Environmental and Experimental Botany 43 2000 73 – 81
The role of ethylene metabolism in the short-term responses to aluminium by roots of two maize cultivars different in
Al-resistance
B. Gunse´, C. Poschenrieder, J. Barcelo´
Laboratorio de Fisiologı´a Vegetal, Facultad de Ciencias, Uni6ersidad Auto´noma de Barcelona, E-
08193
Bellaterra, Spain Received 4 May 1999; received in revised form 14 September 1999; accepted 14 September 1999
Abstract
The possible role of ethylene in the initial signal transduction of Al-induced root growth responses was investigated in two tropical maize Zea mays varieties that differ in Al resistance: ATP SR Yellow and HS 701 B. The intensity
of Al toxicity effects were evaluated after short 4 and 24 h exposure to 50 mM Al in complete low ionic strength nutrient solution. Relative root elongation rates RER and callose formation in root tips were used as stress
indicators. Ethylene production by the root tips and 1-aminocyclopropane-1-carboxylic acid ACC synthase and ACC oxydase activities were analysed. After 24 h exposure to Al, both less callose production and higher RER
indicated that ATP SR Yellow was more Al resistant than HS 701 B. The Al resistance of ATP SR Yellow, however, was not expressed after 4 h exposure to Al, when increased callose and decreased RER were observed. In any of the
varieties and after any of the time-treatments an Al-induced increase of ethylene production was found. Our results indicate that the Al-resistance genes were not constitutively expressed in the absence of Al in the medium, but
activated upon exposure to Al. An efficient protection against Al was achieved after a lag time of more than 4 h. Enhanced ethylene formation does not seem to play a role either in the Al-induced inhibition of root elongation or
in the induction of the resistance mechanism. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Aluminium; Callose; Ethylene; Resistance; Root tip; Tolerance; Toxicity; Zea mays www.elsevier.comlocateenvexpbot
1. Introduction
Aluminium toxicity is considered one of the most important abiotic stress factors limiting crop
production on acid soils in the tropics Foy, 1984. Many investigations have demonstrated
that toxic Al concentrations rapidly affect root elongation and that root tips are the primary site
of Al-induced injury Ryan et al., 1993; Llugany et al., 1995; Sivaguru and Horst, 1998; Va´zquez et
al., 1999. The mechanisms of Al-induced inhibi- tion of root growth are not clearly established
Barcelo´ et al., 1996. Early experiments have demonstrated that Al inhibits root cell division
Clarkson, 1965. However, the fact that Al can
Corresponding author. Fax: + 34-93-581-2003. E-mail address
:
juan.barcelouab.es J. Barcelo´ S0098-847200 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
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inhibit root elongation within minutes, in combi- nation with the formerly common view that Al
enters only slowly into the symplasm, has led to the hypothesis that toxic effects of Al in the
apoplast are responsible for the fast inhibition of root growth Horst, 1995. Recently it was found
that Al can cross the plasmamembrane within minutes or, at least, a few hours Lazov et al.,
1996; Rengel and Reid, 1997; Va´zquez et al., 1999, so that inhibition of cell division or other
toxic effects inside the cells may also be responsi- ble for early root effects. According to Lazov and
Holland 1999 the effects of Al on cell extension would be largely reversible, while cell division is
irreversibly inhibited by Al.
Besides the inhibition of root elongation, early symptoms of Al-toxicity in roots are the depletion
of microtubuli in the distal transition zone of the root apex Sivaguru et al., 1999, apical swelling,
callose deposition Wissemeier et al., 1987; Llu- gany et al., 1994; Massot et al., 1999, the forma-
tion of barrel-shaped root cells Gunse´ et al., 1997 and the swelling of cell walls in the tip
Va´zquez et al., 1999. Similar effects have also been reported for other stress factors and may be
related to ethylene Lynch and Brown, 1997; Morgan and Drew, 1997. Whether or not
ethylene plays a role in the early signal transduc- tion of Al-induced root growth inhibition is not
established. In order to test the hypothesis of ethylene being involved in root responses to Al in
this study we analysed the influence of Al on ethylene production in root tips of maize varieties
that differ in root growth responses to Al. Callose production and morin staining were used in order
to visualize the varietal differences in Al-sensitiv- ity after short-term hours Al exposure.
2. Materials and methods
