G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 184

suggesting a requirement for OsRac1 for the kinase activity response. In rice cell
cultures containing the G α mutation, the MAPK protein level was very much

reduced, indicating that G α is also required for the kinase activity response

(Lieberherr et al. 2005 ).
The Arabidopsis extra-large heterotrimeric G-protein XLG2 overexpression
lines showed constitutive accumulation of transcripts from AtMPK3 (Zhu et al.
2009 ). MPK3 is involved in MAPK signaling cascade (Mészáros et al. 2006 ). A
protein-protein interaction of ) with a

MAPK (PsMPK3) has been reported in Pisum sativum (Bhardwaj et al. 2011 ). The
transcription of these two genes also showed co-regulation under abscisic acid
(ABA) and methyl jasmonate treatments.

β -subunit of heterotrimeric G-proteins (PsG β

-subunit of G-proteins from rice also
showed interaction with PsMPK3, suggesting that the

β

-subunit from a heterolo-
gous system also shows interaction with MPK3. MPK3 may function as an effector
molecule for G

β

subunit of heterotrimeric G-proteins from Pisum sativum
(Bhardwaj et al. 2011 ).

β

3.15 G-Proteins Induce Biosynthesis of Polyamines

Which Act as Second Messengers Triggering
Early Signaling Events

Polyamines are polycationic, ubiquitous aliphatic amines that occur in all plant cells
(Tun et al. 2006 ; Nambeesan et al. 2010 , 2012 ). The diamine putrescine and the
polyamines spermidine and spermine are involved in activation of immune signal-
ing. They play important role as second messengers in immune response signaling
(Walters 2000 ; Takahashi et al. 2003b , 2004 ; Walters 2003 ; Tun et al. 2006 ; Ozawa
et al. 2009 , 2010 ; Szepesi et al. 2011 ). The polyamine biosynthesis is activated dur-
ing pathogenesis (Marini et al. 2001 ; Gardiner et al. 2010 ). Tobacco mosaic virus
(TMV) infection resulted in increased concentration of the putrescine and spermi-
dine in tobacco leaves (Torrigiani et al. 1997 ). The polyamines also may accumulate
during the defense response (Mo and Pus 2002 ; Walters et al. 2002 ; Fujiwara et al.
2006 ). Sugar beet plants treated with methyl jasmonate showed increased resistance
against Beet mosaic virus (BtMV) and the increased resistance was associated with
increased accumulation of polyamines (Haggag et al. 2010 ). Increased levels of
putrescine, spermidine and spermine were observed in barley leaves inoculated with
the powdery mildew fungus Blumeria graminis f. sp. hordei (Cowley and Walters
2002a , b ). Marini et al. ( 2001 ) showed that hypersensitive response to TMV infec-
tion was accompanied by increase in activities of polyamine biosynthetic enzymes.
The polyamines have also been reported to trigger defense responses (Yamakawa
et al. 1998 ; Walters 2003 ). Polyamines have been shown to be involved in resistance
against Ascochyta rabiei in chickpea (Angelini et al. 1993 ). Tobacco leaves infected
with TMV showed accumulation of spermine and the accumulated spermine induced
both acidic PR proteins and resistance to TMV via a salicylic acid-independent path-
way (Yamakawa et al. 1998 ; Hiraga et al. 2000 ). Polyamines may also be involved in