of 48-day-old maize seedlings of the Uzbekskaya Zubovidnaya variety (MNL 76:35-36, 2002) were used as the initial material. To clean the extract we used the column chromatography method with polyvinulpyrrolidone. Gas-liquid chromatography was used to identify and determine ABA and IAA content.

The time period during which the cells achieved their final dimensions was used as a criterion for cell comparison in control and test variants. The cells were shown to finalize extension over 8 hours of growth at 27 C. The temperature of 10 C sharply reduced the root growth speed: only after 96 hours of growth at this temperature did the cells cease the extension process. As a result, the root growth speed decreased by 12 times.

ABA content at 10 C (Table) was shown to increase in all the growth zones compared to control (27 C). At the same time the maximum increase was observed in the cells which started extension at 10 C, which is caused by the increase of these cells' volume and may apparently represent a compensatory response to the increase of IAA content. ABA, apparently, does not prevent extension, but is responsible for other functions associated in particular with its participation in the cells' permeability for water. ABA content increase in root meristem is likely to be connected with the formation induction as well as with the slowed tempo of its basipetal transportation to the extension zone. This conclusion is proven by the observed response of root geotropism at 10 C. Pilet observed a similar georeaction in roots exposed to light.

Plant physiological processes are regulated with the participation of a multi-componental hormonal system, where each hormone is responsible for its own specific function. This is why one should take into account not only the content of an individual phytohormone, but also the balance and interaction of the phytohormones.

Comparison of the data on the dynamics of maize root cell growth with the free IAA dynamics demonstrated that at 10 C, IAA content in the root growth zones undergoes the following changes as compared to control: it increases in the meristem and in the initial extension zone cells and sharply drops in the cells which accomplished their growth. In accordance with the above, it was shown that IAA and ABA proportion in the root cells changes significantly at 10 C. Phytohormone balance in meristem cells and in the cells that started extension shifts towards the decrease of ABA due to an IAA content increase in the cells, thus providing a longer growth impact resulting in the increase of the dimensions of the zone of extending cells. In the cells which accomplished the extension, ABA content grows considerably. In terms of physiological impact, abscisic acid is an antagonist of IAA and inhibits cell extension, which apparently makes a certain impact on the change of these cells growth speed.

Therefore, the change of balance of ABA/IAA in the extending cells of the maize root is one of the ways to control cell extension speed at 10 C.

Table. ABA and IAA content in the maize root cells in optimal conditions and at 10 C (mcg
10-8 /cell)
 
Variant Growth zones  ABA  IAA ABA:IAA proportion 
27 C Meristem 1.180.05 1.730.06 1:1.5
  Extension initiation 3.510.10 3.120.09 1:0.9
  Extension accomplishment 10.240.72 7.360.64 1:0.7
10 C Meristem 1.960.06 4.910.12 1:2.5
  Extension initiation 6.180.25 7.870.34 1:1.3
  Extension accomplishment 15.460.91 3.910.11 1:0.3

 


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