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Flower-Mimicking Rust Fungi

Several fungal plant pathogens infect a host plant and alter its morphology in some fashion, but few alter the flowers of their hosts. Flower-mimicking rust fungi infect their host which results in the creation of pseudoflowers made by the host plant to aid the fungi in gamete dispersal for sexual reproduction. These rust fungi are parasites, requiring a living host plant in which to carry out their five life stages.

Species

Two species of rust fungi are known to infect and alter their plant host's flower morphology, resulting in the formation of structures on the plant that look similar to true flowers. These two species are Puccinia monoica and Uromyces pisi. Both species are in the same order of fungi, Pucciniales.

Puccinia monoica

Puccia monoica is a crown rust that infects a range of species in the botanical genus Arabis, within the family Brassicaceae, affecting 11 Brassicaceae species overall. It also infects species outside of Arabis, notably Boechera stricta, which also is in the family Brassicaceae.

Uromyces pisi

Uromyces pisi infects a perennial ornamental plant, Euphorbia cyparissias, within the family Euphorbiaceae. It is also known to infect plants in the family Fabaceae.

Characteristics

Pseudoflowers

Pathogenic rust fungi infect their host plant and induce the formation of pseudoflowers. These pseudoflowers are modified leaves that resemble flowers of another unrelated plant species. The pseudoflowers differ from the host plant flowers in shape, size, color, scent, and nectar produced. These pseudoflowers are formed as yellow rosettes with embedded fungal spermagonia and produce a sweet-smelling nectar to attract and reward insect pollinators. The mimicry expressed here is similar to Dodsonian mimicry in which reproductive flowers are mimicked by another species. However, in Dodsonian mimicry, nectar is not produced by the impostor flower.

Scent Chemistry

The fragrance of the pseudoflowers produced by rust fungi is one of the main characteristics that attracts insect pollinators. The chemical compounds that form the fragrance of the fungal pseudoflowers differs between species. Mass spectrometry and gas chromatography analyses have allowed identification of different fragrances of these pseudoflowers. Curiously, some fragrances are found to be chemically similar to non-host flowers rather than to host flowers and host vegetation. P. monoica pseudoflowers are mainly composed of aromatic alcohols, aldehydes and esters. In P. monoica, the fungal pseudoflower scent is dominated by aromatic compounds including phenylacetaldehyde and 2-phenylethyl alcohol. Comparatively, the scent of flowers of Arabis hosts is dominated by fatty-acid derived products. P. monoica's pseudoflowers are thought to be most similar to unspecialized Edelwiess flowers due to similarities in scent compounds.

In Uromyces pisi, the fungal pseudoflower scent is made of compounds including indole and jasmine lactone. The scent of E. cyparissias pseudoflowers is known to vary based on geographic location, ranging from a floral-fruity smell to a spermy scent.

Life Cycle

Puccinia monoica life cycle

P. monoica has a macrocyclic, heteroecious life cycle. Macrocyclic refers to the fungus succesively completing 5 spore states in its life. Heteroecious refers to having two complementary mating types housed in separate organisms. The reproduction of a heteroecious species relies upon a dispersal mechanism in order for outcrossing between the mating types to occur. The life cycle of P. monoica is described in more detail here.

Uromyces pisi life cycle

U. pisi also has a macrocyclic, heteroecious life cycle. meaning it successively completes five spore stages in a complete life cycle. The five spore stages created are: Pycniospores (spermatia), Aeciospores, Urediniospores, Teliospores, and Basidiospores. Like P. monoica, outcrossing between mating types depends on a dispersal mechanism by an insect pollinator. U. pisi alternates between two plant hosts to proceed through its life cycle. E. cyparissias hosts the fungal spermatia and aeciospores, while a species in the family Fabaceae is host to the other spore types.

Ecology

Pseudoflower-insect interactions

Rust fungi reproduction depends on insect visitation, analogous to pollination in flowers. Insects transfer spermatia (gametes) between different mating types of infected plants. The presence of the pseudoflower in plant communities may have consequences other than enhance the reproduction of the fungus. Studies in P. monoica demonstrated that insect visitation occurs more often when plants present a mixture of both pseudoflowers and natural occurring flowers, rather than when they occur alone in the plant. Combination of real flowers and pseudoflowers in plants increase the floral display through and apparent higher density, it also produces complementary attraction cues and complementary pollinators rewards, thus it has been proposed that they play a role in the pollination dynamics between insect and plants. In U. pisi, insect pollinator visits are also necessary in order for two mating types of U. pisi to interact and result in fertilization.

Pseudoflowers are visited by halictid, andrenic, bumble and oligolectic bees, muscid flies, and ants.

See also

Rust (fungus)

Parasitism

Mimicry in plants