Introduction to Fungi, Third Edition

UREDINALES: THE RUST FUNGI
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Fig 22.1 The life cycle of Puccinia graminis, with its heterokaryotic phase on cereals and the homokaryotic stage on barberry
(Berberis).The different ways in which the five spore stages are released and dispersed are indicated.Teliospores overwinter after
nuclear fusion, i.e. as diploid cells. During basidiosporogenesis, meiosis is followed by a mitotic division so that each basidiospore is
a homokaryon containing two nuclei of the same mating type.Open and closed circles represent haploid nuclei of opposite mating
type; diploid nuclei are larger and half-filled. Key events in the life cycle are plasmogamy (P), karyogamy (K) and meiosis (M).
Each cell contains two haploid nuclei of opposite
mating-type which fuse to form a diploid nucleus
(see Fig. 22.2c). The teliospore overwinters in the
diploid state, firmly attached to the plant tissue
on which it was produced. In spring, meiosis
occurs while each teliospore cell emits a germ
tube which becomes transversely septate and
forms the promycelium or metabasidium. Each
compartment of the metabasidium produces one
basidiospore which initially contains one haploid
nucleus. This commonly divides so that the
mature basidiospore often contains two genetically
identical haploid nuclei and is thus a
dikaryotic homokaryon (Anikster, 1983). Basidiospores
are actively liberated by the surface
tension catapult mechanism involving Buller’s
drop as described before for other basidiomycetes
(Section 18.5).
The basidiospores of P. graminis are unable to
infect the cereal host but will infect the alternate
host, barberry (Berberis vulgaris). The resulting
primary mycelium is haploid, homo- and monokaryotic.
At the upper (adaxial) surface of the
host leaf, the primary mycelium forms a flaskshaped
fructification known as a spermogonium.
Within the mesophyll, knots of hyphae
form a proto-aecium but do not develop further
at this stage. Minute uninucleate spermatia
are produced from annellide-like structures
(Littlefield & Heath, 1979) within the main body
of the spermogonium, and they aggregate within
a rim of hairs (periphyses) at the spermogonial