untitled

economically important species, especially in North

America. It also displays all of the typical sporing

stages of rust fungi.

Black stem rust of wheat (Puccinia graminis)

The disease cycle of black stem rust is shown in

Fig. 14.22. We will begin this cycle at the dispersal phase,

when rust pustules burst through the epidermis of

cereal or grass leaves in early spring and release large

numbers of the characteristic rust-colored uredospores,

from structures called uredinia(see Fig. 14.23b).

Each uredospore is single-celled but contains two

nuclei, one of each mating compatibility type, so the

spores are dikaryotic as explained in Chapter 1. The

uredospores are dispersed by wind and can undergo

several cycles of infection in the course of a season,

resulting in major epidemics. Towards the end of the

season the production of uredospores ceases, and

instead the pustules (now termed telia) produce two-

celled teliospores(Fig. 14.23c). These also are dikaryotic,

with two nuclei in each cell, but the nuclei fuse to form

diploid cells, and the fungus overwinters in this form.

Early in the spring each cell of the teliospore germin-

ates to produce a short mycelium. The diploid nucleus

migrates into this and undergoes meiosis, leading to

the production of four monokaryotic basidiospores

(spores with only one haploid nucleus). These spores

will only develop further if they land on a barberry leaf.

Then they penetrate the leaf and, a few days later, pro-

duce flask-shaped spermogonia. These have receptive

hyphae and also release many small spermatia(Fig.

14.23,d,f ).

Each spermogonium produces spermatia of a single

mating type and they are exuded from the neck of

the spermogonium in a sugary fluid. Flies and other

insects are attracted to this, and when they visit a sper-

mogonium of a different mating type they transfer

the spermatia. The spermatia of one mating type then

fuse with receptive hyphae of the opposite mating

FUNGI AS PLANT PATHOGENS 303

SPORE

Glycolysis

Glycolysis

Frc

D

Man Man

HYPHA

EXTRAHAUSTORIAL MEMBRANE

HAUSTORIUM

FLUID

MATRIX

PLANT CELL MEMBRANE

PLANT CELL

ATP

ATP+P

H+

H+

AA AA

AA

Suc Suc

AA AA

ATP

ATP+P H+

H+

C

C H+

Frc Frc

B H+

Frc

Glc B Glc

D

A

Fig. 14.21Model for amino acid and sugar uptake and redistribution in rust fungi. Sucrose (Suc) and amino acids (AA)
pass between plant cells and through the extrahaustorial membrane, into the fluid-filled matrix that surrounds the haus-
torium. Invertase (shown as A) cleaves sucrose to glucose (Glc) and fructose (Frc) in the matrix. Then these sugars, and
amino acids cross the haustorial membrane by symport proteins (shown as B) in the membrane. Energy for nutrient
uptake is supplied by the H+gradient generated across the membrane by conversion of ATP to ADP +Pi(shown as C).
Within the haustorium, fructose is converted to mannitol (Man) by the enzyme, major alcohol dehydrogenase (shown
as D). Mannitol is then translocated into the fungal hyphae and provides the nutrients for fungal sporulation. (Based
on a diagram in Voegele & Mendgen 2003.)