Interaction of Cochliobolus heterostrophus with phytoalexin inclusions in Sorghum bicolor

The concentration of 3-deoxyanthocyanidins from inoculated sorghum exhibited a sigmoid curve for accumulation between 10 and 32 h after inoculation. Luteolinidin and apigeninidin were the most abundant deoxyanthocyanidins present. Methoxy-apigeninidin and the caffeic acid ester of arabinosyl 5-O-apigeninidin were detected but their concentrations were very low. At the time of inoculation, conidia of Cochliobolus heterostrophus were brown and cells were turgid. Host tissue was colorless and no symptoms were evident. Spectrophotometry and high performance liquid chromatography (HPLC) analyses showed that the 3-deoxyanthocyanidin phytoalexins were not present at the time of inoculation. Spectrophotometry did not detect deoxyanthocyanidins in plant extracts until 10-12 h post inoculation. In contrast, HPLC analysis showed that inoculated tissue released small amounts of luteolinidin and apigeninidin by 4 h after inoculation. It is the speed of this response that is unusual. After inoculation, both host cells and conidia appeared red. Phytoalexin-containing inclusions were released from the host tissue and became associated with the extracellular sheath of the dead conidia.. (C) 2003 Elsevier Science Ltd. All rights reserved.