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The significance of papillae in plant resistance might
be to delay invading hyphae while other defenses are
activated. The lignin-like precursors in papillae of the
Gramineae may be particularly important, because
lignin can form complexes with the wall polymers,
preventing their enzymatic degradation. Ride & Pearce
(1979) demonstrated this for cereal leaves inoculated
with Botrytis cinerea(not a pathogen of cereals) and
several other fungi that induced the production of
lignified papillae during attempted penetration. The
normal leaf cell walls were digested easily by commercial
wall-degrading enzymes, but the papillae and the
surrounding “haloes” of lignified wall material were
not digested. Thus, lignification and the development
of papillae could have three resistance-related functions:
to increase the wall thickness that a fungus must
penetrate while depending on its endogenous energy
reserves; to render the wall resistant to digestion by the
fungal enzymes; and to confer a locally toxic envir-
onment caused by the phenolic precursors of lignin and
suberin.

The oxidative burst

One of the earliest events in the attempted penetration

of leaves and other above-ground parts of plants is

an oxidative burst, equivalent to the oxidative burst

in phagocytes. It has been studied mainly in relation

to the hypersensitive response – the rapid death of

a group of cells in response to localized invasion

(e.g. Fig. 14.8) – but it seems to be a general reaction

of plant cells to trauma caused by attempted parasitic

invasion or chemical factors. Lamb et al. (1994)

have shown that it involves an extremely rapid (2–3

minute) production of H 2 O 2 at or near the plant cell

surface, by the reaction of oxygen with NADPH to

produce superoxide:

O 2 +NADPH →O 2 −+NADP+

Then O 2 −is transmuted to H 2 O 2 by a plasma membrane

oxidase. H 2 O 2 causes cross-linking of the plant cell

wall proteins, which could strengthen the wall against

attack by wall-degrading enzymes.

Chemical defenses

Plants produce a wide range of secondary metabolites

for defense against invading organisms. Some of

these are preformed compounds, collectively termed

phytoanticipins. They are present either as active

compounds or as precursors that are rapidly converted

to active compounds in response to infection. A

second category of defense compounds are termed

phytoalexins. Again, they are secondary metabolites

but they are synthesized de novoin response to infec-

tion. However, there is not always a clear difference

between these two groups of compounds.

The phytoanticipins and phytoalexins are widely

believed to be significant components of plant defense

systems – at least in the sense that they can prevent

most potential invaders from causing progressive

disease of plants. But host-specialized pathogens can

sometimes overcome the defenses of their specific

hosts, and we consider some representative examples

of this below.

290 CHAPTER 14

Fig. 14.10(a,b) Lignified papillae produced in response to invasion of wheat root cells by the take-all fungus,

Gaeumannomyces graminis. Narrow penetration hyphae are seen growing through the cell walls, but they have been

stopped by the developing papillae.

(a) (b)