Plants show varied cellular responses to salinity that are partly associated with maintaining low cytosolic Na+ levels and a high K+/Na+ ratio. Plant metabolites change with elevated Na+, some changes are likely to help restore osmotic balance while others protect Na+-sensitive proteins. Metabolic responses to salt stress are described for two barley (Hordeum vulgare L.) cultivars, Sahara and Clipper, which differed in salinity tolerance under the experimental conditions used. After 3 weeks of salt treatment, Clipper ceased growing whereas Sahara resumed growth similar to the control plants. Compared with Clipper, Sahara had significantly higher leaf Na+ levels and less leaf necrosis, suggesting they are more tolerant to accumulated Na+. Metabolite changes in response to the salt treatment also differed between the two cultivars. Clipper plants had elevated levels of amino acids, including proline and GABA, and the polyamine putrescine, consistent with earlier suggestions that such accumulation may be correlated with slower growth and/or leaf necrosis rather than being an adaptive response to salinity. It is suggested that these metabolites may be an indicator of general cellular damage in plants. By contrast, in the more tolerant Sahara plants, the levels of the hexose phosphates, TCA cycle intermediates, and metabolites involved in cellular protection increased in response to salt. These solutes remain unchanged in the more sensitive Clipper plants. It is proposed that these responses in the more tolerant Sahara are involved in cellular protection in the leaves and are involved in the tolerance of Sahara leaves to high Na+.

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The plasma membrane of Mycobacterium sp. is the site of synthesis of several distinct classes of lipids that are either retained in the membrane or exported to the overlying cell envelope. Here, we provide evidence that enzymes involved in the biosynthesis of two major lipid classes, the phosphatidylinositol mannosides (PIMs) and aminophospholipids, are compartmentalized within the plasma membrane. Enzymes involved in the synthesis of early PIM intermediates were localized to a membrane subdomain termed PM f , that was clearly resolved from the cell wall by isopyknic density centrifugation and amplified in rapidly dividing Mycobacterium smegmatis. In contrast, the major pool of apolar PIMs and enzymes involved in polar PIM biosynthesis were localized to a denser fraction that contained both plasma membrane and cell wall markers (PM-CW). Based on the resistance of the PIMs to solvent extraction in live but not lysed cells, we propose that polar PIM biosynthesis occurs in the plasma membrane rather than the cell wall component of the PM-CW. Enzymes involved in phosphatidylethanolamine biosynthesis also displayed a highly polarized distribution between the PM f and PM-CW fractions. The PM f was greatly reduced in non-dividing cells, concomitant with a reduction in the synthesis and steady-state levels of PIMs and amino-phospholipids and the redistribution of PM f marker enzymes to non-PM-CW fractions. The formation of the PM f and recruitment of enzymes to this domain may thus play a role in regulating growth-specific changes in the biosynthesis of membrane and cell wall lipids.

Boron (B) is an essential micronutrient that affects plant growth at either deficient or toxic concentrations in soil. The aim of this work was to investigate the adaptation of barley (Hordeum vulgare) plants to toxic B levels and to increase our understanding of B toxicity tolerance mechanisms. We used a metabolomics approach to compare metabolite profiles in root and leaf tissues of an intolerant, commercial cultivar (cv Clipper) and a B-tolerant Algerian landrace (cv Sahara). After exposure to elevated B (200 and 1,000 mM), the number and amplitude of metabolite changes in roots was greater in Clipper than in Sahara. In contrast, leaf metabolites of both cultivars only responded following 1,000 mM treatment, at which B toxicity symptoms (necrosis) were visible. In addition, metabolite levels were dramatically altered in the tips of leaves of the sensitive cultivar Clipper after growth in 1,000 mM B compared to those of Sahara. This correlates with a gradual accumulation of B from leaf base to tip in B-intolerant cultivars. Overall, there were always greater differences between tissue types (roots and leaves) than between the two cultivars. This work has provided insights into metabolic differences of two genetically distinct barley cultivars and information about how they respond metabolically to increasing B levels.Boron (B) is an essential micronutrient for vascular plants. However, when B is present at high concentrations in the soil or ground water, plant growth and reproduction can be affected by B toxicity. B toxicity has been recognized as an important problem limiting crop production in the low rainfall and on highly alkaline and saline soils in regions of Australia, West Asia, and North Africa. Because soil amelioration is impractical, the development of B-tolerant cultivars is a rational solution to the problem.B freely diffuses into the roots as boric acid [B(OH) 3 ; pK a 5 9.25] and accumulates in the cytoplasm as the borate anion [B(OH) 4 2 ] due to pH-dependent interconversion. An inability to exclude B from the roots results in high B concentrations in the tissue. B phytotoxicity manifests itself in a broad range of physiological effects, including decreased shoot and root growth, root cell division and RNA content, reduced leaf chlorophyll, lower photosynthetic rates and stomatal conductance, and reduced levels of lignin and suberin (for review, see Nable et al., 1997). Leaf symptoms of toxicity in barley (Hordeum vulgare) are characterized by interveinal chlorotic and/or necrotic patches, generally at the margins and tips of older leaves. This reflects the accumulation of B at the end of the transpiration stream (Nable et al., 1997). Following long-term exposure to high B concentrations in the soil, overall vegetative plant growth is retarded and this leads to either a reduction in or a complete lack of seed set.B is also an essential nutrient, although its role in plant growth, development, and metabolism remains to be clarified. Originally, B was thought to be essentially immobile in the ...