Early interactions during powdery mildew infection

Abstract

Within minutes of spore deposition, enzymes including cutinase are released by conidia of Erysiphe graminis; these may prepare the infection court and facilitate subsequent fungal development. Germination follows with emergence of the primary germ tube, which contacts the host leaf. Extracellular material is secreted beneath the primary germ tube, which adheres to the leaf. The primary germ tube forms a penetration peg that breaches the host surface and gains access to host cell components including water. The primary germ tube also recognises factor(s) present in the host surface, and this stimulates elongation of the second-formed germ tube. The elongated second tube in turn responds to host surface factor(s) by differentiating an appressorium. Extracellular material, secreted beneath the growing appressorial germ tube, is laid down thickly around the appressorial lobe. An inner ring of extracellular material can often be seen surrounding penetration pores revealed by the removal of primary germ tubes and appressoria. The chemistry of the extracellular material is unknown, but in addition to adhesive properties recent studies show that a monoclonal antibody that recognises purified cutinase and, possibly, hemicellulases, binds to secretions from conidia, primary germ tubes, and appressoria. External factors can also influence germling development. Although light has little effect on the early stages of germling development, it has a profound effect in delaying the formation of haustoria by apparently mature appressoria. Understanding the control of early pathogen development could indicate avenues for genetic engineering and breeding for disease-resistant plants. Key words: Erysiphe graminis, powdery mildew, primary germ tube (PGT), appressoria, extracellular material.