Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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Z . Dajic
The small molecules designated as transcription factors bind to promoter regulatory
elements in salt-inducible genes, whose role is associated with the transcriptional
activation of genes regulated by salt and drought stress (Shinozaki and Yamaguchi-
Shinozaki, 1997). An altered gene expression in salt-tolerant alfalfa seems to be connected
with the involvement of a putative transcriptional factor Alfin1, while
overexpression of the Alfin1 in transgenic alfalfa led to the activation of an additional
salt-inducible gene, the MsPRP2 (Winicov, 1998), suggesting a role of transcription
factors in coordinate gene regulation under salinity conditions.
Salinity tolerance has been considered to be a quantitative trait (Foolad and
Jones, 1993). Hence, the identification of quantitative trait loci (QTL) that contribute to
natural variation in responses to salt stress will be helpful in understanding the complexity
of the genetic control of salt tolerance and provide opportunities for more efficient
breeding for salt tolerance in crops. In the study of quantitative trait loci controlling
various traits of rice seedlings exposed to salinity stress, out of a total of seven
QTLs for traits associated with improved tolerance, four were located on chromosome
6 (Prasad et al., 2000a). It was reported that QTLs for sodium and potassium uptake in
rice were found on different chromosomes (Koyama et al., 2001), which is consistent
with the independent inheritance of Na + and K + uptake in the mapping populations, and
with different uptake pathways (apoplastic leakage and membrane transport, respectively)
for these cations under saline conditions. In rice, the putative locus for osmotic
adjustment and two of five QTL associated with dehydration tolerance were close to
chromosomal regions responsible for the root morphology (Lilley et al., 1996).
Identification of QTLs for salt tolerance during vegetative growth in tomato
led to detection of five genomic regions on chromosomes 1, 3, 5, 6, and 11 (Foolad et al.,
2001). In Arabidopsis, 11 QTLs were detected, for variation in salt tolerance, of which 6
were present during germination and 5 at the vegetative growth stage (Quesada et al.,
2002). A survey of QTLs for salt tolerance in barley seedlings revealed 12 quantitative
trait loci for seven traits (Ellis et al., 2002). A composite genomic map for the Triticeae of
QTLs affecting stress tolerance showed the highest concentration of QTLs and major
loci controlling the plant’s adaptation to environmental stresses located on the chromosome
group 5 (Cattivelli et al., 2002). The QTL analysis performed with 172 recombinant
hybrids between Helianthus annuus and H. petiolaris, revealed a total of 14 QTLs
for mineral uptake and three for survival, suggesting that salt tolerance in Helianthus is
achieved through increased calcium uptake, coupled with greater sodium exclusion
(Lexer et al., 2003). It was reported for several species that QTLs linked to salt tolerance
vary with the developmental stage of the plant (Flowers, 2004).
Significant progress in highlighting the physiological and biochemical responses
of plants to salinity at different levels, as well as the molecular cloning of
genes involved in the different metabolic pathways altered by salts, has enabled the
introduction of genetic engineering technologies in order to improve salt tolerance in
crops.