Herman P. Spaink

Perspectives in Plant Cell Recognition: The Rhizobium trap: root hair curling in root–nodule symbiosis

Introduction The root-nodule bacteria Rhizobium, Bradyrhizobium and Azorhizobium (collectively rhizobia) invade and nodulate the roots of their host plants via either wounds or root hairs. The choice is made by the host plant, e.g. the same rhizobial strain infects Vigna roots via root hairs and Arachis roots via wounds (Sen & Weaver, 1984), whereas another strain infects Parasponia via root epidermal cracks and Macroptilium via root hairs (Marvel et al ., 1985). Shortly before or during root invasion, rhizobia induce cell divisions in the root cortex, resulting in formation of a nodule primordium. Through infection threads (tip-growing tubular structures containing invading rhizobia) and/or between cortical cells the rhizobia migrate towards the growing primordium, are endocytosed by young nodule cells, and differentiate into dinitrogenfixing bacteroids (see also Brewin et al ., this volume). Rhizobial invasion of most agronomically important legumes such as pea ( Pisum sativum ), soybean ( Glycine max ) and bean ( Phaseolus vulgaris ) occurs through root hairs. Infection of a living plant cell is an unusual phenomenon in plant–bacteria interactions. Plants are open organisms. At many sites, the intercellular space of a plant is in direct contact with the environment, e.g. in stomata, hydathodes or wounds resulting from emergence of lateral roots. A plant is used to regular visits of (plant-associated) bacteria to its interior. Therefore, wound-infection by rhizobia is a normal phenomenon whereas root hair infection is special.

Role of rhizobial lipo-oligosacharides in root nodule formation on leguminous plants

During recent years signals leading to the early stages of nodulation of legumes by rhizobia have been identified. Plant flavonoids induce rhizobialnod genes that are essential for nodulation. Most of thenod gene products are involved in the biosynthesis of lipo-oligosaccharide molecules. The commonnodABC genes are minimally required for the synthesis of all lipo-oligosaccharides. Host-specificnod gene products in a givenRhizobium species are responsible for synthesis or addition of various moieties to those basic lipo-oligosaccharide molecules. For example, inR. leguminosarum, thenodFEL operon is involved in the production of lipo-oligosaccharide signals that mediate host specificity. AnodFE-determined highly unsaturated fatty acid (trans-2, trans-4, trans-6, cis-11-octadecatetraenoic acid) is essential for inducing nodule meristems and pre-infection thread structures on the host plantVicia sativa. Lipo-oligosaccharides also trigger autoregulation of nodulation in pea and, if applied in excessive amounts to a legume, can prevent nodulation and thereby might play a role in competition. During our studies on the biosynthesis of lipo-oligosaccharides, we discovered that, besides the lipo-oligosaccharides, other metabolites are synthesizedde novo after induction of thenod genes. These novel metabolites appeared to be phospholipids, containing either one of the three fatty acids which are made by the action of NodFE inR. leguminosarum.