Colonization and control of Striga hermonthica by Fusarium oxysporum f. sp strigae, a mycoherbicide component: An anatomical study

The fungal strain, Fusarium oxysporum f. sp. strigae (Foxy 2) has proved to be specific and effective in controlling Striga hermonthica (Del) Benth. However, little is known on its mode of action. The objective of this study was to evaluate the time and extent of colonization of the parasite by the fungus which is of critical importance considering the mutual interaction between S. hermonthica and its hosts. Anatomical investigations using light and electron microscopy were performed to assess the pattern of colonization and control of S. hermonthica seedlings and Striga shoots by Foxy 2. The connection between the sorghum root and the S. hermonthica seedlings showed that Striga haustorial intrusive cells penetrated the central cylinder of sorghum forming a continuous xylem connection but further growth of the S. hermonthica plant was prevented by Foxy 2. Hyphae invaded and digested all Striga seedling tissues belowground including haustorial intrusive cells, hyaline tissue, vessels, central xylem elements and Striga cortical parenchyma within 26 days after sowing Foxy 2 coated sorghum seeds. In S. hermonthica plants that managed to emerge, hyphae had penetrated and colonized vessels clogging them over long distances, and were identified even in the top of the shoots. In contrast to young seedlings, there was no digestion of vessels in the diseased shoots above-ground. Transmission electron microscopy revealed that some hyphae were embedded in an electron dense wall coating which was formed along the secondary vessel walls in diseased S. hermonthica plants. This electron dense wall layer was uneven in thickness and also occurred along xylem parenchyma pit membranes apparently preventing invasion of xylem parenchyma cells. Penetration of intertracheal pit membranes by hyphae was observed from vessel to vessel while attempted penetration of xylem parenchymal pit membranes was foiled by production of wall apposition beneath the pit. Thus, two mechanisms were identified by which Foxy 2 controls S. hermonthica. (i) Complete digestion of S. hermonthica seedlings inside the host and (ii) clogging of vessels of emerged S. hermonthica plants by hyphae contributing to wilting and subsequent death. (C) 2011 Elsevier Inc. All rights reserved.