Abiotic Stress Responses in Plants: Unraveling the Complexity of Genes and Networks to Survive - Abiotic Stress: Plant Responses and Applications in Agriculture - page 67

In our M. truncatula system, no altered external morphology was observed in adc trans‐
genic plants, that were successfully developed without phenotypic visible alterations and
produced seeds (T 2 generation) [241, 242]. One specific transgenic line (L108) expressing
the heterologous adc transgene had a very high accumulation of Agmatine (22-fold) (the
direct product of the ADC enzyme and intermediate in the Put biosynthesis) and moder‐
ately related increase of Put (1.7-fold) and Spd (1.9-fold) levels, compared to control
plants [242]. These results are consistent with several reports that suggest PAs levels are
under strict homeostatic regulation [227, 243].
Nevertheless, several recent studies have concluded on the feasibility of PA biosynthesis
engineered for the production of stress-tolerant plants. Accumulating experiments and their
main results are summarized in Table 2. The constitutive expression of homologous adc1 and
adc2 in Arabidopsis resulted in freezing and drought tolerance, respectively [244-245]; with a
patent application for “Plant resistance to low-temperature stress and method of production
thereof” by [244]. In another work, transgenic tomato lines transformed with the yeast samdc
fused with a ripening-specific promoter E8, over-accumulate Spd and Spm and, interestingly,
showed phenotypes of agronomical importance such as enhanced phytonutrient content and
fruit quality [246-247]. Polyamine-accumulating transgenic eggplants exhibited increased
tolerance to multiple abiotic stresses (salinity, drought, low and high temperature and heavy-
metal) and also biotic resistance against fungal disease caused by Fusarium oxysporium . These
authors used a construct similar to ours, with the adc gene from oat under the control of the
constitutive CaMV 35S promoter and found that some transgenic eggplants lines showed an
enhanced level of Put, Spd and in some cases also Spd. These lines also showed increase in
ADC and also on the activity of the PA catabolic enzyme, diamine oxidase (DAO) [248].
There are several reports in which the plant response to diverse abiotic stress is associated to
the stimulation of polyamine oxidation [249]. However, the precise role of polyamine catabo‐
lism in the plant response to environmental stress remains elusive [249-250]. Considering these
results, further research concerning the PAs changes and the global response of our M.
truncatula diverse germplasm with altered PA content to multiple stresses should be developed
in the near future.
4.3. Engineering accumulation of photo-protective proteins — ELIPs
To cope with environmental stresses, plants activate a large set of genes, which lead to the
accumulation of specific stress-associated proteins (reviewed in [253]).The stomatal limitation
on photosynthesis imposed by the earlier stages of water deficit (WD) result in a decrease of
primary electron acceptors available for photochemistry [47]. If protection mechanisms are not
activated, the excess of absorbed energy may induce photo-oxidative damage in chloroplast
structures. The nuclear-encoded early-light inducible proteins (ELIPs) may play a relevant role
in the protection mechanisms discussed above.
ELIPs and ELIP-like proteins are pigment-binding components of the thylakoid membrane
widely distributed among plant species and belong to the chlorophyll a/b-binding protein (cab)
family (reviewed in [254, 255]). ELIPs are widely present among different plant species like
pea [256], barley [257], Craterostigma plantagineum [258], Dunaliella bardawil [259], Sporobolus