Ultrastructural and functional similarity of the haustorial neckband of rust fungi and the Casparian strip of vascular plants

In susceptible hosts infected with rust fungi, an osmiophilic band, bridging the interface between host and parasite, is usually observed about midway along the haustorial neck. This neckband bears certain ultrastructural resemblances to the Casparian strip of endodermal cells, and the possible similarity in function as a barrier to apoplastic transport was investigated using 4-acetamido, 4′-isothiocyanostilbene-2, 2′-disulphonic acid (SITS) and uranyl acetate solutions. Both these reagents appeared to travel along host and fungal walls but did not readily penetrate the protoplasts of either organism.When rust-infected cowpea leaves were infiltrated with SITS reagent, haustoria in broken cells fluoresced as brightly as the walls of both intercellular hyphae and host cells. Mature haustoria in intact cells, however, did not fluoresce, suggesting the presence of a barrier to the flow of reagent to the haustorium. When rusted corn leaves were allowed to take up uranyl acetate solution, the ultrastructural distribution of uranyl crystals indicated that this barrier was, indeed, the neckband, and additional circumstantial evidence for the function of this structure was provided by indications from both techniques that the transport barrier was absent from very young haustoria; the latter have previously been shown to lack an ultrastructurally detectable neckband during the early stages of their development.These results appear to confirm the functional, as well as morphological, similarity of the haustorial neckband and the Casparian strip, and these, therefore, present an unusual example of parallel evolution at the structural and functional level. The importance of such a barrier to apoplastic transport in haustoria-producing fungi is discussed.