Communication and Efficiency in the Symbiotic Signal Exchange
Signals and substrates from plant roots are equally as important for the microbial populations as for their interaction with plant roots. Several hundred secondary metabolites from plants have been identified during the past decades in the large groups of
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2.1 Introduction Signals and substrates from plant roots are equally as important for the microbial populations as for their interaction with plant roots. Several hundred secondary metabolites from plants have been identified during the past decades in the large groups of flavonoids, non-proteinogenic amino acids, and alkaloids. With the discovery of flavonoids and isoflavonoids as specific signal molecules in the legume-rhizobia symbiosis, interest in these groups of organic molecules in the rhizosphere has very much increased, since it has transferred this group from the status of secondary plant metabolites to a primary position in developmental biology. Comprehensive overviews about structures and some functions of flavonoids have been published [1, 2, 3]. The symbiosis between rhizobia and legumes can be characterized by a mutual exchange of signal molecules between the two partners (Fig. 2.1) [4, 5, 6]. The coevolution of microsymbionts and legume host plants is considered to be highly specific and very different from pathogenic systems [7]. The communication between the two partners starts with the constitutive exchange of a wide spectrum of flavonoids and isoflavonoids in different composition from the germinating seeds and the emerging root system. From the seeds also betaines are exudated [8]. The pattern in space and time of the signal molecules is very complicated, since along the root system a quantitative gradient of exudation of different flavonoids is found [9]. In the other parts of this chapter, structures of the nod factors produced and production of antibiotics by antagonistic soil bacteria and plants will be discussed. Finally, the biosynthesis and molecular genetics of the various signal molecules will be treated.
, Fachbereich Biologie der Philipps-Universitat, FG Zellbiologie und Angewandte Botanik, 35032 Marburg, Germany
G. Heldmaier et al. (ed.), Environmental Signal Processing and Adaptation © Springer-Verlag Berlin Heidelberg 2003
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Communication and Efficiency in the Symbiotic Signal Exchange
Microsymhionts
o
Host plants
o
nod. noJ..exop.JJjJop.niffJX
- Flavonoids from seed exudation
- Chemotaxis - nod gene induction - Nod factor production
---+ - Flavonoids from root exudation
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r-------------------~
- Phytoalexin resistance induction - Phytoalexin degradation - C-ring cleavage offlavonoids
- EPS and LPS functions for infection and competition
- Phytoalexin induction - Stimulation of flavonoid production - Root hair curling - Meristem induction
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- Lectin and ENOD functions in the early nodule development
Fig. 2. 1. Communication between microsymbiont and legume during the early stages of nodule development (After [4])
2.2 Flavonoids and Isoflavonoids as Signal Molecules from Host Plants The basic structures of flavanones, flavones, and isoflavones together with coumestrol, an intermediate in the phenylpropane metabolism, are given in Fig. 2.2. The 3,5,7,3'-tetrahydroxy-4'-methoxyflavanones is a nod gene inducer in Rhizobium leguminosarum by. viciae, the 3',4'
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