Plant Science 149 1999 95 – 104
Germination, senescence and pathogenic attack in soybean Glycine max. L.: identification of the cytosolic aconitase
participating in the glyoxylate cycle
Joaquim Cots, Franc¸ois Widmer
Laboratory of Plant Biology and Physiology, Biology Building, Uni6ersity of Lausanne, CH-
1015
Lausanne, Switzerland Received 5 May 1999; received in revised form 30 June 1999; accepted 30 June 1999
Abstract
During our study of the glyoxylate cycle in soybean Glycine max. L. var. Maple arrow, two mitochondrial and three cytosolic aconitase molecular species EC 4.2.1.3 were detected, designated as M1, M2, C1, C2 and C3 isoforms, respectively, according
to their intracellular locations and electrophoretic mobilities. Using the glyoxylate cycle marker enzymes isocitrate lyase ICL, EC 4.1.3.1 and malate synthase MS, EC 4.1.3.2, the activity of this pathway providing the essential link between b-oxidation and
gluconeogenesis was confirmed during germination cotyledons and senescence leaves. It was then established that, in both cases, the activity of the C1 aconitase isoform developed concomitantly with the transcription and translation levels of the icl and
ms genes. This strongly suggests that C1 aconitase is constitutive of the glyoxylate cycle. In addition, the same isoform was found to be active during pathogenic attack as well hypocotyls. It might be assumed that in such a case the glyoxylate cycle is
reinitiated as a part of a carbon reallocation system feeding on the diseased tissue cellular components. © 1999 Published by Elsevier Science Ireland Ltd. All rights reserved.
Keywords
:
Glycine max. L.; Aconitase; Germination; Glyoxylate cycle; Pathogenic attack; Senescence www.elsevier.comlocateplantsci
1. Introduction
The glyoxylate cycle was first described in Pseu- domonas spp, where it was shown to allow growth
of these organisms on acetate as the sole carbon source [1,2]. Numerous microorganisms were sub-
sequently shown to operate this carbon-conserving pathway, including archaea [3], with the recent
assumption that it may also allow growth on glycerol, lactate, propionate, oleate and n-alkanes
[4,5]. In higher plants, this 5-enzyme short cut of the citric acid cycle was first detected occurring in
the glyoxysomes of endosperm in castor bean seedlings during early postgerminative growth [6].
Many investigators subsequently reported finding glyoxylate cycle enzyme activities and transcripts
encoding these enzymes in the cotyledons of ger- minating oilseeds, where the cycle is involved in
the mobilisation of lipid reserves for a review, see Ref. [7]. The classical view considers that in this
case the cycle achieves the net conversion of two molecules of acetyl-CoA produced by b-oxidation
to one molecule of succinate. This C4 compound is converted to malate via the citric acid cycle in
the mitochondria and then feeds into the gluco- neogenic pathway in the cytosol to reallocate the
carbon source to the rest of the plant. Such en- zyme activities have also been detected during
embryogenesis in plants [8 – 10], in pollen develop- ment [11], as well as in various animal tissues for
a review and critical comments, see Refs. [12] and [13], respectively. Reallocation of carbon through
the glyoxylate cycle and gluconeogenesis similarly occurs in senescing plant tissues, derived from the
catabolism of structural lipids for a review, see Ref. [12].
Corresponding author. Tel.: + 41-21-692-4190; fax: + 41-21-692- 4195.
E-mail address
:
francois.widmerie-bpv.unil.ch F. Widmer 0168-945299 - see front matter © 1999 Published by Elsevier Science Ireland Ltd. All rights reserved.
PII: S 0 1 6 8 - 9 4 5 2 9 9 0 0 1 2 7 - 2
The postulated exclusive glyoxysomalperoxiso- mal location of the glyoxylate cycle [14] has been
challenged by the experimental evidence that gly- oxysomes are devoid of aconitase activity [15,16],
and that a cytosolic aconitase form might partici- pate in the pathway [17]. A strict organellar com-
partmentation has also been assumed to be dispensable in various unicellular green algae [18 –
20] and in Saccharomyces cere6isiae [21]. In addi- tion, it has been postulated that the in vivo
occurrence of a glyoxylate cycle stricto sensu might be questioned in plant glyoxysomes, in view
of the possible role of the glyoxysomal malate dehydrogenase in the NAD
+
regeneration system sustaining b-oxidation [12]. However, this has no
bearing on the intrinsic involvement of the five considered enzymes in the reactional intricacies
linking lipid catabolism to gluconeogenesis.
The assessment of aconitase isoforms in soybean now indicates that it possesses two mitochondrial
and three cytosolic such molecular species. This distribution pattern is displayed under the physio-
logical conditions of germination and senescence, where in both cases the same aconitase isoform
appears to participate in the glyoxylate cycle. In addition, evidence is also presented that an identi-
cal situation prevails under conditions of patho- genic attack.
2. Materials and methods
