Early Salt Stress Effects on the Changes in Chemical Composition in Leaves of Ice Plant and Arabidopsis. A Fourier Transform Infrared Spectroscopy Study

Abstract

A technique based on Fourier transform infrared (FT-IR) spectrometry was developed to detect the corresponding changes in chemical composition associated with the rapid changes in sodium and water content in 200 mm NaCl-stressed halophyte ice plants (Mesembryanthemum crystallinum). The changes in glycophyte Arabidopsis stressed with 50 mm NaCl were also examined for comparison. The obtained IR spectra were further processed by deconvolution and curve fitting to examine the chemical nature of the responding sources in the leaves. Using three stages of ice plant leaves, absorption bands corresponding to carbohydrates, cell wall pectin, and proteins were identified, with distinct IR spectra representing each developmental stage. Within 48 h of mild salt stress, the absorption band intensities in the fingerprint region increased continuously in both plants, suggesting that the carbon assimilation was not affected at the early stage of stress. The intensities of ester and amide I absorption bands decreased slightly in Arabidopsis but increased in ice plant, suggesting that the cell expansion and protein synthesis ceased in Arabidopsis but continued in ice plant. In both plants, the shift in amide I absorption band was observed hourly after salt stress, indicating a rapid conformational change of cellular proteins. Analyses of the ratio between major and minor amide I absorption band revealed that ice plant was able to maintain a higher-ordered form of proteins under stress. Furthermore, the changes in protein conformation showed a positive correlation to the leaf sodium contents in ice plant, but not in Arabidopsis.High salinity causes pleiotropic effects in plant growth such as reduced cell expansion, decreased protein synthesis, and accelerated cell death. Soil salinity is one of the major limitations of crop productivity worldwide. Halophytes are native flora of saline environments that have the characteristic to overcome the ion and osmotic imbalance caused by high NaCl concentrations. Halophytes possess a set of unique salt adaptation mechanisms. Through comparisons with glycophytes (nonhalophytes), the mechanisms of salt tolerance in halophytes have been studied at physiological, biochemical, and molecular levels (for review, see Hasegawa et al., 2000). In general, three adaptation strategies are commonly found in halophytes: compartmentation of toxic ions, accumulation of osmolytes, and conservation of water (Bohnert et al., 1995). The main strategy for glycophytes is to control of ion flux into root xylem and as the result, restrict ion movement to the shoot (for review, see Hasegawa et al., 2000).Ice plant (Mesembryanthemum crystallinum) has been used as a halophytic model because the mechanisms of salt tolerance can be induced when the plants reach a certain developmental stage. Although ice plant is not suitable for genetic manipulation, comparisons of the physiological, biochemical, and gene expression changes before and after salt stress has provided useful data. In a time course progression of salt-indu...

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