The Avr1b Locus of Phytophthora sojae Encodes an Elicitor and a Regulator Required for Avirulence on Soybean Plants Carrying Resistance Gene Rps1b

Abstract

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.

Microbe-Independent Entry of Oomycete RxLR Effectors and Fungal RxLR-Like Effectors Into Plant and Animal Cells Is Specific and Reproducible

“…confirmed the root cell entry experiments of Dou et al (2008) using full length Avr1b protein and using more specific mutations of RXLR2 (qFLR) that agreed with results of bombardment experiments of Dou et al (2008). Furthermore, validating an observation of Shan et al (2004), showed that full length Avr1b protein could trigger cell death on leaves containing the Rps1b resistance gene, but not on leaves lacking Rps1b. The ability to trigger Rps1b-mediated cell death was abolished by the qFLR mutation and a dEER motif mutation, consistent with RxLR-dependent entry into soybean cells.…”

“…Tyler's lab at the University of California, Davis (Shan et al 2004) that were later replicated in Kale and associates (2010). Lab member B. Gu visited the Tyler lab at Virginia Tech where he carried out numerous root cell entry experiments including replicates of those described in Kale and associates (2010), and particularly experiments with Avr1bNt-GFP.…”

Differential Recognition of Highly Divergent Downy Mildew Avirulence Gene Alleles by RPP1 Resistance Genes from Two Arabidopsis Lines

“…c Noks1 was derived from an oospore of Noco2 (used in Botella et al, 1998), and its interaction phenotype matches that of Noco2 (Holub et al, 1994). oomycete avirulence genes (Allen et al, 2004;Shan et al, 2004 Figure 1). All alleles encoded full-length proteins, and none displayed substitutions rendering them obvious null alleles.…”

“…ATR13 from H. parasitica was cloned by isolating an in planta-expressed sequence that cosegregated with and encoded ATR13 (Allen et al, 2004). By contrast, map-based cloning approaches resulted in the identification of Avr1b-1 from Phytophthora sojae (Shan et al, 2004) and AvrL567 from Melampsora lini (Dodds et al, 2004). All three genes are predicted to encode proteins with N-terminal signal peptides, suggesting that they are secreted from the pathogens.…”

“…Alleles of ATR13 and AvrL567 encode proteins with extreme levels of amino acid polymorphism, which is indicative of diversifying selection and potentially driven by, and driving, the polymorphism seen in the matching resistance proteins (Dodds et al, 2000(Dodds et al, , 2004Allen et al, 2004;Rose et al, 2004). Alleles of Avr1b-1 are also polymorphic, and some virulent P. sojae isolates carry avirulence Avr1b-1 alleles but avoid detection because Avr1b-1 mRNA fails to accumulate in these isolates (Shan et al, 2004).…”