Dealing with Environmental Stresses: Role of Polyamines in Stress Responses - Crop Improvement: New Approaches and Modern Techniques - page 457

Catabolism of Polyamines
PA catabolism is an important process that regulates the intra-cellular levels of
PA. PAs are oxidatively catabolised by copper-binding diamine oxidases (CuAO)/
diamine oxidases (DAO) and FAD-binding polyamine oxidases (PAO) which are
commonly found in all living organisms (Buchanan et al. 2000 ; Bagni and Tassoni
2001 ; Cona et al. 2006 ). Plant CuAO/DAO preferentially catalyses the oxidation of
di-amine put, at the primary amine group producing 4-aminobutanal with concomi-
tant production of NH 3 and H 2 O 2 . Resulting 4-aminobutanal is further converted
to γ-aminobutyric acid (GABA) via Δ 1 -pyrroline. PAOs preferentially catalyse the
oxidation of spd and spm at the secondary amine group producing 4-aminobutanal
and N -(3-aminopropyl)-4 aminobutanal, respectively, with concomitant production
of 1,3-diaminopropane (DAP) and H 2 O 2 (Buchanan et al. 2000 ; Cona et al. 2006 ;
Moschou et al. 2008b , c ) (Fig. 17.2 ). Experimental evidences indicate that CuAO/
DAO and PAO have important regulatory function in plant growth and in stress tol-
erance by modulating the levels of PAs and their reaction products. They are impor-
tant regulators of ROS and GABA synthesis, which are key metabolites involved
in various physiological processes. Both CuAO/DAO and PAO have species and
tissue specific regulatory functions and sptio-temporal expression patterns (Cona
et al. 2006 ). CuAO/DAO encoding genes have been isolated and characterized from
some plants for example, from Arabidopsis (Moller and McPherson 1998 ), chick
pea (Rea et al. 1998 ) and pea (Tipping and McPherson 1995 ). PAOs are identified
from many plant species particularly in monocots (Sebela et al. 2001 ). Gene family
of PAO from maize which consist of three members and proteins are bound to cell
walls (Tavladoraki et al. 1998 ) and symplast localized barley PAO family consisting
of two genes (Cervelli et al. 2001 ) are widely been studied. In Arabidopsis, twelve
CuAO/DAO and five PAO coding genes have been identified and characterized.
The proteins are localized in apoplast and in peroxisomes (Alcázar et al. 2006b ;
Cona et al. 2006 ; Tavladoraki et al. 2006 ). Studies on gain and loss of function mu-
tants especially in Arabidopsis have provided new insights into molecular mecha-
nisms of PA function and PA interaction with other signal transduction pathways.
These findings with special emphasis on abiotic stress tolerance are discussed in
detail in the following sections.
Abiotic Stresses
Experimental evidences indicate that differential regulation of plant PA biosynthe-
sis, conjugation and catabolism are important in dealing with stress tolerance in
response to a variety of environmental stresses ranging from drought, salinity, tem-
perature extremes, mineral deficiency to wounding (reviewed in Cona et al. 2006 ;
Groppa and Benavides 2008 ; Moschou et al. 2008b ; Alcázar et al. 2010a ; Angelini
et al. 2010 ; Wimalasekara and Scherer 2010 ; Wimalasekara et al. 2011a ). The bal-
ance between PA catabolism and anabolism is shown to play a significant role in