Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

334 KARBAN & MYERS
Annu. Rev. Ecol. Syst. 1989.20:331-348. Downloaded from www.annualreviews.org
by ETH- Eidgenossische Technische Hochschule Zurich - BIBLIOTHEK on 03/29/11. For personal use only.
evolutionary time. Most of the studies that point to induced resistance,
assayed as a decrease in herbivore performance, have found that the response
was systemic at least to other parts of the damaged shoot. However, one study
measuring rapid increases in foliage phenols found that this chemical response
was not systemic in birch trees (99). The spatial extent of the induced
response may determine whether the response acts as a defense. A localized
response may encourage herbivores to feed elsewhere on the same plant;
damage to the plant will be spread but not reduced. Surprisingly, no study has
explicitly mapped the spatial extent of induced resistance in all parts of the
entire plant. Despite the exciting suggestion by Rhoades (86) and Baldwin &
Schultz (4) that plants may become more resistant in response to cues released
by damaged neighbors, subsequent experiments have been few and have not
supported the idea (80, 28).
Some responses are known to occur within several hours after damage, as
in the case of proteinase inhibitors in damaged foliage of solanaceous plants
(reviewed in 108) or latex in damaged cucurbits (16). What component of
damage signals rapidly induced responses is generally not known. Damage to
tissues may release cell wall fragments that are translocated to other parts of
the plant where they activate genes that code for enzymes, such as proteinase
inhibitors (91). In this case the signal is transported systemically within
injured tomato plants but is directed primarily up the stem from older leaves to
younger ones (69). The proteinase inhibitors accumulate in vacuoles of
uninjured cells of injured plants and are deleterious to some caterpillars 00).
Induced resistance need not involve de novo synthesis; damage may bring
preformed enzymes and substrates into contact, causing the production of
active agents (21). Enzymatic activation of compartmentalized precursors is
responsible for many reactions, including the cyanogenic response of plants to
herbivores (21, 49). Damage to tissue may release ethylene that stimulates the
production of PAL and increases in phenolics (110, see also 72). Phenolics
are not transported from damaged to undamaged birch leaves; rather, they are
synthesized in undamaged leaves following increases in PAL activity (34).
The mechanisms of responses that occur over several years are also poorly
understood. Plant tissue that dcvelops in the growing season after marked
defoliation often shows increases in phenolics and fiber, declines in nutrient
concentrations, regrowth of juvenile tissue, and changes in plant morphology
(rcvicwed in 102, 79).
MECHANISMS: ACTIVE RESISTANCE OR PASSIVE
DETERIORATION?
While enzymatic activation of precursors and synthesis of phytoalexins and
proteinase inhibitors are clearly active processes, changes in plant chemistry