that are expressed before the onset of nitrogen fixation and are involved in infection and nodule development. The products of the late nodulin genes are involved in the interaction with the endosymbiont and in the metabolic specialization of the nodule (Nap and Bisseling, 1990). In the first part of this review, we describe the early steps of the interaction between rhizobia and legumes that result in the formation of a nitrogen-fixing nodule. We focus on the role of specific lipooligosaccharides secreted by rhizobia in the induction of these early steps. In the second part, we describe nodule functioning and compare actinorhizal and legume nodules.

/pdf/symbiotic-nitrogen-fixation-1by9qh2egl.pdf

Symbiotic Nitrogen Fixation.

SUMMARY
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.

/pdf/developmental-biology-of-legume-nodulation-3ol366huf6.pdf

Developmental biology of legume nodulation

Arabinogalactan-proteins (AGPs) are a family of extensively glycosylated hydroxyproline-rich glycoproteins that are thought to have important roles in various aspects of plant growth and development. After a brief introduction to AGPs highlighting the problems associated with defining and classifying this diverse family of glycoproteins, AGP structure is described in terms of the protein component (including data from molecular cloning), carbohydrate component, processing of AGPs (including recent data on glycosylphosphatidylinositol membrane anchors) and overall molecular shape. Next, the expression of AGPs is examined at several different levels, from the whole plant to the cellular levels, using a variety of experimental techniques and tools. Finally, AGP function is considered. Although the existing functional evidence is not incontrovertible, it does clearly point to roles for AGPs in vegetative, reproductive, and cellular growth and development as well as programmed cell death and social control. In addition and most likely inextricably linked to their functions, AGPs are presumably involved in molecular interactions and cellular signaling at the cell surface. Some likely scenarios are discussed in this context. AGPs also have functions of real or potential commercial value, most notably as emulsifiers in the food industry and as potential immunological regulators for human health. Several important questions remain to be answered with respect to AGPs. Clearly, elucidating the unequivocal functions of particular AGPs and relating these functions to their respective structures and modes of action remain as major challenges in the years ahead.

Arabinogalactan-proteins: structure, expression and function.

Arabinogalactan proteins is an umbrella term applied to a highly diverse class of cell surface glycoproteins, many of which contain glycosylphosphatidylinositol lipid anchors. The structures of protein and glycan moieties of arabinogalactan proteins are overwhelmingly diverse while the "hydroxproline contiguity hypothesis" predicts arabinogalactan modification of members of many families of extracellular proteins. Descriptive studies using monoclonal antibodies reacting with carbohydrate epitopes on arabinogalactan proteins and experimental work using beta-Yariv reagent implicate arabinogalactan proteins in many biological processes of cell proliferation and survival, pattern formation and growth, and in plant microbe interaction. Advanced structural understanding of arabinogalactan proteins and an emerging molecular genetic definition of biological roles of individual arabinogalactan protein species, in conjunction with potentially analogous extracellular matrix components of animals, stimulate hypotheses about their mode of action. Arabinogalactan proteins might be soluble signals, or might act as modulators and coreceptors of apoplastic morphogens; their amphiphilic molecular nature makes them prime candidates of mediators between the cell wall, the plasma membrane, and the cytoplasm.

The biology of arabinogalactan proteins.

We took advantage of a subtractive hybridization procedure to isolate a set of cDNA clones of nodule-specific genes (nodulin genes) from developing soybean root nodules. Single-standed 32P-labelled cDNA synthesized from nodule poly(A)+ RNA was hybridized with a large excess of uninfected root poly(A)+ RNA. Unhybridized cDNA was selected and used to screen nodule cDNA libraries. By this procedure we isolated several novel nodulin cDNA clones together with most of the nodulin cDNAs previously described. Four novel nodulin genes, which were expressed long before the onset of nitrogen fixation, were further characterized. GmN # 36 and GmN # 93 transcripts appeared in the roots less than 3 days after sowing and inoculation with Bradyrhizobium, but GmN # 36 transcripts were also detected at very low levels in the stems of uninfected plants. Transcripts of GmN # 315 and GmN # 70 first appeared at 6–7 days, just before nodule emergence. Amino acid sequences of the predicted products of GmN # 36, GmN # 93 and GmN # 70 exhibited no significant homology to proteins identified so far. The GmN # 315 encoded protein has a limited but significant homology to some plant cyanins, suggesting that it is a metal-binding glycoprotein. In situ hybridization studies revealed that GmN # 36 transcripts first appeared in the pericycle cells of the root stele near the infected site. During nodule emergence they were found in a few cell layers surrounding the vascular strands connecting the nodule meristem with the root stele, and in mature nodules they were present specifically in the pericycle cells in vascular bundles. These observations led us to hypothesize that GmN # 36 gene products play a role in the transport and/or degradation of photosynthate. On the other hand, GmN # 93 transcripts first appeared in the primary nodule meristem just below the root epidermis. In mature nodules they were only present in the infected cells.

Isolation and characterization of novel nodulin cDNAs representing genes expressed at early stages of soybean nodule development

A set of cDNA clones have been characterized that represent early nodulin mRNAs from pea root nodules. By RNA transfer blot analyses, the different early nodulin mRNAs were found to vary in time course of appearance during the development of the indeterminate pea root nodule. In situ hybridization studies demonstrated that the transcripts were located in different zones, representing subsequent steps in development of the central tissue of the root nodule. ENOD12 transcripts were present in every cell of the invasion zone, whereas ENOD5, ENOD3, and ENOD14 transcripts were restricted to the infected cells in successive but partially overlapping zones of the central tissue. We conclude that the corresponding nodulin genes are expressed at subsequent developmental stages. The amino acid sequence derived from the nucleotide sequences of the cDNAs, in combination with the localization data, showed that ENOD5 is an arabinogalactan-like protein involved in the infection process, whereas ENOD3 and ENOD14 have a cysteine cluster suggesting that these are metal-binding proteins. Furthermore, we showed that there is a clear difference in the way Rhizobium induced the infection-related early nodulin genes ENOD5 and ENOD12. A factor acting over a long distance induced the ENOD12 gene, whereas a factor acting over a short distance activated the ENOD5 gene.

E. van der Knaap

Papers

Sequential induction of Nodulin gene expression in the developing Pea Nodule