Lecture #12

Rusts and Smuts


Uredinales - the Rusts; approximately 5000 species in about 140-150 different genera.

Ustilaginales - the Smuts; approximately 1200 species of smut fungi in more than 50 genera

All are parasitic on plants, often causing great losses to many cultivated crops

Heterothallic

Dikaryotic condition in secondary mycelia of rust fungi does not involve clamp connections

Obligate biotrophs that are incapable of surviving saprobically. These pathogens gain entry to their hosts by either growing into stomatal openings or by direct penetration of host epidermal cells.

Rust fungi produce no basidiocarps. The structure in which karyogamy takes place is a specialized spore known as the teliospore. Since it is within this structure that karyogamy takes place, the teliospore is technically part of the basidium. Upon germination the teliospore-or each cell of the spore if it is multicellular-typically gives rise to a short germ tube of determinate growth known as the promycelium. This is the structure in which meiosis occurs and on the surface of which sterigmata and basidiospores develop

Rust fungi may produce as many as five different stages in their life cycles.

Not every species produces all the stages listed above.

Life cycle patterns, the Uredinales may be divided into three categories:

  1. macrocylic forms- all five reproductive stages
  2. demicyclic forms- demicyclic forms the uredinial stage is absent
  3. microcyclic forms- both aeciospores and urediniospores are absent and the teliospore is the only binucleate spore produced

Heteroecism- two taxonomically different host plants in order to complete their life cycles.

Heteroecious species produce Stages 0 and I on one alternate host species and Stages II and III on primary host. Stage IV, the basidial stage, is not parasitic and thus does not arise from the tissues of a specific host

Autoecism- completes its entire life cycle on a single host species

Puccinia graminis produces its spermogonia and aecia (0 and I) on barberry bushes (Berberis vulgaris) and its uredinia and telia (II and III) on various grasses. Barberry is a dicotyledon; the grasses are monocotyledons.

Cronartium ribicola produces Stages 0 and I on white pines (gymnosperms) and Stages II and III on various species of currants and gooseberries (angiosperms)

Uredinopsis osmundae produces Stages 0 and I on the balsam fir (Abies balsamea), a gymnosperm, and Stages II and III on the cinnamon fern (Osmunda cinnamomia)


Spermogonia- the structures that bear the spermatia and receptive hyphae.
spermatia are the male sex organs (cells)
receptive hyphae function as trichogynes

Spermogonia produced from primary, homokaryotic mycelia growing in the tissues of their host
About four days after infection of the host by a basidiospore has taken place, a thin stromatic mass of hyphae consisting of uninucleate compartments develops below the epidermis of the host.
Spermatia, which are produced in tremendous numbers, are then exuded up and out the spermogonial cavity through the ostiole in a droplet of nectar, a thick, sticky, fragrant, sweet liquid
Receptive hyphae are produced from the upper part of the spermogonial wall
Spermatia and receptive hyphae produced by the same spermogonium usually are not compatible
Spermatia and receptive hyphae of opposite mating types do come in contact through the action of insect dispersers or rain, they fuse and the nucleus of the spermatium moves into the receptive hypha

Aecia and Aeciospores- An aecium is a group of typically dikaryotic hyphal cells within the parasitized host that gives rise to chains of dikaryotic aeciospores.

Aecial primordia are formed from primary mycelium prior to dikaryotization; cells are initially uninucleate and become dikaryotic following spermatization, apparently as a result of nuclear migration through receptive hyphae, which would, in this case, function as trichogynes; aecial primordia female reprod. structure?

The surfaces of mature aeciospores of most rust fungi are described as being verrucose. When aecia are produced in a leaf, they generally are located in the lower portion and break through the lower epidermis

Aeciospores of heteroecious species are, of course, produced on the alternate host. They are only capable of infecting the primary host. Aeciospores germinate on tissues of the primary host and give rise to germ tubes that either grow into stomatal openings or directly penetrate the host epidermis. Aeciospores of some species give rise to short germ tubes whose tips differentiate into appressoria that adhere the host surface. Subsequent entry into the host is by direct penetration of the epidermis. Once inside the host, a ramifying, intercellular, dikaryotic mycelium develops.


Uredinia and Urediniospores.

Uredinial cells are formed subepidermally from dikaryotic mycelium originating from the germination of an aeciospore or an earlier urediniospore

Urediniospores several "crops" of spores may be produced in one growing season. borne in structures uredinia

Urediniospores are dikaryotic

Urediniospore lands on the surface of a susceptible host produces a dikaryotic mycelium within the tissues of the host; susceptible host maybe reinfection or another primary host

  1. Recognition phase - germination of the dikaryotic urediniospore and differentiation of an appressorium over a stoma. indirect penetration peg arises from the appressorium and enters the host leaf by growing into the stomatal opening between the guard cells direct penetration in which entry is accomplished by the penetration of epidermal cells. past the guard cells the infection peg expands to form a structure called the substomatal vesicle
  2. Signal phase - substomatal vesicles and infection hyphae are in very close association with host mesophyll cells and it appears that during this time period some signal from the host is required for further development of the pathogen
  3. Parasitic phase begins when the pathogen begins to obtain nutrients from the host plant this third and final phase begins either with the development of the haustorium - D haustoria


Telia and Teliospores

Telia are groups of binucleate cells that give rise to special thick-walled cells called teliospores

In many rust fungi the old uredinia actually are converted to telia.

Teliospores are formed from the tips of binucleate cells of the telium; spore is at first dikaryotic, but eventually karyogamy takes place rendering teliospore diploid and uninucleate.

Karyogamy occurs very soon after teliospore formation.

Most rust fungi overwinter in the teliospore stage, but in some species teliospores germinate soon after they are formed

Teliospores germinate giving rise to a promycelium bearing basidiospores


Basidia and Basidiospores

Rust teliospore is a probasidium

When favorable conditions for germination arrive, the promycelium grows out from each cell of the teliospore

Diploid nucleus now migrates into the metabasidium and undergoes meiosis.

The four resulting haploid nuclei become distributed at more or less equal distances from each other in the metabasidium and septa develop between them to divide the metabasidium into four uninucleate cells.

Each of these cells then produces a sterigma at the tip of which a typically pear- or kidney-shaped basidiospore develops.

The four nuclei then migrate into the developing basidiospores

In some species the basidiospore nucleus then divides mitotically, leaving the spore binucleate

Eventually, basidiospores are forcibly discharged from their sterigma.

Upon germination a basidiospore gives rise either to a germ tube-so called direct gemination - or forms an outgrowth that functions as a sterigma, forming another spore at its tip

This new spore, called either a sporidium or a secondary spore, is virtually identical in appearance to the parent basidiospore. Germination to form a sterigma and a secondary spore is referred to as indirect gemination or repetitive germination and is dependent upon environmental conditions and the host surface

When a basidiospore lands on a suitable host and environmental conditions are suitable, it typically exhibits direct germination.

Basidiospore germlings tips differentiate into appressoria as soon as they contact the host surface

Attached to the host, the appressorium gives rise to a hypha that directly penetrates the underlying host epidermis

Once inside host tissues, hyphae arising from basidiospore germlings develop into homokaryotic mycelia that grow between the cells of the host; once established in the host, the homokaryotic mycelium gives rise to spermogonia distinctive of the species.


Ustilaginales

In remarkable contrast to the dikaryotic phase of the smut life cycle, homokaryons of smut fungi are nonpathogenic and grow readily in culture on simple nutrient media

Most smut fungi examined for mating type appear to be heterothallic and mating of compatible sporidia is required for the production of pathogenic dikaryons.

Unlike Uredinales no sex organs

The teliospores, or so-called smut spores, are the characteristic resting spores of the Ustilaginales

Ustilagninaceae - septate promycelium
Tillitiaceae - nonseptate promycelium


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