We have used map-based approaches to clone a locus containing two genes, Avr1b-1 and Avr1b-2, required for avirulence of the oomycete pathogen Phytophthora sojae (Kaufmann & Gerdemann) on soybean plants carrying resistance gene Rps1b. Avr1b-1 was localized to a single 60-kb bacterial artificial chromosome (BAC) clone by fine-structure genetic mapping. Avr1b-1 was localized within the 60-kb region by identification of an mRNA that is expressed in a race-specific and infection-specific manner and that encodes a small secreted protein. When the Avr1b-1 protein was synthesized in the yeast Pichia pastoris and the secreted protein infiltrated into soybean leaves, it triggered a hypersensitive response specifically in host plants carrying the Rps1b resistance gene. This response eventually spread to the entire inoculated plant. In some isolates of P. sojae virulent on Rps1b-containing cultivars, such as P7081 (race 25) and P7076 (race 19), the Avr1b-1 gene had numerous substitution mutations indicative of strong divergent selection. In other isolates, such as P6497 (race 2) and P9073 (race 25), there were no substitutions in Avr1b-1, but Avr1b-1 mRNA did not accumulate. Genetic complementation experiments with P6497 revealed the presence of a second gene, Avr1b-2, required for the accumulation of Avr1b-1 mRNA. Avr1b-2 was genetically mapped to the same BAC contig as Avr1b-1, using a cross between P7064 (race 7) and P6497. The Avr1k gene, required for avirulence on soybean cultivars containing Rps1k, was mapped to the same interval as Avr1b-1.

http://www.oomycete.cn/pdf/wxshan2004a.pdf

The Avr1b locus of Phytophthora sojae encodes an elicitor and a regulator required for avirulence on soybean plants carrying resistance gene Rps1b.

▪ Abstract Recognition is the earliest step in any direct plant-microbe interaction. Recognition between Phytophthora pathogens, which are oomycetes, phylogenetically distinct from fungi, has been studied at two levels. Recognition of the host by the pathogen has focused on recognition of chemical, electrical, and physical features of plant roots by zoospores. Both host-specific factors such as isoflavones, and host-nonspecific factors such as amino acids, calcium, and electrical fields, influence zoospore taxis, encystment, cyst germination, and hyphal chemotropism in guiding the pathogen to potential infection sites. Recognition of the pathogen by the host defense machinery has been analyzed using biochemical and genetic approaches. Biochemical approaches have identified chemical elicitors of host defense responses, and in some cases, their cognate receptors from the host. Some elicitors, such as glucans and fatty acids, have broad host ranges, whereas others such as elicitins have narrow host ranges. M...

Molecular basis of recognition between phytophthora pathogens and their hosts

1These two authors have contributed equally to the work. Summary Arabidopsis accessions were screened with isolates of Phytophthora porri originally isolated from other crucifer species. The described Arabidopsis‐Phytophthora pathosystem shows the characteristics of a facultative biotrophic interaction similar to that seen in agronomically important diseases caused by Phytophthora species. In susceptible accessions, extensive colonization of the host tissue occurred and sexual and asexual spores were formed. In incompatible combinations, the plants reacted with a hypersensitive response (HR) and the formation of papillae at the sites of attempted penetration. Defence pathway mutants such as jar1 (jasmonic acid-insensitive), etr1 (ethylene receptor mutant) and ein2 (ethylene-insensitive) remained resistant towards P. porri. However, pad2, a mutant with reduced production of the phytoalexin camalexin, was hyper-susceptible. The accumulation of salicylic acid (SA) and PR1 protein was strongly reduced in pad2. Surprisingly, this lack of SA accumulation does not appear to be the cause of the hyper-susceptibility because interference with SA signalling in nahG plants or sid2 or npr1 mutants had only a minor effect on resistance. In addition, the functional SA analogue benzothiadiazol (BTH) did not induce resistance in susceptible plants including pad2. Similarly, the complete blockage of camalexin biosynthesis in pad3 did not cause susceptibility. Resistance of Arabidopsis against P. porri appears to depend on unknown defence mechanisms that are under the control of PAD2.

https://doc.rero.ch/record/6043/files/Roetschi_Alexandra_-_Characterization_of_an_Arabidopsis-Phytophthora_20061019.pdf

Characterization of an Arabidopsis–Phytophthora Pathosystem: resistance requires a functional PAD2 gene and is independent of salicylic acid, ethylene and jasmonic acid signalling

Partial resistance to Phytophthora sojae Kauffmann and Gerdemann in soybean [Glycine max (L.) Merr.] is expressed as a reduced level of root rot and is effective against all populations of the pathogen. The objective of this study was to identify simple sequence repeat (SSR) markers associated with putative quantitative trait loci (QTLs) for partial resistance to P. sojae in the soybean 'Conrad'. Three recombinant inbred soybean populations, Conrad × 'Sloan', Conrad × 'Harosoy', and Conrad × 'Williams', were evaluated for root lesion growth rate in growth chamber experiments following inoculation with P. sojae and with SSR markers to identify putative QTLs. The three populations segregated for root lesion growth rate after root inoculations. Two putative QTLs donated by Conrad were identified in all three populations and were positioned on soybean molecular linkage groups (MLGs) F and D1b+W. The QTL on MLG F explained 32.4, 35.0, and 21.4% of the genotypic variation for Conrad x Sloan, Conrad x Harosoy, and Conrad × Williams populations, respectively. The QTL on MLG D1b+W explained 10.6, 15.9, and 20.7% of the genotypic variation for the same three populations, respectively. The QTL on MLG F appears to be of more value based on the percentage of genotypic variation explained. Because the results indicate that QTLs for partial resistance to P. sojae map to different regions in soybean compared with the known Rps genes poses a challenge to soybean breeders. Marker-assisted selection may expedite the process of combining both Rps genes with partial resistance into high-yielding cultivars.

Quantitative trait loci for partial resistance to Phytophthora sojae in soybean

Simple sequence repeat (SSR) markers with linkages to the Rps1, Rps2, Rps3, Rps4, RpsS, and Rps6 loci that govern soybean [Glycine max (L.) Merr.] resistance to Phytophthora root rot (caused by Phytophthora megasperma Drechs. f. sp. glycinea Kuan and Ervin) are desired. Near-isogenic lines (NILs) of Clark or Williams, homozygous resistant (RpsRps) at just one of those Rps loci, were mated to a NIL of Harosoy homozygous susceptible (rpsrps) at all six loci. From the 100 to 120 F 2:3 progenies per mating, 20 F 3 seedlings were evaluated for resistance (R) or susceptibility (S) following inoculation with the race of P. megasperma affected by the segregating Rps allele. About 15 RpsRps and 15 rpsrps F 2 individuals were used to construct contrasting DNA bulks. Presumptive linkage (i.e., SSR marker polymorphism between two bulks) was confirmed or refuted by SSR assay of 15 to 40 F 2 individuals within each homozygous class. Recombination values were maximum likelihood estimates from the SSR allelic segregation data of both classes, although the rpsrps class was less prone to phenotypic classification error. SSRs on linkage groups (LGs) N, J, F, and G were identified with linkages to Rps1, Rps2, Rps3, and Rps4, respectively. A skewed R:S segregation in the Rps5 population precluded detection of linked SSRs. The Rps6 locus, whose map position was heretofore unknown, was linked with three SSRs in a region of LG-G that contains Rps4 and Rps5. SSR-Rps linkages of P < 0.05 could only be identified for the Rps1 alleles because of a paucity of SSR markers and/or parental monomorphism in the genomic regions surrounding other Rps loci.

Simple sequence repeat markers linked to the soybean Rps genes for phytophthora resistance

An Rps allele in the Harosoy cultivar of soybean (Glycine max) for resistance to races 16, 18, and 19 of Phytophthora megasperma f. sp. glycinea was shown to be at a new locus, designated Rps7, which is linked 12.5±2.7 map units from Rps1 in Linkage Group 10

T. R. Anderson

Papers

Inheritance and linkage of the Rps7 gene for resistance to Phytophthora rot of soybean.