104 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”Plant stress hormones are a group of small organic molecules pro-
duced rapidly in response to injury or infection. Abscisic acid, the best
known of these hormones, promotes tolerance to stress and has been
compared to adrenaline in animals. It is synthesized in all plant parts in
response to biotic and abiotic stress and can be translocated from one
area to another. Other stress hormones include salicylic acid, jasmonate,
and ethylene. Together they promote resistance to microbial and insect
invasion. They amplify the process of gene activation in times of emer-
gency, priming the entire plant to heighten its defense function. Among
the stress hormones, methyl salicylate, methyl jasmonate, and ethylene
are volatile, so they can travel airborne from plant to plant, serving as a
warning signal. Ethylene is unique in that it also has the normal function
of promoting fruit ripening.
“Pathogenesis-related proteins” (PR-proteins) are a group of large
molecules having broad antimicrobial functions. They are produced sys-
tematically to prime the entire plant for defense when only one area
is under attack.^18 Osmotin is a prominent member of the PR-proteins.
It helps plants resist high osmotic pressure, and is also a potent anti-
fungal agent.^19 The osmotin gene can be activated by a wide variety of
factors, including desiccation, NaCl, wounding, ethylene, abscisic acid,
viruses, fungi, and UV light.^20
A sophisticated way for plants to fight viral infection is through RNA
silencing, also called RNA interference. Like animals, plants can cleave
viral double-stranded RNA into small pieces of 21-26 nucleotides long,
called “small interfering RNA,” which are subsequently used to target
and destroy any virally produced RNA of the same species. Spreading of
the silencing signal from cell to cell and through the vasculature imparts
resistance to the whole plant against invasion by the same virus. This
type of antiviral strategy is in common with animals (see next chapter).
5.7 Do Plants have Immunity?
Immunity is the defense of an organism against foreign invasion, based
on molecular recognition of self from non-self. Animals have two kinds of