Calcium transport between tissues and its distribution in the plant

Abstract. The low cytosol concentration of free Ca²⁺ makes the symplast of roots an ineffective pathway for the supply of the calcium needed for healthy growth in the aerial parts of plants. Ca²⁺ moves rapidly across the cortical apoplast by diffusion and mass flow but is probably diverted across the plasmamembranes of endodermal cells by Casparian bands. A proposal is made to account for the movement of calcium across the endodermis and it is estimated that Ca-fluxes are likely to be appreciably greater than in the regulation of cell Ca level by cortical cells.
Ca transport in the xylem occurs by mass flow of free Ca²⁺, and some organically complexed Ca, and by chromatographic movement along Ca-exchange sites in the xylem walls. Delivery of Ca to transpiring leaves and to weakly transpiring meristematic zones is discussed in relation to the two modes of Ca movement in the xylem. Competition between sinks is intensified when [Ca²⁺] in xylem is low and transpiration is great.
Tropic growth responses involve pumping of vacuolar calcium into the apoplast followed by its migration along gradients of electrical potential which develop in the apoplast after geo-stimulation. An attempt is made to estimate plasmalemma efflux during this process.
Redistribution from mature tissues to meristems in the pholem is likely to be small, if it occurs at all, since sieve tubes cannot have more than micro-molar concentrations of free-Ca²⁺ in them.     

Endodermal cell-cell contact is required for the spatial control of Casparian band development in Arabidopsis thaliana

Background and Aims

Apoplasmic barriers in plants fulfil important roles such as the control of apoplasmic movement of substances and the protection against invasion of pathogens. The aim of this study was to describe the development of apoplasmic barriers (Casparian bands and suberin lamellae) in endodermal cells of Arabidopsis thaliana primary root and during lateral root initiation.

Methods

Modifications of the endodermal cell walls in roots of wild-type Landsberg erecta (Ler) and mutants with defective endodermal development – scarecrow-3 (scr-3) and shortroot (shr) – of A. thaliana plants were characterized by light, fluorescent, confocal laser scanning, transmission and cryo-scanning electron microscopy.

Key Results

In wild-type plant roots Casparian bands initiate at approx. 1600 µm from the root cap junction and suberin lamellae first appear on the inner primary cell walls at approx. 7000–8000 µm from the root apex in the region of developing lateral root primordia. When a single cell replaces a pair of endodermal and cortical cells in the scr-3 mutant, Casparian band-like material is deposited ectopically at the junction between this ‘cortical’ cell and adjacent pericycle cells. Shr mutant roots with an undeveloped endodermis deposit Casparian band-like material in patches in the middle lamellae of cells of the vascular cylinder. Endodermal cells in the vicinity of developing lateral root primordia develop suberin lamellae earlier, and these are thicker, compared wih the neighbouring endodermal cells. Protruding primordia are protected by an endodermal pocket covered by suberin lamellae.

Conclusions

The data suggest that endodermal cell–cell contact is required for the spatial control of Casparian band development. Additionally, the endodermal cells form a collet (collar) of short cells covered by a thick suberin layer at the base of lateral root, which may serve as a barrier constituting a ‘safety zone’ protecting the vascular cylinder against uncontrolled movement of water, solutes or various pathogens.     

Cell Death in Cells Overlying Lateral Root Primordia Facilitates Organ Growth in Arabidopsis

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Juncus bulbosus as a pioneer species in acidic lignite mining lakes: interactions, mechanism and survival strategies

Bulbous rush ( Juncus bulbosus ) initiates plant colonization in extremely acid lakes resulting from coal mining operations. Various analytical techniques (methylene blue/agar method, Ti³⁺-citrate solution) X-ray diffraction (XRD), scanning electron microscopy (SEM), and Energy-dispersive X-ray (EDX) were used to assess the mechanisms and strategies employed by J. bulbosus to overcome the extreme conditions. The plant releases oxygen into the rhizosphere in turn increasing the redox potential and inducing iron oxide plaque formation. XRD showed that the iron oxide of the plaque is mainly goethite that has been developed in the presence of CO₂; SEM showed that there is a micro-space between the roots and sand grains which is inhabited by microorganisms. Furthermore, SEM-EDX studies on internal iron distribution demonstrate that iron toxicity is delayed by the physiological and biochemical structure of the plant. It is suggested that J. bulbosus uses a variety of mechanisms and strategies (morphological, physiological and biochemical adaptation) which are mainly complementary and which interact with each other to help J. bulbosus to manage its growth and survival in an extreme environment.     

X-Ray Microanalytical Studies on Mineral Concentrations in Vacuoles of Needle Tissues from Larix decidua (L.) Mill.

Elemental concentrations in the vacuoles of mesophyll, endodermis and transfusion parenchyma cells were determined by means of electron probe microanalysis on bulk frozen hydrated needles of an outdoor-grown larch tree. The investigations were done routinely throughout a life cycle of the deciduous larch needle, starting with bud burst in March and ending with leaf abscission in November. It could be shown that the endodermal vacuoles preferentially accumulate S, CI and Ca while the transfusion parenchyma vacuoles transiently store Mg, K and S. Between May and September the inorganic ion level of mesophyll cells is constantly lower than in the endodermal and transfusion parenchyma vacuoles. In autumn, when the degradation of chlorophylls becomes detectable, the elements Mg, S, CI and Ca are found increasingly in the vacuoles of the mesophyll cells. It is concluded that, in mature and intact larch needles, the vacuoles of the endodermis in cooperation with the transfusion parenchyma vacuoles act as efficient ion storage buffers which maintain a constant low ion level in the vacuoles of mesophyll cells.     

木贼类营养器官凯氏带类型的新观察

该研究用石蜡切片法比较观察了5种木贼科植物营养器官的内皮层及凯氏带,首次报道了2层内皮层及其凯氏带的形态特征及分布规律,并讨论各种类型的凯氏带及其与厚壁组织的协作防御机制。结果表明:(1)5种木贼的地下茎和根都只有1条凯氏带,其中4种木贼的地上茎有2条凯氏带。(2)木贼类营养器官具有3种凯氏带类型,即2层公共内皮层上各具有1条凯氏带、1层散生内皮层上的1条凯氏带、1层公共内皮层上的1条凯氏带。(3)木贼类地下茎和根都有发达的厚壁组织或致密的表皮。(4)问荆地上茎外侧内皮层具有复合内皮细胞。研究认为,木贼类植物凯氏带数量不能作为分类的依据;地下茎和根虽然只有1条凯氏带,但地下部分都有发达的厚壁组织或(和)与其紧密相连的表皮,推测厚壁组织或(和)表皮可能具有与凯氏带相同的功能;3种类型凯氏带的防御能力由强到弱依次是:2层公共内皮层上的凯氏带 1层散生内皮层上的凯氏带 1层公共内皮层上的凯氏带。     

Radial transport of salt and water in roots of the common reed (Phragmites australis Trin. ex Steudel)

To understand the root function in salt tolerance, radial salt and water transport were studied using reed plants growing in brackish habitat water with an osmotic pressure (π_M) of 0.63 MPa. Roots bathed in this medium exuded a xylem sap with NaCl as the major osmolyte and did so even at higher salt concentration (π_M up to 1.3 MPa). Exudation was stopped after a small increase of π_M (0.26 MPa) using polyethylene glycol 600 as osmolyte. The endodermis of fine lateral roots was found to be the main barrier to radial solute diffusion on an apoplastic path. Apoplastic salt transfer was proven by rapid replacement of stelar Na⁺ by Li⁺ in an isomolar LiCl medium. Water fluxes did not exert a true solvent drag on NaCl. Xylem sap concentrations of NaCl in basal internodes of transpiring culms were more than five times higher than in medial and upper ones. It was concluded that the radial NaCl flux was mainly diffusion through the apoplast, and radial water transport, because of the resistance of the cell wall matrix to convective mass flow, was confined to the symplast. Radial salt permeation in roots reduced the water stress exerted by the brackish medium.     

Comparative vegetative anatomy and systematics of the Oncidiinae (Maxillarieae, Orchidaceae)

Subtribe Oncidiinae comprises a vegetatively heterogeneous assemblage of species that has persistently been incapable of organization. Anatomy was considered to be a possible means to resolve the perplexity of relationships amongst the constituent taxa. The consistent occurrence of a foliar hypodermis, homogeneous mesophyll, conical silica bodies in stegmata, and ubiquitous fibre bundles in leaves provides a matrix for linking the taxa, as do the parenchymatous pith and O-thickened endodermal cell walls in roots. However, the strict consensus of the 40 genera studied was completely unresolved, suggesting that vegetative characters alone are insufficient to assess the relationships amongst these taxa, a conclusion also reached for the remainder of Maxillarieae.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 152 , 91–107.     

Histolocalization of chemotaxonomic markers in Brazilian Vernonieae (Asteraceae)

Vernonieae are well represented in South America by subtribe Lychnophorinae, with 104 Brazilian species, some of them rare and endangered. Lychnophorinae are well known for producing metabolites of high pharmaceutical and chemotaxonomic value. Despite the importance of secondary metabolites in Lychnophorinae, there is still little evidence regarding the secretory structures responsible for producing these compounds. Therefore, in this study we investigated 15 species representative of the main lineages of Lychnophorinae, with the following objectives: to identify where the chemotaxonomic markers of Lychnophorinae are secreted and stored and in which developmental stage of the organ these metabolites are found. Samples of stems and leaves were processed according to the usual techniques in plant anatomy. It was found that the main sites of secondary metabolite biosynthesis are glandular trichomes, epidermal cells and parenchyma tissues. Metabolites from glandular trichomes, especially sesquiterpene lactones (STL), are prevalent in the early developmental stages of organs. The metabolite compounds stored in parenchyma tissues are mainly terpenoids, flavonoids and other phenolic compounds; young and expanded leaves are equally rich in metabolites. Thus, the information obtained in this study is essential for conducting chemotaxonomic studies in this group, helping to promote selective collection and conservation of species.     

Stagnant deoxygenated growth enhances root suberization and lignifications, but differentially affects water and NaCl permeabilities in rice (Oryza sativa L.) roots

It has been shown that rice roots grown in a stagnant medium develop a tight barrier to radial oxygen loss (ROL), whereas aerated roots do not. This study investigated whether the induction of a barrier to ROL affects water and solute permeabilities. Growth in stagnant medium markedly reduced the root growth rate relative to aerated conditions. Histochemical studies revealed an early deposition of Casparian bands (CBs) and suberin lamellae (SL) in both the endodermis (EN) and exodermis, and accelerated lignification of stagnant roots. The absolute amounts of suberin, lignin and esterified aromatics (coumaric and ferulic acid) in these barriers were significantly higher in stagnant roots. However, correlative permeability studies revealed that early deposition of barriers in stagnant roots failed to reduce hydraulic conductivity (Lp(r) ) below those of aerated roots. In contrast to Lp(r) , the NaCl permeability (P(sr) ) of stagnant roots was markedly lower than that of aerated roots, as indicated by an increased reflection coefficient (σ(sr) ). In stagnant roots, P(sr) decreased by 60%, while σ(sr) increased by 55%. The stagnant medium differentially affected the Lp(r) and P(sr) of roots, which can be explained in terms of the physical properties of the molecules used and the size of the pores in the apoplast.