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This review focuses on the regulation of root water uptake in plants which are exposed to salt stress. Root water uptake is not considered in isolation but is viewed in the context of other potential tolerance mechanisms of plants-tolerance mechanisms which relate to water relations and gas exchange. Plants spend between one third and half of their lives in the dark, and salt stress does not stop with sunset, nor does it start with sunrise. Surprisingly, how plants deal with salt stress during the dark has received hardly any attention, yet any growth response to salt stress over days, weeks, months and years is the integrative result of how plants perform during numerous, consecutive day/night cycles. As we will show, dealing with salt stress during the night is a prerequisite to coping with salt stress during the day. We hope to highlight with this review not so much what we know, but what we do not know; and this relates often to some rather basic questions.

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BACKGROUND: Mineral nutrient limitation affects the water flow through plants. We wanted to test on barley whether any change in root-to-shoot ratio in response to low supply of nitrogen and phosphate is accompanied by changes in root and cell hydraulic properties and involves changes in aquaporin (AQP) gene expression and root apoplastic barriers (suberin lamellae, Casparian bands). METHODS: Plants were grown hydroponically on complete nutrient solution or on solution containing only 3.3 % or 2.5 % of the control level of nutrient. Plants were analysed when they were 14-18 d old. RESULTS: Nutrient-limited plants adjusted water flow to an increased root-to-shoot surface area ratio through a reduction in root hydraulic conductivity (Lp) as determined through exudation analyses. Cortex cell Lp (cell pressure probe analyses) decreased in the immature but not the mature region of the main axis of seminal roots and in primary lateral roots. The aquaporin inhibitor HgCl2 reduced root Lp most in nutrient-sufficient control plants. Exchange of low-nutrient for control media caused a rapid (20-80 min) and partial recovery in Lp, though cortex cell Lp did not increase in any of the root regions analysed. The gene expression level (qPCR analyses) of five plasma membrane-localized AQP isoforms did not change in bulk root extracts, while the formation of apoplastic barriers increased considerably along the main axis of root and lateral roots in low-nutrient treatments. CONCLUSIONS: Decrease in root and cortex cell Lp enables the adjustment of root water uptake to increased root-to-shoot area ratio in nutrient-limited plants. Aquaporins are the prime candidate to play a key role in this response. Modelling of water flow suggests that some of the reduction in root Lp is due to increased formation of apoplastic barriers.

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BACKGROUND AND AIMS: Contractile roots are known and studied mainly in connection with the process of shrinkage of their basal parts, which acts to pull the shoot of the plant deeper into the ground. Previous studies have shown that the specific structure of these roots results in more intensive water uptake at the base, which is in contrast to regular root types. The purpose of this study was to find out whether the basal parts of contractile roots are also more active in translocation of cadmium to the shoot. METHODS: Plants of the South African ornamental species Tritonia gladiolaris were cultivated in vitro for 2 months, at which point they possessed well-developed contractile roots. They were then transferred to Petri dishes with horizontally separated compartments of agar containing 50 µmol Cd(NO3)2 in the region of the root base or the root apex. Seedlings of 4-d-old maize (Zea mays) plants, which do not possess contractile roots, were also transferred to similar Petri dishes. The concentrations of Cd in the leaves of the plants were compared after 10 d of cultivation. Anatomical analyses of Tritonia roots were performed using appropriately stained freehand cross-sections. KEY RESULTS: The process of contraction required specific anatomical adaptation of the root base in Tritonia, with less lignified and less suberized tissues in comparison with the subapical part of the root. These unusual developmental characteristics were accompanied by more intensive translocation of Cd ions from the basal part of contractile roots to the leaves than from the apical-subapical root parts. The opposite effects were seen in the non-contractile roots of maize, with higher uptake and transport by the apical parts of the root and lower uptake and transport by the basal part. CONCLUSIONS: The specific characteristics of contractile roots may have a significant impact on the uptake of ions, including toxic metals from the soil surface layers. This may be important for plant nutrition, for example in the uptake of nutrients from upper soil layers, which are richer in humus in otherwise nutrient-poor soils, and also has implications for the uptake of surface-soil pollutants.

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Suberin is a highly persistent cell wall polymer, predominantly composed of long-chain hydroxylated fatty acids. Apoplastic suberin depositions occur in internal and peripheral dermal tissues where they generate lipophilic barriers preventing uncontrolled flow of water, gases, and ions. In addition, suberization provides resistance to environmental stress conditions. Despite this physiological importance the knowledge about suberin formation has increased slowly for decades. Lately, the chemical characterization of suberin in Arabidopsis enabled the proposal of genes required for suberin biosynthesis such as ß-ketoacyl-CoA synthases (KCS) for fatty acid elongation and cytochrome P450 oxygenases (CYP) for fatty acid hydroxylation. Advantaged by the Arabidopsis molecular genetic resources the in silico expression pattern of candidate genes, concerted with the tissue-specific distribution of suberin in Arabidopsis, led to the identification of suberin involved genes including KCS2, CYP86A1, and CYP86B1. The isolation of mutants with a modified suberin composition facilitated physiological studies revealing that the strong reduction in suberin in cyp86a1 mutants results in increased root water and solute permeabilities. The enhanced suberin 1 mutant, characterized by twofold increased root suberin content, has increased water-use efficiency and is affected in mineral ion uptake and transport. In this review the most recent findings on the biosynthesis and physiological importance of suberin in Arabidopsis are summarized and discussed.

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• The structure and development of the cortical layers, especially the endodermis and exodermis, and changes in the cortex caused by the secondary growth of vascular tissues are described in the adventitious roots of gentian (Gentiana asclepiadea). • Sections along the whole axis of the soil-grown roots were observed using light microscopy; fluorescence microscopy was used to determine developmental stages of the endodermis and exodermis. • Both endodermis and exodermis develop in three stages: Casparian band formation, suberin lamellae deposition and secondary thickening of walls. After the onset of cambial activity (20 mm from apex) cortical cells expand tangentially and subdivision of individual cells starts between 20 mm and 60 mm from apex. Highly differentiated endodermal cells are divided by 0-19 new anticlinal walls, exodermal cells by 0-3 and parenchymatous mid-cortex by 0-1. • The additional anticlinal cell walls of the endodermis and exodermis possess neither Casparian bands nor suberin lamellae. Suberin lamellae remain continuous on the surface of extended tangential walls of both layers. There is a correlation between increasing diameter of the secondary vascular tissues and the number of endodermal cells created by subdivision of the original cells.

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Following our publication of a new method of calculating rates of water uptake by roots from measurements of the rate of accumulation on the roots of a marker solute, this paper describes the sites of accumulation of the solute, which indicate the sites where the water entered the symplast. Sulphorhodamine G (SR) was supplied in aeroponic mist culture to large maize plants with fully developed root systems. Root samples were collected after 4 to 8 h of transpiration in the dye-mist from both axes and branches of the main roots, and from non-transpiring (detopped) controls, frozen rapidly, freeze-substituted, and embedded and sectioned by an anhydrous procedure that preserves the SR in place. Whole mounts and sections were examined by bright-field, polarizing and epifluorescence microscopy. Major accumulations of SR were all at the outer surface of the roots, on Epidermal or root hair cell walls, or, in older roots where the epidermal cells were separating or dead, on the outer wall of the hypodermis. On some branch roots, though not on any main axes, the accumulations of SR were conspicuously aligned in the grooves over anticlinal cell walls of the epidermis. Non-transpiring plants showed very slight accumulations. Diffusion of SR into the cell wall apoplast was limited by the suberized lamellae and Casparian bands of the hypodermis, except in some branch roots, where SR diffused throughout the cortical cell walls. In parts of roots where the epidermis and hypodermis had been damaged, SR diffused through cell walls of the cortex from the wound site. These patterns of accumulation show that water enters the symplast of roots at the outermost cell membranes of the root, whether they are epidermal or hypodermal cells. Water enters roots with fully developed hypodermises at high rates. The rote of the hypodermal suberization is to limit solute movement in the wall apoplast. A symplastic path for water throughout the cortex, endodermis and living cells of the stele is suggested.