Publications
Walter, M. H.; Hans, J.; Strack, D.; Two distantly related genes encoding 1-deoxy-D-xylulose 5-phosphate synthases: differential regulation in shoots and apocarotenoid-accumulating mycorrhizal roots Plant J. 31, 243-254, (2002) DOI: 10.1046/j.1365-313X.2002.01352.x
Isopentenyl diphosphate, the universal precursor of isoprenoids, is synthesized by two separate routes, one in the cytosol and the other in plastids. The initial step of the plastidial pathway is catalysed by 1‐deoxy‐d ‐xylulose 5‐phosphate synthase (DXS), which was previously thought to be encoded by a single‐copy gene. We have identified two distinct classes of DXS‐like cDNAs from the model legume Medicago truncatula . The deduced mature MtDXS1 and MtDXS2 proteins, excluding the predicted plastid‐targeting peptides, are similar in size (72.7 and 71.2 kDa) yet share only 70% identity in their amino acid sequences, and both encode functional DXS proteins as shown by heterologous expression in Escherichia coli. Available DXS sequences from other plants can easily be assigned to either class 1 or class 2. Partial sequences of multiple DXS genes in a single genome may be found in the databases of several monocot and dicot plants. Blot analyses of RNA from M. truncatula , maize, tomato and tobacco demonstrate preferential expression of DXS1 genes in many developing plant tissues except roots. By contrast, DXS2 transcript levels are low in most tissues but are strongly stimulated in roots upon colonization by mycorrhizal fungi, correlated with accumulation of carotenoids and apocarotenoids. Monoterpene‐synthesizing gland cells of leaf trichomes appear to be another site of DXS2 gene activity. The potential importance of DXS1 in many housekeeping functions and a still hypothetical role of DXS2 in the biosynthesis of secondary isoprenoids is discussed.
Publications
Pflanzen und bestimmte Pilze haben im Laufe ihrer Entwicklungsgeschichte „gelernt”︁, in einer engen Assoziation im Boden, der Mykorrhiza, eine äußerst erfolgreiche Symbiose miteinander einzugehen. Arbuskuläre Mykorrhizapilze helfen Pflanzen sich auf nährstoffarmen Böden ausreichend mit Wasser, Nährsalzen und Spurenelementen zu versorgen und fördern entscheidend Diversität und Produktivität von Pflanzengesellschaften. Darüber hinaus zeigen mykorrhizierte Pflanzen eine erhöhte Widerstandsfähigkeit gegen Pathogenbefall. Im Gegenzug „bezahlt”︁ die Pflanze den Pilz für diesen Gewinn mit Kohlenhydraten in Form einfacher Zucker (Glucose, Fructose). Durch manche Erfolge in der Erforschung der Mykorrhiza auf Metaboliten‐ und Genebene beginnen wir allmählich zu erahnen, wie komplex die molekularen Interaktionen dieser Symbiose sind. Es ist zu erwarten, dass das steigende Interesse an der Mykorrhizaforschung zu neuen Einsichten in die Strategien von Pflanzen und Pilzen in der Entwicklung mutualistisch‐symbiontischer Assoziationen führen wird.
Publications
Walter, M. H.; Fester, T.; Strack, D.; Arbuscular mycorrhizal fungi induce the non-mevalonate methylerythritol phosphate pathway of isoprenoid biosynthesis correlated with accumulation of the 'yellow pigment' and other apocarotenoids Plant J. 21, 571-578, (2000) DOI: 10.1046/j.1365-313x.2000.00708.x
Plants and certain bacteria use a non‐mevalonate alternative route for the biosynthesis of many isoprenoids, including carotenoids. This route has been discovered only recently and has been designated the deoxyxylulose phosphate pathway or methylerythritol phosphate (MEP) pathway. We report here that colonisation of roots from wheat, maize, rice and barley by the arbuscular mycorrhizal fungal symbiont Glomus intraradices involves strong induction of transcript levels of two of the pivotal enzymes of the MEP pathway, 1‐deoxy‐D‐xylulose 5‐phosphate synthase (DXS) and 1‐deoxy‐D‐xylulose 5‐phosphate reductoisomerase (DXR). This induction is temporarily and spatially correlated with specific and concomitant accumulation of two classes of apocarotenoids, namely glycosylated C13 cyclohexenone derivatives and mycorradicin (C14) conjugates, the latter being a major component of the long‐known ‘yellow pigment’. A total of six cyclohexenone derivatives were characterised from mycorrhizal wheat and maize roots. Furthermore, the acyclic structure of mycorradicin described previously only from maize has been identified from mycorrhizal wheat roots after alkaline treatment of an ‘apocarotenoid complex’ of yellow root constituents. We propose a hypothetical scheme for biogenesis of both types of apocarotenoids from a common oxocarotenoid (xanthophyll) precursor. This is the first report demonstrating (i) that the plastidic MEP pathway is active in plant roots and (ii) that it can be induced by a fungus.
Publications
Vierheilig, H.; Maier, W.; Wyss, U.; Samson, J.; Strack, D.; Piché, Y.; Cyclohexenone derivative- and phosphate-levels in split-root systems and their role in the systemic suppression of mycorrhization in precolonized barley plants J. Plant Physiol. 157, 593-599, (2000) DOI: 10.1016/S0176-1617(00)80001-2
In a split-root system root colonization by the arbuscular mycorrhizal fungus Glomus mosseae on one side is reduced when roots on the other side are already colonized by G. mosseae. Root colonization by arbuscular mycorrhizal fungi enhances the P-status of plants, thus the observed suppressional effect on further root colonization in precolonized barley plants could be P-level regulated. Split-root systems allow to separate plant mediated P-effects on root colonization by arbuscular mycorrhizal fungi from direct P-effects on arbuscular mycorrhizal fungi. By adding a KH2PO4-solution to one side of the split-root system of non-mycorrhizal control plants, higher P-levels were obtained as in split-root systems of G. mosseae precolonized plants. Subsequent inoculation with G. mosseae of the P-supplied and the precolonized plants resulted in an inhibition of root colonization in the precolonized plants, but not in the P-supplied plants, discarding the enhanced P-level as the responsible factor for the observed suppression. Cyclohexenone derivatives are secondary plant compounds only found in roots of mycorrhizal plants. Analysis of cyclohexenone derivatives in mycorrhizal and non-mycorrhizal roots in split-root systems revealed that cyclohexenone derivatives can be detected in mycorrhizal roots, but not in non-mycorrhizal roots of mycorrhizal plants. The presented results show clearly that cyclohexenone derivatives are not systemically accumulated and that the P-levels are not the responsible factors for the observed systemic suppression of mycorrhization in roots of precolonized barley plants.
Publications
Maier, W.; Schmidt, J.; Wray, V.; Walter, M. H.; Strack, D.; The arbuscular mycorrhizal fungus, Glomus intraradices , induces the accumulation of cyclohexenone derivatives in tobacco roots Planta 207, 620-623, (1999) DOI: 10.1007/s004250050526
Tobacco (Nicotiana tabacum L.) plants were grown with and without the arbuscular mycorrhizal fungus, Glomus intraradices Schenk & Smith. High-performance liquid chromatographic analyses of methanolic extracts from mycorrhizal and non-mycorrhizal tobacco roots revealed marked fungus-induced changes in the patterns of UV-detectable products. The UV spectra of these products, obtained from an HPLC photodiode array detector, indicated the presence of several blumenol derivatives. The most predominant compound among these derivatives was spectroscopically identified as 13-hydroxyblumenol C 9-O-gentiobioside (“nicoblumin”), i.e. the 9-O-(6′-O-β-glucopyranosyl)-β-glucopyranoside of 13-hydroxy-6-(3-hydroxybutyl)-1,1,5-trimethyl-4-cyclohexen-3-one, a new natural product. This is the first report on the identification of blumenol derivatives in mycorrhizal roots of a non-gramineous plant.