Developmental pattern of aquaporin expression in barley (Hordeum vulgare L.) leaves

Aquaporins are multifunctional membrane channels which belong to the family of major intrinsic proteins (MIPs) and are best known for their ability to facilitate the movement of water. In the present study, earlier results from microarray experiments were followed up. These experiments had suggested that, in barley (Hordeum vulgare L.), aquaporin family members are expressed in distinct patterns during leaf development. Real-time PCR and in situ hybridization were used to analyse the level and tissue-distribution of expression of candidate aquaporins, focusing on plasma membrane and tonoplast intrinsic proteins (PIPs, TIPs). Water channel function of seven aquaporins, whose transcripts were the most abundant and the most variable, was tested through expression in yeast and, in part, through expression in oocytes. All PIP1 and PIP2 subfamily members changed in expression during leaf development, with expression being much higher or lower in growing compared with mature tissue. The same applied to those TIPs which were expressed at detectable levels. Specific roles during leaf development are proposed for particular aquaporins.

Casparian bands occur in the periderm of Pelargonium hortorum stem and root

BACKGROUND AND AIMS

Casparian bands are characteristic of the endodermis and exodermis of roots, but also occur infrequently in other plant organs, for example stems and leaves. To date, these structures have not been detected in phellem cells of a periderm. The aim of this study was to determine whether Casparian bands occur in phellem cells using tests that are known to detect Casparian bands in cells that also contain suberin lamellae. Both natural periderm and wound-induced structures were examined in shoots and roots.

METHODS

Using Pelargonium hortorum as a candidate species, the following tests were conducted: (1) staining with berberine and counterstaining with aniline blue, (2) mounting sections in concentrated sulphuric acid and (3) investigating the permeability of the walls with berberine as an apoplastic, fluorescent tracer.

KEY RESULTS

(1) Berberine-aniline blue staining revealed a modification in the radial and transverse walls of mature phellem cells in both stems and roots. Three days after wounding through to the cortex of stems, the boundary zone cells (pre-existing, living cells nearest the wound) had developed vividly stained primary walls. By 17 d, staining of mature phellem cells of wound-induced periderm was similar to that of natural periderm. (2) Mature native phellem cells of stems resisted acid digestion. (3) Berberine was excluded from the anticlinal (radial and transverse) walls of mature phellem cells in stems and roots, and from the wound-induced boundary zone.

CONCLUSIONS

Casparian bands are present in mature phellem cells in both stems and roots of P. hortorum. It is proposed that Casparian bands act to retard water loss and pathogen entry through the primary cell walls of the phellem cells, thus contributing to the main functions of the periderm.

Permeability of Iris germanica's multiseriate exodermis to water, NaCl, and ethanol

The exodermis of Iris germanica roots is multiseriate. Its outermost layer matures first with typical Casparian bands and suberin lamellae. But as subsequent layers mature, the Casparian band extends into the tangential and anticlinal walls of their cells. Compared with roots in which the endodermis represents the major transport barrier, the multiseriate exodermis (MEX) was expected to reduce markedly radial water and solute transport. To test this idea, precocious maturation of the exodermis was induced with a humid air gap inside a hydroponic chamber. Hydraulic conductivity (Lp(pc)) was measured on completely submerged roots (with an immature exodermis) and on air-gap-exposed root regions (with two mature exodermal layers) using a pressure chamber. Compared with regions of roots with no mature exodermal layers, the mature MEX reduced Lp(pc) from 8.5×10(-8) to 3.9×10(-8) m s(-1) MPa(-1). Puncturing the MEX increased Lp(pc) to 19×10(-8) m s(-1) MPa(-1), indicating that this layer constituted a substantial hydraulic resistance within the root (75% of the total). Alternatively, a root pressure probe was used to produce pressure transients from which hydraulic conductivity was determined, but this device measured mainly flow through the endodermis in these wide-diameter roots. The permeability of roots to NaCl and ethanol was also reduced in the presence of two mature MEX layers. The data are discussed in terms of the validity of current root models and in terms of a potential role for I. germanica MEX during conditions of drought and salt stress.

An improved method for clearing and staining free-hand sections and whole-mount samples

BACKGROUND AND AIMS

Free-hand sectioning of living plant tissues allows fast microscopic observation of internal structures. The aim of this study was to improve the quality of preparations from roots with suberized cell walls. A whole-mount procedure that enables visualization of exo- and endodermal cells along the root axis was also established.

METHODS

Free-hand sections were cleared with lactic acid saturated with chloral hydrate, and observed with or without post-staining in toluidine blue O or aniline blue. Both white light and UV light were used for observation. Lactic acid was also used as a solvent for berberine, and fluorol yellow for clearing and staining the samples used for suberin observation. This procedure was also applied to whole-mount roots with suberized celllayers.

KEY RESULTS

Clearing of sections results in good image quality to observe the tissue structure and cell walls compared with non-cleared sections. The use of lactic acid as a solvent for fluorol yellow proved superior to previously used solvents such as polyethylene glycol-glycerol. Clearing and fluorescence staining of thin roots such as those of Arabidopsis thaliana were successful for suberin visualization in endodermal cells within whole-mount roots. For thicker roots, such as those of maize, sorghum or tea, this procedure could be used for visualizing the exodermis in a longitudinal view. Clearing and staining of peeled maize root segments enabled observation of endodermal cell walls.

CONCLUSIONS

The clearing procedure using lactic acid improves the quality of images from free-hand sections and clearings. This method enhances the study of plant root anatomy, in particular the histological development and changes of cell walls, when used in combination with fluorescence microscopy.