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The NLP Toxin Family in Phytophthora sojae Includes Rapidly Evolving Groups That Lack Necrosis-Inducing Activity

July 2012 , Volume 25 , Number  7
Pages  896 - 909

Suomeng Dong,1,3 Guanghui Kong,1 Dinah Qutob,2 Xiaoli Yu,1 Junli Tang,1 Jixiong Kang,1 Tingting Dai,1 Hai Wang,1 Mark Gijzen,2 and Yuanchao Wang1,3

1College of Plant Protection, Nanjing Agricultural University, Nanjing 210095, China; 2Agriculture and Agri-Food Canada, London, ON, N5V 4T3, Canada; 3Key Laboratory of Integrated Management of Crop Diseases and Pests (Nanjing Agricultural University), Ministry of Education, Nanjing 210095, China

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Accepted 29 February 2012.

Necrosis- and ethylene-inducing-like proteins (NLP) are widely distributed in eukaryotic and prokaryotic plant pathogens and are considered to be important virulence factors. We identified, in total, 70 potential Phytophthora sojae NLP genes but 37 were designated as pseudogenes. Sequence alignment of the remaining 33 NLP delineated six groups. Three of these groups include proteins with an intact heptapeptide (Gly-His-Arg-His-Asp-Trp-Glu) motif, which is important for necrosis-inducing activity, whereas the motif is not conserved in the other groups. In total, 19 representative NLP genes were assessed for necrosis-inducing activity by heterologous expression in Nicotiana benthamiana. Surprisingly, only eight genes triggered cell death. The expression of the NLP genes in P. sojae was examined, distinguishing 20 expressed and 13 nonexpressed NLP genes. Real-time reverse-transcriptase polymerase chain reaction results indicate that most NLP are highly expressed during cyst germination and infection stages. Amino acid substitution ratios (Ka/Ks) of 33 NLP sequences from four different P. sojae strains resulted in identification of positive selection sites in a distinct NLP group. Overall, our study indicates that expansion and pseudogenization of the P. sojae NLP family results from an ongoing birth-and-death process, and that varying patterns of expression, necrosis-inducing activity, and positive selection suggest that NLP have diversified in function.

This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2012.