The Plant Self 105“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityimmunity: (1) the innate type, which is genetically prescribed as a result
of evolutionary history; (2) the acquired type, which is determined by the
individual exposure in the lifetime of an organism. Plants only have innate
immunity, but it is more robust and better developed than that in animals.
Figure 5.5 depicts the two strategies of plant innate immunity. First,
plants possess cell surface proteins called R-proteins (“R” for resistance)
that can recognize molecules on invading microbial cell wall, called “elic-
itors.” The interaction results in a plant response called “PTI,” leading to
signal transduction and gene activation, and ending with the production of
antimicrobial substances. Simultaneously, some infected cells are encour-
aged to undergo cell death in order to contain the site of infection. Over
the course of evolution, many pathogens developed special molecules
Fig. 5.5. Schematic of plant innate immunity. The dotted-line rectangle outlines the
boundary of a plant cell. Microbial pathogens possess special molecules (elicitors) on
their cell wall that can be recognized by the surface proteins of a plant cell (R-proteins).
The interaction elicits a host response called PTI, which leads to the production of anti-
microbial substances and defensive cell destruction. To overcome this, microbial patho-
gens produce a kind of protein called “effectors” to block the PTI response. To override
the block, plant cells also produce a type of protein called NB-LRR. If the NB-LRR
matches the “effector” protein, the ETI response is elicited, which bypasses the PTI
pathway. The two pathways lead to a common defense outcome. PTI, PAMP-triggered
immunity; ETI, effector-triggered immunity; NB-LRR, nucleotide-binding leucine-rich
repeat. [See Note 21.]