Abiotic Stress Responses in Plants: Unraveling the Complexity of Genes and Networks to Survive - Abiotic Stress: Plant Responses and Applications in Agriculture - page 61

Presently, numerous genes associated to plant responses to abiotic stress have been identified
and characterized in laboratory studies (reviewed in [157, 162-163]). Engineered overexpres‐
sion of biosynthetic enzymes for osmoprotectants such as glycine betaine [164,165]; stress
induced proteins such as LEA proteins [166-167]; scavengers of reactive oxygen species
[168,169]; transcription factors [170, 171] or signal transduction components [172-173] were
reported. Since stress resistance is a complex trait regulated by several genes acting in a
concerted way during the process, it is not surprising that transgenic approaches using a single
stress-related gene will only lead to marginal stress improvement [174]. One of the major
challenges is the introduction of multiple genes by pyramiding strategies or co-transformation
[175-176].
It is also expected that several areas, such as post-transcriptional regulation involving protein
modification, protein degradation and RNA metabolism will emerge [163]. An example is the
application of miRNAs in the improvement of stress resistance. The discovery of miRNAs
involved in the regulation of stress responses and discovering the potential use of these
miRNAs to modulate or even increase stress resistance in plants is an open field of research
as previously discussed in section 3.2 of this chapter. As an example, Sunkar and co-workers
[110] have generated transgenic Arabidopsis thaliana plants overexpressing a miR398-resistant
form of a plastidic Cu/Zn Super Oxide Dismutase (Cu/Zn-SOD;CSD2) and confirmed that
transgenic plants accumulate more CSD2 mRNA than plants overexpressing a regular CSD2
and are consequently much more tolerant to high light, heavy metals, and other oxidative
stresses. These results suggest that understanding posttranscriptional gene regulation is
important to widen our ability to manipulate stress tolerance in plants and offer an improved
strategy to engineer crop plants with enhanced stress tolerance.
The process of generating transgenic lines requires success in the transformation method and
proper incorporation of stress resistance genes into plants. The most used method to transfer
foreign genes into plant cells and the subsequent regeneration of transgenic plants is based on
the natural system, the Agrobacterium- mediated plant transformation [177]. Particle bombard‐
ment has also been exploited extensively for plant transformation especially in species
recalcitrant to Agrobacterium infection such as maize. The development of new plant transfor‐
mation vectors namely using new-plant associated bacteria (such as from the Rhizobiacea
family) has also proved to be an effective approach to generate transgenic plants from explants/
genotypes unsuitable for Agrobacterium- mediated transformation methodology [178].
The promoters that have been most commonly employed in the production of abiotic stress-
tolerant plants include the cauliflower mosaic virus (CaMV) 35S promoter (mostly used for
dicot crops) and the actin 1 promoter (Act-1) (used for expression of transgenes in monocot
crops) [155]. As these promoters are constitutive, the downstream transgenes are expressed in
all organs and at all stages which is unnecessary as well as demanding on the energy reserves
of the cell [170]. In some cases, constitutive expression of a gene normally only induced by
stress can have negative effects on growth and development when stress is not present
(pleiotropic effects). The use of inducible promoters that allow the expression of a transgene
only when it is required could therefore be the ideal solution [179, 180]. There is a strong need
to obtain an increased array of inducible promoters, which are expressed only when exposed