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some of the partial breakdown products of pectin –
the oligomers composed of 4 or 5 galacturonic acid
residues that are generated by endo-PG – are especially
toxic, causing rapid cell death.
Plant cells also contain a PG-inhibiting protein,
perhaps as a general resistance mechanism. As the
plant tissues age, the pectic compounds are increas-
ingly cross-linked with calcium, which renders them
more resistant to degradation, perhaps by restricting
access by the endo-acting enzymes. For example,
the hypocotyls (basal stem tissues) of bean seedlings
(Phaseolus vulgaris) show a marked increase in resistance
to infection by Rhizoctonia solaniabout 2 weeks after
seed germination, coinciding with calcification of the
middle lamella. The pathogens that rot older plant
tissues (Athelia rolfsii, and Sclerotinia sclerotiorumwhich

rots the seed heads of crops such as sunflower) over-

come this by producing large amounts of oxalic acid,

which they also produce in laboratory culture. Oxalic

acid can combine with calcium, removing it from

association with the pectic compounds and rendering

these susceptible to enzymatic attack.

Host-specialized necrotrophic pathogens

In contrast to the pathogens of immature or com-

promised hosts, many necrotrophic pathogens are

host-specialized. They have evolved mechanisms for

invading the normal healthy tissues of plants by

overcoming the specific defense mechanisms of their

hosts. These defense mechanisms can include physical

FUNGI AS PLANT PATHOGENS 287

Fig. 14.7Organization of the middle lamella (cementing layer) between plant cell walls, and summary of the pectic
compounds and pectic enzymes that degrade them.