Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

The Plant Self 101

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

to drought. However, plant B, which is not drought-stressed, also closes
its stomata, suggesting that the stress cues put out from the roots are car-
ried from one plant to the next (airborne transmission is excluded in this
setup). What is of interest is that, after some delay, plant C also responds
to the drought signal, providing evidence that root-mediated messages,
like the airborne counterparts, can be serially passed on.^10
Root exudation is one of the mechanisms of root-root communica -
tion. Exudates act within a few millimeters of soil space called
“rhizosphere”. The rhizosphere represents a highly dynamic front for
interaction not only between plants but also between plants and soil
microbes and parasites. Root exudation constitutes a significant portion
of photosynthetic product and is of considerable carbon cost to the plant.
The exudates cover a diverse array of chemicals ranging from small mol-
ecules (such as amino acids, organic acids, sugars, phenolics, and other
primary and secondary metabolites) to high molecular weight compounds
(polysaccharides and proteins). Whether an exudate is beneficial or harm-
ful to the recipient depends on how the message is interpreted. Some
exudates are growth facilitators, supporting the growth of other plants.
Others induce defense response, either by reducing the susceptibility to
pathogens, or by causing the release of volatiles that attract predators
of plant-eating insects. (In  this sense a parallelism exists between the
leaf-warning system above ground and the root-warning system below
ground.)^11 When an exudate is toxic to the neighbors, the phenomenon
is called allelopathy. The toxic substance mediates competition among
plant species. For example, conifers release a mono-terpenoid called
alpha-pinene, which inhibits cell proliferation in plants of other species.^12
Other phytotoxins are anti- microbial and insecticides. Ironically, some
plants exhibit auto-inhibition by secreting toxins that delay seed germina-
tion and growth of seedlings of the same species. It is believed that auto-
inhibition is a way for population control in densely populated areas.^11 In
addition, root exudates also mediate kin recognition among plants.^13
Roots evolve mutualistic relationships with certain species of bac-
teria (rhizobacteria) and fungi (mycorrhizae). Nitrogen-fixing bacteria