1. Discuss and evaluate mechanisms plant scientists can
employ to enhance crop performance under conditions
of abiotic stress
Environmental conditions incurring limitations on plant performance are termed abiotic
stresses (Cramer et al. 2011). Such adverse impacts are of concern in the agricultural
community, being the main cause of crop performance limitation (Jenks & Hasegawa,
2005). Evolved mechanisms allow plants to grow and perform under stress (Thomashow,
1999). Prior genomic and molecular research has attributed this to the stress-induced
expression of specific genes (Jenks & Hasegawa, 2005). This suggestion is reinforced in
Figure 1, a simplified network of signalling pathways associated with abiotic stress
response (Cramer et al. 2011). Studies of transcriptomics, proteomics and metabolomics
have enabled comprehensive analysis of plants to explore how these stress responses
occur (Cramer et al. 2011).
Figure 1: A working model of plant signalling network following response to abiotic stress.
Ovals represent metabolites, proteins or processes (Cramer et al. 2011)
Whilst some desirable crops may already have the genomic characteristics necessary to
tolerate the abiotic stresses of the regions in which they’re cultivated, others won’t.
Having been cultivated there for over two millennia, Chenopodium quinoaWilld tackles
drought in the Andes using a variety of tolerance, avoidance and escape strategies

2. (Jacobsen et al. 2003). Various genomic methods allow stress-induced genes to be
transferred from one species to another (Jenks & Hasegawa, 2005). Such practices are
paramount to global food sustainability, as regions considered low yielding could be
cultivated with genetically modified crops. This would be of particular beneficial application
in regions suffering with poor food sustainability and low economic development. Prior
applications of transgenic plants have already benefitted in the field, both ecologically and
economically, and this review serves to explore existing and further potential (Schmitz &
Schütte. 2000).
The impending threats of global warming and climate change are predicted to adversely
impact crop performance (Bita & Gerats, 2013). The grain, Triticum spp. (wheat), a
fundamental food crop, exhibits optimum performance at ~25°C, but the quantity and
quality of wheat yield is known to decrease substantially >31°C, as the stress reduces grain
filling rate (Curtis, 2004; Ferris et al. 1998). In many plants, heat stress induces the
accumulation of compounds, such as glycine betaine and amino acids. These low
molecular compounds protect important complexes that could become inactivated or
inhibited (Bita & Gerats, 2013; Schmitz & Schütte. 2000). The Arthrobacter globiformis
genome includes a gene that expresses choline oxidase, necessary for the production of
glycine betaine, which can be inserted into the crop genome to give a superior heat
tolerance (Schmitz & Schütte. 2000). Considering this, the chloroplast protein synthesis
elongation factor (EF-Tu) would also be worth investigating for use in similar transgenic
methods, due to its’ accumulation having a high correlation with improved heat tolerance
(Fu et al. 2008).
Plants that are not adapted to cold temperatures exhibit consistency alterations in lipid
membranes, causing loss of compartmentalisation, when exposed (Schmitz & Schütte,
2000). Furthermore, cold exposure raises the instance of frost and ice crystal formation.
Frost can lessen crop yield by up to 40% or cause entire loss (Jacobsen et al. 2003).
Quinoa resists freezing due to a high concentration of solutes, such as sugars and amino
acids, resulting in a decreased freezing temperature (Jacobsen et al. 2003). As well as
proline, glycine betaine is a regular example of such accumulated solutes (Verslues et al.
2006). With this in mind, the same gene responsible for choline esterase expression, as
aforementioned, could also be applied in transgenic practices, as it has been for heat
tolerance. Fortunately, many species are equipped with cold regulator genes (COR genes),
which the transcription factor, CBF1, induces (Thomashow, 1999). Gene transfer has
enabled continuous expression of COR genes, improving membrane stabilisation against
freeze-induced injury, in non-adapted plants, without compromising optimum performance
(Thomashow, 1999). The progression of cold stress and its’ relationship with CBF is
depicted in Figure 1 (Cramer et al. 2011).
Water availability can become a stressor in a variety of circumstances, namely drought,
flood, salinity and freezing. Adequate moisture availability is detrimental during wheat
growth but an excess of water can cause root complications and disease susceptibility
(Curtis, 2004). Flooding can also cause root rot and subsequent yield reduction (Jacobsen
et al. 2003). As agreed in Figure 1, ethylene and abscisic acid (ABA) play important roles in
stress responses brought about by anoxia (Cramer et al. 2011). Flooding depletes soil
oxygen content and a reduction of yield is often attributed to the loss of symbiotic nitrogen
fixation (Collins et al. 2008). Collins et al. (2008) investigated the roles of quantitative trait
loci (QTLs) in heritable variability and their potential manipulation to improve crop

3. performance in the presence of abiotic stressors. Collins et al. highlighted that Oryza sativa
(rice), whilst dependent on traditional flooded rice systems, can be subjected to
submergence stress. Collins et al. also commended Neeraja et al.’s research (2007), in
which submergence tolerance the rice cultivar, Swarna, was improved using marker
assisted back-crossing (MABC) with flood-resistant donors. The region on chromosome 9,
named Sub1, expressed traits of submersion tolerance, which could be incorporated into
Swarna. The three factor genes that controlled the Sub1 phenotype were also found to be
ethylene induced. Since its introgression, the Sub1 QTL has been incorporated into several
other cultivars, appropriate for various flood-prone regions and substantially improving crop
yield (Collins et al. 2008).
When water deficit occurs, growth limitation occurs due to the distorted ability to adjust
water potential, which requires solutes to alter, but such solutes are products of
photosynthesis, which would usually be supplying energy for growth (Cramer et al. 2011).
The majority of crops lack the ability to tolerate desiccation as they cannot recover from a
substantial reduction in water or enter a dormant state (Verslues et al. 2006). Drought
stress often occurs hand in hand with another factor, such as heat or salinity (Collins et al.
2008). Genetic engineering is promising, as several hundred genes involved in drought
response have been identified (Schmitz & Schütte. 2000). The bean, Vigna aconitfolia,
contains the gene pyrroline-5-carboxylsynthetase, essential for the expression of proline,
used in osmotic regulation (Schmitz & Schütte. 2000). Proline is accumulated in the
presence of very low temperatures to prevent water reduction through freezing (Verslues et
al. 2006). Transgenic rice cultivars, having gained this gene, have indicated improved crop
performance in drought and salinity-prone territories (Zhu et al. 1998). Tolerant species of
plants have adapted root architectural traits, for which several relevant QTLs have been
associated (Collins et al. 2008). MACB has also allowed introgression of such alleles for
improve root length development, such as in the rice variety, Kalinga III, indicating that the
utilisation of QTLs in transgenic applications can not only improve crop fitness at a
biochemical level, but also a morphological one (Steele, 2007). Grain yield and root length
has also been connected with ABA concentration in leaves, which has undergone some
investigation (Collins et al. 2008). The increase of ABA levels associated with both drought
and salinity tolerance brings about reversible responses, such as stomatal closure, and the
gene ERA1 is necessary for the signaling pathway resulting in ABA increase (Global
Knowledge Center on Crop Biotechnology (KC), 2008). However, This gene cannot be
continuously expressed in transgenic plants as ABA down-regulation is necessary for the
reverse mechanism and yield is greatly compromised (KC, 2008). The addition of a drought
inducible promoter was successfully used to stimulate ERA1 antisense expression in
transgenic Arabidopsis sp., resulting in controlled drought tolerance without yield sacrifice.
Salinity incurs damage on crops through dehydration, mineral disturbances and salt ion
toxicity (Schmitz & Schütte. 2000). Tolerance is usually manifested in the form of specific
peroxidase activity (Schmitz & Schütte. 2000). Incorporation and overexpression of the
genes responsible for such peroxidases could be transferred to non-tolerant crops. Some
crops, including quinoa, can transfer Na+
ions into vacuoles via the Na+
/H+
antiport protein
to better adjust water potential (Jacobsen et al. 2003; Schmitz & Schütte. 2000). The
incorporation of the responsible gene(s), either through breeding sexually compatible
species or genetic manipulation, could create salt-resistant cultivars of desired crops. The
incorporation of salt tolerance into wheat from wild relatives has previously been achieved
in research funded by the International Maize & Wheat Improvement Centre (KC), 2008). t

4. Despite plants having the ability to accumulate a higher content of reactive oxygen species
(ROS) without incurring damage, there is still potential for adverse impacts on bioenergetics
under the influence of certain abiotic factors, such as high light intensity, as demonstrated
in Figure 1, as it drives the transport of electrons (Cramer et al. 2011; Gill & Tuteja, 2010).
ROS-induced damage includes lipid peroxidation, protein oxidation and DNA damage and it
is established that the improvement of in vivo antioxidant concentration enhances
detoxification mechanisms against ROS (Gill & Tuteja, 2010). The superoxide dismutase
(SOD) enzyme, Mn SOD, was successfully expressed in transgenic Triticum aestivum cv.
Oasis protoplast, increasing tolerance to photooxidative stress and reducing potential
oxidative damage (Melchiorre et al. 2009). Other transgenic advances have produced
plants tolerant to other ROS-instigating stresses in the presence of intense light as well. In
transgenic Nicotiana tabacum cv. Xanthi, the expression of glutathione peroxidase (GPX)
resulted in tolerance to chilling under high light intensity (Yoshimura et al. 2004).
Light energy is harvested by complexes LHCI and LHCII, the absorption of which initiates
photophosphorylation (Baker & Rosenqvist, 2004). It can therefore be assumed that light
deprivation results in a reduction of growth and subsequently crop performance. Baker &
Rosenqvist (2004) suggested that the key to improved crop yield is the detailed evaluation
of performance, which could be reflected in chlorophyll fluorescence. They stated that this
is because a change in chlorophyll fluorescence indicates altered photosynthetic activity,
deduced through the measurement of the electron transports’ operated quantum efficiency
throughout photosystem II (PSII). Plants found to have impaired growth or metabolism
could indicate numerous stressors and allow for superior plant selection. Low light could
also be enhanced with applications of dynamic climate control, such as supplementary
lighting or glass housing, and measuring fluorescence could also ensure effective regimes
and performance (Baker & Rosenqvist, 2004).
At a low pH, oxygen (O2) is dismutated and hydrogen peroxide (H2O2) is inevitably
generated (Gill & Tuteja, 2010). Additionally, an acidic pH (<5) limits crop performance due
to the generation of the Al3+
cation and promoting aluminium (Al) toxicity, which retards root
growth and reduces nutrient uptake (Collins et al. 2008; Schmitz & Schütte 2000). In Al-
tolerant species, citrate accumulation generates complexes with Al that are not absorbable
to the plant (Schmitz & Schütte 2000). Transgenic tobacco and papaya plants have
successfully expressed genes that code for citrate synthetase, having been transferred
from Pseudomonas aeruginosa, and produced up to six times more citric acid than controls
(de la Fuente et al. 1997). ROS-induced damage due to low pH and Al toxicity have been
tolerated in transgenic Arabidopsis species by incorporating genes responsible for
expressing Glutathione S-transferases (GSTs), such as NtPox ParB, sourced from N.
tabacum (Gill & Tuteja, 2010).
Nutrient deficit can be combatted with fertilizer use, but this potentially contributes to
adverse impacts to the environment (eutrophication), therefore, the efficiency at which
crops absorb nutrients can be improved instead (Collins et al. 2008). QTLs containing
enzymes associated with nutrient uptake and metabolism, such as Glu-ammonia ligase, β-
fructofuranosidase and cytosolic GS, are among candidates for transgenic crops with the
aim of increasing productivity (Collins et al. 2008). A potential source for such QTLs could
be Zea mays, as this species tolerates low N availability due to it’s ability to accumulate and
store N, to then remobilize it for grain filling (Collins et al. 2008). A QTL on chromosome 12,
Pup1, has also been identified to assist in Phosphorus (P) uptake efficiency, for which the

5. beneficial allele was successfully expressed in transgenic rice after MABC, resulting in a
large increase of P uptake (Wissuwa et al. 2002).
Considering yield exclusively, genetic potential dictates that maximum yield possible for
wheat could be as much as 20 tonnes per hectare (tonnes/ha), compared to the current
highest attainable, 14 tonnes/ha (Curtis, 2004). It’s often a collection of factors that
characterises a region. For example, the Andes encounters frost, drought, heat, flood and
salinity (Jacobsen et al. 2003). Despite this, some transgenic alterations are of benefit in
the presence of multiple stressors. For example, the gene that expresses choline esterase,
a precursor for glycine betaine, would prove useful for tolerance to drought, freezing, heat
and salinity. Plants prepared for dehydration stress will also fare well in regions that incur
freezing, chilling, heat, water deficit and salinity. Collins et al. (2008) agrees that the future
of improving performance of crops suffering stress relies on the use of QTLs. A growing
understanding of the plant genome has opened potential for transgenic practices and
breeding strategies that improve crop performance in conditions of abiotic stress. There are
still QTLs that have yet to been explored fully, such as those that improve Zinc (Zn) uptake
efficiency (Collins et al. 2008). Further investigation and practical experimentation into the
utilisation of applicable genes will shed more light on potential transgenic crop variants,
serving to provide maximum yield in the presence of most stresses and diminish concerns
of food scarcity.
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