NaCl effects in Zea mays L. x Tripsacum dactyloides (L.) L. hybrid calli and plants
del Carmen Molina
Financial support: FCAyF-UNLP (grant A129), FCA-UNLZ and CONICET (grant 4650).
Keywords: intergeneric hybrid, plant regeneration, organogenesis, salinity tolerance.
High salt concentrations
in soils negatively affect maize growth. Techniques such as remote
hybridization and in vitro selection have been extensively
used to accelerate breeding processes. In order to determine the usefulness
of Tripsacum to improve salt tolerance in maize, the effects
of NaCl, in vitro and in vivo, were evaluated in an
intergeneric hybrid (MT) obtained from crossing Zea mays with
Tripsacum dactyloides. Organogenic calli, induced from immature
MT hybrid embryos, were exposed to different NaCl concentrations and
the survival and regeneration percentages were calculated. Plants
of the MT hybrid, obtained from the organogenic calli, were exposed
to NaCl concentrations considered harmful for maize. The shoot dry
weights of plants exposed to
High salt concentrations in soils negatively affect maize growth and, consequently, produce a large drop in yield (Pasternak et al. 1995). In many countries of the world soil salinity is a serious problem for agriculture and, consequently, the development of salinity tolerant genotypes is considered an important research subject for genetic improvement (Flowers et al. 1997).
Salinity causes both, hyperionic and hyperosmotic stress effects, and the consequence of these can be plant demise. Most common stress is caused by high Na+ and Cl- concentrations in soil solution (Hasegawa et al. 2000). Maize soil salinity effects have been widely studied. On one hand, NaCl presence in soil solution affects crop water relations, which becomes in an osmotic stress for maize plants (Cramer et al. 1994; Hasegawa et al. 2000). And on the other hand, shoot Na+ concentration in maize increases with NaCl increments in soil solution, which involves ionic balance alterations (Shabala et al. 1998).
Resistance to abiotic stresses in general, and to salt stress in particular is under polygenic control, which have hindered the improvement in this aspect (Flowers and Yeo, 1995; Winicov, 1996). Techniques such as remote hybridization and in vitro selection have been greatly used to accelerate the breeding process. Hybridization of two phylogenetically distant species offers a great potential to increase the genetic variability, whether by introgression of desirable characters in the cultivated species (introduction of simple genes or addition/substitution/translocation of chromosomes or chromosome segments), or by the generation of new allopolyploids with one or more genomes and several useful characteristics of both parental species (Matzk, 1997).
Tripsacum dactyloides is a highly palatable and productive perennial grass (Faix et al. 1980), which shows tolerance to different environmental stresses (Foy, 1997; Clark et al. 1998; Ray et al. 1999). Long-term organogenic calli have been obtained from tetraploid maize (2n = 40) x Tripsacum dactyloides (2n = 72) hybrid embryos. Regenerated hybrid plants showed a somatic chromosome number 2n = 56 (García et al. 2000) and high tolerance to salinity (Pesqueira et al. 2003) and low temperatures (Jatimliansky et al. 2004). Even though maize x T. dactyloides F1 hybrid plants exhibit very low fertility, a few viable seeds have been obtained (Leblanc et al. 1995; Sokolov et al. 2000; Molina et al. 2005). Fertility and seed production increased in subsequent generations by reducing the number of Tripsacum chromosomes (Khatypova et al. 2002; Molina et al. 2005). Further, meiotic cells of these hybrids showed maize and Tripsacum chromosomes pairing, which suggest the possibility of genetic recombination between parental species (Molina et al. 2005). In this context, maize x T. dactyloides F1 hybrids could be a source of salinity tolerance to use in a maize improvement program.
Although methods such as remote hybridization and in vitro selection have been widely used to accelerate plant breeding process, some frequent limitations observed have been the difficulty to regenerate plants from the selected material (Lutts et al. 1999) or the lack of correlation between in vitro and in vivo tolerance (McCoy, 1987).
The aims of the present study were to evaluate the effects of different NaCl concentrations on the following parameters of the MT hybrid: i) regeneration capacity and survival of organogenic calli; ii) growth of the regenerated plants exposed to a NaCl concentration considered to be harmful to cultivated maize; iii) plant shoot concentrations of sodium, potassium and calcium.
MT hybrid organogenic calli, obtained from crossing the maize line N107B (2n = 40) and Tripsacum dactyloides (2n = 72); and MT hybrid plants, obtained from regenerated shoots.
MT hybrid calli growth and shoot regeneration, were carried out in a basic medium (García et al. 1992) with 1 mg L-1 2,4-D. The regenerated shoots were separated and individually transferred to the basic medium to induce root production. After 45 days, the plantlets were transplanted into 200 ml pots with a mixture of equal parts of earthworm compost and sterile soil, and covered with plastic bags. After 50 days rusticated plants were transferred to bigger pots (1700 ml) and watered with tap water.
hybrid calli were exposed to the following NaCl concentrations: 0;
70; 140 and
plants with an average height of
hybrid calli survival and regeneration decreased as NaCl concentration
increased in the culture medium (Figure 1).
After 35 days in culture, 40% of the calli exposed to
hybrid plants watered with
content in the MT hybrid plants significantly increased respect to
the control, when they were watered with
conclusion, our results and those previously reported, suggest that
MT hybrid salt tolerance is based, firstly on the strategy to accumulate
sodium, and consequently to lower leaves water potential, maintaining
the turgor pressure required for vegetative growth; and secondly,
on the capacity to lower shoot/root rate, which is a favourable aspect
for plant water balance. Compared to the controls, MT hybrid plants
We’d like to thank Dr. Ma. Elena Dallorso for her contributions in spectrophotometry.
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