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may nullify the negative action of LeEIX1 in signal transduction. These studies
suggest the potential role of BAK1 in signal transduction through the PAMP-
PRR signaling net work.
2.11.10
Glucan—Binding Proteins
2.11.10.1
Glucan-Binding Proteins in Soybean and Medicago truncatula
Many
-glucan components of fungal and oomycete cell walls act as PAMPs
(Shibuya and Minami 2001 ; Silipo et al. 2010 ). A
β
-glucan-binding protein (GBP)
has been identifi ed as a PRR in soybean. It lacks a transmembrane domain and pre-
dominantly localizes to the cytoplasmic face of the plant cell wall (Fliegmann et al.
2004 ). In Medicago truncatula , a high-affi nity
β
-glucan-binding site was character-
ized biochemically. Four full-length clones encoding putative
β
-glucan-binding
proteins from M . truncatula , Mt GBP1, 2, 3, and 4, composing a multigene family
encoding GBP-related proteins have been identifi ed (Leclercq et al. 2008 ). The
GBP has been detected in vesicles at the plasma membrane and in the cytoplasm
and it indicates that GBP may interact with a transmembrane RLK or RLP
(Fliegmann et al. 2004 ).
β
2.11.10.2
Glucan-Binding Protein Contains Two Different Activities:
Releases
β
Glucan from
β
Glucan Polysaccharides and Also
Acts as a Receptor of
β
Glucan Signaling
GBP represents a soluble extracellular binding protein. It is a member of family 81
glycoside hydrolases (Fliegmann et al. 2005 ). GBP has been shown to contain two
different activities. As part of the plasma membrane-localized pathogen receptor
complex, it binds the PAMP
-glucan, triggering the activation of defense responses.
Additionally, the GBP is able to hydrolyze
β
-1,3-glucans present in the cell walls of
potential pathogens (Fliegmann et al. 2005 ). GBP initially acts on oomycete/fungus-
derived heptaglucosides as a glucan hydrolase, releasing
β
-glucans that subse-
quently are perceived by a different domain of GBP (Fliegmann et al. 2004 ).
β
2.11.11
Mannose-Binding Lectin Receptors
Some mannose-binding lectin (MBL) receptor kinases have been identifi ed as
PRRs for perception of PAMPs. Function of MBL appears to be pattern recognition.
MBL recognizes carbohydrate patterns, found on the surface of a large number of
pathogenic microorganisms. MBL binds to carbohydrates (specifi cally mannose
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