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

Salt Stress
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tolerance from halophytic species via wide-hybridization and single gene transfer
through genetic engineering, improvement within existing crop genomes by screening
and breeding based on natural genetic variability, use of mutants and cell and tissue
culture (to generate novel genotypes) and breeding only for yield potential and yield
stability.
Conventional breeding techniques (such as interspecific hybridization, screening
and recurrent selection) rely on existing genetic variability of plants in responses to
salinity. There are many reports referring to screening for salt tolerance of varieties,
genotypes and lines of crops, such as barley (Wei et al., 2003), rice (Zeng et al., 2003),
wheat (Saneoka et al., 1999, Rivelli et al., 2002a), sugarcane (Plaut et al., 2000), soybean
(Essa, 2002, An et al., 2002), citrus (Moya et al., 2002), canola (Qasim et al., 2003),
grapevine (Storey et al., 2003), bean (Bayuelo-Jimenez et al., 2002) and alfalfa (Al-Khatib
et al., 1993).
The International Rice Research Institute (IRRI) has developed a number of
salt-tolerant varieties in rice. (Moeljopawiro and Ikehashi, 1981). High- and low-sodium
transporting lines of rice were developed by intravarietal selection (Yadav et al., 1996).
Four rice varieties which were tested at the National Centre for Genetic Engineering in
Bangkok have been chosen for further tests, as they expressed an ability to survive
irrigation with water containing 2% to 3% NaCl (Öko- Institut, http://
www.plantstress.com/Articles/up_general_files/GE_Tol.pdf). Breeding for salt tolerance
carried out at the Central Soil Salinity Research Institute (CSSRI, Karnal, India) resulted
in the development of CSR10, the first salt-tolerant early maturing rice variety of the
country, capable to withstand highly alkaline and saline conditions (EC e
of up to 10 dS
m -1 ) under transplanted irrigated management system (http://www.plantstress.com/
admin/files/Salt_Karnal.htm). Furthermore, CSSRI has released several other salt-tolerant
varieties, including two fine and one superfine grain rice variety, two wheat varieties
and one of Indian mustard.
For certain crops, wild relatives can offer higher level of salt tolerance and help
in wide hybridization. Several wild relatives of tomato were tested and evaluated for
salt tolerance, including Lycopersicon pimpinellifolium, L. cheesmanii, L. peruvianum
and L. pennellii (Cuartero et al., 1992). Recurrent selection for salt tolerance of the
interspecific hybrids resulting from backcrosses to a domestic tomato cultivar, led to
the appearance of plants able to survive in up to 70% of the seawater salt concentration
(Subbarao and Johansen, 1994). Interspecific crosses between species of the same
genus had certain success in improving salt tolerance within some cross-pollinating
species, such as rice, wheat, tomato and barley (Biosalinity Awareness Project, http://
www.biosalinity.org/halophytes.htm).
In spite of high expectations, conventional breeding programs led so far, to
only about 30 cultivars in just 12 with improved salt tolerance (Flowers and Yeo, 1995,
Flowers, 2004). This may be associated with extremely difficult and time consuming
testing of salt tolerance of crops in the field, due to: a) enormous ability of salt-affected
soil to interact with other environmental factors, b) changeable salinity within fields