Recognition of pathogens by plants is mediated by several distinct families of functionally variable but structurally related disease resistance ( R ) genes. The largest family is defined by the presence of a putative nucleotide binding domain and 12 to 21 leucine-rich repeats (LRRs). The function of these LRRs has not been defined, but they are speculated to bind pathogen-derived ligands. We have isolated a mutation in the Arabidopsis RPS5 gene that indicates that the LRR region may interact with other plant proteins. The rps5-1 mutation causes a glutamate-to-lysine substitution in the third LRR and partially compromises the function of several R genes that confer bacterial and downy mildew resistance. The third LRR is relatively well conserved, and we speculate that it may interact with a signal transduction component shared by multiple R gene pathways. INTRODUCTIONThe molecular recognition of pathogens by plants is often characterized by a gene-for-gene relationship that requires a specific plant resistance ( R ) gene and a corresponding pathogen avirulence ( avr ) gene (Flor, 1971). Genetic evidence from a wide diversity of plant pathosystems suggests that when an appropriate R-avr gene pair is present, the result is host resistance, whereas absence or inactivation of either member of the gene pair results in susceptibility of the host to the pathogen. A common explanation for the molecular basis of this gene-for-gene relationship is an elicitorreceptor model (Gabriel and Rolfe, 1990). According to this model, avr genes directly or indirectly produce an elicitor that is recognized by the corresponding R gene-encoded receptor. This molecular interaction then triggers downstream signaling events that result in the activation of plant defenses and the limitation of pathogen growth.R genes have been cloned from several plant species (reviewed in Bent, 1996; Baker et al., 1997; Hammond-Kosack and Jones, 1997). These include R genes that mediate resistance to bacterial, fungal, oomycete, viral, and nematode pathogens. Many of these R gene products share structural motifs, which indicates that disease resistance to diverse pathogens may operate through similar pathways. For example, leucine-rich repeats (LRRs) are common to most of the R genes that have been characterized (Bent et al., 1994;Jones et al., 1994;Mindrinos et al., 1994; Whitham et al., 1994; Grant et al., 1995;Lawrence et al., 1995;Song et al., 1995; Dixon et al., 1996; Anderson et al., 1997;Parker et al., 1997). LRRs have been shown to play a role in protein-protein interactions (Kobe and Deisenhofer, 1994). This fact, along with the common occurrence of LRRs in R gene proteins, has led to speculation that LRRs serve as the binding domain for the pathogen-produced elicitor (Bent, 1996; Baker et al., 1997).Despite recent work in this area, it remains to be proven that LRR-containing R gene products function as receptors. In tomato, high-affinity binding sites from intact membranes have been found for an elicitor produced by races of Cladosporium fu...

Malus sieversii (Lebed.) M. Roem. is a wild progenitor species of the domesticated apple. It is found across a mountainous region of central Asia and has been the focus of several collection expeditions by the USDA-ARS-National Plant Germplasm System. This study used microsatellite variation at seven loci to estimate diversity and differentiation within M. sieversii using several complimentary approaches. Multilocus genotypes were amplified from 949 individuals representing seedling trees from 88 half-sib families from eight M. sieversii populations collected in Kazakhstan. Apportioning of genetic variation was estimated at both the family and site level. Analyses using a hierarchical model to estimate F st showed that differentiation among individual families is more than three times greater than differentiation among sites. In addition, average gene diversity and allelic richness varied significantly among sites. A rendering of a genetic network among all sites showed that differentiation is largely congruent with geographical location. In addition, nonhierarchical Bayesian assignment methods were used to infer genetic clusters across the collection area. We detected four genetic clusters in the data set. The quality of these assignments was evaluated over multiple Markov Chain Monte Carlo runs using both posterior likelihood and stability of the assignments. The spatial pattern of genetic assignments among the eight collection sites shows two broadly distributed and two narrowly distributed clusters. These data indicate that the southwestern collection sites are more admixed and more diverse than the northern sites.

The improvement bottleneck in domesticated apples appears to be mild or nonexistent, in contrast to improvement bottlenecks in many annual and perennial fruit crops, as documented from the literature survey. The low diversity of the subset of cultivars used for commercial production, however, indicates that an improvement bottleneck may be in progress for this perennial crop.

Recognition of pathogens by plants is mediated by several distinct families of functionally variable but structurally related disease resistance ( R ) genes. The largest family is defined by the presence of a putative nucleotide binding domain and 12 to 21 leucine-rich repeats (LRRs). The function of these LRRs has not been defined, but they are speculated to bind pathogen-derived ligands. We have isolated a mutation in the Arabidopsis RPS5 gene that indicates that the LRR region may interact with other plant proteins. The rps5-1 mutation causes a glutamate-to-lysine substitution in the third LRR and partially compromises the function of several R genes that confer bacterial and downy mildew resistance. The third LRR is relatively well conserved, and we speculate that it may interact with a signal transduction component shared by multiple R gene pathways. INTRODUCTIONThe molecular recognition of pathogens by plants is often characterized by a gene-for-gene relationship that requires a specific plant resistance ( R ) gene and a corresponding pathogen avirulence ( avr ) gene (Flor, 1971). Genetic evidence from a wide diversity of plant pathosystems suggests that when an appropriate R-avr gene pair is present, the result is host resistance, whereas absence or inactivation of either member of the gene pair results in susceptibility of the host to the pathogen. A common explanation for the molecular basis of this gene-for-gene relationship is an elicitorreceptor model (Gabriel and Rolfe, 1990). According to this model, avr genes directly or indirectly produce an elicitor that is recognized by the corresponding R gene-encoded receptor. This molecular interaction then triggers downstream signaling events that result in the activation of plant defenses and the limitation of pathogen growth.R genes have been cloned from several plant species (reviewed in Bent, 1996; Baker et al., 1997; Hammond-Kosack and Jones, 1997). These include R genes that mediate resistance to bacterial, fungal, oomycete, viral, and nematode pathogens. Many of these R gene products share structural motifs, which indicates that disease resistance to diverse pathogens may operate through similar pathways. For example, leucine-rich repeats (LRRs) are common to most of the R genes that have been characterized (Bent et al., 1994;Jones et al., 1994;Mindrinos et al., 1994; Whitham et al., 1994; Grant et al., 1995;Lawrence et al., 1995;Song et al., 1995; Dixon et al., 1996; Anderson et al., 1997;Parker et al., 1997). LRRs have been shown to play a role in protein-protein interactions (Kobe and Deisenhofer, 1994). This fact, along with the common occurrence of LRRs in R gene proteins, has led to speculation that LRRs serve as the binding domain for the pathogen-produced elicitor (Bent, 1996; Baker et al., 1997).Despite recent work in this area, it remains to be proven that LRR-containing R gene products function as receptors. In tomato, high-affinity binding sites from intact membranes have been found for an elicitor produced by races of Cladosporium fu...

Seeds and scionwood of Malus sieversii Lebed. have been collected from wild populations of apple trees in Kazakhstan. Seedlings and grafted trees were planted in the orchards at the U.S. Dept. of Agriculture Plant Genetic Resources Unit in Geneva, N.Y. We developed core collections to capture the genetic and phenotypic diversity represented in the trees from each of two of the Kazakhstan collection sites. These core collections capture more than 90% of the genetic diversity of the original populations, as determined using seven unlinked simple sequence repeat markers and 19 quantitative traits. Since phenotypic evaluations of these materials have been completed, the 35 trees within each population will be used as parents in crosses so that the genetic diversity in the orchard populations can be captured as seed for long-term ex situ conservation. This strategy of storing seeds, rather than maintaining costly field collections, could be applied to other collections of wild plant materials in the National Plant Germplasm System.

Pasteurella multocida is a mucosal pathogen that colonizes the respiratory system of susceptible hosts. Most isolates of P. multocida produce sialidase activity, which may contribute to colonization of the respiratory tract or the production of lesions in an active infection. We have cloned and sequenced a sialidase gene, nanH, from a fowl cholera isolate of P. multocida. Sequence analysis of NanH revealed that it exhibited significant amino acid sequence homology with many microbial sialidases. Insertional inactivation of nanH resulted in a mutant strain that was not deficient in sialidase production. However, this mutant exhibited reduced enzyme activity and growth rate on 2-3 sialyl lactose compared to the wild type. Subsequently, we demonstrated the presence of two sialidases by cloning another sialidase gene that differed from nanH in DNA sequence and substrate specificity. NanB demonstrated activity on both 2-3 and 2-6 sialyl lactose, while NanH demonstrated activity only on 2-3 sialyl lactose. Neither enzyme liberated sialic acid from colominic acid (2-8 sialyl lactose). Recombinant E. coli containing the sialidase genes were able to utilize several sialoconjugants when they were provided as sole carbon sources in minimal medium. These data suggest that sialidases have a nutritional function and may contribute to the ability of P. multocida to colonize and persist on vertebrate mucosal surfaces.Pasteurella multocida is a gram-negative coccobacillus of the family Pasteurellaceae and is a normal inhabitant of the upper respiratory system of many animals (24). The organism has a broad host range and is commonly a secondary pathogen in upper respiratory infections. Serotype D virulent isolates are toxigenic, but all serotypes produce capsules which confer serum resistance and resistance to phagocytosis (42). However, it is unusual to isolate a P. multocida strain that does not produce sialidase activity (40). Sialidases (neuraminidases; EC 3.2.1.18) are enzymes that liberate sialic acid from sialylconjugated glycoproteins, glycolipids, or colominic acids by cleaving alphaketosidic linkages. It is hypothesized that sialidase contributes to the virulence of some pathogenic organisms, especially those that inhabit and invade mucosal surfaces (7). Drzeniek (14) found sialidase activity in bacterial isolates that belong to the orders Pseudomonadales and Eubacteriales, and sialidases have been cloned from Clostridium species (35,36,37), Vibrio cholerae (48), Streptococcus pneumoniae (4, 5), Micromonospora viridifaciens (38), and Salmonella enterica serotype Typhimurium (21). Many of these bacterial sialidases have about 20% similarity at the amino acid level (21).Sialidases have been implicated as bacterial virulence factors (7, 34). It has been shown that a sialidase-deficient mutant of S. pneumoniae was less able to colonize and persist on mucosal surfaces than the wild type (46). In addition, a Bacteroides fragilis sialidase-deficient mutant was attenuated in the rat abscess model (18). The role of sialidase in ...

Garlic (Allium sativum L.) has been clonally propagated for thousands of years because it does not produce seed under standard cultivation conditions. A single garlic accession frequently displays a high degree of phenotypic plasticity that is likely to be dependent upon soil type, moisture, latitude, altitude, and cultural practices. The diversity observed by collectors has occasionally led to the renaming of varieties as they are exchanged among growers and gardeners. As a result, there are numerous garlic varieties available both commercially and within the USDA National Plant Germplasm System (NPGS) that may be identical genotypically, yet have unique cultivar names. To address this possibility, we performed amplified fragment-length polymorphism (AFLP) analysis on a comprehensive selection of 211 Allium sativum and Allium longicuspis accessions from NPGS and commercial sources. We used several statistical approaches to evaluate how these clonal lineages are genetically differentiated and how these patterns of differentiation correspond to recognized phenotypic classifications. These data suggest that while there are extensive duplications within the surveyed accessions, parsimony and distance based analyses reveal substantial diversity that is largely consistent with major phenotypic classes.

Alleles or tightly linked genes at the soybean (Glycine max L. Merr.) Rpg1 locus confer resistance to strains of Pseudomonas syringae pv. glycinea that express the avirulence genes avrB or avrRpm1. We have previously mapped Rpg1-b (the gene specific for avrB) to a cluster of resistance genes (R genes) with diverse specificities in molecular linkage group F. Here, we describe the high-resolution physical and genetic mapping of Rpg1-b to a 0.16-cM interval encompassed by two overlapping BAC clones spanning approximately 270 kilobases. Rpg1-b is part of a complex locus containing numerous genes related to previously characterized coiled coil-nucleotide binding site-leucine rich repeat (CC-NBS-LRR)-type R genes that are spread throughout this region. Phylogenetic and Southern blot analyses group these genes into four distinct subgroups, some of which are conserved in the common bean, Phaseolus vulgaris, indicating that this R gene cluster may predate the divergence of Phaseolus and Glycine. Members from different subgroups are physically intermixed and display a high level of polymorphism between soybean cultivars, suggesting that this region is rearranging at a high frequency. At least five CC-NBS-LRR-type genes cosegregate with Rpg1-b in our large mapping populations.