Role of Root Hairs and Lateral Roots in Silicon Uptake by Rice

The rice plant (Oryza sativa L. cv Oochikara) is known to be a Si accumulator, but the mechanism responsible for the high uptake of Si by the roots is not well understood. We investigated the role of root hairs and lateral roots in the Si uptake using two mutants of rice, one defective in the formation of root hairs (RH2) and another in that of lateral roots (RM109). Uptake experiments with nutrient solution during both a short term (up to 12 h) and relatively long term (26 d) showed that there was no significant difference in Si uptake between RH2 and the wild type (WT), whereas the Si uptake of RM109 was much less than that of WT. The number of silica bodies formed on the third leaf in RH2 was similar to that in WT, but the number of silica bodies in RM109 was only 40% of that in WT, when grown in soil amended with Si under flooded conditions. There was also no difference in the shoot Si concentration between WT and RH2 when grown in soil under upland conditions. Using a multi-compartment transport box, the Si uptake at the root tip (0-1 cm, without lateral roots and root hairs) was found to be similar in WT, RH2, and RM109. However, the Si uptake in the mature zone (1-4 cm from root tip) was significantly lower in RM109 than in WT, whereas no difference was found in Si uptake between WT and RH2. All these results clearly indicate that lateral roots contribute to the Si uptake in rice plant, whereas root hairs do not. Analysis of F(2) populations between RM109 and WT showed that Si uptake was correlated with the presence of lateral roots and that the gene controlling formation of lateral roots and Si uptake is a dominant gene.     

The role of nitrate reduction in the anoxic metabolism of roots I. Characterization of root morphology and normoxic metabolism of wild type tobacco and a transformant lacking root nitrate reductase

Two tobacco lines with (Nicotiana tabacum cv. Gatersleben, WT) or without (transformant LNR-H) nitrate reductase in roots were chosen as model systems to re-evaluate the role of root nitrate reduction for survival of anoxia. In this first paper, the two hydroponically grown lines were compared with respect to their root morphology, root respiration and the root content of inorganic cations, anions, and metabolites. Leaf transpiration in relation to root morphology was also determined. In comparison to WT roots containing NR, the NR-free LNR-H transformants had slightly shorter and thicker roots with a lower root surface area per g leaf FW. Consistent with that, LNR-H leaves had lower transpiration rates than WT. LNR-H-roots also showed consistently higher respiration and higher contents of ATP, starch and hexose monophosphates than WT roots. Concentrations of free sugars were only slightly higher in LNR-H roots. Total soluble protein content was identical in both lines, whereas amino acids were higher in LNR-H. Contents of major inorganic cations and anions were also almost identical in both lines. We conclude that WT versus LNR-H plants are a suitable tool to re-evaluate the role of nitrate reduction in flooding tolerance.     

The role of nitrate reduction in the anoxic metabolism of roots II. Anoxic metabolism of tobacco roots with or without nitrate reductase activity

The effects of root anoxia on a tobacco (Nicotiana tabacum) wild type (WT) and a transformant (LNR-H) lacking root nitrate reductase were compared. LNR-H plants were visibly more sensitive to oxygen deprivation than WT, showing rapid and heavy wilting symptoms. LNR-H roots also produced substantially more ethanol and lactate than WT roots under anoxia, and their sugar and sugar-P content, as well as their ATP levels, remained higher. The fermentation rates of WT and LNR-H roots were unaffected by sugar feeding and the higher fermentation rate in the LNR-H roots was associated with a greater acidification of the cytoplasm under anoxia. From these observations it is concluded: (i) that the absence of NR activity in the LNR-H roots does not necessarily limit NADH recycling; and (ii) that nitrate reduction in the WT roots results in a more acidifying metabolism. It is the higher metabolic rate in the LNR-H roots that leads to the greater cytoplasmic acidification under anoxia despite the absence of a contribution from the metabolism of nitrate. Competition for NADH cannot explain this difference in metabolic rate, and it remains unclear why the NR-free LNR-H, and tungstate-treated WT roots, had much higher fermentation rates than WT roots. The difference in anaerobic metabolism could still be due to the presence or absence of nitrate reductase and the possibility that this could occur through the production of nitric oxide is discussed.     

Supply and partitioning of assimilates to roots of Medicago sativa L. and Lotus corniculatus L. under anoxia

Abstract A current explanation of the mechanism of flooding injury to roots suggests that oxygen deficiency depresses the supply of respirable carbohydrates sufficiently to inhibit fermentation. However, even though it has been shown that phloem transport of assimilate is sharply reduced to anaerobic roots, inhibition of assimilate metabolism has also been suggested to be an important factor. This study examines these hypotheses by relating assimilate supply and metabolic activity in anoxic roots of alfalfa (Medicago sativa L.), a flood-intolerant species, and birdsfoot trefoil (Lotus corniculatus L.), a flood-tolerant plant. Roots were made anoxic (severe O₂ deficiency) for 2, 4 or 6 d and shoots were labelled with ¹⁴CO₂. Assimilate transport to the roots and metabolism to structural components were significantly decreased in both species in response to anoxia. Trefoil exhibited significantly greater ¹⁴C incorporation into the residue fraction at 4 d anoxia than did alfalfa, and this was consistent with the greater flooding tolerance of trefoil. When assimilate supply to O₂-deficient roots was decreased by shoot shading, shoot fresh weight was reduced by both anoxia and light treatments. Root-soluble sugars were significantly decreased by shading but were greatly increased in response to anoxia. Root starch concentration also increased under anoxia. Root K⁺ concentration was reduced by anoxia only. The energy status (ATP/ADP) of roots was significantly decreased by shading; however, anoxia reduced the energy status only in unshaded plants. The data indicate that carbohydrate supply to anaerobic roots does not appear to be a limiting factor in the metabolic response of alfalfa roots. Alternatively, metabolism of assimilate in anoxic roots may be an important determinant of survival.     

水稻超短根突变体ssr1的遗传分析和基因定位

该研究从甲基磺酸乙酯(EMS)诱变的籼稻‘Kasalath’突变体库中筛选到1个根系超短的突变体,命名为ssr1(super short root 1),8d苗龄突变体的主根和不定根长度分别只有野生型的8.89%和2.29%,其不定根发生正常,但侧根的发生和伸长都受到严重抑制,且根毛也非常短。此外,ssr1植株整体矮小,株高不到野生型的一半。遗传分析结果表明,该突变性状由1对隐性核基因控制。利用图位克隆技术将SSR1基因定位在第9染色体的STS(sequence tagged site)分子标记9g7047K和9g7290K之间,物理距离约为243kb,在定位区间共发现39个预测基因,经分析其中没有已克隆的根系发育基因。对SSR1的定位为进一步克隆该基因和阐明水稻根构型的分子机理奠定了基础。     

Anoxia tolerance and ethanol sensitivity of rice and oat coleoptiles

Anoxia tolerance and ethanol sensitivity of rice (Oryza sativa L.) and oat (Avena sativa L.) seedlings were evaluated to clarify their growth habit in anoxia. Anoxic stress inhibited elongation and dry weight gain of coleoptiles of the oat and rice seedlings; however, the inhibition of the oat coleoptiles was much greater than that of the rice coleoptiles. Anoxic stress increased endogenous ethanol concentration and alcohol dehydrogenase activity in oat and rice coleoptiles and their increases in the rice coleoptiles were much greater than those in the oat coleoptiles. At concentrations greater than 30 mM and 300 mM, exogenously applied ethanol inhibited the elongation and weight gain for the oat and the rice coleoptiles, respectively, and the inhibition was increased with increasing ethanol concentrations with marked inhibition being achieved on the oat coleoptiles. These results suggest that anoxia tolerance and induction of ethanolic fermentation in anoxia may be greater in rice than oat, and ethanol sensitivity of rice may be lower than that of oat.     

Intensity of hydrostimulation for the induction of root hydrotropism and its sensing by the root cap

Roots of Pisum sativum L. and Zea mays L. were exposed to different moisture gradients established by placing both wet cheesecloth (hydrostimulant) and saturated aqueous solutions of various salts in a closed chamber. Atmospheric conditions with different relative humidity (RH) in a range between 98 and 86% RH were obtained at root level, 2 to 3mm from the water-saturated hydrostimulant. Roots of Silver Queen corn placed vertically with the tips down curved sideways toward the hydrostimulant in response to approximately 94% RH but did not respond positively to RH higher than approximately 95%. The positive hydrotropic response increased linearly as RH was lowered from 95 to 90%. A maximum response was observed at RH between 90 and 86%. However, RH required for the induction of hydrotropism as well as the responsiveness differed among plant species used; gravitropically sensitive roots appeared to require a somewhat greater moisture gradient for the induction of hydrotropism. Decapped roots of corn failed to curve hydrotropically, suggesting the root cap as a major site of hydrosensing.     

Mitochondrial ultrastructure in roots of mesophyte and hydrophyte at anoxia and after glucose feeding

Summary In order to investigate the nature of the tolerance of mesophytes and hydrophytes to root anaerobiosis, changes in the mitochondrial ultrastructure of excised roots (with and without added glucose under anoxia) were studied in plants from two ecologically opposite types-pumpkin and rice.A 12-hour exposure to anoxia led to mitochondrial degradation in roots of adult rice and pumpkin plants. The addition of glucose preserved cell ultrastructure for up to 72–96 hours. During this period mitochondrial ultrastructure changed. In rice roots this primarily involved an increased number of cristae and a change in their arrangement into parallel rows. Cells of pumpkin roots displayed long mitochondria (up to 55 m) of different profiles which fused to form a complex mitochondrial network that was in close association with parts of the endoplasmic reticulum carrying a large number of ribosomes. This may be regarded as an adaptive development that facilitates the transport of glycolytic energy along mitochondrial membranes to the sites of protein synthesis.It is concluded that root cells of a hydrophyte are not more tolerant to anoxia than mesophyte. Thus, the ability of hydrophytes to grow on anaerobic soils should be attributed not so much to peculiar features of the roots' metabolism but to the ability of these plants to perform an easy transport of O₂ from leaves to roots. With respect to mesophytes it is stressed that the supply of assimilates is important for the resistance of roots to soil anaerobiosis.     

Relationship between the Accumulation of Putrescine and the Tolerance to Oxygen-Deficit Stress in Gramineae Seedlings

The accumulation of putrescine under anoxia was studied in seedlingsof 6 Gramineae species showing tolerance to the stress in thefollowing order: rice, barnyard grass>maize>rye, barleyand wheat. The accumulation of putrescine in shoots and rootsduring 6 h of oxygen deprivation was different among the speciesand correlated with their tolerance to anaerobic conditions.In both tissues, rice and barnyard grass accumulated more than0.6 µmol/g fresh weight. The lower increase in the putrescinetiter was observed in wheat tissues with less than 0.1 µmol/gfresh weight. Feeding experiments with [^l4C]arginine showedthat rice tissues have a greater capacity than wheat tissuesto synthesize putrescine under anoxia. The better tolerant speciesto anaerobic conditions showed high arginine decarboxylase activityin shoots and roots than the sensitive species. The presenceof exogenous Put partially increased the survival of wheat rootsafter 7.5 h of oxygen-deficit stress. The role of putrescinein determining differences in the tolerance to anoxia in Gramineaeseedlings is discussed. (Received December 20, 1989; Accepted March 12, 1990)     

Auxin and its transport play a role in plant tolerance to arsenite‐induced oxidative stress in Arabidopsis thaliana

The role of auxin in plant development is well known; however, its possible function in root response to abiotic stress is poorly understood. In this study, we demonstrate a novel role of auxin transport in plant tolerance to oxidative stress caused by arsenite. Plant response to arsenite [As(III)] was evaluated by measuring root growth and markers for stress on seedlings treated with control or As(III)‐containing medium. Auxin transporter mutants aux1, pin1 and pin2 were significantly more sensitive to As(III) than the wild type (WT). Auxin transport inhibitors significantly reduced plant tolerance to As(III) in the WT, while exogenous supply of indole‐3‐acetic acid improved As(III) tolerance of aux1 and not that of WT. Uptake assays using H³‐IAA showed As(III) affected auxin transport in WT roots. As(III) increased the levels of H₂O₂ in WT but not in aux1, suggesting a positive role for auxin transport through AUX1 on plant tolerance to As(III) stress via reactive oxygen species (ROS)‐mediated signalling. Compared to the WT, the mutant aux1 was significantly more sensitive to high‐temperature stress and salinity, also suggesting auxin transport influences a common element shared by plant tolerance to arsenite, salinity and high‐temperature stress.     

Effect of hexokinase activity on tomato root metabolism during prolonged hypoxia

Hypoxically induced tolerance to anoxia in roots of tomato (Solanum lycopersicum) was previously shown to depend on sucrose and the induction of sucrose synthase. In contrast to maize, root hexokinase (HXK) activities did not increase during hypoxia and glucose was unable to sustain glycolytic flux under anoxia. In this paper, we asked whether hypoxic metabolism in roots would be altered in transgenic tomato plants overexpressing either a plant (Arabidopsis) or a yeast (Saccharomyces cerevisiae) HXK and whether such modifications could be related to improved energy metabolism and consequently root tolerance under anoxia. Tomato plants grown hydroponically with shoots always maintained in air were submitted to a 7 d hypoxic treatment applied by stopping air bubbling. A combination of techniques including (1)H-nuclear magnetic resonance spectroscopy, RT-PCR and enzyme analyses was used to obtain a broad picture of hypoxic root metabolism. In normoxic conditions, HXK overexpression resulted in higher ADP and AMP levels only in roots of AtHXK1 transgenic plants. During hypoxic treatment, oxygen levels in the hydroponic tank decreased rapidly to 5 kPa within the first 2 d and then remained at 5 kPa throughout the 7 d experiment. Oxygen levels were similar at 5 and 20 cm below the water surface. A decline of the adenylate energy status was observed after 2 d of hypoxic treatment, with a further decrease by 7 d in roots of non-transgenic (WT) and ScHXK2, but not in AtHXK1 transgenic plants. Sucrose synthase activity increased to comparably higher levels at 7 d of hypoxic treatment in WT and ScHXK2 compared with AtHXK1 roots. Differences between WT and the transgenic plants are discussed with respect to the metabolic response to low (hypoxia) but not zero (anoxia) oxygen.     

Silicon suppresses zinc uptake through down-regulating zinc transporter gene in rice

One of the beneficial effects of silicon (Si) is to improve nutrient imbalance including deficiency and excess of nutrients, however the molecular mechanisms underlying this effect are still poorly understood. In this study, we investigated the interaction between Si and zinc (Zn) in rice by using a mutant (lsi1) defective in Si uptake and its wild-type (WT, cv. Oochikara) at different Zn levels. High Zn inhibited the root elongation of both WT and lsi1 mutant, but Si did not alleviate this inhibition in both lines. By contrast, Si supply decreased Zn concentration in both the roots and shoots of the WT, but not in the lsi1 mutant. A short-term (24 h) labeling experiment with stable isotope ⁶⁷Zn showed that Si decreased ⁶⁷Zn uptake, but did not affect the root-to-shoot translocation and distribution ratio to different organs of ⁶⁷Zn in the WT. Furthermore, Si accumulated in the shoots, rather than Si in the external solution, is required for suppressing Zn uptake, but this was not caused by Si-decreased transpiration. A kinetic study showed that Si did not affect K_m value of root Zn uptake, but decreased V_max value in the WT. Analysis of genes related with Zn transport showed that among ZIP family genes, the expression of only OsZIP1 implicated in Zn uptake, was down-regulated by Si in the WT, but not in the lsi1 mutant. These results indicate that Si accumulated in the shoots suppresses the Zn uptake through down-regulating the transporter gene involved in Zn uptake in rice.     

Heterologous expression of Arabidopsis H+‐pyrophosphatase enhances salt tolerance in transgenic creeping bentgrass (Agrostis stolonifera L.)

The Arabidopsis vacuolar H⁺‐pyrophosphatase (AVP1), when over‐expressed in transgenic (TG) plants, regulates root and shoot development via facilitation of auxin flux, and enhances plant resistance to salt and drought stresses. Here, we report that TG perennial creeping bentgrass plants over‐expressing AVP1 exhibited improved resistance to salinity than wild‐type (WT) controls. Compared to WT plants, TGs grew well in the presence of 100 mm NaCl, and exhibited higher tolerance and faster recovery from damages from exposure to 200 and 300 mm NaCl. The improved performance of the TG plants was associated with higher relative water content (RWC), higher Na⁺ uptake and lower solute leakage in leaf tissues, and with higher concentrations of Na⁺, K⁺, Cl^‐ and total phosphorus in root tissues. Under salt stress, proline content was increased in both WT and TG plants, but more significantly in TGs. Moreover, TG plants exhibited greater biomass production than WT controls under both normal and elevated salinity conditions. When subjected to salt stress, fresh (FW) and dry weights (DW) of both leaves and roots decreased more significantly in WT than in TG plants. Our results demonstrated the great potential of genetic manipulation of vacuolar H⁺‐pyrophosphatase expression in TG perennial species for improvement of plant abiotic stress resistance.     

Phytochrome B control of total leaf area and stomatal density affects drought tolerance in rice

We report that phytochrome B (phyB) mutants exhibit improved drought tolerance compared to wild type (WT) rice (Oryza sativa L. cv. Nipponbare). To understand the underlying mechanism by which phyB regulates drought tolerance, we analyzed root growth and water loss from the leaves of phyB mutants. The root system showed no significant difference between the phyB mutants and WT, suggesting that improved drought tolerance has little relation to root growth. However, phyB mutants exhibited reduced total leaf area per plant, which was probably due to a reduction in the total number of cells per leaf caused by enhanced expression of Orysa;KRP1 and Orysa;KRP4 (encoding inhibitors of cyclin-dependent kinase complex activity) in the phyB mutants. In addition, the developed leaves of phyB mutants displayed larger epidermal cells than WT leaves, resulting in reduced stomatal density. phyB deficiency promoted the expression of both putative ERECTA family genes and EXPANSIN family genes involved in cell expansion in leaves, thus causing greater epidermal cell expansion in the phyB mutants. Reduced stomatal density resulted in reduced transpiration per unit leaf area in the phyB mutants. Considering all these findings, we propose that phyB deficiency causes both reduced total leaf area and reduced transpiration per unit leaf area, which explains the reduced water loss and improved drought tolerance of phyB mutants.     

Ion uptake and respiration of dry bean roots subjected to localized anoxia

Summary Ion uptake by dry bean root systems was examined during a three day treatment period. Three aeration treatments were applied to split root systems where both halves were aerated, both halves were nonaerated and one half aerated and the remaining half nonaerated (localized anoxia). Ion absorption was similar for the aerated control and localized anoxia treatments. The nonaerated control absorbed 2, 40, and 60 percent of the aerated control for K⁺, Ca⁺⁺, and NO₃ ⁻, respectively. Ion absorption by stressed plants appeared to increase directly with root growth in the aerated portions of the localized anoxia treatments. Localized anoxia resulted in greater potassium ion uptake per unit root weight and in greater root respiration rates of the aerated half of the Pinto III cultivar root system. Transpiration rates of Seafarer subjected to localized anoxia were 135% of the aerated control. The additional water use may have contributed to greater ion uptake, by mass flow, in the nonaerated portion of the localized anoxia treatment. Nutrient solutions of the nonaerated controls became more alkaline during stress than did the nonaerated portions of the localized anoxia treatments, indicating a possible direct or indirect effect of the aerated portions of the localized anoxia treatments on the corresponding nonaerated half. Compensation in ion uptake by dry bean roots subjected to localized anoxia appeared to be the result of increased root growth, greater respiration rates, greater transpiration rates and, for Pinto III, an increase in the ion uptake rate per unit root weight. This compensatory uptake of water and nutrients by the root system may be one mechanism by which roots overcome localized stress within a soil profile.     

Anoxia tolerance and α-amylase activity in four rice cultivars

The relationship between anoxia tolerance and reserved carbohydrate catabolism was investigated in four rice (Oryza sativa L.) cultivars subjected to a 48-h anoxic stress. The coleoptile elongation of all cultivars was suppressed by anoxic stress, however, the elongation of cvs Koshihikari and Awa-akamai was much greater than that of cvs Touzoumochi and Asahimochi. The anoxic coleoptiles of cvs Koshihikari and Awa-akamai contained about 2-fold as much ATP as those of cvs Touzoumochi and Asahimochi. Ethanol production in the anoxic coleoptiles of cvs Koshihikari and Awa-akamai was about 2-fold as much as that of cvs Touzoumochi and Asahimochi, which suggests that ethanolic fermentation is probably more active in cvs Koshihikari and Awa-akamai than in cvs Asahimochi and Touzoumochi. Activity of α-amylase, which catabolizes starch to soluble sugars, in endosperms of cvs Koshihikari and Awa-akamai was about 2-fold that of cvs Touzoumochi and Asahimochi, and soluble sugar concentration in the coleoptiles of cvs Koshihikari and Awa-akamai was about 3-fold greater than that of cvs Touzoumochi and Asahimochi. Soluble sugar concentration and ethanol production rate in the coleoptiles of rice seedlings were correlated well with α-amylase activity in their endosperms, which were also correlated well with anoxia tolerance with respect to the coleoptile elongation and ATP concentration in the coleoptiles. These results suggest that the ability to degrade starch to soluble sugar by α-amylase in endosperm may be important for the anoxia tolerance in rice coleoptiles and it may serve to distinguish the anoxia tolerance of rice coleoptiles.