We conducted a coordinated biochemical and morphometric analysis of the effect of saline conditions on the differentiation zone of developing soybean (Glycine max L.) roots. Between d 3 and d 14 for seedlings grown in control or NaCl-supplemented medium, we studied (a) the temporal evolution of the respiratory alternative oxidase (AOX) capacity in correlation with the expression and localization of AOX protein analyzed by tissue-print immunoblotting; (b) the temporal evolution and tissue localization of a peroxidase activity involved in lignification; and (c) the structural changes, visualized by light microscopy and quantified by image digitization. The results revealed that saline stress retards primary xylem differentiation. There is a corresponding delay in the temporal pattern of AOX expression, which is consistent with the xylemspecific localization of AOX protein and the idea that this enzyme is linked to xylem development. An NaCl-induced acceleration of the development of secondary xylem was also observed. However, the temporal pattern of a peroxidase activity localized in the primary and secondary xylem was unaltered by NaCl treatment. Thus, the NaCl-stressed root was specifically affected in the temporal patterns of AOX expression and xylem development.

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We used tissue printing and specific immunostaining to examine the localization of the alternative oxidase (AOX) protein in correlation with measurements of AOX capacity. Selected root and hypocotyl regions were analyzed during the first 14 d of growth. It is shown that AOX protein is localized in the apical meristem and in developing xylem. The temporal pattern of expression is coincident with the evolution of AOX capacity. Data suggest that AOX expression is linked to xylem differentiation. Since heat is a major product of the alternative pathway, we speculate that thermogenesis i s implicated in morphogenesis.In higher plants the mitochondrial electron transport is branched and there are two terminal oxidases: the usual Cyt oxidase and a KCN-insensitive AOX, which is not coup\led to ATP production (Moore and Siedow, 1991;Siedow and Umbach, 1995). The capacity for AOX respiration has been shown to depend on the organ, developmental stage, growth conditions, and treatments (Bingham and Farrar, 1989;Obenland et al., 1990;Kearns et al., 1992;Vanlerberghe and McIntosh, 1992a). This capacity for AOX respiration, measured as the O, consumption blocked by the addition of an AOX inhibitor in the presence of an inhibitor of the Cyt path, correlates with the amount of AOX protein as determined by western-blot analysis (Hiser and McIntosh, 1990;Obenland et al., 1990;Kearns et al., 1992; Rhoads and McIntosh, 1992; McIntosh, 1992a, 1992b).No information is available concerning the spatial localization or possible tissue specificity of AOX protein within the root or the hypocotyl. In floral buds high levels of AOX protein were found in the younger tapetal cells of fertile anthers (Conley and Hanson, 1994), and this pattern of expression was considered possible evidence supporting the hypothesis that AOX is needed for NAD+ regeneration in tissues with high biosynthetic activity.In this study we used tissue printing and specific immunostaining to examine the expression and localization of AOX protein in developing soybean (GZycine mux L.) roots and hypocotyls, in correlation with measurements of AOX capacity. Selected root and hypocotyl regions were analyzed during the first 14 d of growth. To our knowledge, this is the first report of a comparison of the temporal evolution of AOX capacity in the apical meristem and the differentiation zone of primary and secondary roots, since previous studies were performed at the whole-organ level. It is shown that AOX protein is specifically expressed in meristematic and xylematic tissues. The temporal pattern of AOX expression, which coincides with the temporal evolution of AOX capacity, appears to be linked to developmental processes associated with xylem differentiation. The information presented here could serve as a basis for studies aimed at unveiling the still-unknown function of this enzyme. MATERIALS A N D METHODS Plant GrowthSoybean (Glycine max L.) seeds of variety UFV-8 were germinated at 28°C in sterile sand moistened with tap water for 3 d. Then, the seedlings were transfer...

Branched nodal roots comprise the largest portion of the root system mass in Chloris gayana Kunth and the effects of high salinity on nodal root appearance and elongation rates were analysed in cv. Boma in greenhouse experiments. Roots from salt‐treated plants (0·2 mol l–1 NaCl) were smaller than controls, and accumulated higher concentrations of soluble sugars and reserve lipids. The number of nodal roots was reduced by the saline treatment. Leaf, tiller and nodal root appearance were delayed by salinity but the correlation among these processes was maintained, indicating that the developmental pattern was not altered by this level of salinity, only its rate. Initial nodal root growth rates varied as a function of plant size and were decreased by salinity only after 2 weeks of treatment. When shoots of non‐salinized plants were cut, a drastic reduction in nodal root appearance was observed, suggesting emerging leaves were stronger sinks than roots for available reserves. However, when the shoots of salt‐treated plants were cut, the already depressed root appearance rate was not further reduced. This suggests that, under salinity, alterations in root ability to metabolize reserves could have been more significant than reserve availability for controlling elongation.

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