Agriculture Reference
In-Depth Information
to stress situations, and to pair such promoters with the stress tolerance-related genes in the
adequate cloning vectors [181]. Additional tests need to be performed to guarantee that
obtained stress-inducible promoters work in heterologous plant systems.
Concerning the improvement of stress resistance, the past decade has witnessed the utilization
of transgenic approaches for experimental purposes, mainly in model plant systems but not
in important agricultural species or crops. Nevertheless, the creation of stress-tolerant crops
either by genetic engineering or through conventional breeding has covered almost all aspects
of plant science, and is pursued by both public and private sector researchers [161]. One of the
major goals of transgenic technology is to produce plants not only able to survive stress, but
also capable to grow under adverse conditions with substantial biomass production, thus
overcoming the negative correlation between drought resistance traits and productivity, which
was often present in past breeding programs [155, 182]. In the case of crop plants, it is ultimately
the yield of genetically altered plants under specific field conditions that will determine
whether or not a specific gene, or metabolic or signaling pathway, is of technologic importance
[3]. One successful case in releasing tolerant plants to abiotic stresses is the transgenic maize
line resistant to drought developed by the Monsanto company. This maize line (MON87460)
was recently approved in the USA and is able to growth in soils with reduced water content
due to the presence of a cold shock protein -CSPB- from Bacillus subtilus [183].
During the last decade, our group has engineer model species like tobacco and Medicago
truncatula with improved abiotic stress traits (drought and salinity), using different stress
related genes.
4.1. Engineering trehalose accumulation
Trehalose is a disaccharide, containing two glucose molecules. Trehalose was first discovered
in 1832 from the Ergot of rye [184-186] and since then isolated from numerous organisms,
including algae, fungi, bacteria, insects and crustaceans. Trehalose is nevertheless considered
non-occurring in measurable amounts in plants, with the exception of a few species [184],
notably the so called “resurrection plants”, able of surviving the loss of most of their water
content until a quiescent stage is achieved and upon watering rapidly revive and restored to
their former state [187].
Trehalose can be synthesized by three different pathways [188] and the most frequent in nature
involves the enzyme trehalose-6-phosphate synthase (TPS; EC 2.4.1.15) that catalyzes the
transfer of glucose from UDP-glucose to glucose-6-phosphate to produce trehalose-6-phos‐
phate plus UDP. Another enzyme, trehalose-6-phosphate phosphatase (TPP; EC 3.1.3.12)
converts trehalose-6-phosphate to free trehalose [184, 186, 189, 190]. Genes codifying both
enzymes have been isolated in several species including Sacharomyces cerevisiae and Escherichia
coli and several plant species such as Arabidopsis and rice [191]. Trehalose may be degraded
by the enzyme trehalase (EC 3.2.1.28) [186, 191].
In living organisms, several functional properties have been proposed for trehalose: energy
and carbon reserve, protection from dehydration, protection against heat, protection from
damage by oxygen radicals and protection from cold [186]. As trehalose, sucrose is one of the
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