Environmental and Economical Opportunities for the Valorisation of the Genus Atriplex: New Insights - Crop Improvement: New Approaches and Modern Techniques - page 445

(NHX1) (Shi and Zhu 2002 ). In particular, the vacuolar Na + /H + antiporter had been
demonstrated to play a key role in salt tolerance of plants (Blumwald et al. 2000 ).
The greater salt tolerance in Atriplex species is related to the transport to shoots of
high quantities of Na + concomitant to an efficient vacuolar compartmentation of
this ion, which prevents the ionic damage to the cytoplasm. The vacuolar Na + /H +
antiporter genes has been characterized and identified from A. gmelili ( AgNHX1 )
(Hamada et al. 2001 ). The analysis and comparison of these genes showed that
they were highly homologous with similar structural and conserved domains. Ohta
et al. ( 2002 ) demonstrated that transgenic rice plants overexpressing AgNHX1 gene
could survive after short period of high concentration salt exposure (300 mM NaCl
for 3 days). Evacuation of Na + from the cytoplasm is energy-dependent. A partial
sequence of an isoform of the plasma membrane PM-H + -ATPase was been isolated
from A. nummularia . Increased H + -ATPase mRNA abundance was reported in A.
nummularia when NaCl adapted (342 mM NaCl) cells were re-exposed to NaCl
after having been grown in media without additional NaCl (Niu et al. 1993 ). Which
provide evidence that enhanced H + -transport activity by NaCl in A. nummularia
is mediated at least in part by transcriptional or post-transcriptional processes that
result in higher mRNA accumulation.
Exposure to saline and drought stress results in the accumulation in the cytosol of
low-molecular mass compounds, termed as compatible solutes, which do not inter-
fere with normal biochemical reactions. It has been frequently reported that GB acts
as the main stress-induced agent involved in the osmotic adjustment and protection
of cellular structure in plant species belonging to the Chenopodiaceae (Rhodes and
Hanson 1993 ). GB facilitates osmotic adjustment by lowering the internal osmotic
potential that contributes to the water stress tolerance ability. In addition, it stabi-
lizes both PSII complex and RuBisCO during photosynthesis under stress condi-
tions (Sakamoto and Murata 2000 ). Yang et al. ( 2007 ) reported that genetically
engineered tobacco with the ability to accumulate GB showed a higher content of
ascorbate and reduced glutathione as well as an increase in the activity of superox-
ide dismutase (SOD). The positive effect of exogenous glycine betaine application
in plant growing under salinity stress has been proven. Plant cell could be pro-
tected from the adverse effect of salinity induced oxidative stress by the exogenous
application of glycine betaine (Demiral and Türkan 2004 ). In A. nummularia GB
play a major role in cytosol osmotic adjustment in both leaves and roots, regardless
of NaCl presence (Silveira et al. 2009 ). In higher plants, the first and second steps in
the biosynthesis of GB are catalyzed by a rate-limiting enzyme choline monooxy-
genase (CMO) and betaine aldehyde dehydrogenase (BADH), respectively (Saka-
moto and Murata 2000 ). CMO gene from A. hortensis ( AhCMO ) has been isolated
and used for GB production in tobacco (Shen et al. 2002 ) and cotton plants (Zhang
et al. 2009 ) to improve its abiotic stress tolerance. CMO homologs have been also
identified in A. prostrate (Wang and Showalter 2004 ) and A. nummularia (Tabuchi
et al. 2005 ). The gene encoding the second enzyme, BADH , has been cloned from
A. hortensis ( AhBADH ) and introduced into rice, wheat, and turf grass (Xiao et al.
1995 ; Guo et al. 1997 , 2000) and improvement of salt tolerance in transgenic plants