decreased photoassimilation rates, and reduction in yield per plant [recently
reviewed by 67]. SAR provides an example of integrated responses to light stimuli
where photoreceptors (such as the phytochromes) rapidly interfere with transcriptional regulators and phytohormones to regulate plant growth and biomass production [68]. Similar mechanisms of shade avoidance are common in both monocots
and dicots, where auxin and strigolactones inhibit axillary bud growth and play
antagonistic roles with respect to growth-promoting cytokinins [69]. Therefore,
perception of shade has a relatively rapid impact on the levels of hormones known
to stimulate hypocotyl elongation. For instance, short-term (4 h) simulated shade
treatments resulted in higher levels of the auxin IAA and brassinosteroids in a
dynamic fashion, along with a mild but sustained increase in the levels of GA 4 ,
the major bioactive gibberellic acid [70].
Plants can respond rapidly to the emission of volatile compounds from damaged
or undamaged neighbor tissues and may alter their physiology in response to
allelochemicals in their surroundings [71]. While soil allelochemicals necessitate a
direct contact with the roots of neighboring plants (see above), volatiles emitted by
neighbor plants have different lifetimes, can act at considerable distances, and
require both root and shoot perception sites to deliver their information [72]. Volatile
perception may involve different mechanisms of reception, including glycosylation,
as recently reported [73]. There are several evidences of plant-plant signaling
molecules, among which methyl salicylate and the green leaf volatile (GLV) Z-3hexenyl acetate can induce long-distance responses. Methyl salicylate (MeSA) can
be converted to salicylic acid upon plant uptake, and recently several plant proteins
able to bind salicylate have been characterized [74] which could be related to MeSA
signal perception. Z-3-hexenyl acetate was shown to induce long-distance responses
in several plant species. Chrysanthemum cinerariaefolium wounded plants emit,
among other volatiles, Z-3-hexenyl acetate and induce the biosynthesis of pyrethrins
in volatile-exposed neighboring plants [75]. Z-3-hexenyl acetate emitted by
herbivore-damaged tomato plants was also found to induce a rapid Vm depolarization and a significant influx on Ca
2+ in leaves of receiver conspecific plants [76]. At
the whole-plant level, plant volatiles can induce rapid decisions in plants in need to
find their source of nutrition. For instance, parasitic plants can be guided toward host
plant volatiles [66].
However, long-distance communication through volatiles is complicated by the
presence of atmospheric oxidant (e.g., oxygen, ozone, and reactive N and S species)
and temperature that may drastically change or degrade emitted molecules [77, 78].
Neighboring plants can also rapidly perceive root exudates that may provide a
detailed layer of information regarding the competitive environment [79]. Among
root exudates, a growing interest is toward strigolactones that plants exude into the
rhizosphere. These molecules can stimulate interactions with arbuscular mycorrhizal
(AM) fungi and improve plant fitness, but on the other hand, these molecules are
exploited by parasitic plants and rapidly stimulate the germination of their seeds
resulting in parasitization of the host plant [80, 81].
586
S. A. Zebelo
reviewed by 67]. SAR provides an example of integrated responses to light stimuli
where photoreceptors (such as the phytochromes) rapidly interfere with transcriptional regulators and phytohormones to regulate plant growth and biomass production [68]. Similar mechanisms of shade avoidance are common in both monocots
and dicots, where auxin and strigolactones inhibit axillary bud growth and play
antagonistic roles with respect to growth-promoting cytokinins [69]. Therefore,
perception of shade has a relatively rapid impact on the levels of hormones known
to stimulate hypocotyl elongation. For instance, short-term (4 h) simulated shade
treatments resulted in higher levels of the auxin IAA and brassinosteroids in a
dynamic fashion, along with a mild but sustained increase in the levels of GA 4 ,
the major bioactive gibberellic acid [70].
Plants can respond rapidly to the emission of volatile compounds from damaged
or undamaged neighbor tissues and may alter their physiology in response to
allelochemicals in their surroundings [71]. While soil allelochemicals necessitate a
direct contact with the roots of neighboring plants (see above), volatiles emitted by
neighbor plants have different lifetimes, can act at considerable distances, and
require both root and shoot perception sites to deliver their information [72]. Volatile
perception may involve different mechanisms of reception, including glycosylation,
as recently reported [73]. There are several evidences of plant-plant signaling
molecules, among which methyl salicylate and the green leaf volatile (GLV) Z-3hexenyl acetate can induce long-distance responses. Methyl salicylate (MeSA) can
be converted to salicylic acid upon plant uptake, and recently several plant proteins
able to bind salicylate have been characterized [74] which could be related to MeSA
signal perception. Z-3-hexenyl acetate was shown to induce long-distance responses
in several plant species. Chrysanthemum cinerariaefolium wounded plants emit,
among other volatiles, Z-3-hexenyl acetate and induce the biosynthesis of pyrethrins
in volatile-exposed neighboring plants [75]. Z-3-hexenyl acetate emitted by
herbivore-damaged tomato plants was also found to induce a rapid Vm depolarization and a significant influx on Ca
2+ in leaves of receiver conspecific plants [76]. At
the whole-plant level, plant volatiles can induce rapid decisions in plants in need to
find their source of nutrition. For instance, parasitic plants can be guided toward host
plant volatiles [66].
However, long-distance communication through volatiles is complicated by the
presence of atmospheric oxidant (e.g., oxygen, ozone, and reactive N and S species)
and temperature that may drastically change or degrade emitted molecules [77, 78].
Neighboring plants can also rapidly perceive root exudates that may provide a
detailed layer of information regarding the competitive environment [79]. Among
root exudates, a growing interest is toward strigolactones that plants exude into the
rhizosphere. These molecules can stimulate interactions with arbuscular mycorrhizal
(AM) fungi and improve plant fitness, but on the other hand, these molecules are
exploited by parasitic plants and rapidly stimulate the germination of their seeds
resulting in parasitization of the host plant [80, 81].
586
S. A. Zebelo