|Home | About | Journals | Submit | Contact Us | Français|
During the first days of development, maize roots showed considerable variation in the production of ethylene and the rate of elongation. As endogenous ethylene increases, root elongation decreases. When these roots are treated with the precursor of ethylene aminocyclopropane- 1-carboxylic acid (ACC), or inhibitors of ethylene biosynthesis 2-aminoethoxyvinyl glycine (AVG) or cobalt ions, the root elongation is also inhibited. Because of root growth diminishes at high or reduced endogenous ethylene concentrations, it appears that this phytohormone must be maintained in a range of concentrations to support normal root growth. In spite of its known role as inhibitor of ethylene action, silver thiosulphate (STS) does not change significantly the root elongation rate. This suggests that the action of ethylene on root elongation should occur, at least partially, by interaction with other growth regulators.
Seedling root is a suitable experimental system for Developmental and Cell Biology research because it has a relatively simple histological organisation, a high physiological dynamism, and a certain predictability of cell activities. Maize seedling roots can be obtained after the germination of caryopsides transferred to hydroponic medium supplemented with calcium and potassium.1 At 30°C and after a short acclimation period, maize roots 60–80 mm long grow at a rate of about 2.4 mm/h during the first 24 h of culture. During the following 24 h, growth rate declined to 2.0 mm/h. To analyse the role played by ethylene in the change of growth rate during this 48 h ethylene production by root tips was monitored. Roots of 70 mm (0 h), 80 mm (4 h), 130 mm (24 h) and 180 mm (48 h) produced ethylene at rates of 1.05, 1.11, 1.68 and 4.56 nL ethylene/h g FW, respectively. The increase of ethylene production coincides with a decrease of root elongation rate.
Previous results indicate that exogenous 1-aminocyclopropane-1-carboxylic acid (ACC) is efficiently converted to ethylene by maize roots and, consequently, increasing ethylene endogenous concentration (reviewed in ref. 1). This treatment usually reduces root growth. Only very low ACC concentrations (0.5 μM) resulted in a stimulation of root growth by 8.5%, and only in the period 24–48 h. All these results suggest that ethylene is mainly an inhibitor of maize root elongation. Nevertheless, there is not a linear correlation between the internal ethylene concentration and the elongation rate obtained under different developmental conditions. Consequently, the root sensitivity to this growth regulator apparently changes with time. On the other hand, the ethylene could interact with other phytohormones to achieve a final effect on root growth changing with time. These two notions have some evidence favorable in diverse plant materials.2,3,6,9
To know how root growth is affected by low ethylene concentrations, a suitable approach is to test the effect of ethylene biosynthesis inhibitors such as 2-aminoethoxyvinyl glycine (AVG). At the first and second day of our study, AVG diminished the root elongation rate (Fig. 1) and the endogenous ethylene levels comparatively to the corresponding control (Fig. 2). Another ethylene biosynthesis inhibitor is the cobalt ion. A concentration of cobalt 10 µM strongly inhibited root growth and ethylene production after 4, 24 and 48 h (Figs. 1 and 2),2), thus confirming the results obtained with AVG treatment. Therefore, an adequate production of ethylene seems to be necessary to sustain a rapid root growth.
Moreover, our observations indicate that the inhibition of ethylene action by silver thiosulphate (STS) apparently does not affect maize root elongation (Fig. 1). This observation supports the results of Clark et al.3 in a study of the tomato ethylene-insensitive mutant never ripe. The root development of this mutant was anomalous when the plant grew in sand but correct when kept under greenhouse conditions. This suggests that ethylene sensitivity is necessary for normal seedling root growth in response to mechanical impedance but not when under unimpeded circumstances.
If we consider that reduced ethylene concentration by cobalt ions or AVG inhibits normal root growth but inhibition of ethylene action by STS does not, hence it is possible to assume an indirect effect of ethylene upon root elongation. In this sense, ethylene could interact with other growth regulators to affect to this process. In fact, ethylene has various cross-talks with other growth regulators controlling diverse physiological processes such as root growth,2,8,9 root gravitropism and lateral root formation,5 root hair development,4,7,10 etc.
The precursor of ethylene ACC and two inhibitors of ethylene biosynthesis tested in our study reduced maize root elongation. Hence, when the endogenous ethylene level is experimentally raised or lowered, the result is the same: an inhibition of root growth. This implies that an adequate endogenous concentration of ethylene is necessary for root growth.
Addendum to: Alarcón MV, Lloret PG, Iglesias DJ, Talón M, Salguero J. Response of maize seedling roots to changing ethylene concentra- tionsRuss J Plant Physiol200956488494 doi: 10.1134/S1021443709040074.
Previously published online: www.landesbioscience.com/journals/psb/article/9948