Agriculture Reference
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compatible osmolytes such as, proline, mannitol, sorbitol, and glycine betaine. The latter also
acts as an antioxidant and thus detoxifies reactive oxygen species (ROS) [227]. Ionic toxicity,
osmotic stress, and nutritional defects under salinity lead to metabolic imbalances and
oxidative stress. From a practical point, salt stress can be imposed more easily and precisely
in laboratory settings. Although the importance of salt and drought stress signaling was
recognized long ago, few molecular components were known until recently.
Also, drought, salinity, extreme temperatures and oxidative stress are often interconnected,
and may induce similar cellular damage. For example, drought and/or salinization are
manifested primarily as osmotic stress, resulting in the disruption of homeostasis and ion
distribution in the cell [5-6]. Oxidative stress, which frequently accompanies high temperature,
salinity, or drought stress, may cause denaturation of functional and structural proteins [7].
As a consequence, these diverse environmental stresses often activate similar cell signaling
pathways [8] and cellular responses, such as production of stress proteins, up-regulation of
anti-oxidants and accumulation of compatible solutes [9-11]. Compatible solutes are small
organic metabolites that are very soluble in water and are non-toxic at high concentrations.
Therefore, breeding for drought and salinity stress tolerance in agronomy and horticultural
crops (for food supply) and in forest trees (a central component of the global ecosystem) should
be given high research priority. Molecular control mechanisms for abiotic stress tolerance are
based on the activation and regulation of specific stress related genes. These genes are involved
in the whole sequence of stress responses, such as signaling, transcriptional control, protection
of membranes and proteins, and free-radical and toxic-compound scavenging. Recently,
research into the molecular mechanisms of stress responses has started to bear fruit and, in
parallel, genetic modification of stress tolerance has also shown promising results that may
ultimately apply to agriculturally and ecologically important plants [180].
Persian walnut (
Juglans regia
L.) is one of the most economically valuable tree species of
northwest, northeast and central regions of Iran. Natural distribution of this species is quite
sensitive to site water status [213]. Walnut trees need large amounts of water for optimum
growth and productivity and are among the more sensitive plants to abiotic stresses [14]. The
majority of walnut trees in the world are propagated by seed or by grafting onto seedling
rootstocks [101]. Hence, there is huge genetic diversity among rootstock traits. For example,
there are many old Persian walnut trees in Iran that have been planted on the banks of rivers.
The survival of these trees for hundreds years may indicate possession of valuable stress
resistance genes that help them cope with unfavorable environmental conditions [101].
Finding genetic resources tolerant to abiotic stress at different growth stages is important
for such arid and semiarid regions. In studies of fruit trees, half-sib progeny of several
species (for example, 'Serr' walnut, 'Texas' almond, 'Lovell' and 'Missouri' peach) have
been used for producing rootstocks [180]. Because half-sibs are individuals that have one
parent in common and differ in the other parent, the mean genotypic value of the group
of half-sibs is by definition half the breeding value of the common parent. This reduces
the number of seedlings of half-sib families needed as replicates for studying tolerance
genes in rootstock breeding programs [213; 219].
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