Characterization and cloning of pathogen-inducible genes and promotersof Carica papaya to improve resistance to Phytophthora palmivora

Abstract: Phytophthora species cause devastating diseases to important crop plants world-wide.The Carica papaya, the fifth sequenced angiosperm (Ming et al., 2008; Porter et al., 2009), hasrelatively few genes and is highly susceptible to the broad-host-range pathogen, P. palmivora.These qualities make this tropical fruit tree useful for comparative genomics of compatiblePhytophthora-plant interactions.As a first step toward engineering resistance of C. papaya to P. palmivora, defense-relatedgenes and inducible promoters in Carica papaya in response to P. palmivora were characterizedin this study. A survey of the root transcriptome and the expression of genes isolated from theroots of C. papaya (cultivar ‘SunUp’) seedlings were evaluated for regulation by P. palmivoraafter infection with this pathogen. Twenty-three genes exhibiting predominant root expressionwere isolated from a cDNA library created from infected root tissues. Sequence analysis revealeda number of genes associated with stress, pathogen and defense-related response. An openreading frame (ORF) encoding a predicted ascorbate peroxidase was found to be up-regulated inleaves, but not in roots. Another peroxidase ORF was down-regulated in roots, while genespredicted to encode a β-1,3-glucanase and ferulate 5-hydroxylase (F5H) were up-regulated inroots. An ORF encoding a hypersensitive-induced response protein was induced by P. palmivorain both roots and leaves. Finally, an ORF predicted to encode an aquaporin with normally highroot expression was down-regulated following inoculation. Although many host genes regulatedduring Phytophthora infection are associated with the host defense, others are required forpathogenicity. These genes have significance roles in the plant-pathogen interactions, and severalearly and strong pathogen-inducible promoters for papaya roots will be useful for engineeringnovel pathogen resistance. Collectively, expression patterns revealed in this study and similarstudies can be used to identify host genes regulated by Phytophthora for pathogenicity and hostdefenses with their associated pathways to provide fundamental knowledge on the mechanismsby which papaya metabolically responds to this pathogen.