Transcription of resistance in the transgenic lines was

Transcription factors are of importance for plants to adjustgene 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 tobe involved in plant defense and abiotic stress responses (Abe et al. 2003; Mengisteet al.

2003; Rahaie et al. 2010). Isolationof an Aeluropus-specific MYB gene responsive to a range of abioticstresses contributes to a better understanding of regulatory mechanisms of MYB genes in response to environmentalchanges in cereals and grasses. In thisstudy, a full-length genomic AlMYB gene was isolated from halophytegrass Aeluropus littoralis root tissues subjected to a salt stress.

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The deduced AlMYB protein contains typical R2and R3 domains of the R2R3-MYB subfamily. Based on phylogenetic analysis, AlMYBappears to be a novel member of the plant R2R3-MYB subfamily. Transient expressionof a GFP-AlMYB fusion protein in onionepidermal cells proved that AlMYB islocated onthe nucleus. We have shown that AlMYB gene was induced by salt, drought,cold and heat stresses. In the model plants, Arabidopsisand rice, a number of MYB genes were characterized to function askey factors in the signaling pathways for plant resistance to abiotic stresses(Vannini et al. 2004; Dai et al.

2007; Dubos etal., 2010). The expression patterns of 60wheat MYB genes in response to abiotic stress conditions have beenrecently determined. Thirty-two genes that responded to different stress treatmentswere discovered, of which 20 genes responded to multiple stress treatments,indicating that they were major factors involving cross-talk among differentsignal transduction pathways in response to abiotic stresses (Zhang et al. 2012a). However, multiplewheat MYB genes appeared to participate in responding to one stress stimulus,suggesting that there are multiple signaling pathways implicated in theresponse to abiotic stress treatment (Zhang etal. 2012a).

To further analyze thefunction of AlMYB in planta, transgenictobacco plants expressing AlMYB were generated by the Agrobacterium-mediatedtransformation method and were molecularly characterized. The enhanced level ofresistance in the transgenic lines was associated with accumulation of AlMYBtranscript. Under greenhouse conditions, AlMYBtransgenic tobacco exposed to continuoussalt or drought stress showed a higher levelof tolerance. Alltogether the results suggested animportant role of AlMYB in regulating drought or salt stress response andthat AlMYB was involved in the tolerance to bothstresses. The results obtained on RWC of detached leaves suggest that theconstitutive expression of AlMYB gene in plant tissues allows amaintenance of water uptake under severe deficit. The mechanism by which AlMYB conferstolerance in plants subjected to salt anddrought stresses and the effect ofoverexpression in tobacco needs further investigation. The overexpression of some MYB genesincreased tolerance of transgenic plants to biotic or abiotic stresses (Feller et al. 2011; Ma et al.

2009; Dubos et al. 2010; Zhanget al. 2012). In addition, AlMYB conferred leaf tissue tolerance to H2O2-inducedoxidative stress. Antioxidant metabolism and oxidative stress caused by ROSconstitute a major component of temperature stress in plants although lowtemperature signals appear to be also transduced by non-overlapping andindependent pathway components at the level of perception and signaltransduction (Sung et al.

2003). Thepresent results suggest that some of the observed stress tolerance in AlMYB linesmight be due to an improved intracellular control of ROS. This may convey a generalprotection against intra-cellular stress damage and thus help maintain manyvital 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 ofabiotic stresses, thereby stimulating the expression of other genes known to contributeto stress responses. Over-expression of a stress response gene in homologousvs. heterologous systems is not always effective for the same types ofstresses.

For example, constitutive over-expression of Arabidopsis DREB1A resultedin improved tolerance to drought, salinity and freezing but caused severegrowth retardation under control conditions (Kasuga etal. 1999). Engineering inducible expression of DREB genestherefore appeared to be a more promising strategy than engineeringconstitutive expression in plants.

Expression of the same gene in rice improvedtolerance to drought and salinity but to a very little extent to chilling (Oh et al. 2005). Wehave shown that the steady state level of transcripts of some stress associatedgenes encoding proteins involved in anti-oxidative and protection activitiesare higher in unstressed AlMYB tobacco than in WT plants. It is well knownthat antioxidant enzyme activity is increased in plants in response to variousenvironmental and chemical stresses (Baek etal. 2006). ‘Late Embryogenesis Abundant’ orLEA proteins are thought to protect macromolecules and membranes under stressconditions (Grover et al.

2001). In conclusion, in this study, a novel R2R3-AlMYBgene from Aeluropus littoralis was isolated, that functions as positiveregulator of abiotic stress tolerance. The overexpression of AlMYB inheterologous (tobacco) system leads to an increase in stress tolerance, asdetermined by salt, drought, and oxidative tolerance assays. It was found thatthe chlorophyll retention, germination percentage, fresh and dry weight, rootsand shoot elongation were much better in tobacco transgenic lines, whencompared to control plants, under stress conditions.

The results of this workprovide significant information for improving the stress tolerance of crops throughmolecular breeding.


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