Introduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 1

Chapter 1
Introduction
Abstract Innate immunity is the fi rst line of defense against invading microorganisms
in plants. Pathogen-associated molecular patterns (PAMPs) are the classical acti-
vators of immune responses. These are alarm signal molecules are perceived as
'nonself' by plant pattern recognition receptors (PRRs) to switch on the plant
immune responses. PAMPs are not only detected in pathogens, but also detected in
nonpathogens and even in saprophytes. The PAMPs are often called as microbe-
associated molecular patterns (MAMPs). MAMPs are molecular signatures typical
of whole classes of microbes and their recognition by PRRs activates the plant
innate immunity. Most of the PRRs are receptor-like kinases (RLKs) and RLKs are
proteins with a “receptor” and a “signaling domain” in one molecule. The extracel-
lular domains of RLKs bind directly to legands to perceive extracellular signals,
whereas the cytoplasmic kinase domains transduce these signals into the cell. PRRs
interact with additional transmembrane proteins which act as “signaling amplifi -
ers”. PAMPs induce autophosphorylation of the kinase domain of PRRs and the
autophosphorylated PRRs are translocated to endosomes. The biogenesis of trans-
membrane PRRs occurs through endoplasmic reticulum (ER) with the aid of
ER-resident chaperones. The PRR in ER is transported from ER to plasma mem-
brane and N -glycosylation of PRRs is required for the transport of PRRs. Second
messengers deliver the information generated by the PAMP/PRR signaling complex
to the proteins which decode/interpret signals to initiate defense gene expression.
Calcium ion is a ubiquitous intracellular second messenger involved in various
defense signaling pathways. Ca 2+ is a master regulator of gene expression in plants.
Calcium signatures are recognized by calcium sensors to transduce calcium-
mediated signals into downstream events. Guanosine triphosphate (GTP)-binding
proteins (G-proteins) act as molecular switches in signal transduction system.
Mitogen-activated protein kinase (MAPK) cascades transduce extracellular stimuli
into intracellular responses in plants. Reactive oxygen species is a second messen-
ger in transmitting the PAMP signal. Nitric oxide (NO) is a diffusible second
messenger acting in cellular signal transduction through stimulus-coupled
S-nitrosylation of cysteine residues. The plant hormones salicylic acid, jasmonate,