G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 185

Rice G-protein OsRac1

S-adenosyl-
methionine

Arginine

Arginine

Arginase

S-adenosyl-
methionine
decarboxylase

Arginine
decarboxylase

Ornithine

Ornithine
decarboxylase

Decarboxylated
S-adenosyl-
methionine

Putrescine

Spermidine synthase

Spermidine

Spermine synthase

Spermine

Fig. 3.8 Small G-protein OsRac1-mediated polyamine biosynthesis in rice cells (Adapted from
Fujiwara et al. 2006 )

conferring susceptibility to some pathogens. Polyamines induced susceptibility to
the necrotrophic pathogen Botrytis cinerea in tomato (Nambeesan et al. 2012 ).
The starting point for polyamine biosynthesis is the basic amino acids ornithine
and arginine, which are decarboxylated by ornithine decarboxylase (ODC) and argi-
nine decarboxylase (ADC), respectively (Walters 2003 ). Decarboxylation of orni-
thine by ODC or arginine by ADC leads to the synthesis of putrescine, which is
converted to spermidine by spermidine synthase. Spermidine, in turn, is then con-
verted to spermine by spermine synthase (Nambeesan et al. 2012 ). Fujiwara et al.
( 2006 ) showed the induction of the two key enzymes in biosynthesis of polyamines,
arginase and spermidine synthase, in rice cells by OsRac1, besides induction of
S-adenosylmethionine decarboxylase. Arginase produces ornithine which is trans-
formed into putrescine via ornithine decarboxylase, while spermidine synthase con-
verts putrescine to spermidine. S-adenosylmethionine decarboxylase is involved in
decarboxylation of S-adenosyl methionine. In these reactions, both sperimidine
synthase and spermine enzymes use aminopropyl residues derived from decarbox-
ylated S-adenosyl-methionine (Fig. 3.8 ; Fujiwara et al. 2006 ; Kresge et al. 2007 ;
Nambeesan et al. 2012 ).