A model for water transport in the stele of plant roots

Summary The mechanism of water movement across roots is, as yet, not well understood. Some workable black box theories have already been proposed. They, however, assumed unrealistic cell membranes with low values of , or were based on a poor anatomical knowledge of roots. The role of root stele in solute and water transport seems to be especially uncertain. An attempted explanation of the nature of root exudation and root pressure by applying the apoplast canal theory (Katou andFurumoto 1986 a, b) to transport in the root stele is given. The canal equations are solved for boundary conditions based on anatomical and physiological knowledge of the root stele. It is found that the symplast cell membrane, cell wall and net solute transport into the wall apoplast are the essential constituents of the canal system. Numerical analysis shows that the canal system enables the coupled transport of solutes and water into a xylem vessel, and the development of root pressure beyond the level predicted by the osmotic potential difference between the ambient medium and the exudate. Observations on root exudation and root pressure previously reported seem to be explained quite well. It is concluded that the movement of water in the root stele although apparently active is essentially osmotic.Abbreviations J _v ^ex volume exudation per root surface - J₀ non-osmotic exudation - L^r overall radial hydraulic conductivity of an excised root - reflection coefficient - C_s difference in the osmotic concentration between the bathing medium and the exudate - R gas constant - T absolute temperature - C_K molar concentration of K⁺ - C_Cl molar concentration of Cl^– - C_j molar concentration of ion species j - P_j membrane permeability of ion j - z_j valence of ion j - F Faraday constant - V_ix intracellular electric potential with reference to the canal     

A model for radial water transport across plant roots

Summary A model based on the canal theory (Katou andFurumoto 1986 a, b) is proposed for the absorption of solute and water at the root periphery. The present canal model in the periphery and the model which was previously proposed for the exudation in the stele (Katou et al. 1987), are organized into a model for radial transport across excised plant roots, in the light of anatomical and physiological knowledge of maize roots. The canal equations for both canals are numerically solved to give quite a good explanation for the observed exudation of maize roots. It is found that the regulation of solute transport has a primary importance in the regulation of water transport across excised roots. The internal cell pressure of the symplast adjusts the water absorption at the root periphery to the water secretion into the vessels. There seems no need for this explanation of the radial water transport across roots to assume cell membranes with low reflection coefficient or variable water permeability. It would seem that the apoplast wall layers play a crucial role in metabolic control of water transport in roots as well as in hypocotyls.Abbreviations J _s ^ex* the theoretically estimated rate of solute exudation per unit surface area of model maize roots - J that of volume exudation per unit surface area of model maize roots - the reflection coefficient of the cell membrane against solutes     

Experimental evaluation of an efflux-influx model of C exudation by individual apical root segments

The aim of this study was to evaluate if a model describing the efflux and the influx of C through the root surface could be fitted to experimental short-term kinetics of carbon (C) exudation by individual apical root segments in maize (Zea mays L.). The efflux of C was set constant or modelled by a power function of the distance from the apex to simulate the greater release of C around the root tip commonly reported in the literature. The influx was proportional to the C concentration in the external solution to simulate the active re-uptake of exudates by the root. Plants were exposed to full light or to shade to manipulate C allocation to roots. The model with a constant efflux gave satisfactory fits to the kinetics of exudation (average R(2)=0.66). The average gross efflux was then 2.1 mug C cm(-2) root surface h(-1). The model was improved if exudation was set more intense towards the root apex (average R(2)=0.74). The estimated gross efflux decreased then from 5.2 mug C cm(-2) h(-1) at the apex to 1.8 mug C cm(-2) h(-1) for the region located 5-25 cm from the root tip. The decrease in net exudation of individual roots due to the shading of plants was weak, which may indicate that the import of C by the primary roots studied was not reduced significantly. By describing the exudation of an apical root segment of variable length and diameter, the model is a first step in linking exudation to root system architecture models and to whole plant functioning.     

Laticifers and secretory ducts: two other tube systems in plants

The plant kingdom has elaborated several conducting systems. Three are primarily for mass transport: the aerenchyma (for gas exchange in submerged parts), the phloem (for exchange of nutrients within the plant), and the xylem (largely for transport of water from soil to transpiring leaves). Two others are believed to be primarily defensive and to store under pressure aversive contents which they exude when punctured: the laticifer and the secretory duct. This review provides for the latter two systems the highlights of what is known about their general physiology and ecophysiology but not their metabolism and their molecular biology. It is argued that, given the importance of laticifers and secretory ducts to plant defense against insect herbivory, these structures are under-investigated and deserve more intensive study.     

Target sites of aluminum phytotoxicity

The primary phytotoxic effect of aluminum (Al) is confined to the root apex. It is a matter of debate whether the primary injury of Al toxicity is apoplastic or symplastic. This review paper summarizes our current understanding of the spatial and metabolic sites of Al phytotoxicity. At tissue level, the meristematic, distal transition, and apical elongation zones of the root apex are most sensitive to Al. At cellular and molecular level, many cell components are implicated in Al toxicity including DNA in nucleus, numerous cytoplastic compounds, the plasma membrane, and the cell wall. Although it is difficult to distinguish the primary targets from the secondary effects so far, understanding of the target sites of Al toxicity is helpful for elucidating the mechanisms by which Al exerts its deleterious effects on root growth.This work was partly supported by fund from the Huoyingdong Foundation, Education Ministry of China and Natural Science Foundation of China (Contact No. 30170548).     

Changes in the redox status of chickpea roots in response to infection by Fusarium oxysporum f. sp. ciceris: apoplastic antioxidant enzyme activities and expression of oxidative stress-related genes

Activity levels of oxidative stress-related enzymes in the root apoplast during the interaction of WR315 (resistant) and JG62 (susceptible) chickpeas ( Cicer arietinum L.) with the highly virulent race 5 of Fusarium oxysporum f. sp. ciceris were compared. Because this fungus develops asymptomatic infections in the chickpea root cortex in both susceptible and resistant plants, but only intrudes into the root xylem in the susceptible variety, the interactions were compared at three specific stages during disease development in JG62: (i) before symptom development (10 days after inoculation); (ii) at the time of appearance of the first disease symptoms (15–17 days after inoculation) and (iii) when all plants had developed disease symptoms (20–22 days after inoculation). Diamine oxidase (DAO), ascorbate peroxidase (APX), glutathione reductase (GR), guaiacol-dependent peroxidase and superoxide dismutase (SOD), but not catalase (CAT), were found in the apoplast of chickpea roots. In terms of APX activity, infection by the pathogen caused a different response in the incompatible compared to the compatible plant. In the case of GR, SOD and DAO activities, the pathogen caused the same response, but it developed earlier ( i.e. GR and SOD) or to higher levels ( i.e. DAO) in the incompatible interaction. Expression of apx , cat , sod , lipoxygenase ( lox ) and actin genes was also analysed in infected roots. Infection by F. oxysporum f. sp. ciceris race 5 only caused a significant change in the root expression of lox and actin genes. This up-regulation was earlier ( lox ) or higher ( actin ) in the incompatible than in the compatible interaction. Thus, changes in oxidative metabolism differ in compatible and incompatible interactions in Fusarium wilt of chickpea.     

Does aluminium affect root growth of maize through interaction with the cell wall – plasma membrane – cytoskeleton continuum?

The mechanism of aluminium-induced inhibition of root elongation is still not well understood. It is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic. The present paper summarises experimental evidence which offers new avenues in the understanding of Al toxicity and resistance in maize. Application of Al for 1 h to individual 1 mm sections of the root apex only inhibited root elongation if applied to the first 3 apical mm. The most Al-sensitive apical root zone appeared to be the 1–2 mm segment. Aluminium-induced prominent alterations in both the microtubular (disintegration) and the actin cytoskeleton (altered polymerisation patterns) were found especially in the apical 1–2 mm zone using monoclonal antibodies. Since accumulation of Al in the root apoplast is dependent on the properties of the pectic matrix, we investigated whether Al uptake and toxicity could be modulated by changing the pectin content of the cell walls through pre-treatment of intact maize plants with 150 mM NaCl for 5 days. NaCl-adapted plants with higher pectin content accumulated more Al in their root apices and they were more Al-sensitive as indicated by more severe inhibition of root elongation and enhanced callose induction by Al. This special role of the pectic matrix of the cell walls in the modulation of Al toxicity is also indicated by a close positive correlation between pectin, Al, and Al-induced callose contents of 1 mm root segments along the 5 mm root apex. On the basis of the presented data we suggest that the rapid disorganisation of the cytoskeleton leading to root growth inhibition may be mediated by interaction of Al with the apoplastic side of the cell wall – plasma membrane – cytoskeleton continuum. This revised version was published online in June 2006 with corrections to the Cover Date.     

秸秆质外体蛋白对纤维素酶活力的影响

研究了玉米秸秆在储存过程中质外体蛋白含量的变化情况,及其对Penicilllumexpansum纤维素酶活力的影响。结果表明随着储存时间的延长,可抽提的玉米秸秆质外体蛋白的数量逐渐减少,与P.expansum纤维素酶的协同作用也随之减弱。储存玉米秸秆质外体蛋白没有内源性纤维素酶活力,而新鲜玉米秸秆有内源性EG活性,但它的稳定性较差,储存半年后基本降解或失活。储存秸秆质外体蛋白对FPA酶活力、棉花酶活力、β-葡萄糖苷酶活力有明显的增效作用,最大增效分别达到95.32%、102.06%和96.6%。而对CMCase却表现出抑制作用,最大抑制率为49.52%,质外体蛋白-EG-βG表现出明显的协同关系,而它对CBH酶活力没有影响。消除内源性EG的影响后,新鲜玉米秸质外体蛋白对FPA活力、棉花酶活力、β-葡萄糖苷酶活力的促进率,以及CMCase的抑制率均高于储存秸秆的质外体蛋白。可见质外体蛋白是天然纤维素酶解研究不可忽视的影响因素。     

植物根系分泌及其在林业中的意义

本文简要介绍了植物根系分泌物的成分、数量, 充分讨论了根系分泌的影响因素、根系分泌的部位及根系分泌物在土壤中扩散的范围, 并阐明了树木通过根系分泌对土壤营养条件、土壤微生物及其它相邻植物均有显著的影响。     

The effect of local application of fertilizer on the content of cytokinins in the xylem sap of maize

Cytokinin content in xylem sap was higher in plants grown under local supply of fertilizers as compared to those grown under homogenous distribution of nutrients in soil. The separate assay of cytokinins in xylem exudate from split root system showed that roots, which were in contact with fertilizer mainly contributed to cytokinins transported from roots to shoots. This revised version was published online in July 2006 with corrections to the Cover Date.     

Acidic cell elongation drives cell differentiation in the Arabidopsis root

In multicellular systems, the control of cell size is fundamental in regulating the development and growth of the different organs and of the whole organism. In most systems, major changes in cell size can be observed during differentiation processes where cells change their volume to adapt their shape to their final function. How relevant changes in cell volume are in driving the differentiation program is a long‐standing fundamental question in developmental biology. In the Arabidopsis root meristem, characteristic changes in the size of the distal meristematic cells identify cells that initiated the differentiation program. Here, we show that changes in cell size are essential for the initial steps of cell differentiation and that these changes depend on the concomitant activation by the plant hormone cytokinin of the EXPAs proteins and the AHA1 and AHA2 proton pumps. These findings identify a growth module that builds on a synergy between cytokinin‐dependent pH modification and wall remodeling to drive differentiation through the mechanical control of cell walls.     

In vitro molecular weight increase in xyloglucans by an apoplastic enzyme preparation from epicotyls of Vigna angularis

In order to gain insight into the mechanism of cell extension growth, enzymic processes involved in structural modification of cell wall xyloglucans were investigated, using an apoplastic enzyme preparation from epicotyls of dark grown Vigna angularis Ohwi et Ohashi cv. Takara and purified xyloglucans derived from cell walls of Vigna. The reaction of Vigna xyloglucan (mass average molecular weight=420 kDa) with the apoplastic enzyme preparation gave three fractions: (1) a waterinsoluble high molecular weight (820 kDa) xyloglucan fraction (WI), (2) a watersoluble low molecular weight (149 kDa) xyloglucan fraction (WS), and (3) an 80% ethanol-soluble monosaccharide fraction (ES). WI and WS were chiefly composed of t -galactosyl-, t -xylosyl-, 2-xylosyl-, 4-glucosyl- and 4,6-glucosyl residues, whereas ES was composed of fucose, galactose, glucose and xylose monomers. The data indicate that WI is generated by the linking of xyloglucan molecules by some alkali stable linkages, probably of glycosidic nature. The optimal pH for the WI-producing activity of the apoplastic enzyme preparation was 5.4. Higher WI-producing activity was detected in the upper juvenile than in the lower non-elongating regions of the epicotyl. Our data suggest the possible involvement of a transglycosylation reaction in the structural changes of the xyloglucans that are responsible for cell extension growth of the Vigna angularis epicotyl. The data are also consistent with the idea that the enzymic processes are regulated by hydrogen ions in the apoplastic space.     

A new concept of root exudation1

After discussing numerous models for exudation from the xylem of roots, we present a new biphasic exudation model based on osmoregulation of the root symplast by stretch-activated ion channels (SA channels). We tested some features of the model in maize roots. (1) Using a microdrop recorder we showed that bathing the roots in 50 mmol m^?3 gadolinium ions, known to inhibit some SA channels, inhibited xylem exudation by over 80% after 24h application. (2) Measuring xylem exudation from single roots into an attached micropipette revealed the capacity of the roots to perform strong autonomous exudation pulses. (3) In partially encased roots, the rhizodermis exuded water concurrently to xylem exudation. These results were regarded as supporting our model. An interesting observation with the microdrop recorder, which does not address the theory, is that addition of a variety of inorganic ions to distilled water as the roots' bathing medium instantaneously and reversibly increases xylem exudation, evidently nonosmotically.     

根分泌物及其生态效应

根分泌物是个古老而年轻的研究领域。早在18,19世纪,人们(Plenk,1795;Decandolle,1830)就观察到根分泌物对邻近植株的促生和抑制作用,1904年Hilter提出“根际”的概念,标志着人们对根分泌物及其生态效应的进一步认识。此后人们对根分泌物研究逐步展开。Lyon和Willson(1920)发现,生长于无菌水培液中的植物能释放有机物,为深入研究奠定了基础。但很长一段时间这个研究领域一     

Does hydraulic lift exist in shallow-rooted species? A quantitative examination with a half-shrub Gutierrezia sarothrae

Hydraulic lift occurs in some deep-rooted shrub and herbaceous species. In this process, water taken up by deep roots from the moist subsoil is delivered to the drier topsoil where it is later reabsorbed by shallow roots. However, little is known about the existence of hydraulic lift in shallow-rooted xeric species. The objectives of this study were 1) to ascertain whether hydraulic lift exists in Gutierrezia sarothrae (broom snakeweed), a widespread North American desert species with a shallow root system, grown in pot and field conditions and 2) if it does, how much water can be transferred from the subsoil to the 30 cm topsoil during the night. Snakeweed seedlings were transplanted in buried pots allowing the deeper roots to grow into the subsoil 30 cm below the surface. Soil water content inside and outside of the pot was measured seasonally and diurnally with time domain reflectometry technique (TDR). An increase in water content was detected in the pot after the plant was covered for 3 h by an opaque plastic bag during the day, suggesting hydraulic lift from deeper depths and exudation of water into the drier topsoil. Root exudation was also observed on native range sites dominated by snakeweed. Water efflux in the pot was 271 g per plant per night. which was equivalent to 15.3% of the extrapolated, porometer-derived whole-plant daily transpiration. Hydraulic lift observed in Gutierrezia improved water uptake during the day when evaporative demand is high and less water is available in the topsoil. We concluded that hydraulic lift might help snakeweed to alleviate the effect of water stress.