| The basic elements of papaya systemic acquired resistance (SAR) pathway have been reported to resemble that of Arabidopsis thaliana. An Arabidopsis NPR1 (nonexpresser of pathogenesis - related gene) homolog (CpNPR1) was previously isolated from papaya and partially characterized but the expression of pathogenesis-related (PR) genes remained to be done. This research produced plants transformed to over-express CpNPR1, or to express the Arabidopsis NPR1 gene (AtNPR1), as a way to characterize the expression of PR genes in papaya and potentially to increase the resistance of papaya to Phytophthora palmivora.; Transgenic plants and non-transformed plants of the cultivar Kapoho were treated with known elicitors of disease reactions including a synthesized oligopeptide Pep-13 from Phytophthora, benzothiadiazole (BTH), P. palmivora zoospores, or distilled water as a non-eliciting control. Changes in the level of NPR1 and PR-1d gene expression were measured using northern blots and quantitative RT-PCR. The Pep-13 elicitor and BTH activated NPR1 and PR-1d gene expression in the non-transformed Kp and transgenic lines, but induction was strongest and fastest in CpNPR1 transgenic lines. P. palmivora suppressed NPR1 gene expression in the non-transformed Kp, but not in transgenic lines. When transformed lines in tissue culture were challenged with P. palmivora zoospores, pathogen growth, quantified by DAS ELISA, was reduced showing that the CpNPR1 and AtNPR1 lines were more resistant than the non-transformed Kp.; Measurements of PDF1.2 gene expression, a marker for the jasmonic acid - dependent pathway, showed that the Arabidopsis and papaya NPR1 homologs may act at different points in a generalized pathogen defense response pathway. Based on these results, a modified defense signaling pathway for papaya is proposed. Essential elements of the papaya defense signaling pathway are the increased NPR1 and PR-1d gene expression after P. palmivora infection, probably through Phytophthora avr factors recognized by the plant, and the action of the papaya NPR1 gene in a synergistic fashion between the SA- and JA-dependent signaling pathways. Future studies investigating the regulatory factors involved in this interaction that govern the expression of defense in this system should contribute further to our knowledge of the elaborate mechanisms that regulate defense responses in plants. |
