The Plant Self 103“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityclosely related.^14 Transfer of stress signal through the root system has
been demonstrated in an experiment in which an aphid-infested plant
was able to confer resistance to a neighboring tree when the only con-
nection between the two was the fungal network.^15
A case of nutrition transfer among con-specific plants has been
reported for the Douglas fir of the Pacific Northwest. Because of the
exceptional height of these trees, younger trees growing in the shade
of the older ones have little chance of catching sunlight for photosyn-
thesis. To mitigate this situation, the older trees are able to subsidize
their young with nutrients they make. This “mothering” phenomenon
was studied with the use of radiocarbon tracer, and the mechanism is
believed to involve the underground fungal network.^16
5.6 How Plants Respond to Pathogen Infection
Plant pathogens include viruses, bacteria, fungi, and nematodes. The
following happens rapidly at the site of infection: (a) rapid release of
strong oxidative agents to kill the microbes; (b) local accumulation of
antimicrobial chemicals; (c) self-induced cell death surrounding the
infection site to limit the spread of the disease.
Phytoalexins are small organic molecules that can damage the
microbial cell wall, disturb their metabolism, delay their growth, or inter-
fere with their reproduction. Examples of phytoalexins are flavonoids,
resveratrol and alpha-pinene. Flavonoids are a diverse group (over 4,000
of them) of plant chemicals called polyphenols that, in the normal state,
impart characteristic color, odor, and taste to the leaves, flowers, and
fruits. Their peculiar taste deters some potential herbivores. They give
pigments to flowers to attract insect pollinators. They also serve as light
filters to selectively permit useful light to pass through for photosyn-
thesis. During dry seasons, they impart drought resistance to plants.
After injury by pathogens or pests, flavonoids accumulate in the lesion
margins of leaves to contain the damage.^17 (Humans consume flavonoids
for their nutrition value as antioxidants — protection against harmful
free radicals.)