Dealing with Environmental Stresses: Role of Polyamines in Stress Responses - Crop Improvement: New Approaches and Modern Techniques - page 454

of PAs in abiotic stress responses in plants and possible means of manipulating PAs
in the crop plants for enhanced stress tolerance.
Introduction
The impact of extreme climatic conditions such as atmospheric warming, desertifi-
cation and soil salinisation on agriculture is growing, causing substantial crop loss
worldwide. Climate change also modifies the risks of pest and pathogen outbreak,
negatively affecting crop productivity. Devastating climate coupled with population
growth exerts constant pressure on crop production, demanding more attention to
find ways and means of crop adaptability to challenging environmental conditions.
In order to secure the food requirements in changing climate scenario new technolo-
gies, efficient and sustainable farming practices have to be taken into consideration.
Plants are continuously being exposed to abiotic stress conditions such as
drought, heat, chilling, freezing and high salinity and biotic stresses like pathogen
invasion, insect predations and weeds. Understanding the mechanisms that trigger
stress damage and adaptive machinery of plants to various stress situations is of
prime importance in the progress of agriculture industry. Adaptation and acclima-
tion to stresses is a result of a combination of events occurring at the anatomical
and morphological levels to the cellular, biochemical and molecular levels. Signal
transduction pathways that link the perception of stress signals with the appropri-
ate cellular responses leading to stress tolerance are extensively studied (Taiz and
Zeiger 2006 ) and profound knowledge in all these aspects is needed in production
of plants with improved stress tolerance.
Among array of components involved, polyamines (PAs) are one of the compo-
nents that play significant functions in plant stress responses. PAs are small organic
cations containing two or more amino groups and known to be essential growth
regulators present ubiquitously in both prokaryotic and eukaryotic cells (Evans and
Malmberg 1989 ; Buchanan et al. 2000 ; Martin-Tanguy 2001 ). The most abundant
free PAs in plants are di-amine putrescine (put), tri-amine spermidine (spd) and tetra-
amine spermine (spm). PAs occur as free form or as conjugated forms attached to
proteins, nucleic acids, hydroxycinnamic acid forming phenol amides, anionic com-
ponents of phospholipids and cell wall components such as pectic polysaccharides
(Buchanan et al. 2000 ; Kakkar et al. 2000 ; Martin-Tanguy 2001 ; Kakkar and Sawh-
ney 2002 ; Takahashi and Kakehi 2010 ). The equilibrium between free and conju-
gated forms of PAs is crucial in development and their titer fluctuates in response
to environmental conditions (Torrigiani et al. 1987 ; Galston and Sawhney 1990 ). In
plants the PA content varies from micromolar to more than milimolar amounts.
The functions of PAs in growth and development (Kakkar et al. 2000 ; Martin-
Tanguy 2001 ; Kakkar and Sawhney 2002 ; Kusano et al. 2008 ; Hussain et al. 2011 )
and in biotic and abiotic stress responses (Walter 2003 ; Alcázar et al. 2006b , 2010a ;
Groppa and Benavides 2008 ; Hussain et al. 2011 ; Marco et al. 2011 ; Nambeesan