Resistance gene analogs are conserved and clustered in soybean.

Sequences of cloned resistance genes from a wide range of plant taxa reveal significant similarities in sequence homology and structural motifs. This is observed among genes conferring resistance to viral, bacterial, and fungal pathogens. In this study, oligonucleotide primers designed for conserved sequences from coding regions of disease resistance genes N (tobacco), RPS2 (Arabidopsis) and L6 (flax) were used to amplify related sequences from soybean [Glycine max (L.) Merr.]. Sequencing of amplification products indicated that at least nine classes of resistance gene analogs (RGAs) were detected. Genetic mapping of members of these classes located them to eight different linkage groups. Several RGA loci mapped near known resistance genes. A bacterial artificial chromosome library of soybean DNA was screened using primers and probes specific for eight RGA classes and clones were identified containing sequences unique to seven classes. Individual bacterial artificial chromosomes contained 2-10 members of single RGA classes. Clustering and sequence similarity of members of RGA classes suggests a common process in their evolution. Our data indicate that it may be possible to use sequence homologies from conserved motifs of cloned resistance genes to identify candidate resistance loci from widely diverse plant taxa.

Further Studies of the Role of Cyclic β-Glucans in Symbiosis. An ndvC Mutant of Bradyrhizobium japonicum Synthesizes Cyclodecakis-(1→3)-β-Glucosyl1

The cyclic β-(1→3),β-(1→6)-d-glucan synthesis locus of Bradyrhizobium japonicum is composed of at least two genes, ndvB and ndvC. Mutation in either gene affects glucan synthesis, as well as the ability of the bacterium to establish a successful symbiotic interaction with the legume host soybean (Glycine max). B. japonicum strain AB-14 (ndvB::Tn5) does not synthesize β-glucans, and strain AB-1 (ndvC::Tn5) synthesizes a cyclic β-glucan lacking β-(1→6)-glycosidic bonds. We determined that the structure of the glucan synthesized by strain AB-1 is cyclodecakis-(1→3)-β-d-glucosyl, a cyclic β-(1→3)-linked decasaccharide in which one of the residues is substituted in the 6 position with β-laminaribiose. Cyclodecakis-(1→3)-β-d-glucosyl did not suppress the fungal β-glucan-induced plant defense response in soybean cotyledons and had much lower affinity for the putative membrane receptor protein than cyclic β-(1→3),β-(1→6)-glucans produced by wild-type B. japonicum. This is consistent with the hypothesis presented previously that the wild-type cyclic β-glucans may function as suppressors of a host defense response.

Antioxidant Defenses in the Peripheral Cell Layers of Legume Root Nodules1

Ascorbate peroxidase (AP) is a key enzyme that scavenges potentially harmful H2O2 and thus prevents oxidative damage in plants, especially in N2-fixing legume root nodules. The present study demonstrates that the nodule endodermis of alfalfa (Medicago sativa) root nodules contains elevated levels of AP protein, as well as the corresponding mRNA transcript and substrate (ascorbate). Enhanced AP protein levels were also found in cells immediately peripheral to the infected region of soybean (Glycine max), pea (Pisum sativum), clover (Trifolium pratense), and common bean (Phaseolus vulgaris) nodules. Regeneration of ascorbate was achieved by (homo)glutathione and associated enzymes of the ascorbate-glutathione pathway, which were present at high levels. The presence of high levels of antioxidants suggests that respiratory consumption of O2 in the endodermis or nodule parenchyma may be an essential component of the O2-diffusion barrier that regulates the entry of O2 into the central region of nodules and ensures optimal functioning of nitrogenase.