The Ins and Outs of Rust Haustoria

Rust diseases caused by fungi of the order Pucciniales afflict a wide range of plants, including cereals, legumes, ornamentals, and fruit trees, and pose a serious threat to cropping systems and global food security. The obligate parasitic lifestyle of these fungi and their complex life cycles, often involving alternate hosts for the sexual and asexual stages, also make this group of pathogens of great biological interest. One of the most remarkable adaptations of rust fungi is the specialized infection structure that underpins the sustained biotrophic association with hosts; the haustorium (Figure 1A and C). This organ forms after penetration of the wall of a live host cell, expanding on the inner side of the cell wall while invaginating the surrounding host plasma membrane (Figure 1C). Through haustoria, the pathogen derives nutrients from the host and secretes virulence proteins called effectors, which are believed to be the key players that manipulate the physiological and immune responses of host cells [1–4]. Analogous terminal feeding structures have independently evolved in other organisms such as the haustorium in powdery mildews (ascomycetes) and downy mildews (oomycetes, not true fungi), and the arbuscules in arbuscular mycorrhizae, suggesting that such architecture represents a successful adaptation of these organisms to interact with their respective host plants [5,6].