Transcription of resistance in the transgenic lines was

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Transcription factors are of importance for plants to adjust
gene transcription under environmental changes (Chen and Zhu, 2004).
The MYB gene superfamily has versatile functions in plants (Dubos et al. 2010). R2R3-MYB proteins are known to
be involved in plant defense and abiotic stress responses (Abe et al. 2003; Mengiste
et al. 2003; Rahaie et al. 2010). Isolation
of an Aeluropus-specific MYB gene responsive to a range of abiotic
stresses contributes to a better understanding of regulatory mechanisms of MYB genes in response to environmental
changes in cereals and grasses. In this
study, a full-length genomic AlMYB gene was isolated from halophyte
grass Aeluropus littoralis root tissues subjected to a salt stress. The deduced AlMYB protein contains typical R2
and R3 domains of the R2R3-MYB subfamily. Based on phylogenetic analysis, AlMYB
appears to be a novel member of the plant R2R3-MYB subfamily. Transient expression
of a GFP-AlMYB fusion protein in onion
epidermal cells proved that AlMYB is
located on
the nucleus. We have shown that AlMYB gene was induced by salt, drought,
cold and heat stresses. In the model plants, Arabidopsis
and rice, a number of MYB genes were characterized to function as
key factors in the signaling pathways for plant resistance to abiotic stresses
(Vannini et al. 2004; Dai et al. 2007; Dubos et
al., 2010). The expression patterns of 60
wheat MYB genes in response to abiotic stress conditions have been
recently determined. Thirty-two genes that responded to different stress treatments
were discovered, of which 20 genes responded to multiple stress treatments,
indicating that they were major factors involving cross-talk among different
signal transduction pathways in response to abiotic stresses (Zhang et al. 2012a). However, multiple
wheat MYB genes appeared to participate in responding to one stress stimulus,
suggesting that there are multiple signaling pathways implicated in the
response to abiotic stress treatment (Zhang et
al. 2012a). To further analyze the
function of AlMYB in planta, transgenic
tobacco plants expressing AlMYB were generated by the Agrobacterium-mediated
transformation method and were molecularly characterized. The enhanced level of
resistance in the transgenic lines was associated with accumulation of AlMYB
transcript. Under greenhouse conditions, AlMYB
transgenic tobacco exposed to continuous
salt or drought stress showed a higher level
of tolerance. Alltogether the results suggested an
important role of AlMYB in regulating drought or salt stress response and
that AlMYB was involved in the tolerance to both
stresses. The results obtained on RWC of detached leaves suggest that the
constitutive expression of AlMYB gene in plant tissues allows a
maintenance of water uptake under severe deficit. The mechanism by which AlMYB confers
tolerance in plants subjected to salt and
drought stresses and the effect of
overexpression in tobacco needs further investigation. The overexpression of some MYB genes
increased tolerance of transgenic plants to biotic or abiotic stresses (Feller et al. 2011; Ma et al. 2009; Dubos et al. 2010; Zhang
et al. 2012). In addition, AlMYB conferred leaf tissue tolerance to H2O2-induced
oxidative stress. Antioxidant metabolism and oxidative stress caused by ROS
constitute a major component of temperature stress in plants although low
temperature signals appear to be also transduced by non-overlapping and
independent pathway components at the level of perception and signal
transduction (Sung et al. 2003). The
present results suggest that some of the observed stress tolerance in AlMYB lines
might be due to an improved intracellular control of ROS. This may convey a general
protection against intra-cellular stress damage and thus help maintain many
vital processes involved in energy metabolism, development and organ expansion.

The present results establish that AlMYB,
when expressed in tobacco, conveys adaptive responses to a wide range of
abiotic stresses, thereby stimulating the expression of other genes known to contribute
to stress responses. Over-expression of a stress response gene in homologous
vs. heterologous systems is not always effective for the same types of
stresses. For example, constitutive over-expression of Arabidopsis DREB1A resulted
in improved tolerance to drought, salinity and freezing but caused severe
growth retardation under control conditions (Kasuga et
al. 1999). Engineering inducible expression of DREB genes
therefore appeared to be a more promising strategy than engineering
constitutive expression in plants. Expression of the same gene in rice improved
tolerance to drought and salinity but to a very little extent to chilling (Oh et al. 2005). We
have shown that the steady state level of transcripts of some stress associated
genes encoding proteins involved in anti-oxidative and protection activities
are higher in unstressed AlMYB tobacco than in WT plants. It is well known
that antioxidant enzyme activity is increased in plants in response to various
environmental and chemical stresses (Baek et
al. 2006). ‘Late Embryogenesis Abundant’ or
LEA proteins are thought to protect macromolecules and membranes under stress
conditions (Grover et al. 2001).

In conclusion, in this study, a novel R2R3-AlMYB
gene from Aeluropus littoralis was isolated, that functions as positive
regulator of abiotic stress tolerance. The overexpression of AlMYB in
heterologous (tobacco) system leads to an increase in stress tolerance, as
determined by salt, drought, and oxidative tolerance assays. It was found that
the chlorophyll retention, germination percentage, fresh and dry weight, roots
and shoot elongation were much better in tobacco transgenic lines, when
compared to control plants, under stress conditions. The results of this work
provide significant information for improving the stress tolerance of crops through
molecular breeding.

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