Carotenoid biosynthesis and the evolution of carotenogenesis genes in rust fungi

Diseases caused by rust fungi pose a significant threat to global plant production. Although carotenoid pigments are produced in spores of nearly all rust species, the corresponding biosynthesis pathway(s) have not been investigated. Here, candidate genes for carotenoid biosynthesis in Puccinia graminis f. sp. tritici (Pgt) were identified, cloned and functionally complemented using specifically engineered strains of Escherichia coli. A part of the carotenoid biosynthesis pathway in rust fungi was elucidated, with only two genes, CrtYB and CrtI, catalysing the reactions from geranyl-geranyl diphosphate (GGPP) to γ-carotene. The CrtYB gene encodes a bi-functional lycopene cyclase/phytoene synthase, which catalyses the condensation of two GGPP into phytoene, as well as the cyclisation of the ψ-end of lycopene to form γ-carotene. The CrtI gene encodes a phytoene desaturase that carries out four successive desaturations of phytoene, through the intermediates phytofluene and neurosporene to lycopene. The evolution of carotenoid pigmentation in rust fungi, including Pgt, P. graminis avenae, P. graminis secalis (Pgs), P. graminis lolli, P. striiformis f. sp. tritici, P. striiformis f. sp. pseudohordei, P. striiformis f. sp. hordei, the "scabrum" rust (putative hybrids between Pgt and Pgs), P. triticina, and P. hordei, was investigated by phylogenetic analysis. Both CrtYB and CrtI were found to be closely related among rust fungi, other pathogenic fungi, and some aphids. Our results provide a springboard to increase the understanding of the physiological role(s) of carotenoid pigments in rust fungi, to better understand evolution within the Pucciniales, and to develop robust molecular diagnostics for rust fungi.