Host-parasite interaction during subepidermal sporulation and pustule opening in rust fungi (Pucciniales)

In late infection stages, rust fungi sporulate by building blister-like structures (pustules) in the mesophyll under the epidermal layer of their hosts. I investigated the host-pathogen interaction during pustule development and pustule opening in three different host-parasite combinations with different types of sori: Puccinia lagenophorae developing aecia, Phakopsora pachyrhizi developing uredinia, and Puccinia malvacearum developing telia. Light microscopy as well as scanning and transmission electron microscopy was applied. Although the development of the three host-parasite combinations varied in their soral development, there were common features detectable. During late infection stages, middle lamellae of mesophyll cells were dissolved locally to clear space for the pustule-building hyphae. Host cells were shoved aside, and hyphae adhering to host cells and to other hyphae by an extracellular matrix built new compact pseudoparenchyma, normally without killing host cells. The epidermal cells were separated from the mesophyll cells, and the growing pustule lifted up the covering tissue. The dissolution of the middle lamella of the anticlinal walls of the epidermal cells became visible. Scanning electron microscopy revealed that the epidermal cells collapsed over the bulging pustules, fissures appeared along cell boundaries, and finally the epidermal layer ruptured and the spores were set free. The interaction between hosts and pathogens is discussed.