Colonization of corn, Zea mays, by the entomopathogenic fungus Beauveria bassiana

Abstract

Light and electron microscopy were used to describe the mode of penetration by the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin into corn, Zea mays L. After inoculation with a foliar spray of conidia, germinating hyphae grew randomly across the leaf surface. Often a germ tube formed from a conidium and elongated only a short distance before terminating its growth. Not all developing hyphae on the leaf surface penetrated the cuticle. However, when penetration did occur, the penetration site(s) was randomly located, indicating that B. bassiana does not require specific topographic signals at an appropriate entry site as do some phytopathogenic fungi. Long hyphal structures were observed to follow the leaf apoplast in any direction from the point of penetration. A few hyphae were observed within xylem elements. Because vascular bundles are interconnected throughout the corn plant, this may explain how B. bassiana travels within the plant and ultimately provides overall insecticidal protection. Virulency bioassays demonstrate that B. bassiana does not lose virulence toward the European corn borer, Ostrinia nubilalis (Hübner), once it colonizes corn. This endophytic relationship between an entomopathogenic fungus and a plant suggests possibilities for biological control, including the use of indigenous fungal inocula as insecticides.

FIG. 1

(A) Leaf surface and growing hyphae. Bar, 100 μm. (B) Conventionally prepared leaf with conidia and developing germ tube. Bar, 2 μm. (C) Conventionally prepared leaf with elongating germ tube. Bar, 10 μm. (D) Germinating conidia with short germ tube and penetration site. Bar, 2 μm. (E) Frozen preparation illustrating hyphal penetration site at cell wall junction and waxy surface of leaf cuticle. Bar, 1 μm. (F) TEM section parallel to leaf surface showing conidium inside epidermal cell. Arrow indicates epidermal leaf surface (wax cuticular layer has been removed during processing). Bar, 500 nm. (G) Cross section parallel to leaf surface illustrating stomata with invading hyphae. Bar, 2 μm. (H) Perpendicular section of epidermal cell wall showing cross section of hyphae within wall. Epidermal cell surface at top. Bar, 500 nm. (I) Penetration site showing hypha moving through cell wall. Bar, 500 nm. (J) Penetration site showing thin hyphal neck and fungal cytoplasm. Bar, 200 nm. (K) Hyphae in corn leaf xylem vessel. Bar, 1 μm.

FIG. 1

(A) Leaf surface and growing hyphae. Bar, 100 μm. (B) Conventionally prepared leaf with conidia and developing germ tube. Bar, 2 μm. (C) Conventionally prepared leaf with elongating germ tube. Bar, 10 μm. (D) Germinating conidia with short germ tube and penetration site. Bar, 2 μm. (E) Frozen preparation illustrating hyphal penetration site at cell wall junction and waxy surface of leaf cuticle. Bar, 1 μm. (F) TEM section parallel to leaf surface showing conidium inside epidermal cell. Arrow indicates epidermal leaf surface (wax cuticular layer has been removed during processing). Bar, 500 nm. (G) Cross section parallel to leaf surface illustrating stomata with invading hyphae. Bar, 2 μm. (H) Perpendicular section of epidermal cell wall showing cross section of hyphae within wall. Epidermal cell surface at top. Bar, 500 nm. (I) Penetration site showing hypha moving through cell wall. Bar, 500 nm. (J) Penetration site showing thin hyphal neck and fungal cytoplasm. Bar, 200 nm. (K) Hyphae in corn leaf xylem vessel. Bar, 1 μm.