Dealing with Environmental Stresses: Role of Polyamines in Stress Responses - Crop Improvement: New Approaches and Modern Techniques - page 462

water stress tolerance was attributed to PA-mediated regulation of inward K + chan-
nel in the plasma membrane of guard cells and modulation of stomatal aperture.
Further, these findings indicate that PAs target a putative guard cell K + channel
KAT1-like inward K + channels in guard cells and modulate stomatal movements,
providing evidence for presence of link among stress conditions, polyamine levels,
and stomatal regulation (Lie et al. 2000 ). When dehydrated in vitro grown Citrus
reticulata plants were treated with exogenous spm, stomatal closure was promoted
and exhibited a less wilted phenotype, reduction of water loss and electrolyte leak-
age compared to spm untreated plants (Shi et al. 2010 ), again indicating a function
of PA in stomatal regulation.
The importance of PA catabolism in water stress responses is described in many
plant species (reviewed in Cona et al. 2006 ; Moschou et al. 2008b ; Angelini et al.
2010 ). Osmotic stress tolerance of rape leaf discs and whole rape seedlings sub-
jected to drought is linked to increased amounts of put and DAP which is a product
of PAO activity (Aziz et al. 1997 ). Experimental evidence of Toumi et al. ( 2010 )
indicated that drought-tolerant grapevine has increased PAO activity as compared
to drought-sensitive variety. H 2 O 2 produced by PA oxidation reaction was involved
in signalling cascade of drought tolerance (Toumi et al. 2010 ). DAO/CuAO and
PAO are considered as important controllers of ABA signalling pathway in stomatal
regulation (Lie et al. 2000 ; An et al. 2008 ; Moschou et al. 2008a , b ). In Arabidop-
sis , induction of AtPAO2, AtPAO3 and AtPAO4 by ABA suggests a role of PAO in
ABA signalling (Moschou et al. 2008a ). Gene expression analysis demonstrated
that PAO2 of Arabidopsis is upregulated during drought stress and shows similar
expression kinetics as the ABA-inducible RD29A and RD22 genes supporting a role
of PAO2 in drought resistance (Alcázar et al. 2011 ). An et al. ( 2008 ) showed that
exogenous application of ABA to the Vicia faba leaf epidermis caused stimulation
of apoplast CuAO activity followed by increased H 2 O 2 production finally leading
to induced stomatal closure. When CuAO inhibitors were applied these processes
were impaired. These factors indicate that CuAO in V. faba guard cells is an es-
sential enzymatic source for H 2 O 2 production in ABA-induced stomatal closure.
For the ABA-induced stomatal closure, second messengers such as Ca 2 + , ROS,
and nitric oxide (NO) are also important. CuAO/DAO and PAO may be important
in regulating the other signalling substances probably via generation of H 2 O 2 in
stress situations like water stress. Involvement of DAO in the ABA induced H 2 O 2
production in roots of rice seedling is reported (Lin and Kao 2001 ). Biosynthe-
sis of H 2 O 2 by ABA-stimulated CuAO activity resulted in root growth inhibition
by cell wall stiffening by peroxidases (Lin and Kao 2001 ). Involvement of a
copper amine oxidase gene, COPPER AMINE OXIDASE1 ( CuAO1 ) of Arabidop-
sis was tested for its role in ABA mediated stress responses using the knockouts
cuao1-1 and cuao1-2 (Wimalasekara et al. 2011b ). Compared to WT, the knock-
outs showed less sensitivity to ABA during germination, seedling establishment
and root growth inhibition characterizing knockouts as ABA-insensitive. Fur-
ther, PA-induced and ABA-induced NO production in cuao1-1 and cuao1-2 were
impaired suggesting a function of CuAO1 in PA and ABA-mediated NO production
(Wimalasekara et al. 2011b ).