Abstract
The Phytophthora sojae avirulence gene Avr3a encodes an effector that is capable of triggering immunity on soybean plants carrying the resistance gene Rps3a. P. sojae strains that express Avr3a are avirulent to Rps3a plants, while strains that do not are virulent. To study the inheritance of Avr3a expression and virulence towards Rps3a, genetic crosses and self-fertilizations were performed. A cross between P. sojae strains ACR10 X P7076 causes transgenerational gene silencing of Avr3a allele, and this effect is meiotically stable up to the F5 generation. However, test-crosses of F1 progeny (ACR10 X P7076) with strain P6497 result in the release of silencing of Avr3a. Expression of Avr3a in the progeny is variable and correlates with the phenotypic penetrance of the avirulence trait. The F1 progeny from a direct cross of P6497 X ACR10 segregate for inheritance for Avr3a expression, a result that could not be explained by parental imprinting or heterozygosity. Analysis of small RNA arising from the Avr3a gene sequence in the parental strains and hybrid progeny suggests that the presence of small RNA is necessary but not sufficient for gene silencing. Overall, we conclude that inheritance of the Avr3a gene silenced phenotype relies on factors that are variable among P. sojae strains.
Highlights
The genus Phytophthora includes some 120 species of plant pathogenic organisms [1,2]
Virulence assays were performed on test (Rps3a) and control soybean plants, and the cultures were assessed for the presence of Avr3a mRNA transcript by reverse transcriptase polymerase chain reaction (RT-Polymerase chain reaction (PCR))
Recent research has indicated that the variation of Avr3a mRNA transcript levels determines the virulence of P. sojae towards soybean plants carrying the resistance gene Rps3a
Summary
The genus Phytophthora includes some 120 species of plant pathogenic organisms [1,2]. Self-fertilized oospores can develop from one strain, while out-crossing between strains results in F1 hybrids. It is not possible to distinguish hybrid progeny from self-fertilized progeny without the use of strain-specific markers. The first experimental hybridizations of P. sojae were identified by crossing strains differing in drug resistance, but DNA markers are routinely used for tracking parentage [5,6,7,8,9,10]. Segregation patterns usually follow Mendelian rules but loss of heterozygosity in F1 or F2 progeny commonly occurs in P. sojae, and in other Phytophthora species, and can result in unusual inheritance patterns [9,11,12,13,14,15]
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