PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 51

2.11.6 CEBiP, the Second PRR for the PAMP Chitin

Another PRR protein for the perception of chitin has been recognized in rice and
designated chitin elicitor-binding protein (CEBiP) (Kaku et al. 2006 ; Shinya et al.
2010 ). It is a receptor-like protein (RLP), unlike the PRR OsCERK1, which is a
receptor-like kinase (RLK). CEBiP contains extracellular LysM motifs for chitin-
binding but lacks an intracellular kinase domain, which is characteristically present
in OsCERK1. CEBiP is a transmembrane protein with two extracellular LysM
domains and a short cytoplasmic tail. It is predicted to have the two LysM domains
in its extracellular part and a single transmembrane domain. It directly binds the
fungal PAMP chitin (Kaku et al. 2006 ).
CEBiP specifi cally binds the chitin oligosaccharide. Its knock-down transfor-
mants exhibited the suppression of chitin-induced defense responses, suggesting
that CEBiP functions as receptor for the PAMP (Kaku et al. 2006 ). RNAi experiment
showed that CEBiP is required for chitin-induced defenses in rice (Kaku et al.
2006 ). CEBiP functions as a cell surface receptor for chitin elicitor in rice (Miya
et al. 2007 ). The predicted structure of CEBiP does not contain any intracellular
domains, suggesting that an additional component(s) is required for signaling
through the plasma membrane into the cytoplasm (Miya et al. 2007 ). Although
CeBiP possesses two LysM domains and a transmembrane region, it does not have
any domain that could function as a signal transduction module (Kaku et al. 2006 ).
Since CEBiP lacks a signifi cant intracellular domain, it likely is only a part of the
chitin receptor complex in rice (Kaku et al. 2006 ). An obvious partner for CEBiP
would be a membrane-associated receptor-like kinase (Wan et al. 2008a ).
Several studies have indicated that rice requires both the types of plasma mem-
brane PRRs CEBiP and OsCERK1 for chitin signaling. The extracellular domain
of OsCERK1 can interact with that of CEBiP. It appears that CEBiP plays a major
role in chitin elicitor binding and that OsCERK1 functions as a signal transducer
through its Ser/Thr kinase activity in rice (Shimizu et al. 2010 ). In the absence of
chitin oligosaccharides, CEBiP and OsCERK1 mostly exist separately from each
other, although a major portion of CEBiP appears to exist as homo-oligomers.
CERK1 may form a heterodimer with CEBiP to bind chitin (Shimizu et al. 2010 ).

2.11.7 NbLRK1, the PRR for the PAMP INF1 Elicitin

The PRR for the PAMP elicitin INF1 of Phytophthora infestans has been identifi ed
as a lectin-like receptor kinase and it was designated NbLRK1. NbLRK1 is a typical
RD kinase (Kanzaki et al. 2008 ). The gene encoding this receptor, NbLRK1 , has
been isolated from Nicotiana benthamiana (Kanzaki et al. 2008 ). The structure of
lectin-like receptor kinases (LRKs) is similar to other plant receptor-like kinases
with an N-terminal targeting signal, an extracellular domain, a single transmem-
brane (TM) spanning helix, and a highly conserved cytosolic kinase domain. The