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Systemic acquired resistance and induced
systemic resistance

Earlier in this chapter, we noted that plants can
respond to attack from necrotrophic pathogens by
mounting a hypersensitive response, in which the
infected cells and their immediate neighbors die
rapidly and accumulate fungitoxic compounds.
Although this response is localized, it leads to the
production of signalling molecules that are mobilized
throughout the plant, so that the plant is, in effect,
immunized. Any subsequent infection will lead to
increased resistance or reduced expression of disease.
This phenomenon is termed systemic acquired resist-
ance(SAR). It involves the activation of a set of genes
that synthesize pathogenesis-related proteins, includ-
ing β-glucanases and chitinase which can attack the cell
walls of invading fungi. The aspirin-like compound
salicylic acid(Fig. 14.13) has been identified as the
signal molecule involved in SAR, and this is supported
by the finding that salicylic acid can be applied exogen-
ously and will induce the synthesis of pathogenesis-
related proteins, leading to enhanced plant resistance.
In fact, salicylic acid is marketed commercially for this
purpose. The remarkable point is that SAR confers a
generalized resistance to a wide range of pathogens,
including fungi, bacteria, and viruses.
A second type of induced resistance develops in
response to colonization of plant roots by specific
nonpathogenic strains of rhizosphere bacteria, such as
Pseudomonas fluorescens. But this defense system is
mediated by a different set of signalling compounds,
including jasmonic acidand ethylene. This type of
induced resistance is termed rhizobacteria-mediated
induced systemic resistance(ISR). Like SAR, induced
systemic resistance acts against several different types
of pathogen, including strains of Fusarium oxysporum,
the downy mildew pathogen Peronospora parasitica
(Oomycota), and the bacterial pathogens Xanthomonas
campestrisand Pseudomonas syringae. Thus, there are
two distinct signalling pathways, one mediated by
salicylic acid and one by jasmonic acid/ethylene, and

both confer resistance to pathogenic attack. Recent

studies suggest that both of these pathways can act

simultaneously, because there is no significant cross-

talk between them. Therefore, the combination of

SAR and ISR could confer an enhanced level of plant

resistance.

Vascular wilt diseases

Vascular wilt pathogens cause some of the most

devastating plant diseases. Their characteristic mode

of infection is to enter the water-conducting xylem

vessels, either by means of vectors such as bark beetles

in the case of Dutch elm disease (Ophiostoma ulmi and

O. novo-ulmi– see Chapter 10) or via wounds, or by

invasion of the young root tips. Then these fungi pro-

liferate in the xylem as spores or yeast-like budding cells

and are carried upwards in the water flow. The spores

become trapped on the perforated end walls of the

xylem vessels, where they germinate, grow through the

pores, and produce further spores that are carried

progressively upwards (Fig. 14.14).

Plants respond to invasion by the vascular wilt

fungi in several ways (Fig. 14.14):

• Balloon-like swellings (called tyloses) can bulge into
  the xylem from parenchyma cells adjacent to the
  xylem vessels; this seems to be an initial response
  before other defense mechanisms come into play.


• Pectic gels are extruded into the xylem, through
  bordered pits where the vessel wall is thin because
  it consists only of the primary wall, not overlaid by
  the secondary cellulosic wall.


• Phenolic compounds are released into the vessels from
  phenol store cells. The phenolic compounds are
  then oxidized and polymerize, helping to stabilize the
  gels and to create a fungitoxic environment.


• Phytoalexins accumulate in the vessels, presumably
  as part of a general stress response.

If the host responds fast enough to invasion of the

xylem vessels, the infection will be contained and the

FUNGI AS PLANT PATHOGENS 293

Salicylic acid Jasmonic acid

HO C O

OH

HH

H

H

OH

CH (^2) CH
2

H 2 C

O C

C

CH 3

O C

Fig. 14.13Salicylic acid and jasmonic acid – two
key signalling compounds that enhance the
resistance of plants to pathogenic attack.