Pigments in rust fungi: biosynthesis, role and evolution

E Wang, T Roberts, CM Dong and R Park

Faculty of Agriculture and Environment, the University of Sydney, 2015, NSW, Australia

The rust diseases are caused by fungi and are among the most damaging of all plant diseases. The most characteristic feature of all rust fungi is the pigments that are produced in one or more spore forms, which give them a rusty appearance. The pigments are thought to protect rust fungi against UV radiation and oxidative stress, and possibly act as a virulence factor. Phytoene desaturase (585 aa, encoded by the CrtI gene) and lycopene cyclase (707 aa, CrtY) were identified as candidate genes for cytoplasmic carotenoid pigment biosynthesis in rust fungi. In the postulated pathway, phytoene desaturase conducts the desaturation of phytoene (colourless) leading to lycopene (red-coloured), and then lycopene cyclase cyclizes lycopene to generate γ-carotene or β-carotene (yellow-coloured). To obtain a better understanding of the pigment biosynthetic pathway in rust fungi, cytoplasmic pigment profiles of 32 wild-type and colour mutants of Puccinia graminis f. sp. tritici (Pgt), P. g. f. sp. avenae (Pga), P. g. f. sp. secalis, P. triticina, P. coronate f. sp. avenae, P. hordei, P. striiformis and P. psidii were determined by high performance liquid chromatography. Four carotenoids, including phytoene, lycopene, γ-carotene and β-carotene, were identified in the urediniospores of wild-type spores. Compared with the wild-type parents, yellow mutants in P. graminis, P. triticina and P. hordei contained less lycopene, but more γ-carotene and β-carotene. An albino mutant of P. striiformis accumulated only phytoene, indicating that the pathway was blocked between phytoene and lycopene. The candidate genes were amplified and sequenced. Variations of the sequences between Pgt and Pga were 1.54% (CrtI) and 3.25% (CrtY), respectively. Function of candidate genes will be verified by complementation studies.