Ammonium accumulation in relation to growth of rice cell suspension under phosphate deprivation

Suspension-cultured rice cells growth was markedly inhibited and ammonium content increased when rice cells were deprived of phosphate. When rice cells were cultured at increasing concentrations of ammonium chloride, ammonium content increased, however, no significant inhibition of cell growth was observed. Addition of D-arginine, an inhibitor of putrescine biosynthesis, resulted in a complete recovery of growth in rice cells under phosphate deprivation, but did not decrease the content of ammonium. Our results indicate that the growth inhibition induced by phosphate deprivation is not associated with ammonium accumulation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparative analysis of anoxic coleoptile elongation in rice varieties: relationship between coleoptile length and carbohydrate levels, fermentative metabolism and anaerobic gene expression

Rice ( Oryza sativa L.) seeds can germinate under anoxia and can show coleoptile elongation. The anoxic coleoptile is usually longer than aerobic coleoptiles. Although several hypotheses have been proposed to explain the ability of rice to elongate coleoptiles under anoxia, conclusive experimental evidence explaining this physiological trait is lacking. In order to investigate whether metabolic and molecular markers correlate with anoxic coleoptile length, we screened 141 Italian and 23 Sri Lankan rice cultivars for their ability to elongate coleoptiles under anoxia. Differences in anoxic coleoptile length were used to evaluate whether a correlation exists between coleoptile length and biochemical and molecular parameters. The expression of genes coding for glycolytic and fermentative enzymes showed a very low correlation with anoxic coleoptile length. Although differences were found in carbohydrate content between the varieties tested, this parameter also does not appear to be critical in terms of coleoptile elongation. Efficient ethanol fermentation does, however, correlate well with the elongation of coleoptiles under anoxic conditions.     

Changes at panicle emergence in the water relations of a wetland and a dryland Japonica rice cultivar under wetland conditions

The diurnal and seasonal changes in plant water relations of two Japonica rice ( Oryza sativa L.) cultivars, Nipponbare and Tachiminori, were studied under flooded conditions at Kyoto University. The dryland cv. Tachiminori maintained higher predawn and midday leaf osmotic potentials relative to the wetland cv. Nipponbare during the vegetative stage, but the ranking was reversed after flowering. The relationship between leaf water potential and leaf osmotic potential showed that prior to panicle emergence Nipponbare was able to adjust osmotically to maintain turgor, whereas after heading there was little turgor maintenance. Tachiminori showed little difference in osmotic adjustment before and after panicle emergence. Fertilizer treatment during panicle development also helped to maintain the degree of osmotic adjustment in both cultivars.     

A loss‐of‐function allele of OsHMA3 associated with high cadmium accumulation in shoots and grain of Japonica rice cultivars

Excessive cadmium (Cd) accumulation in rice poses a risk to food safety. OsHMA3 plays an important role in restricting Cd translocation from roots to shoots. A non‐functional allele of OsHMA3 has been reported in some Indica rice cultivars with high Cd accumulation, but it is not known if OsHMA3 allelic variation is associated with Cd accumulation in Japonica cultivars. In this study, we identified a Japonica cultivar with consistently high Cd accumulation in shoots and grain in both field and greenhouse experiments. The cultivar possesses an OsHMA3 allele with a predicted amino acid mutation at the 380^th position from Ser to Arg. The haplotype had no Cd transport activity when the gene was expressed in yeast, and the allele did not complement a known nonfunctional allele of OsHMA3 in F₁ test. The allele is present only in temperate Japonica cultivars among diversity panels of 1483 rice cultivars. Different cultivars possessing this allele showed greatly increased root‐to‐shoot Cd translocation and a shift in root Cd speciation from Cd―S to Cd―O bonding determined by synchrotron X‐ray absorption spectroscopy. Our study has identified a new loss‐of‐function allele of OsHMA3 in Japonica rice cultivars leading to high Cd accumulation in shoots and grain.     

Heavy metal accumulation in rice (Oryza sativa) near electronic waste dumps and related human health risk assessment

This research was conducted to assess heavy metal contamination in the environment and within Oryza sativa. The translocation factors (TFs) and bioaccumulation factors (BAFs) for heavy metals in O. sativa and estimated daily intake (EDI) and health risk index (HRI) were measured. The samples were analyzed for heavy metals using inductively coupled plasma optical emission spectrometry (ICP-OES). Pb and Cr concentrations in water samples within and near the electronic-waste dumping area exceeded water quality standards for surface water sources from the Pollution Control Department in the Ministry of Natural Resources and Environment of Thailand (PCD). The Pb concentration in soil samples within the area also exceeded soil quality standards for habitat and agriculture from PCD. Most of the metals were highly concentrated in roots, except for Mn which has the highest concentration in leaves. Pb concentrations in rice grains exceeded the FAO/WHO standard (0.2 mg/kg). The average TF values for heavy metals from the soil to roots, roots to stems, stems to leaves, and stems to grains were Mn > Pb > Ni > Cr, Mn > Cr > Ni > Pb, Ni > Pb > Mn > Cr, and Pb > Ni > Cr > Mn, respectively. The average BAF values in O. sativa were Mn > Ni > Pb > Cr. The EDI for Cr, Pb, Mn, and Ni via O. sativa consumption were 6.19, 6.02, 370.57, and 3.80 µg/kg/day, respectively. The HRI for Cr, Pb, Mn, and Ni via O. sativa consumption were 0.30, 1.50, 2.60, and 0.002, respectively.     

NaCl-induced changes in cytosolic free Ca2+ in Arabidopsis thaliana are heterogeneous and modified by external ionic composition

Increases in cytosolic free Ca²⁺ ([Ca²⁺]_cyt) are common to many stress-activated signalling pathways, including the response to saline environments. We have investigated the nature of NaCl-induced [Ca²⁺]_cyt signals in whole Arabidopsis thaliana seedlings using aequorin. We found that NaCl-induced increases in [Ca²⁺]_cyt are heterogeneous and mainly restricted to the root. Both the concentration of NaCl and the composition of the solution bathing the root have profound effects on the magnitude and dynamics of NaCl-induced increases in [Ca²⁺]_cyt. Alteration of external K⁺ concentration caused changes in the temporal and spatial pattern of [Ca²⁺]_cyt increase, providing evidence for Na⁺-induced Ca²⁺ influx across the plasma membrane. The effects of various pharmacological agents on NaCl-induced increases in [Ca²⁺]_cyt indicate that NaCl may induce influx of Ca²⁺ through both plasma membrane and intracellular Ca²⁺-permeable channels. Analysis of spatiotemporal [Ca²⁺]_cyt dynamics using photon-counting imaging revealed additional levels of complexity in the [Ca²⁺]_cyt signal that may reflect the oscillatory nature of NaCl-induced changes in single cells.     

Production and organ distribution of succinate in rice seedlings during anoxia

Anaerobic production of succinate, a common feature in animals able to sustain anoxia, has seldom been reported in plants. By the use of ¹H-nuclear magnetic resonance spectroscopy we show here that succinate is produced by rice seedlings (Oryza sativa L. cv. Arborio) subjected to anoxic conditions. Starting from levels below I μmol (g fresh weight)⁻¹ in air, after 48 h of anoxia the levels of alanine, succinate and lactate had increased to 23.8, 5.2 and 1.0 μmol (g fresh weight) ⁻¹, respectively, in shoot tissues. Succinate was accumulated in shoots, notably in the coleoptiles, but not in roots of the rice seedlings, suggesting its involvement in rice coleoptile elongation under anoxia. Other possible functions of succinate production in rice seedling, an organism highly tolerant to anoxia, are discussed.     

低磷胁迫不同磷效率水稻苗期根系的生理适应性研究

以P效率差异显著的IR74（P高效型）、IR71331（中间型）及IR71379（P低效型）等3个品种为供试材料,采用水培法研究了这3类水稻在低P胁迫下P的吸收、转运及利用效率等P效率差异的原因以及P吸收动力学特征参数、R／S、酸性磷酸酯酶（acid phosphoesterase,简称APase）与核糖核酸酶（ribonuclease,简称RNase）等表现。结果表明,在低P胁迫下,P效率的高低是由水稻对P的吸收率、转运率以及利用效率综合作用的结果,存在基因型差异,P高效基因型IR74和P效率中间型的IR71331具有高的P吸收率,而IR712379P的吸收率低。不同P效率水稻在低P胁迫下,其Km、Cmin、Imax、R／S与Apase相对活性等参数表现出基因型的差异,小的Km和Cmin,大的Imax和R／S及高的APase升幅是水稻对P胁迫的生理适应性特征,也是植株在低P胁迫下较为敏感生理指标。其中各类型水稻叶片中RNase活性在低P胁迫一均表现大幅度上升,在品种间无显著差异,说明该酶是逆境胁迫并非低P胁迫的特征反应。     

Gaseous factors involved in the enhanced elongation of rice coleoptiles under water

Abstract. Elongation responses of intact coleoptiles of rice (Oryza sativa L. ev. Sasanishiki) explants to various gases were examined under submerged conditions in continuously flowing gas-saturated incubation media. Reduced O₂ tension (hypoxia). CO₂ and especially C₂H₄ significantly stimulated coleoptile elongation; the optimal concentrations of O₂, CO₂ and C₂H₄ when applied singly were 0.07 m³ m^-3, 0.10 m³ m^-3, and 3 cm³, respectively. However, in addition to these gases other as yet unknown factors were involved in the enhanced elongation of rice coleoptiles under water. The actions of CO₂ and C₂H₄, unlike that of hypoxia, were accompanied by increases in dry weight of the coleoptiles. The effect of C₂H₄ occurred independently of O₂ concentrations, whereas that of CO₂ occurred above 0.08 m³ m^-3O₂. Maximum elongation of rice coleoptiles under submerged conditions was obtained when the flowing medium was saturated with a gas mixture containing 0.10 m³ m^-3 O₂, 0.10 m³ m^-3 CO₂ and 10 cm³ m^-3 C₂H₄, greatly surpassing elongation in static media. However, elongation in static media was greater than that in a closed atmosphere. The intercellular C₂H₄ concentration in explants growing in static media was higher than that in a closed atmosphere. These results showed that the coleoptile elongation of rice seedlings under water may be regulated by the accumulation of CO₂ and C₂H₄ in and around the seedlings under hypoxic conditions.     

Differential expression of cell wall related genes in the elongation zone of rice roots under water deficit

Growth in the apical elongation zone of plant roots is central to the development of functional root systems. It has been known that rice seminal root elongation could be enhanced by water stress. In the present study, 17 cell-wall related genes were identified by cDNA-amplified fragment length polymorphism (cDNA-AFLP) technique. Five genes encoded cell-wall loosening enzymes and six genes were involved in the lignin biosynthesis. The six other genes were related to the metabolism of polysaccharide and protein matrices in cell wall. Northern blot analysis confirmed that they were differentially expressed in the elongation zone of rice seminal roots under water stress, and none of them was root-specific. The results indicated that the activity of cell-wall loosening enzymes was enhanced in the early stage (within 16 h), and some cell wall matrices were synthesized rapidly in the middle stages (from 16 to 48 h), while lignin biosynthesis was enhanced in the middle and late stages of water stress (from 48 to 72 h). Published in Russian in Fiziologiya Rastenii, 2006, Vol. 53, No. 3, pp. 437–443. The text was submitted by the authors in English.     

Cadmium retention in rice roots is influenced by cadmium availability, chelation and translocation

Analysis of rice plants exposed to a broad range of relatively low and environmentally realistic Cd concentrations showed that the root capacity to retain Cd ions rose from 49 to 79%, corresponding to increases in the external Cd2+ concentration in the 0.01-1 μM range. Fractioning of Cd ions retained by roots revealed that different events along the metal sequestration pathway (i.e. chelation by thiols, vacuolar compartmentalization, adsorption) contributed to Cd immobilization in the roots. However, large amounts of Cd ions (around 24% of the total amount) predictable as potentially mobile were still found in all conditions, while the amount of Cd ions loaded in the xylem seemed to have already reached saturation at 0.1 μM Cd2+, suggesting that Cd translocation may also play an indirect role in determining Cd root retention, especially at the highest external concentrations. In silico search and preliminary analyses in yeast suggest OsHMA2 as a good candidate for the control of Cd xylem loading in rice. Taken as a whole, data indicate Cd chelation, compartmentalization, adsorption and translocation processes as components of a complex 'firewall system' which acts in limiting Cd translocation from the root to the shoot and which reaches different equilibrium positions depending on Cd external concentration.     

Interactive effects of Al, Ca and other cations on root elongation of rice cultivars under low pH

BACKGROUND AND AIMS: As with other crop species, Al tolerance in rice (Oryza sativa) is widely different among cultivars, and the mechanism for tolerance is unknown. The Ca2+-displacement hypothesis, that is, Al displaces Ca2+ from critical sites in the root apoplast, was predicted to be the essential mechanism for causing Al toxicity in rice cultivars. If displacement of Ca is an essential cause of Al toxicity in rice, Al toxicity may show the same trend as toxicities of elements such as Sr and Ba that are effective in displacing Ca. METHODS: The interactive effects of Al, Ca, Sr and Ba on root elongation of rice cultivars with different Al tolerances were evaluated in hydroponic culture. Al and Ca accumulation in root tips was also investigated. KEY RESULTS AND CONCLUSIONS: Not only Al but also Sr and Ba applications inhibited root growth of rice cultivars under low Ca conditions. As expected, rice cultivars more tolerant of Sr and Ba were also tolerant of Al (japonica > indica). Although Mg application did not affect Sr or Ba toxicity, Mg alleviated Al toxicity to the same level as Ca application. In addition, Ca application decreased the Al content in root tips without displacement. These results suggest that Ca does not have a specific, irreplaceable role in Al toxicity, unlike Sr and Ba toxicities. Alleviation of Al toxicity with increasing concentrations of Ca in rice cultivars is due to increased ionic strength, not due to decreased Al activity. The difference in Al tolerance between indica and japonica cultivars disappears under high ionic strength conditions, suggesting that different electrochemical characteristics of root-tip cells are related to the significant difference in Al tolerance under low ionic strength conditions.     

Ion accumulation in the cell walls of rice plants growing under saline conditions: evidence for the Oertli hypothesis

Abstract. When plants of rice ( Oryza saliva L.) are subjected to mildly saline (50mol m⁻³ NaCl) conditions, the leaves show symptoms of water deficit, even though ion accumulation has been more than sufficient to adjust to the decrease in external water potential. After a few days of exposure to salt, there is a negative correlation, in a population of leaves, between the leaf water concentration (g water per g dry weight) and their sodium concentration (mmol Na per g dry weight). Ion concentrations in the cell walls and the cytoplasm of cells of plants grown in low salinity were measured by X-ray microanalysis. The NaCl concentration in solution in the apoplast was calculated to be around 600mol m⁻³ in leaves of plants whose roots were exposed to only 50 mol m⁻³ NaCl. This constitutes strong evidence that an important factor in salt damage in rice is dehydration due to the extracellular accumulation of salt as suggested in the Oertli hypothesis. The implication, that changes in tissue ion concentration and solute potentials equivalent to the external medium is not evidence of plant osmotic adjustment to salinity, is discussed.     

Photosynthetic characteristics of five rice cultivars grown under increased solar UV-B radiation

The impact of increased solar UV-B radiation on photosynthetic characteristics of rice (Oryza sativa L.) cultivars ADT36, IR20, IR50, J13 and MDU4 has been studied. In all the cultivars concentrations of photosynthetic pigments decreased under increased UV-B radiation. Even low enhancement of UV-B reduced the photochemical activities in all the cultivars except MDU4 and changed chlorophyll a fluorescence. Enhanced UV-B radiation caused a dose-dependent changes in chloroplast proteins in most of the cultivars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gibberellin-induced elongation and osmoregulation in internodes of floating rice

Internodal elongation in floating rice ( Oryza sativa L. cv. Habiganj Aman II) is known to be enhanced by treatment with ethylene or gibberellic acid (GA₃) at high relative humidity (RH). However, ethylene-induced internodal elongation is inhibited at low RH. while GA₃-induced internodal elongation is hardly affected by humidity. We examined the possible involvement of osmoregulation in the stimulation by GA₃ of the elongation of internodes at low RH. Submergence and treatment with ethylene or GA3₃ at 100% RH increased the osmotic potential in internodes of excised stem segments, while GA₃ at 20% RH maintained the osmotic potential at a low level. In internodes of stem segments that had been treated with GA₃ at 20% RH, the activity of invertase and the level of soluble sugars were almost 2- and 1.5-fold higher, respectively, than those in internodes that had been treated with GA₃ at 100% RH. These results indicate that one of the possible mechanisms by which GA₃ promotes elongation of internodes at low RH involves the osmoregulation that is achieved by promotion of the synthesis of invertase.     

Effect of cadmium on proline accumulation and ribonuclease activity in rice seedlings: role of proline as a possible enzyme protectant

When seedlings of two rice (Oryza sativa L.) cultivars Ratna and Jaya were raised under 100 and 500 μM cadmium nitrate in the medium, a high proline content was noted in Cd²⁺ stressed seedlings compared to controls. Seedlings grown under 500 μM Cd(NO₃)₂ maintained increased proline level compared to non-stressed seedlings. Kinetic properties of RNase extracted from control grown and Cd²⁺ stressed seedlings showed a marked alteration in Km due to Cd²⁺ treatment. The RNase isoforms were purified from 15-d-old rice seedlings with a total purification of 22.25 fold and 74.75 % yield using conventional biochemical techniques. Three RNase isoforms, namely I, II and III were eluted from DEAE-Sephacel column. The isoform RNase II had Km value of 3.2 mg(RNA) cm^-3. The in vitro osmotic stress created by incorporation of PEG in the enzyme assay medium led to decreased affinity of enzyme towards its substrate with increase in Km. This loss in affinity was partially restored by the addition of 1 M proline in the assay medium, suggesting the possible protective role of proline on RNase under osmotic stress. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enhancement of NADP-malic enzyme in transgenic rice induced the accumulation of reactive oxygen species

It had been demonstrated that the photosynthetic photodamage, such as photoinhibition and photooxidation, was enhanced in transgenic rice plants overexpressing NADP-malic enzyme (ME). However, its physiological base has not been investigated. In order to elucidate the physiological elements contributed to the enhancement of photodamage in NADP-ME transgenic rice plants, some physiological indices related to reactive oxygen species (ROS) accumulation were studied using the T₁ progeny of transgenic rice plants. The results showed that more ROS such as O ₂ ^−. and H₂O₂ were accumulated in transgenic rice plants, which enhanced photooxidation, while the accumulation of malondialdehyde in transgenic rice plants was not evident as compared with the wild-type plants. The measurement of NADPH/NADP ratios in leaves showed that transgenic rice plants had a higher ratio than untransformed rice plants. Based on these data, we speculated that overexpression of NADP-ME led to the deficiency of NADP and overreduction of photosystem I, which induced the accumulation of ROS in the transgenic rice plants, and just ROS were accounted for plant sensitivity to photooxidation. Published in Russian in Fiziologiya Rastenii, 2006, Vol. 53, No. 3, pp. 364–370. The text was submitted by the author in English.     

Crystal structure of nucleoside diphosphate kinase required for coleoptile elongation in rice (Oryza sativa L.)

Nucleoside diphosphate kinase (NDK) is a ubiquitous enzyme found in all organisms and cell types, and catalyzes the transfer of the phosphoryl group from a nucleoside triphosphate to a nucleoside diphosphate. The enzyme is involved in and required for coleoptile elongation in rice as the level of the rice NDK (rNDK) changes during seed germination and the early stages of seedling growth. The expression of rice NDK gene is up-regulated in the growing coleoptiles when the anaerobic stress persists. The rNDK structure determined at 2.5 A resolution consists of a four-stranded anti-parallel beta-sheet, of which the surfaces are partially covered with six alpha-helices; its overall and active site structures are similar to those of homologous enzymes except the major conformation variations of residue 132-138 regions, involving significant structural contacts. The model contains 148 residues of 149 residues in total and averaged 19 water molecules per monomer for 12 molecules in an asymmetric unit. A mold of 12 superimposed molecules shows that the alphaA-alpha2 area has greater variations and higher temperature factors, indicating the flexibility for a substrate entrance. Hexameric molecular packing in both crystal and solution implies that rNDK functions as hexamers. This rNDK structure, which is the first NDK structure from a higher plant system, provides the structural information essential to understand the functional significance of this enzyme during growth and development in both rice and other plants.