Variation in structure and activity among elicitins from Phytophthora sojae

Transcripts encoding elicitin-like protein domains were identified from similarity searches of Phytophthora sojae expressed sequence tags and were characterized with regard to molecular structure and elicitor activity. The A sojae elicitin family consists of at least nine genes with products similar to previously described elicitins (SOJA-2, SOJB, SOJ2, SOJ3, SOJ5, SOJ6 and SOJ7) or highly diverged from known sequences (SOJX and SON). The predicted structural features of seven (SOJA-2, SOJB, SOJ2, SOJ3, SOJ6, SOJ6 and SOJY) of the elicitin preproteins were compared. All of the predicted elicitins possess a leader signal sequence and a core elicitin domain. Five (SOJ2, SO.13, SOJ6, SOJX and SON) of the characterized elicitins also contain a variable C-terminal region. In addition, SOJX and SON contain a C-terminal hydrophobic membrane-spanning domain. An analysis of expression patterns of the elicitin transcripts showed that SOJA-2, SOJB, SOJ2, SOJ3 and SOJ6 were expressed in axenically grown mycelia and during infection, but not in zoospores. In contrast, SOJX and SON were predominantly and specifically expressed in zoospores. Selected elicitin domains were also tested for the induction of the hypersensitive response (HR) in Nicotiana spp. All of the elicitin protein domains tested induced the HR, except for SOJX and SOJY. Overall, the results show that the P. sojae elicitin gene family is large and diverse, with varying patterns of expression and HR-inducing activity.