PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 53

that plant recognition of capsicein occurs in the extracellular and intracellular
spaces (Kim et al. 2010 ). NgRLK1 is a new type of plant RLK that recognizes
capsicein (Kim et al. 2010 ).

2.11.9 LeEIX1 and LeEIX2, the PRRs for the PAMP

EIX Proteins

2.11.9.1 Two Different EIX Receptors Exist for Perception of the PAMP

EIX Signals

Two PRRs have been identifi ed in tomato for the perception of the fungal PAMP
EIX (Ethylene-Inducing Xylanase elicitor). These include LeEIX1 and LeEIX2,
which contain a leucine zipper, an extracellular LRR domain, a transmembrane
domain, and a C-terminal domain with an endocytosis signal (Ron and Avni
2004 ). These PRRs have been identifi ed as cell-surface receptors without kinase
domain in plants (Ron and Avni 2004 ; Kaku et al. 2006 ). The two EIX proteins
are highly similar to each other and have extracellular domains of 31 LRRs. The
structure of these EIX receptors is similar to a family of receptor-like proteins
(RLPs) (He et al. 2007 ). These EIX receptors belong to a superclade of leucine-
rich repeat receptor-like proteins with a signal for receptor-mediated endocyto-
sis, which was shown to be essential for proper induction of defense responses
(Bar et al. 2010 ). LeEIX2 contains the conserved endocytosis signal Yxx

within the short cytoplasmic domain, and mutation in this endocytosis motif
resulted in abolishment of hypersensitive response (HR) induction in response
to the PAMP EIX, suggesting that endocytosis plays a key role in mediating the
signal generated by EIX that leads to hypersensitive response induction (Ron
and Avni 2004 ).

Φ

2.11.9.2 LeEIX2 Transmits EIX-Induced Signals, Whereas LeEIX1

Attenuates EIX-Signaling of LeEIX2

The tomato PRRs LeEIX1 and LeEIX2 appear to act in distinctly different ways
in the PAMP-PRR signaling complex. It has been observed that both the PRRs
are able to bind the PAMP EIX, but only LeEIX2is involved in triggering defense
responses (Bar et al. 2011 ). LeEIX1 heterodimerizes with LeEIX2 upon applica-
tion of the EIX elicitor. LeEIX1 attenuates EIX-induced internalization and
signaling of the LeEIX2 receptor. The brassinosteroid co-receptor, BAK1, binds
LeEIX1 but not LeEIX2. In BAK1-silenced plants, LeEIX2 was no longer able
to attenuate plant responses to EIX, indicating that BAK1 is required for this
attenuation. It is suggested that LeEIX1 functions as a decoy receptor for LeEIX2,
a function which requires BAK1 (Bar et al. 2010 ). For effective signal transduc-
tion, the effective PRR LeEIX2 requires action of the co-receptor BAK1, which