|Kim, Soo-hyung Kim|
Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2009
Publication Date: 6/29/2009
Publication URL: http://jxb.oxfordjournals.org/cgi/content/full/erp165
Citation: Bae, H., Kim, S., Sicher, Jr., R.C., Kim, M.S., Strem, M.D., Bailey, B.A., Melnick, R. 2009. The beneficial endophyte, Trichoderma hamatum, isolate DIS 219B promotes growth and delays the onset of the drought response in Theobroma cacao. Journal of Experimental Botany. 60:3279-3295. Interpretive Summary: Cacao (Theobroma cacao), the source of chocolate, is a small tree grown on small farmer in tropical regions around the world. In order to produce the crop, farmers must over come many stresses including drought. Fungi growing inside tree tissues provide drought tolerance. We studied the effects of the beneficial fungus Trichoderma hamatum on the responses of cacao to drought. Drought conditons affected gene expression, amino acid profiles, and photosynthesis in cacao. The majority of these responses were delayed in cacao seedlings treated with Trichoderma. It may be possible to improve the tolerance of cacao to drought by treating plants with Trichoderma in the field. Cacao trees with improved tolerance to drought 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: Theobroma cacao (cacao), the source of chocolate, is cultivated in tropical climates where the crop is exposed to many biotic and abiotic stresses including plant diseases and drought. Endophytic Trichoderma isolates are being studied for their potential in controlling cacao diseases, but endophytes have also been characterized in other plants for their potential to induce drought tolerance. The impact of the fungal endophyte Trichoderma hamatum isolate DIS 219b on cacao’s molecular, physiological, and biochemical responses to drought stress were studied. Drought-induced changes in stomatal conductance, net photosynthesis, photosystem II (PS II) efficiency and leaf water potential were delayed approximately 3 days when seedlings were first treated with the DIS 219b. Drought-induced changes in emission of green fluorescence from cacao leaves were also delayed. Drought- induced changes in the soluble amino acids serine, glutamate, glycine, alanine, and proline were also altered by endophyte colonization. Changes in gene expression were analyzed using a real-time reverse transcription PCR (QPCR). The expression of 30 ESTs was altered by drought. Enhanced expression of ESTs putatively encoding a histidine kinase, a protein phosphatase 2C, a sorbitol transporter, a trehalose-6-phosphate phosphatase, an ABC transporter, an arginine decarboxylase, and a spermidine synthase was observed in cacao roots after 7 days of withholding water. Changes in gene expression in the leaves were not observed, with limited exceptions, until after10 days of withholding water. The expression patterns of fourteen ESTs were altered in droughted leaves. The drought-induced expression of 11 ESTs was delayed approximately 3 days in seedlings colonized by DIS 219b. In cacao roots the drought altered expression of 24 ESTs. Drought induced expression of 15 of the ESTs being studied in roots were not influenced by colonization with 219b. The colonization of cacao seedlings by T. hamatum isolate 219b delayed the onset of the cacao drought response in leaves at the molecular, physiological, and biochemical levels. The majority of drought induced changes in gene expression in the roots were not impacted by DIS 219b colonization leading us to suggest the impact of 219b on the drought response occurs after perception of drought by the roots.