Isolation of strigol from Striga host plants and the role of flavonoids in sorghum leaf disease reaction
Abstract
The germination of Striga asiatica, a root parasite of many cereal and leguminous crops, is stimulated by several host and non-host plant derived signals. HPLC revealed the presence of three active compounds in root exudates from Striga host plants, maize and sorghum, and also from proso millet. A fourth active compound was present in sorghum exudates. Acetate and heptafluorobutyrate derivatives were prepared and analyzed by HPLC and Mass Spectrometry. Each step involved in the isolation, chromatographic purification, and derivatization was followed by a sensitive Striga seed germination bioassay. We report the isolation of strigol as the major Striga seed germination stimulant in maize and proso millet root exudates and as a minor component of the total activity in sorghum root exudates. Strigol was previously isolated only from cotton, a non-host plant. Sorghum plant pigments have been associated with resistance to leaf diseases and grain deterioration. Several sorghum lines, four near-isogenic pairs (tan and red) and six non-isogenic (2 tan, 2 red and 2 purple plants) were evaluated for their reaction to the leaf diseases, rust and anthracnose. Mature plant tissues were then assayed for their phenolic content. No proanthocyanidins were detected in all tissues although other genotypes can have high levels of these compounds in seed coats. The red plants accumulated the 3-deoxyanthocyanidin, apigeninidin, in the glumes and leaf sheath which was not detected in the tan plants. Tan plants accumulated the flavone, apigenin. Tan and red near-isogenic pairs, which differ significantly in phenolic composition, did not show significant differences in their reaction to leaf diseases. The six non-isogenic sorghum lines showed marked differences in their reaction to leaf diseases as well as in their phenolic composition. However, there was no correlation between phenolic composition and resistance or susceptibility to leaf diseases. When the pigments produced by the red plants were compared to those in the purple plants, I found that the two plant colors are mostly due to the 3-deoxyanthocyanidins, apigeninidin (red plants) and luteolinidin (purple plants). Two naturally occurring O-methyl derivatives of apigeninidin (red plants) and luteolinidin (purple plants) were isolated and characterized. Pigment and/or flavan-4-ol production beyond the seedling stage may not be a good indicator of resistance or susceptibility of sorghum to leaf diseases.
Degree
Ph.D.
Advisors
Butler, Purdue University.
Subject Area
Biochemistry|Plant pathology
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