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Roles of pre-formed inhibitors in plant resistance

to pathogens

The take-all fungus of cereals, Gaeumannomyces graminis
(Fig. 9.11), grows on the surface of cereal roots by
dark “runner hyphae” then penetrates the root cortex
and enters the vascular system, causing disruption of
the sugar-conducting phloem cells, and blockage of the
water-conducting xylem vessels. If enough roots are
killed in this way during the growing season then the
plants die prematurely, with serious loss of grain yield.
G. graminishas two main pathogenic forms – variety
tritici(GGT) which attacks wheat roots but not oat roots,
and variety avenae(GGA) which attacks both wheat and
oats. This difference was explained in the 1960s when
oat roots were found to contain pre-formed inhibitors.
The most potent of these inhibitors is avenacin A
(Fig. 9.12), a saponin (soap-like compound) which
combines with sterols in the fungal membrane, creat-
ing ion-permeable pores. In laboratory culture both
GGT and GGA grow readily in aqueous extracts of
wheat roots, but GGT is totally suppressed by aqueous
extracts of oat roots whereas GGA is unaffected by them.
The reason was suggested to be that GGA detoxifies
avenacin, cleaving the terminal sugars from the mole-
cule (Fig. 9.12), by producing a glycosidase enzyme
termed avenacinase. This enzyme would thus be a key

pathogenicity determinant, allowing GGA to extend

its host range to oats.

This was shown to be the case by targeted disrup-

tion of the avenacinase gene (Osbourne et al. 1994).

GGA was transformed with a marked, disrupted cDNA

of the gene, which inserted at the gene site by homo-

logous recombination and caused the fungus to lose

its pathogenicity to oats, while retaining its normal

pathogenicity to wheat. But when the marked cDNA

had inserted elsewhere in the genome (nonhomologous

recombination) the fungus was still fully pathogenic to

oats. Extending from this work, Osbourneet al. exam-

ined other host–pathogen systems where saponins

have been implicated as plant resistance factors. In par-

ticular, tomatoes are known to contain the saponin

αα-tomatine (Fig. 9.12), and a pathogen of tomatoes,

Septoria lycopersici, is known to detoxify this by cleav-

ing a single sugar from the molecule. The enzyme

responsible (tomatinase) was found to be very similar to

avenacinase: it was recognized by an anti-avenacinase

antibody, and cDNA of avenacinase hybridized with

DNA components of S. lycopersici, presumably by

recognizing the gene for tomatinase. This might be

explained by the fact that all the saponin-detoxifying

enzymes are β-glycosidases, with perhaps some common

structure, reflected in the DNA sequences. However,

avenacinase (from GGA) had very little ability to

172 CHAPTER 9

Fig. 9.11A wheat root at different magnifications, infected by the take-all fungus, Gaeumannomyces graminis. The
fungus grows on the root surface as dark “runner hyphae” (rh) then invades the root cortex by infection hyphae (ih),
destroying the phloem (phl) and causing intense discoloration and blockage of the xylem.