Rapid changes in levels of mineral nutrients in root-tip cells following short-term exposure to aluminium

We investigated the rapid modification of plasma membrane and changes in mineral nutrients in root-tip cells of Al-tolerant rice and Al-sensitive barley following short-term exposure to Al (20 M Al, 1 h). The plasma membrane of the barley cells was significantly permeabilized when re-elongated in an Al-free Ca solution following a 1-h pre-treatment with Al, while that of rice cells was not affected at all. The elemental distribution and concentration in a 2-mm portion of the root apex were determined by electron probe X-ray microanalysis. Al was localized primarily to the epidermis and outer cortex cells in both species, and was much more abundant in barley than in rice. Al increased and decreased remarkably the intracellular K concentration in whole root-tip cells of rice and barley, respectively. In barley, the decrease in the concentration of Ca coincided with the accumulation of Al. Conversely, the intracellular concentration of P in the surface layers of root-tip cells increased with the accumulation of Al. The distribution and concentration of Ca and P in rice did not change after 1-h treatment with Al. These results suggest that the rapid modification of the plasma membrane of root-tip cells induced by Al affects the nutritional homeostasis in the cells.     

One Novel Mitochondrial Citrate Synthase from <Emphasis Type="Italic">Oryza sativa</Emphasis> L. can Enhance Aluminum Tolerance in Transgenic Tobacco

Rice exhibits the greatest aluminum (Al) tolerance compared with other cereals such as wheat, barley, maize, etc. A full-length gene, OsCS1, encoding citrate synthase, which is highly induced by aluminum toxicity in rice (Oryza sativa L.), was isolated. Sequence analysis and the sub-cellular localization of OsCS1 in yeast revealed that it is a mitochondrial citrate synthase. OsCS1 was induced by Al toxicity. Several independent transgenic tobacco lines expressing OsCS 1 exhibitted increased citrate efflux and extraordinary Al tolerance. Possible outlook for OsCS1 to be applied to enhance plant tolerance to Al toxicity was also discussed.     

Ethylene Production by Fe-deficient Roots and its Involvement in the Regulation of Fe-deficiency Stress Responses by Strategy I Plants

Species that showed marked morphological and physiological responsesby their roots to Fe-deficiency (Strategy I plants) were comparedwith others that do not exhibit these responses (Strategy IIplants). Roots from Fe-deficient cucumber (Cucumis sativusL.‘Ashley’), tomato (Lycopersicon esculentumMill.T3238FER) and pea (Pisum sativumL. ‘Sparkle’) plantsproduced more ethylene than those of Fe-sufficient plants. Thehigher production of ethylene in Fe-deficient cucumber and peaplants occurred before Fe-deficient plants showed chlorosissymptoms and was parallel to the occurrence of Fe-deficiencystress responses. The addition of either the ethylene precursorACC, 1-aminocyclopropane-1-carboxylic acid, or the ethylenereleasing substance, Ethephon, to several Fe-sufficient StrategyI plants [cucumber, tomato, pea, sugar beet (Beta vulgarisL.),Arabidopsis(Arabidopsis thaliana(L.) Heynh ‘Columbia’), plantago(Plantago lanceolataL.)] promoted some of their Fe-deficiencystress responses: enhanced root ferric-reducing capacity andswollen root tips. By contrast, Fe-deficient roots from severalStrategy II plants [maize (Zea maysL. ‘Funo’), wheat(Triticum aestivumL. ‘Yécora’), barley (HordeumvulgareL. ‘Barbarrosa’)] did not produce more ethylenethan the Fe-sufficient ones. Furthermore, ACC had no effecton the reducing capacity of these Strategy II plants and, exceptin barley, did not promote swelling of root tips. In conclusion,results suggest that ethylene is involved in the regulationof Fe-deficiency stress responses by Strategy I plants.Copyright1999 Annals of Botany Company. Arabidopsis (Arabidopsis thaliana(L.) Heynch), barley (Hordeum vulgareL.), cucumber (Cucumis sativusL.), ethylene, iron deficiency, maize (Zea maysL.), pea (Pisum sativumL.), plantago (Plantago lanceolataL.), ferric-reducing capacity, sugar beet (Beta vulgarisL.), tomato (Lycopersicon esculentumMill.), wheat (Triticum aestivumL.).     

Blue light activates a specific protein kinase in higher plants

Blue light mediates the phosphorylation of a membrane protein in seedlings from several plant species. When crude microsomal membrane proteins from dark-grown pea (Pisum sativum L.), sunflower (Helianthus annuus L.), zucchini (Cucurbita pepo L.), Arabidopsis (Arabidopsis thaliana L.), or tomato (Lycopersicon esculentum L.) stem segments, or from maize (Zea mays L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), wheat (Triticum aestivum L.), or sorghum (Sorghum bicolor L.) coleoptiles are illuminated and incubated in vitro with [γ-³²P]ATP, a protein of apparent molecular mass from 114 to 130 kD is rapidly phosphorylated. Hence, this system is probably ubiquitous in higher plants. Solubilized maize membranes exposed to blue light and added to unirradiated solubilized maize membranes show a higher level of phosphorylation of the light-affected protein than irradiated membrane proteins alone, suggesting that an unirradiated substrate is phosphorylated by a light-activated kinase. This finding is further demonstrated with membrane proteins from two different species, where the phosphorylated proteins are of different sizes and, hence, unambiguously distinguishable on gel electrophoresis. When solubilized membrane proteins from one species are irradiated and added to unirradiated membrane proteins from another species, the unirradiated protein becomes phosphorylated. These experiments indicate that the irradiated fraction can store the light signal for subsequent phosphorylation in the dark. They also support the hypothesis that light activates a specific kinase and that the systems share a close functional homology among different higher plants.     

Aluminum-tolerant Protoplasts from Roots Can be Collected with Positively Charged Silica Microbeads: a Method Based on Differences in Surface Negativity

A new technique was developed for the collection of Al-tolerantplant cells. The procedure takes advantage of the differencein surface charge between Al-tolerant and Al-sensitive protoplasts.Equal volumes of freshly prepared, positively charged silicamicrobeads (0.05% w/v) and purified protoplasts derived fromroot tips (2x10⁵ ml^-1) of rice, maize or pea were mixed andthen centrifuged on a discontinuous Ficoll gradient. Intactprotoplasts from the Al-tolerant plant were recovered mostlyin the bottom fraction, those from the Al-sensitive plant wererecoverd at the uppermost interface, and those from the plantwith intermediate tolerance to Al were collected at the middleinterface. Moreover, the pattern of fractionation was unrelatedto the diameter of the protoplasts. The technique should beuseful in future investigations of the physiological and biochemicalproperties of Al-tolerant cells or their plasma membranes. (Received April 19, 1995; Accepted September 1, 1995)     

Comparative mapping of HKT genes in wheat, barley, and rice, key determinants of Na+ transport, and salt tolerance

Salt tolerance of plants depends on HKT transporters (High-affinityK⁺ Transporter), which mediate Na⁺-specific transport or Na⁺-K⁺co-transport. Gene sequences closely related to rice HKT geneswere isolated from hexaploid bread wheat (Triticum aestivum)or barley (Hordeum vulgare) for genomic DNA southern hybridizationanalysis. HKT gene sequences were mapped on chromosomal armsof wheat and barley using wheat chromosome substitution linesand barley–wheat chromosome addition lines. In addition,HKT gene members in the wild diploid wheat ancestors, T. monococcum(A^m genome), T. urartu (A^u genome), and Ae. tauschii (D^t genome)were investigated. Variation in copy number for individual HKTgene members was observed between the barley, wheat, and ricegenomes, and between the different wheat genomes. HKT2;1/2-like,HKT2;3/4-like, HKT1;1/2-like, HKT1;3-like, HKT1;4-like, andHKT1;5-like genes were mapped to the wheat–barley chromosomegroups 7, 7, 2, 6, 2, and 4, respectively. Chromosomal regionscontaining HKT genes were syntenic between wheat and rice exceptfor the chromosome regions containing the HKT1;5-like gene.Potential roles of HKT genes in Na⁺ transport in rice, wheat,and barley are discussed. Determination of the chromosome locationsof HKT genes provides a framework for future physiological andgenetic studies investigating the relationships between HKTgenes and salt tolerance in wheat and barley. Key words: Barley, comparative mapping, HKT, rice, salt tolerance, sodium transport, wheat     

Sink Activity Estimation by Sink Size and Dry Matter Increase During the Ripening Stage of Barley (Hordeum vu/gare) and Rice (Oryza sativa)

During the ripening stage of barley and rice, the sink activitywas defined as the dry matter increase per units sink size,leaf area and time, as follows: NAR = A.SinkW+NAR', where NAR is the net assimilation rate (g d.wt dm^–2d^–1);A is the sink activity [g d.wt g^–1d.wt (ear) dm^–2d^–1]; Sink W is ear wt per plant at heading (g d.wt);and NAR' is net assimilation rate excluding the assimilate ofsink organ (g d.wt dm^–2 d^–1). Plant material with 16 combinations of mutually different sink(ear) and source (leaf) size were produced at heading for eachcrop: parts of each leaf and ear were removed to produce fourgrades in barley, and also a part of each leaf was removed producingfour grades for four rice varieties showing different ear size.NAR and NAR' were determined during 26 and 21 d in barley andrice after heading, respectively. Sink activity (A), representedas the assimilation rate induced by the sink organ, was estimatedfrom the relationship between SinkW and NAR using the previousequation. The sink activity was significantly higher in ricewith a value of 0–0.028 g d.wt g^–1 d.wt (ear) dm^–2d^–1 vs. 0–0.0017 in barley, suggesting that therelative role of leaves for grain filling is considerably higherin rice than in barley. The sink activity obtained in the studymight be introduced into a model to predict the yields of barleyand rice. Hordeum vulgare L, barley, Oryza saliva L, rice, dry matter, NAR, sink, source, sink activity, model     

Organic Acid Exudation by Wild Herbs in Response to Elevated Al Concentrations

In acidic soils, monomeric aluminium (Al³⁺) can reach levelsthat are toxic to plants, thus preventing many species fromgrowing there. Organic acids chelate Al and render it non-toxic.It has been shown that exudation of organic acids by Al-tolerantcrops increases their tolerance to Al. We have extended thisobservation to wild plants by comparing the ability of ten herbsto exude organic acids in response to elevated Al levels. Wehypothesized that exudation of organic acids was related tothe ability of plants to grow on Al-rich soils. Two grasseswere grown in rhizotrons in soils with 41 and 63 µM reactiveAl. Organic acids were sampled from root tips connected to anintact plant-root system.Deschampsia flexuosa (L.) Trin. exudedmore malic acid when grown in the soil with the highest Al content.Five forbs and five grasses were also exposed to three Al levels(0, 25 and 75 µM) in a hydroponic system.Rumex acetosellaL. and Viscaria vulgaris Bernh. increased exudation of oxalicacid and Galium saxatile auct. non L. and Veronica officinalisL. increased exudation of citric acid in response to elevatedAl. The distribution of the forbs in the field as describedby soil pH was negatively related to the amount of organic acidsexuded in response to Al. In contrast, none of the grasses exudedhigher amounts of organic acids with increasing Al concentrationin the hydroponic experiment. Copyright 2001 Annals of BotanyCompany Citrate, malate, aluminium detoxification, rhizotron, hydroponics, rhizosphere, Carex pilulifera,Deschampsia flexuosa , Festuca gigantea, Galium saxatile, Geum urbanum, Holcus mollis,Milium effusum , Rumex acetosella, Veronica officinalis, Viscaria vulgaris     

The spontaneous growth response of maize coleoptile segments and the change in tissue 8

Decapitated segments from maize (Zea mays L.) coleoptiles orientedvertically in an upright position show a strong spontaneousgrowth response (SGR) 3 h after decapitation. The latent periodof the SGR is markedly reduced when these segments are orientedin an inverted position. Coleoptile segments with intact tipsexhibit a weak and transient SGR in the vertical upright orientation.However, in the inverted orientation, these segments show atypical SGR. The data are inconsistent with the current hypothesisthat the SGR is caused by a time-dependent increase in tissuesensitivity to auxin. The parallel increase in membrane potentialdifference and growth rate during the time-course of the SGRindicates a possible role for PM H⁺-ATPase in the establishmentof the SGR in maize coleoptile segments. Key words: Auxin, spontaneous growth response, membrane potential, plasma membrane H⁺-ATPase, Zea mays L.     

Root Growth and Carbohydrate Metabolism at Low Temperatures

A study of carbohydrate metabolism in the roots of pea and maizeshows that the differing ability of these two species to growat low temperatures is associated with the maintenance of adequatesugar supplies to the root tip. In the maize root reducing theambient temperature to 2 °C causes a sharp and continuingfall in the soluble sugar content of the growing tip. A similartreatment with pea roots causes only a temporary reduction insugar content lasting no more than 24 h. The fall in root sugarsin maize is accompanied by a reduction in respiration rate andthe cessation of growth. During the periods of sugar shortagecaused by low temperatures both respiration and growth can bestimulated in root tips by a supply of exogenous glucose. Pearoots also show an additional ability to adapt to low temperaturesby lowering the K^m value for invertase after pretreatment ata low temperature. This effect is not seen in maize.     

Apoplastic Sugar Concentration and pH in Barley Leaves Infected with Brown Rust

Soluble sugars were extracted by low speed centrifugation fromthe apoplast of leaves of barley (Hordeum distichum L.) infiltratedwith water. Infection of the leaf with the brown rust fungus(Puccinia hordeii) resulted in a reduction in the concentrationof sucrose, glucose and fructose in the apoplast. Sugars werepresent in an apoplastic space occupying 12 and 17 cm³ m^–2of leaf area in healthy and infected tissue, respectively. Uptakeof hexoses by intercellular hyphae is suggested as a cause ofthis reduction. The pH of apoplastic sap extracted from rust-infectedleaves was increased to pH 7·3 from pH 6·6 incontrols. The effect of a reduced apoplastic sugar pool andincreased pH on export from infected leaves is discussed. Key words: Apoplast, barley (Hordeum distichum L.), brown rust (Puccinia hordeii Otth.), pH, sucrose, hexose     

Species-dependent variation in the interaction of substrate-bound ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) and rubisco activase

Purified spinach (Spinacea oleracea L.) and barley (Hordeum vulgare L.) ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase supported 50 to 100% activation of substrate-bound Rubisco from spinach, barley, wheat (Triticum aestivum L.), soybean (Glycine max L.), pea (Pisum sativum L.), Arabidopsis thaliana, maize (Zea mays L.), and Chlamydomonas reinhardtii but supported only 10 to 35% activation of Rubisco from three Solanaceae species, tobacco (Nicotiana tabacum L.), petunia (Petunia hybrida L.), and tomato (Lycopersicon esculentum L.). Conversely, purified tobacco and petunia Rubisco activase catalyzed 75 to 100% activation of substrate-bound Rubisco from the three Solanacee species but only 10 to 25% activation of substrate-bound Rubisco from the other species. Thus, the interaction between substrate-bound Rubisco and Rubisco activase is species dependent. The species dependence observed is consistent with phylogenetic relationships previously derived from plant morphological characteristics and from nucleotide and amino acid sequence comparisons of the two Rubisco subunits. Species dependence in the Rubisco-Rubisco activase interaction and the absence of major anomalies in the deduced amino acid sequence of tobacco Rubisco activase compared to sequences in non-Solanaceae species suggest that Rubisco and Rubisco activase may have coevolved such that amino acid changes that have arisen by evolutionary divergence in one of these enzymes through spontaneous mutation or selection pressure have led to compensatory changes in the other enzyme.     

Auxin and Ethylene Control of Growth in Seedlings of Zea mays L. and Avena sativa L

The effects of applied ethylene on the growth of coleoptilesand mesocotyls of etiolated monocot seedlings (oat and maize)have been compared with those on the epicotyl of a dicot seedling(the etiolated pea). Significant inhibition of elongation by ethylene (10 µll^–1for 24 h) was found in intact seedlings of all three species,but lateral expansion growth was observed only in the pea internodeand oat mesocotyl tissue. The sensitivity of the growth of seedlingparts to ethylene is in the decreasing order pea internode,oat coleoptile and oat mesocotyl, with maize exhibiting theleast growth response. Although excised segments of mesocotyland coleoptile or pea internode all exhibit enhanced elongationgrowth in IAA solutions (10^–6–2 ? 10^–5 moll^–1), no consistent effects were found in ethylene. Ethyleneproduction in segments was significantly enhanced by applicationof auxin (IAA, 10^–5 mol l^–6 or less) in all tissuesexcept those of the eat mesocotyl. Segments of maize show a slow rate of metabolism of applied[2-¹⁴C]IAA (30 per cent converted to other metabolites within9 h) and a high capacity for polar auxin transport. Ethylene(10 µl l^–1 for 24 h) has little effect on eitherof these processes. The oat has a smaller capacity for polartransport than maize and the rate ef metabolism of auxin isas fast as in the pea (90 per cent metabolized in 6 h). Althoughethylene pretreatment does not change the rate of auxin metabolismin oat, there is a marked reduction in auxin transport. It is proposed that the insensitivity of maize seedlings toethylene is related to the supply and persistence of auxin whichcould protect the seedling against the effects of applied orendogenously produced ethylene. Although the mesocotyl of oatis sensitive to applied ethylene it may be in part protectedagainst ethylene in vivo by the absence of an auxin-enhancedethylene production system. The results are discussed in relationto a model for the auxin and ethylene control of cell growthin the pea.     

The Development of Desiccation-tolerance and Maximum Seed Quality During Seed Maturation in Six Grain Legumes

‘Physiological maturity’, i.e. the time when seedsreach their maximum dry weight during development, occurredwhen maturation drying on the parent plant in the field hadreduced seed moisture content to approximately 60 per cent infaba bean (Vicia faba L.), lentil (Lens culinaris Medic.), chickpea(Cicer arietinum L.), white lupin (Lupinus albus L.), soya bean(Glycine max [L.] Merr.) and pea (Pisum sativum L.) The onsetof desiccation-tolerance, i.e. the ability of seeds to germinatefollowing harvest and rapid artificial drying, coincided withphysiological maturity, except in pea where it occurred a littleearlier at about 70 per cent moisture content. Maximum seedquality as determined by maximum viability, minimum seedlingabnormalities and maximum seedling size occurred in pea, chickpeaand lupin when seeds were harvested for rapid drying at physiologicalmaturity; but for maximum seed quality in the other speciesmaturation drying had to proceed further - to about 45 per centmoisture content in soya bean and to about 30 per cent moisturecontent in lentil and faba bean seed crops. Much of this variationamongst the six species, however, was due to differences inthe variation in maturity within each seed crop. Results forindividual pods showed that peak maturity, i.e. maximum seedquality following harvest and rapid artificial drying, was achievedin all six species once maturation drying had reduced the moisturecontent of the seeds to 45–50 per cent. In pea, faba beanand soya bean there was a substantial decline in viability andan increase in seedling abnormalities when harvest was delayedbeyond the optimal moisture content for harvest.     

Evidence for a general light-dependent negative control of NADPH-protochlorophyllide oxidoreductase in angiosperms

The effect of light on NADPH-protochlorophyllide oxidoreductase and its mRNA has been studied in five different species of dicotyledonous plants, bean (Phaseolus vulgaris L.), pea (Pisum sativum L.), tomato (Lycopersicon esculentum Mill.), sunflower (Helianthus annuns L.) and mustard (Sinapis alba L.), and in two monocotyledonous plant species, maize (Zea mays L.) and barley (Hordeum vulgare L.). In all these species, illumination of etiolated seedlings led to a rapid decline of both the activity and the content of the enzyme protein. These results indicate that there may be a general light-dependent regulation of the enzyme common to higher plants.Abbreviation Pchlide reductase NADPH-protochlorophyllide oxidoreductase Didicated to Professor H. Mohr on the occasion of his 60th birthdayWe are grateful to P. Westhoff and H. Schrubar (Botanisches Institut, Universität Düsseldorf, FRG) for making available to us their data on the Pchlide reductase of Sorghum bicolor prior to publication and for sending us seeds of this plant species. This work has been supported by the Deutsche Forschungsgemeinschaft.     

Effects of Prolonged Darkness on the Sensitivity of Leaf Respiration to Carbon Dioxide Concentration in C3and C4Species

Predicting responses of plant and global carbon balance to theincreasing concentration of carbon dioxide in the atmosphererequires an understanding of the response of plant respirationto carbon dioxide concentration ([CO₂]). Direct effects of thecarbon dioxide concentration at which rates of respiration ofplant tissue are measured are quite variable and their effectsremain controversial. One possible source of variation in responsivenessis the energy status of the tissue, which could influence thecontrol coefficients of enzymes, such as cytochrome-c oxidase,whose activity is sensitive to [CO₂]. In this study we comparedresponses of respiration rate to [CO₂] over the range of 60to 1000 µmol mol^-1in fully expanded leaves of four C₃andfour C₄herbaceous species. Responses were measured near themiddle of the normal 10 h dark period, and also after another24 h of darkness. On average, rates of respiration were reducedabout 70% by the prolonged dark period, and leaf dry mass perunit area decreased about 30%. In all species studied, the relativedecrease in respiration rate with increasing [CO₂] was largerafter prolonged darkness. In the C₃species, rates measured at1000 µmol mol^-1CO₂averaged 0.89 of those measured at 60µmol mol^-1in the middle of the normal dark period, and0.70-times when measured after prolonged darkness. In the C₄species,rates measured at 1000 µmol mol^-1CO₂averaged 0.79 of thoseat 60 µmol mol^-1CO₂in the middle of the normal dark period,and 0.51-times when measured after prolonged darkness. In threeof the C₃species and one of the C₄species, the decrease in theabsolute respiration rate between 60 and 1000 µmol mol^-1CO₂wasessentially the same in the middle of the normal night periodand after prolonged darkness. In the other species, the decreasein the absolute rate of respiration with increase in [CO₂] wassubstantially less after prolonged darkness than in the middleof the normal night period. These results indicated that increasingthe [CO₂] at the time of measurement decreased respiration inall species examined, and that this effect was relatively largerin tissues in which the respiration rate was substrate-limited.The larger relative effect of [CO₂] on respiration in tissuesafter prolonged darkness is evidence against a controlling roleof cytochrome-c oxidase in the direct effects of [CO₂] on respiration.Copyright 2001 Annals of Botany Company Carbon dioxide, respiration, Abutilon theophrasti(L.), Amaranthus retroflexus(L.),Amaranthus hypochondriacus (L.), Datura stramonium(L.), Helianthus annuus(L.), Solanum melongena(L.), Sorghum bicolor(L. Moench), Zea mays     

Nodule Function in Symbiotic Bambara Groundnut (Vigna subterraneaL.) and Kersting's Bean (Macrotyloma geocarpumL.) is Tolerant of Nitrate in the Root Medium

Nitrogen-fixing activity in two nodulated African legumes, Bambaragroundnut (Vigna subterraneaL.) and Kersting's bean (MacrotylomageocarpumL.), was assessed in the presence of nitrate (NO₃^-)ions in the rooting medium. Nitrogenase activity was unimpairedby the supply of 5 mol m^-3NO₃to both species. Also, large concentrationsof ureides dominated the transpiration stream of NO₃-fed plants.Compared to other symbiotic legumes cultured with similar NO₃concentrations,nodule functioning in the tested landraces of Bambara groundnutand Kersting's bean is tolerant of NO₃ions in the rhizosphere.The potential benefits of such naturally occurring NO₃-tolerantsymbioses are substantial, as they would permit inorganic Nfertilizer application in intercropping systems without inhibitingN₂fixation in the associated legumes.Copyright 1998 Annals ofBotany Company NO₃tolerance, Bambara groundnut, Kersting's bean, nitrogenase activity, xylem ureides.     

Amino Acid and Peptide uptake in the scutella of germinating grains of barley, wheat, rice, and maize

Scutella separated from germinating grains of barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), rice (Oryza sativa L.), and maize (Zea mays L.) took up the four amino acids and the three peptides tested from incubation media. The uptake of amino acids by wheat scutella was similar to that of barley scutella and was via at least four uptake systems: two nonspecific amino acid uptake systems, one system specific for proline, and another system specific for basic amino acids. The scutellum of rice apparently has two nonspecific systems and a system specific for the basic amino acids, but the proline-specific system is lacking. The scutellum of maize seems to have the same systems as the scutellum of rice, but one (or both) of the nonspecific systems differs from that of the other species studied in taking up arginine only slowly. No great differences were observed in the uptake of peptides in the four species studied. The rates of uptake of different amino acids and peptides were of the same order of magnitude in the four cereals. The fact that carboxypeptidase activities in the endosperms of wheat and barley are 20-to 100-fold higher than those in rice and maize, does thus not seem to be reflected in the uptake properties of the scutella.     

Two functionally distinct members of the MATE (multi‐drug and toxic compound extrusion) family of transporters potentially underlie two major aluminum tolerance QTLs in maize

Crop yields are significantly reduced by aluminum (Al) toxicity on acidic soils, which comprise up to 50% of the world’s arable land. Al‐activated release of ligands (such as organic acids) from the roots is a major Al tolerance mechanism in plants. In maize, Al‐activated root citrate exudation plays an important role in tolerance. However, maize Al tolerance is a complex trait involving multiple genes and physiological mechanisms. Recently, transporters from the MATE family have been shown to mediate Al‐activated citrate exudation in a number of plant species. Here we describe the cloning and characterization of two MATE family members in maize, ZmMATE1 and ZmMATE2, which co‐localize to major Al tolerance QTL. Both genes encode plasma membrane proteins that mediate significant anion efflux when expressed in Xenopus oocytes. ZmMATE1 expression is mostly concentrated in root tissues, is up‐regulated by Al and is significantly higher in Al‐tolerant maize genotypes. In contrast, ZmMATE2 expression is not specifically localized to any particular tissue and does not respond to Al. [¹⁴C]‐citrate efflux experiments in oocytes demonstrate that ZmMATE1 is a citrate transporter. In addition, ZmMATE1 expression confers a significant increase in Al tolerance in transgenic Arabidopsis. Our data suggests that ZmMATE1 is a functional homolog of the Al tolerance genes recently characterized in sorghum, barley and Arabidopsis, and is likely to underlie the largest maize Al tolerance QTL found on chromosome 6. However, ZmMATE2 most likely does not encode a citrate transporter, and could be involved in a novel Al tolerance mechanism.     

Time of Flowering of Pea (Pisum sativum L.) as a Function of Leaf Appearance Rate and Node of First Flower

In order to assess the influence of environmental conditionson time of flowering of pea (Pisum sativum L.), a serial sowingtrial was conducted over 2 years at Dijon, France, on two wintercultivars Frisson and Frilene. Time of flowering was analysedaccording to two variables: the leaf appearance rate R_L andthe node of first flower N_I. R_L was linearly related to temperature (r² = 0·94). Thebase temperature was 2°C for both varieties. Growth rateaccounted for the residual variability of R_L . Photoperiod andtemperature acted on N_I in an additive way. Frilene, the latergenotype, was more responsive than Frisson. A model for predicting time of flowering based upon these resultsis proposed. Deviations from this model were related to N nutritionin interaction with the plant water relations. Steps for improvingthe model are then discussed.Copyright 1993, 1999 Academic Press Pisum sativum L., pea, flowering, temperature, photoperiod, phyllochron, model