Agriculture Reference
In-Depth Information
few free disaccharides in nature. Both are non-reducing sugars and synthesized by similar
pathways. Contrary to trehalose, sucrose synthesis is mainly limited to photosynthetic
organisms [192], where it holds a central position as the major product of photosynthesis and
as a transport molecule involved in growth, development, storage, signal transduction and
acclimation to environmental stress. Sucrose transport is finally energetically superior to
trehalose transport making it more “preferred” to plants metabolism. It is hence often
suggested that trehalose is evolutionary more ancient than sucrose [192].
As trehalose is present in so low or in undetectable amounts in most plants, it is unlikely that
under natural conditions and with the exception of desiccation tolerant plants, this sugar might
play a role in stress protection in plants [193]. Nevertheless, other roles have been proposed
for trehalose and trehalose-6-phosphate synthase: regulation of plant growth and develop‐
ment; broad spectrum agent preventing symbiosis between susceptible plants and trehalose
producing microorganisms [193-194]; the regulation of carbohydrate metabolism or the
perception of carbohydrate availability [190,194-197]; the regulation of embryo maturation
[197-199]; implication on vegetative growth and transition to flowering [200]; implication on
seedling development [201-202]; and regulation of glucose, abscisic acid and stress signaling
[203-205]. According to [190], trehalose plays several roles in carbohydrate metabolism, with
a number of processes and pathways being affected.
For all that was stated above, trehalose is one of the most studied osmoprotectants and in recent
years there has been a growing interest in trehalose metabolism as a means of engineering
stress tolerance in crop plants [191]. Several experiments have been conducted to obtain
transgenic plants over-expressing genes codifying enzymes of the trehalose biosynthetic
pathway of
E. coli
and
S. cerevisiae
, using both model plants like tobacco (
Nicotiana tabacum
)
and crop plants such as potato (
Solanum tuberosum
), rice (
Oryza sativa
) and more recently
tomato (
Lycopersum esculentum
). Additional, attempts have been made using an alternative
approach: the inhibition of the expression of trehalase gene. Those experiments and their main
results are summarized in Table 1.
The previously mentioned genetic engineering obtained a variable degree of success. Gener‐
ally speaking, transgenic plants were found to have higher tolerance than controls to some
form of water stress imposed, following in most cases, confirmed trehalose accumulation.
Albeit such fact, trehalose engineered plants frequently had altered phenotypes, particularly
dwarfism and leaf abnormalities. Such fact was particularly true for the first transformation
events in which genes of microbial origin were used. Later events, in which endogenous or
plant origin genes were used seem to counter that tendency [217, 218]. Genetic engineering of
plants with trehalose biosynthesis genes seems therefore to be of extreme pertinence to the
increase of abiotic stress tolerance in plants, particularly plants of agricultural importance such
as cereals and legumes.
4.2. Engineering polyamine accumulation
Polyamines (PAs) are small (low-molecular-weight), positively charged, aliphatic amines that
are found in all living organisms. The major forms of PAs are putrescine (Put), spermidine
(Spd) and spermine (Spm), although plants also synthesized a variety of other related com‐
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