|Natarajan, Savithiry - Savi|
Submitted to: Plant Physiology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/22/2007
Publication Date: 2/1/2008
Citation: Bae, H., Kim, S., Kim, M.S., Sicher, Jr., R.C., Strem, M.D., Natarajan, S.S., Bailey, B.A. 2008. Coordinated regulation of polyamine biosynthesis in Theobroma cacao (cacao) tissues responding to stress. Plant Physiology and Biochemistry. 46:174-188. Interpretive Summary: Cacao (Theobroma cacao), the source of chocolate, is a small tree grown on small farms in tropical regions around the world. In order to produce the crop, farmers must overcome many stresses including plant diseases and drought. Polyamines are small compounds produced by plants in response to many types of stress and in some cases have been shown to protect plants from damages due to stress. We studied the effects of drought and infection by the pathogen Phytophthora megakarya on the production of polyamines in cacao. Both drought and infection induced expression of genes involved in production of polyamines. It may be possible to improve the tolerance of cacao to stresses by modifying polyamine production in cacao through plant breeding or genetic engineering. Cacao trees with improved tolerance to stress would be of benefit to the farmer by stabilizing crop yields and profitability. In addition, United States farmers who produce commodities used in chocolate production and the United States chocolate industry would benefit by the stabilization of global cocoa bean supplies and markets.
Technical Abstract: We have studied the relationship between polyamine synthesis and various stress treatments in cacao (Theobroma cacao). Polyamines (PAs) have been associated with the response to drought and many other biotic and abiotic stresses in plants in addition to their roles in physiological and developmental processes. The most common PAs in higher plants are putrescine (Put), spermidine (Spd) and spermine (Spm). In higher plants, there are two pathways for PA biosynthesis; 1) Put synthesis from ornithine by ornithine decarboxylase (ODC), and 2) Put synthesis from arginine by arginine decarboxylase (ADC). Spd and Spm are synthesized by either spermidine synthase (SPDS) or spermine synthase (SPMS) from Put and decarboxylated S-adenosylmethionine, which donates the aminopropyl groups. S-adenosylmethionine decarboxylase (SAMDC) decarboxylates S-adenosylmethionine (SAM). In this study, a full-length ODC (TcODC) was cloned from Theobroma cacao (cacao) and expression of TcODC, and ESTs for 4 other genes associated with PA biosynthesis including cacao ADC (TcADC), SAMDC (TcSAMDC), SPDS (TcSPDS), and SPMS (TcSPMS) were studied. Expression analysis using quantitative real-time reverse transcription-PCR (QPCR) results showed that the PA biosynthesis genes were expressed in all plant tissues examined. Constitutive expression of PA biosynthesis genes was generally highest in mature leaves and open flowers. Expression of TcODC, TcADC, and TcSAMDC was induced with the onset of drought as verified by changes in leaf water potential and altered emission of blue-green fluorescence from cacao leaves. Elevated levels of Put, Spd, and Spm were detected in cacao leaves 13 days after the onset of drought. Genes encoding ODC, ADC, and SAMDC were also responsive to mechanical wounding, infection by Phytophthora megakarya (a causal agent of black pod disease in cacao), the necrosis- and ethylene-inducing protein (Nep1) of Fusarium oxysporum, and flower abscission. TcODC was induced approximately 100-fold by Nep1 and P. megakarya and was constitutively expressed at much lower levels than TcADC, TcSAMDC, TcSPDS, and TcSPMS. In comparison, genes encoding SPDS and SPMS were induced 3.5-fold after 24 h treatment with P. megakarya. The results indicate TcODC, TcADC, and TcSAMDC are co-regulated by both abiotic and biotic stresses. The stresses responsible for the induction TcODC, TcADC, and TcSAMDC may share signal transduction pathways and/or the stress induced signal induction pathways may converge at these three genes leading to their coordinated induction. It is possible alteration of PA levels in cacao will result in enhance the tolerance of cacao to multiple stresses including drought and disease.