Stress related responses in soybean.
Environmental stresses such as drought, salinity and low temperature have been major selective forces throughout plant evolution and are important factors which limit crop plant distribution and agricultural productivity. An understanding of how crops adapt to adverse conditions is not only of theoretical interest, but also has considerable practical value . Low-temperature stress subtraction libraries were constructed in a pBluescript vector with the two-step-PCR amplified cDNAs using subtractive hybridization. One insert cs18 was obtained and the sequence analysis of insert cs18 revealed that the insert cDNA had a 76% homology with the sequences of the corresponding portion of glucose dehydrogenase from Thermoplasma acidophilum and 62.0% homology with a genomic DNA of Arabidopsis. Four clones, cs18-13, cs18-14, cs18-15, and cs18-16 from low-temperature stress soybean root conventional cDNA library have been confirmed to have inserts that could hybridize to the cs18 insert. One cDNA with a Xba I and Xho I fragment of approximately 3,500 bp in length corresponded to the insert cs18 , which probably encodes for glucose dehydrogenase, was obtained. Northern blot analysis indicated that cs18 mRNA was highly expressed in soybean root but moderately expressed in leaves under low temperature. Changes in the nuclei of meristematic root cells in response to severe salinity were studied. Roots are in direct contact with the surrounding solution . Thus, they are the first to encounter the saline medium and are potentially the first site of damage or line of defence under salt stress. Nuclear deformation or degradation in the soybean root meristem with 150 mM or higher NaCI led to sequential cell degradation, cell death and cessation of plant growth . However, this study indicates that an increase in CaCI concentration up to 5 mM could partially prevent salt injury to the cells. Tissue culture is an excellent tool for elucidat ing the correlation between plant organizational levels and salt tolerance because of the possibility it offers for studying the physiology of intact plantlets together with that of organs and single cells using homogenous plant material under uniform environmental conditions. One NaCI-tolerant cell line (R100) was isolated during this study. The R100 callus cell line was significantly more tolerant to salt than the salt-sensitive line (S100) during exposure to salt stress. Salt tolerance in this culture was characterized by an altered growth behaviour, reduced cell volume and relative water content, and accumulation of Na+, Cl ¯, K+, proline and sugars when grown under salt stress and with its subsequent relief. The selection of this salt tolerant cell line has potential for contributing new genetic variability to plant breeders. Sugars are not only important energy sources and structural components in plants , they are also central regulatory molecules controlling physiology, metabolism, cell cycle , development, and gene expression in plants. The concentrations of glucose and fructose increased during salt stress and after relieving salt stress, at a rate closely corresponding to the increase in relative water content. Their accumulation was the earliest response detected during the removing of salt stress indicating that glucose and fructose may play important roles during salt-stress.