The Rust Fungi

Abstract

The rust fungi are a monophyletic group of approximately 7000 species in the Basidiomycota and are highly specialised obligate parasites of plants. The life cycle of rusts can be complex. Some rusts have up to five spore stages that alternate between haploid and dikaryotic nuclear conditions and that can occur on two taxonomically unrelated host plants. The rusts have evolved specialised structures that allow them to penetrate and obtain nutrients from living host cells. Biologic forms of a single rust species may differ in ability to attack different genera of host plants. Furthermore, within a single rust species they can be highly variable in ability to attack different genotypes of one host species. At the species level, genes that condition avirulence/virulence in rusts interact in a specific relationship with rust resistance genes in plants. Many of the most important plant diseases in the world are caused by rust fungi.

Key Concepts

  • The rust fungi are a monophyletic group of approximately 7000 species within the Basidiomycota, the group of fungi that includes mushrooms and shelf fungi.
  • Rust fungi have complex life cycles with up to five spore stages that alternate between haploid and dikaryotic nuclear condition on two unrelated host plants.
  • The rust fungi are obligate parasites of plants and have evolved specialised structures to infect and obtain nutrients from plants.
  • Rusts have a highly specific host range. Genes in rust that express avirulence or virulence interact with rust resistance genes in host plants to condition a compatible or incompatible interaction. Some rust species are highly variable for biologic forms that vary in their ability to attack host genotypes with different rust resistance genes.
  • Many of the most important plant diseases worldwide are caused by rust fungi.

Keywords: Basidiomycota; Pucciniomycotina; Pucciniales; Uredinales; plant disease; fungi

Figure 1. Life cycle of a macrocyclic‐heteroecious rust. (a) mature, diploid teliospore; (b) basidia with basidiospores; (c) spermogonial stage; (d) aecial stage; (e) uredinial stage; (f) telial stage. Reproduced with from permission Alexopoulus et al. . © Wiley.
Figure 2. Diagrammatic representation of infection structures of a cereal rust fungus. (a) Uredinial infection structures at 60 h. U = urediniospore; GT = germ tube; A = appresoria; GC = stomatal guard cell; PP = penetration peg; SV = substomatal vesicle; IH = infection hyphae; PH = primary haustoria; ICH = intercellular hyphae; H = additional haustoria. Reproduced with permission from Harder . © Elsevier. (b) Cross section of invaded host cell at site of penetration to show three dimensional representation of a mature haustoria and association with host cell organelles. E, EH = extrahaustorial matrix; EM = extrahaustorial membrane; ER = endoplasmic reticulum; FN = fungal nucleus; G = golgi body; HB = haustorial body; HMC = haustorial mother cell; HN = haustorial neck; M = mitochondria; N = host nucleus; P = plasmalemma; R = ring neck; T = tubule complex; V = vesicle; W = host cell wall. Reproduced with permission from Harder and Chong . © Elsevier. (c) Urediniospore formation on a cereal leaf. U = urediniospores; SC = sporogenous cell; SB = spore bud; UI = urediniospore initial; PD = pedicel; N = nucleus; V = vacuole. Reproduced with permission from Harder . © Elsevier.
Figure 3. Uredinial infection of Hemileia vastatrix (coffee rust). Photo courtesy of Howard Schwartz.
Figure 4. (a) Aecial infection of Cronartium ribicola (white pine blister rust) on pine tree. Source: USDA Forest Service, Ogden UT. (b) Uredinia of Cronartium ribicola on Ribes sp. Source: Robert Anderson, USDA Forest Service.
Figure 5. (a) Stem rust caused by Puccinia graminis f.sp. tritici on wheat. (b) leaf rust caused by P. triticina on wheat. (c) Stripe rust caused by P. striiformis f. sp. tritici on wheat. Source: USDA‐ARS.
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Further Reading

Aime MC, McTaggart AR, Mondo SJ, et al. (2017) Phylogenetics and phytogenomics of rust fungi. Advances in Genetics 100: 267–307.

Bolton MD, Kolmer JA and Garvin DF (2008) Wheat leaf rust caused by Puccinia triticina. Molecular Plant Pathology 9: 563–575.

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Duplessis S, Bakkeren G and Hamelin R (2014) Advancing knowledge on biology of rust fungi through genomics. In: Martin FM (ed.) Advances in Botanical Research Vol 70. Elsevier Ltd.

Goellner K, Loehrer M, Langebach C, et al. (2010) Phakopsora pachyrhizi, the causal agent of Asian soybean rust. Molecular Plant Pathology 11: 169–177.

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Talinhas P, Batista D, Diniz I, et al. (2017) The coffee leaf rust pathogen Hemileia vastatrix: one and a half centuries around the tropics. Molecular Plant Pathology 18: 1039–1051.

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Kolmer, James A, Ordoñez, Maria E, and Groth, James V(Nov 2018) The Rust Fungi. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021264.pub2]