Metabolism DL-C14-tryptophan in the maize root cells at low temperature --Akimova, GP, Sokolova, MG, Nechaeva, LV, Luzova, GB One of the factors regulating the synthesis of endogenous indolylacetic acid (IAA) may be the amount of available tryptophan (Trp), which is known to convert into IAA in plants, with the acid level regulated by the content of free Trp inside the tissue.

The present research was aimed at the demonstration of temperature impact on the content of free Trp in the roots of maize seedlings in connection with the change of IAA level at low positive temperature, and to determine the impact of a temperature of 10 C on metabolism of DL-C14-tryptophan.

The study was carried out on the primary roots of 2 day old seedlings of maize varieties differing in resistance to low temperature - Omskaya-2 (resistant to low temperature) and Uzbeksksya tooth-like (non-resistant). Quantitative determination of free tryptophan was conducted using an amino acid analyser. Growth zones of maize root seedlings (meristem, commencement and termination of cell expansion) with the view to determine the impact of a temperature of 10 C on DL-C14-Trp metabolism were cut immediately prior to the introduction of DL-C14-Trp into the incubation medium. 0.05 ml of DL-C14-Trp were placed into small Petri dishes with 5 ml of H2O; incubation was conducted at 27 C for 4 hours. The plant material was separated from the medium, washed three times with distilled water and put into Petri dishes on wet filter paper. Some of the dishes were transferred in a thermostate with the temperature of 10 C (test), the other part was left at the temperature 27 C (control) for 20 hours. The cut pieces were thoroughly ground, 10-fold volume of boiling ethanole was poured onto them; the samples were used to C14-IAA and its metabolites.

Table 1 demonstrates a gradual increase of free Trp in the course of cell growth. At a temperature of 10 C Trp content in the growth zones of maize seedling roots of Uzbekskaya non-resistant variety goes through the following changes: it increases in the expanding cells and reduces in the cells, which have terminated expansion. The resistant variety Omskaya-2 shows Trp content close to the control.

Radio-isotope analysis showed that meristem cells and expansion zones actively adsorb Trp (Table 2). Marked Trp is incorporated into water and ether fractions and in the zones of chromatograms with Rf IAA (Table 3). Radioactivity of chromatogram zones with Rf IAA in percentage of radioactive ether fraction in the options with a temperature of 10 C in the cells of resistant variety reduces, and increases in the cells of non-resistant variety.

The data acquired allow the supposition that in the resistant variety IAA metabolism at 10 C takes place with a higher speed, that is why C14-IAA content amounts to a lower percentage as compared to control. In the non-resistant variety C14-IAA metabolism slows down at 10 C, which is likely to account for IAA accumulation.

Therefore, the results of the investigation show a certain dependence between Trp content and the level of endogenous IAA in the maize root cells: low temperature increases tryptophan levels and IAA content compared to controls. At the same time, at 10 C the proportion between the rate of synthesis and IAA metabolism changes, and is obviously one of the critical factors of growth regulation in maize varieties differing in cold-resistance.

Table 1. Free tryptophan content in maize root growth zones in optimal growth conditions and at low temperature.
 
Variety Root growth zones
Tryptophan, mcg10-6/cell
   
27 C
10 C
Omskaya-2 Meristem

Expansion commencement

Expansion termination

0.240.01

1.660.03

3.860.15

0.160.00

1.760.04

3.770.06

Uzbekskaya

tooth-like

Meristem

Expansion commencement

Expansion termination

0.600.01

5.800.09

21.600.22

0.420.01

12.360.25

14.900.30

Table 2. DL-C14-tryptophan absorption speed by root growth zones.
 
Variety
Impulse/min per 1 g of raw material
 
Meristem
Expansion commencement
Omskaya-2
5952120
597297
Uzbekskaya tooth-like
438392
437487

Table 3. Radioactivity of water residue (1), ether fraction (2) and chromatogram zones with Rf IAA (3) (103 impulse/min/g of the raw material).
 
Growth

zones

Control, 27 C
Test, 10 C
 
1
2
3
1
2
3
Omskaya - 2 (resistant)
Meristem
55.51.1
27.70.6
0.440.09
66.41.2
31.40.6
0.310.06
Expansion commencement
48.01.0
24.00.5
0.240.06
66.51.2
52.91.6
0.310.06
Uzbekskaya tooth-like (non-resistant) 
Meristem
62.21.3
32.10.6
0.270.06
64.41.3
26.40.5
0.460.01
Expansion commencement
58.11.2
42.20.8
0.320.07
94.421.9
47.51.2
0.540.01

 


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