Involvement of Rhizobium leguminosarum nodulation genes in gene expression in pea root hairs

Recruitment of alkaloid-specific homospermidine synthase (HSS) from ubiquitous deoxyhypusine synthase: Does Crotalaria possess a functional HSS that still has DHS activity?

Quinolizidine alkaloids are the most prominent group of alkaloids occurring in legumes, except for many members of the tribe Crotalarieae that accumulate pyrrolizidine alkaloids (PAs). To study the evolution of PA biosynthesis as a typical pathway of plant secondary metabolism in this tribe, we have searched for a cDNA coding for homospermidine synthase (HSS), the enzyme catalyzing the first specific step in this biosynthesis. HSS was shown to have been recruited from deoxyhypusine synthase (DHS) by independent gene duplication in several different angiosperm lineages during evolution. Except for a cDNA sequence coding for the DHS of Crotalaria retusa, no data is available concerning the origin of PA biosynthesis within this tribe of the Fabaceae. In addition to several pseudogenes, we have identified one functional DHS in C. scassellatii and two in C. juncea. Despite C. juncea plants under study being devoid of PAs, we have found that the two sequences of C. juncea are different with respect to their genomic organization, their tissue-specific expression, and their biochemical activities. Supported by the branching pattern of a maximum likelihood analysis of these sequences, they have been classified as "class 1" and "class 2" DHS. It remains open whether the duplicated DHS belonging to class 2 is involved in the biosynthesis of PAs.

Localization of a Nod factor-binding protein in legume roots and factors influencing its distribution and expression

The roots of the legume Dolichos biflorus contain a lectin/nucleotide phosphohydrolase (Db-LNP) that binds to the Nod factor signals produced by rhizobia that nodulate this plant. In this study we show that Db-LNP is differentially distributed along the surface of the root axis in a pattern that correlates with the zone of nodulation of the root. Db-LNP is present on the surface of young and emerging root hairs and redistributes to the tips of the root hairs in response to treatment of the roots with a rhizobial symbiont or with a carbohydrate ligand. This redistribution does not occur in response to a non-symbiotic rhizobial strain or a root pathogen. Db-LNP is also present in the root pericycle where its level decreases upon initiation of nodule formation. Maximum levels of Db-LNP are found in 2-d-old roots, and the expression of this root protein is increased when the plants are grown in the absence of NO(3)(-) and NH(4)(+). These results support the possibility that Db-LNP is involved in the initiation of the Rhizobium legume symbiosis.

The root epidermis-specific pea gene RH2 is homologous to a pathogenesis-related gene

Two-dimensional gel electrophoresis of pea root and root hair proteins revealed the existence of at least 10 proteins present at elevated levels in root hairs. One of these, named RH2, was isolated and a partial amino acid sequence was determined from two tryptic peptides. Using this sequence information oligonucleotides were designed to isolate by PCR an RH2 cDNA clone. In situ hybridization studies with this cDNA clone showed that rh2 is not only expressed in root hairs, but also in root epidermal cells lacking these tubular outgrowths. During post-embryonic development the gene is switched on after the transition of protoderm into epidermis and since rh2 is already expressed in a globular pea embryo in the protoderm at the side attached to the suspensor, we conclude that the expression of rh2 is developmentally regulated. At the amino acid level RH2 is 95% homologous to the pea PR protein I49a. These gene encoding I49a is induced in pea pods upon inoculation with the pathogen Fusarium solani [12]. We postulate that rh2 contributes to a constitutive defence barrier in the root epidermis. A similar role has been proposed for chalcone synthase (CHS) and chitinase, pathogenesis-related protein that are also constitutively present in certain epidermal tissues.

Developmental biology of legume nodulation

Many legumes respond to Rhizobium inoculation by developing unique structures known as nodules on their roots. The development of a legume nodule in which rhizobia convert atmospheric N₂ into ammonia is a finely tuned process. Gene expression from both partners of the symbiosis must be temporally and spatially coordinated. Exactly how this coordination takes place is an area of intense study. Nodule morphogenesis appears to be elicited by at least two distinct signals: one from Rhizobium, a product of the nod genes (Nod factor), and a second signal, which is generated within plant tissues after treatment with Nod factor. The identity of the second signal is unknown but changes in the balance of endogenous plant hormones or the sensitivity of plant tissues to these hormones are likely to be involved. These hormonal changes may be triggered by endogenous flavonoids produced by the root in response to inoculation with Rhizobium. There is some controversy as to whether the legume nodule is an organ sui generis or a highly derived lateral root. A resolution of this question may become more critical as attempts to induce nodules on non-legume hosts, such as rice or maize, increase in number and scope. CONTENTS Summary 211 I. Introduction 211 II. Nodule development 213 III. Nodule initiation 220 IV. The second signal for nodule morphogenesis: role for the plant hormones ? 225 V. Lateral root development 229 VI. Are nodules modified lateral roots ? 229 VII. Conclusions and future prospects 231 Acknowledgements and dedication 232 References 232.

Developmental Biology of a Plant-Prokaryote Symbiosis: The Legume Root Nodule

The development of nitrogen fixing root nodules on the roots of leguminous plants is induced by soil bacteria (for example, from the genus Rhizobium). The formation of this plant organ involves specific activation of genes in both plant and bacterium. Analysis of these genes gives insight into the way in which plant and bacterium succeed in coordinating plant development.

Exopolysaccharide production in Rhizobium and its role in invasion

A complex interaction between rhizobia and specific legume plants results in the formation of nitrogen-fixing root nodules. The necessity for signal exchange and a chemically based recognition system between the symbiotic partners has been appreciated for some time, but the details are only gradually being elucidated. The two basic nodule ontogenies exhibit different requirements for Rhizobium exopolysaccharides. These surface exopolysaccharide molecules of Rhizobium are synthesized at a membrane complex, which is regulated by both transcriptional and post-translational controls. The acidic exopolysaccharide probably plays both a passive and an active role during the invasion process.

The ENOD12 gene product is involved in the infection process during the pea-Rhizobium interaction

The pea cDNA clone pPsENOD12 represents a gene involved in the infection process during Pisum sativum L.-Rhizobium leguminosarum bv. viciae symbiosis. The ENOD12 protein is composed of pentapeptides containing two hydroxyprolines. The expression of the ENOD12 gene is induced in cells through which the infection thread is migrating, but also in cells that do not yet contain an infection thread. Soluble compounds from Rhizobium are involved in eliciting ENOD12 gene expression. Rhizobium common and host-specific nodulation genes are essential for the production of these compounds. Two ENOD12 genes are expressed in nodules and in stem tissue of uninoculated plants. The gene represented by the cloned ENOD12 mRNA is also expressed in flowers, but a different transcription start may be used.