|De Mayolo, Gabriela|
Submitted to: Plant Physiology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2005
Publication Date: 6/4/2005
Citation: Bailey, B.A., Bae, H., Strem, M.D., De Mayolo, G.A., Guiltinan, M.J., Verica, J.A., Maximova, S.N., Bowers, J.H. 2005. Nep1 induced gene expression in theobroma cacao leaves and the relationship to a compatible infection by phytophthora megakarya, causal agent of black pod. Plant Physiology and Biochemistry 43: 611-622. Interpretive Summary: The plant pathogen P. megakarya attacks the tropical tree Theobroma cacao (cacao) causing black pod. P. megakarya poses a major threat to cacao production in western Africa. Susceptibility to P. megakarya is characterized by tissue necrosis. Nep1 and related proteins, some produced by Phytophthora species, cause plant cell death and are thought to promote susceptibility to disease. The protein Nep1 caused necrotic flecks when applied to the underside of cacao leaves. Altered gene expression was observed in cacao leaves after treatment with Nep1 and the response closely resembled a general wound response. P. megakarya expressed the gene for a necrosis inducing protein closely related to Nep1 during infection of cacao leaves, and insensitivity to that protein could lead to resistance to the P. megakarya in the field. By understanding how P. megakarya causes disease in cacao, resistance to the disease can be developed. Controlling black pod through use of resistant varieties will allow sustainable production of cacao with reduced inputs assuring stable yields for farmers dependent upon the crop for their lively hood and assuring the supply required for dependent industries and associated consumers.
Technical Abstract: The developmental expression and Phytophthora megakarya, and Nep1 induced expression of eight genes from Theobroma cacao were studied in leaves. Evidence is presented that P. megakarya expresses an orthologue to the gene for Nep1 during infection of cacao leaves. Leaf development was separated into four stages: unexpanded (UE), young red (YR), immature green (IG), and mature green (MG). TcWRKY-1 and TcChiB were highly expressed in MG leaves, and TcPer-1, TcGlu-1, and TcCou-1 were highly expressed in YR leaves. TcCaf-1 was highly expressed in IM leaves, and TcLhca-1 was highly expressed in IG and MG leaves. Nep1 (5 ug mL-1 plus 0.2% Silwet-L77) caused necrotic flecks on the under side of MG leaves within 24 hr of treatment. Necrotic lesions were observed on YR leaves 10 days after Nep1 treatment. Expression of TcWRKY-1, TcORFX-1, TcPer-1, and TcGlu-1 were enhanced and TcLhca-1 was repressed in MG leaves after treatment with Nep1. Expression of TcWRKY-1 and TcORFX-1 was enhanced in YR leaves after Nep1 treatment. A leaf disk assay was used to study gene expression during infection by P. megakarya, the causal agent of Block Pod. Cutting leaf disks had profound effects on the expression of TcWRKY-1, TcORFX-1, TcLhca-1, TcChiB and TcPer-1. There was minimal change in the expression of the eight genes studied in response to infection by P. megakarya. RT-PCR was used to amplify a NEP1 orthologue of P. megakarya from cacao leaf disks 48 h after inoculation with P. megakarya yielding a 451 bp cDNA fragment with homology to orthologues of the Nep1 gene found in other Phytophthora species. P. megakarya expresses a Nep1 orthologue during infection of cacao leaves. Nep1 causes cell death in cacao leaves but the altered gene expression observed more closely resembles a wound response than functional induced resistance.