Cytochemical Aspects of the Plant–Rust Fungus Interface during the Compatible Interaction Coffea arabica (cv. Caturra)–Hemileiavastatrix (race III)
Abstract
The infection process of Hemileia vastatrix within leaf tissues of susceptible Coffea arabica plants was investigated ultrastructurally and cytochemically and also by light microscopy. An exoglucanase‐gold complex, anti–galacturonic acid monoclonal antibodies (JIM7), anti‐β‐1,3‐glucans polyclonal antibodies, and a WGA‐ovomucoid‐gold complex were used to localize β‐1,4‐glucans, pectins, callose, and chitin, respectively. After urediospore germination and appressoria differentiation on leaf stomata, the fungus penetrated and colonized the mesophyll tissues inter‐ and intracellularly. The intercellular hyphae, including haustorial mother cells and haustoria, contained β‐1,3‐glucans and chitin in their walls. The interface of the pathogen with the host cell wall was characterized by the occurrence of adhesive material that was labeled for pectin. Also, labeled plant cell wall fragments for β‐1,4‐glucans were detected close to the intercellular fungal cell wall. Plant cell wall degradation during haustorium formation was restricted only to the site of host cell penetration as judged by the use of the exoglucanase‐gold complex. In advanced stages of the infection process, haustoria were encased by a material that positively reacted for callose and β‐1,4‐glucans. Pectins were not detected in the encasement material around the haustorial body but only in the encasement material around the penetration peg. This haustorium encasement is a plant defense response but occurred too late to be efficient in preventing fungal growth and sporulation.
