Australian Centre for Plant Functional Genomics publications

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Selection of monoploids for protoplast fusion and generation of intermonoploid somatic hybrids of potato
(Springer, 2001) Johnson, Alexander Arthur Theodore; Piovano, Suzanne M.; Ravichandran, Vidya; Veilleux, Richard E.; Australian Centre for Plant Functional Genomics (ACPFG)
The union of potato monoploid genotypes (2n=1x =12) through protoplast fusion may result in vigorous somatic hybrids due to a reduction of the genetic load normally present in this highly heterozygous tetraploid (2n=4x=48) crop. More than 100 androgenic monoploids derived from diploid (2n=2x=24) Solanum phureja Juz. & Buk. and S. chacoense Bitt. x S. phureja clones were evaluated in field trials during 1996 and 1997 to identify the most promising genotypes for protoplast fusion experiments. Compared to the total population, the 1996 selected genotypes had higher means for tuber number (30.1 vs 11.2 tubers/plant), average tuber weight (3.0 vs 1.8 g/tuber) and total yield (66.1 vs 20.4 g/plant). Similarly, the 1997 selected genotypes had higher means for tuber number (42.8 vs 25.4 tubers/plant), average tuber weight (3.6 vs 2.5 g/tuber) and total yield (114.0 vs 63.4 g/plant) compared to the total population. The 31 selected monoploid genotypes from 1996-97 varied in their response to protoplast isolation and culture from no growth (9), cell enlargement (5), limited cellular divisions (8), callus formation (5) to plant regeneration from callus (4). Chemical fusion and electrofusion produced three groups of intermonoploid somatic hybrids. Polymorphic simple sequence repeat (SSR) loci enabled distinction of somatic hybrids from parental somaclones. Rapid DNA extraction with SSR analysis enabled screening of calluses to identify somatic hybrid tissue prior to plant regeneration. The somatic hybrids were highly polyploid, mostly hexaploid (2n=6x=72), possibly due to fusion of endopolyploid protoplasts and/or chromosomal doubling during plant regeneration.
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Somatic hybridization and application in plant breeding
(John Wiley & Sons, 2001) Johnson, Alexander Arthur Theodore; Veilleux, Richard E.; Australian Centre for Plant Functional Genomics (ACPFG)
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Root exudates in phosphorus acquisition by plants
(Springer-Verlag, 2001) Randall, P.; Hayes, J.; Hocking, P.; Ae, N.; Arihara, J.; Okada, K.; Ancha, S.; Australian Centre for Plant Functional Genomics (ACPFG)
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Recombination between paralogues at the rp1 rust resistance locus in maize
(Genetics, 2001) Sun, Q.; Collins, N.; Ayliffe, M.; Smith, S.; Drake, J.; Pryor, A.; Hulbert, S.
Rp1 is a complex rust resistance locus of maize. The HRp1-D haplotype is composed of Rp1-D and eight paralogues, seven of which also code for predicted nucleotide binding site-leucine rich repeat (NBS-LRR) proteins similar to the Rp1-D gene. The paralogues are polymorphic (DNA identities 91–97%), especially in the C-terminal LRR domain. The remaining family member encodes a truncated protein that has no LRR domain. Seven of the nine family members, including the truncated gene, are transcribed. Sequence comparisons between paralogues provide evidence for past recombination events between paralogues and diversifying selection, particularly in the C-terminal half of the LRR domain. Variants selected for complete or partial loss of Rp1-D resistance can be explained by unequal crossing over that occurred mostly within coding regions. The Rp1-D gene is altered or lost in all variants, the recombination breakpoints occur throughout the genes, and most recombinant events (9/14 examined) involved the same untranscribed paralogue with the Rp1-D gene. One recombinant with a complete LRR from Rp1-D, but the amino-terminal portion from another homologue, conferred the Rp1-D specificity but with a reduced level of resistance.
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Expression of phytase in plants as a method of modifying plant productivity
(2001) Richardson, A.; Hayes, J.; Simpson, R.; Australian Centre for Plant Functional Genomics (ACPFG); Australian Centre for Plant Functional Genomics (ACPFG)
The present invention provides a method of modifying plant productivity comprising expressing in a plant cell, tissue or organ one or more genes capable of facilitating a plant's ability to utilise soil phosphorus, and, in particular, a phytase gene, wherein said phytase gene expresses a phytase polypeptide in secretable form. More particularly, the present invention provides a method of increasing plant productivity comprising expressing in the root of a plant an isolated nucleic acid molecule encoding a phytase polypeptide for a time and under conditions sufficient for said phytase to be secreted from the root. In a preferred embodiment of the invention the chemistry of the soil around the root is modified by the application of an organic acid. The invention also provides novel phytase-encoding genes; genetic constructs which are useful for performing the inventive method; and transgenic plants produced therewith having improved productivity compared to their otherwise isogenic counterparts.
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Sequence haplotypes revealed by sequence-tagged site fine mapping of the Ror1 gene in the centromeric region of barley chromosome 1H
(Amer Soc Plant Physiologists, 2001) Collins, N.; Lahaye, T.; Peterhansel, C.; Freialdenhoven, A.; Corbitt, M.; Schulze-Lefert, P.
We describe the development of polymerase chain reaction-based, sequence-tagged site (STS) markers for fine mapping of the barley (Hordeum vulgare) Ror1 gene required for broad-spectrum resistance to powdery mildew (Blumeria graminis f. sp. hordei). After locating Ror1 to the centromeric region of barley chromosome 1H using a combined amplified fragment length polymorphism/restriction fragment-length polymorphism (RFLP) approach, sequences of RFLP probes from this chromosome region of barley and corresponding genome regions from the related grass species oat (Avena spp.), wheat, and Triticum monococcum were used to develop STS markers. Primers based on the RFLP probe sequences were used to polymerase chain reaction-amplify and directly sequence homologous DNA stretches from each of four parents that were used for mapping. Over 28,000 bp from 22 markers were compared. In addition to one insertion/deletion of at least 2.0 kb, 79 small unique sequence polymorphisms were observed, including 65 single nucleotide substitutions, two dinucleotide substitutions, 11 insertion/deletions, and one 5-bp/10-bp exchange. The frequency of polymorphism between any two barley lines ranged from 0.9 to 3.0 kb, and was greatest for comparisons involving an Ethiopian landrace. Haplotype structure was observed in the marker sequences over distances of several hundred basepairs. Polymorphisms in 16 STSs were used to generate genetic markers, scored by restriction enzyme digestion or by direct sequencing. Over 2,300 segregants from three populations were used in Ror1 linkage analysis, mapping Ror1 to a 0.2- to 0.5-cM marker interval. We discuss the implications of sequence haplotypes and STS markers for the generation of high-density maps in cereals.
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Extracellular secretion of Aspergillus phytase from Arabidopsis roots enables plants to obtain phosphorus from phytate
(Blackwell Publishing Ltd, 2001) Richardson, A.; Hadobas, P.; Hayes, J.; Australian Centre for Plant Functional Genomics (ACPFG)
Phosphorus (P) deficiency in soil is a major constraint for agricultural production worldwide. Despite this, most soils contain significant amounts of total soil P that occurs in inorganic and organic fractions and accumulates with phosphorus fertilization. A major component of soil organic phosphorus occurs as phytate. We show that when grown in agar under sterile conditions, Arabidopsis thaliana plants are able to obtain phosphorus from a range of organic phosphorus substrates that would be expected to occur in soil, but have only limited ability to obtain phosphorus directly from phytate. In wild-type plants, phytase constituted less than 0.8% of the total acid phosphomonoesterase activity of root extracts and was not detectable as an extracellular enzyme. By comparison, the growth and phosphorus nutrition of Arabidopsis plants supplied with phytate was improved significantly when the phytase gene (phyA) from Aspergillus niger was introduced. The Aspergillus phytase was only effective when secreted as an extracellular enzyme by inclusion of the signal peptide sequence from the carrot extensin (ex) gene. A 20-fold increase in total root phytase activity in transgenic lines expressing ex::phyA resulted in improved phosphorus nutrition, such that the growth and phosphorus content of the plants was equivalent to control plants supplied with inorganic phosphate. These results show that extracellular phytase activity of plant roots is a significant factor in the utilization of phosphorus from phytate and indicate that opportunity exists for using gene technology to improve the ability of plants to utilize accumulated forms of soil organic phosphorus.
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QTL analysis of photosynthesis and water status traits in sunflower (Helianthus annuus L.) under greenhouse conditions
(Oxford Univ Press, 2001) Fleury, D.; Fabre, F.; Berrios, E.; Chaarani, G.; Planchon, C.; Sarrafi, A.; Gentzbittel, L.; Australian Centre for Plant Functional Genomics (ACPFG)
The identification of QTL for several physiological traits in sunflower is described. Traits related to photosynthesis (leaf chlorophyll concentration, net photosynthesis and internal CO2 concentration) and water status (stomatal conductance, transpiration, predawn leaf water potential, and relative water content) were evaluated in a population of recombinant inbred lines under greenhouse conditions. Narrow-sense heritabilities were low to average. Using an AFLP linkage map, 19 QTL were detected explaining 8.8–62.9% of the phenotypic variance for each trait. Among these, two major QTL for net photosynthesis were identified on linkage group IX. One QTL co-location was found on linkage group VIII for stomatal movements and water status. Coincident locations for QTL regulating photosynthesis, transpiration and leaf water potential were described on linkage group XIV. These results lead to the first description of the organization of genomic regions related to yield in sunflower.
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Induction of late maturity α-amylase in wheat by cool temperature
(C S I R O Publishing, 2001) Mrva, K.; Mares, D.
Wheat genotypes prone to late maturity α-amylase (LMA) produced high levels of germination-type (high pI isozymes) α-amylase following exposure to cool temperature during grain development. Plants grown in the glasshouse, plants grown in the field and transplanted to the glasshouse after flowering, and tillers taken from field or glasshouse grown plants all responded in a similar manner. Plants or detached tillers can therefore be used in screening tests to identify germplasm with the LMA genotype. The cool temperature treatment was effective when applied continuously from shortly after flowering until near-ripeness, or when limited to the phase of grain development (26–35 days after anthesis) in LMA-prone genotypes that appears to be most sensitive to cool temperature. Based on these observations, guidelines for screening wheat germplasm are proposed and the advantages of using detached tillers discussed. The detached tiller method was successfully applied to a range of genotypes, some of which were known to be prone to LMA, and to a doubled haploid population derived from the cross Janz (low amylase) BD159 (LMA genotype). The preliminary data from this population were consistent with control by a single gene and similar therefore to the model proposed previously for cv. Spica.
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Resistance gene analogs in barley and their relationships to rust resistance genes.
(Natl Research Council Canada, 2001) Collins, N.; Park, R.; Spielmeyer, W.; Pryor, A.; Ellis, J.
Regions of amino acid conservation in the NBS domain of NBS-LRR resistance proteins facilitated the PCR isolation of eight resistance gene analog (RGA) sequences from genomic DNA of rice, barley, and Aegilops tauschii. These clones and other RGAs previously isolated from maize, rice, and wheat were assigned to 13 classes by DNA-sequence comparison and by their patterns of hybridisation to restricted barley DNA. Using a doubled-haploid mapping population, probes from 12 RGA classes were used to map 17 loci in the barley genome. Many of these probes have been used for mapping in wheat, and the collective data indicate that the positions of orthologous RGAs are conserved between barley and wheat. RGA loci were identified in the vicinity of barley leaf rust resistance loci Rph4, Rph7, and Rph10. Recombinants were identified between RGA loci and Rph7 and Rph10, while a cluster of RGA sequences detected by probe 5.2 cosegregated with Rph4 in 55 F2 lines.Key words: barley, Hordeum vulgare, rust, Puccinia, resistance gene analog, RGA, resistance.
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Simultaneous measurement of ammonium, nitrate and proton fluxes along the length of eucalypt roots
(Kluwer Academic Publ, 2001) Garnett, T.; Shabala, S.; Smethurst, P.; Newman, I.; Australian Centre for Plant Functional Genomics (ACPFG)
Knowledge of the preferred source of N for Eucalyptus nitens will lead to improved fertiliser management practices in plantations. Ion selective microelectrodes were used non-invasively to measure simultaneously net fluxes of NH4⁺, NO3^- and H⁺ along the tap root of solution-cultured E. nitens. Measurements were conducted in solutions containing 100 μm NH4NO3. The pattern of fluxes was such that there was a large influx of NH4⁺ a smaller influx of NO3^- and large H⁺ efflux. The ratio of these fluxes was constant, according to the ratio 3:1:-6 (NH4⁺:NO3^- :H⁺). Within the region 20-60 mm from the root apex of E. nitens seedlings there was spatial and temporal variation in fluxes but flux patterns remained constant. Root hair density did not affect fluxes nor did proximity to lateral roots. Variation was less than that found in previous studies of localised root fluxes using similar high-resolution measurement techniques. It was concluded that small-scale spatial variation in fluxes may have confounded previous studies. There were associations between fluxes of all three ions, the strongest associations being between NH4⁺ and H⁺, and NH4⁺ and NO3^-. Overall, these results are consistent with NH4⁺ being the preferred source N for E. nitens.
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Utilization of phosphorus by pasture plants supplied with myo-inositol hexaphosphate is enhanced by the presence of soil micro-organisms
(Kluwer Academic Publ, 2001) Richardson, A.; Hadobas, P.; Hayes, J.; O'Hara, C.; Simpson, R.; Australian Centre for Plant Functional Genomics (ACPFG)
A range of pasture grass (Danthonia richardsonii and Phalaris aquatica) and legume (Medicago polymorpha, M. sativa, Trifolium repens and T. subterraneum) species showed limited capacity to obtain phosphorus (P) from inositol hexaphosphate (IHP), when grown in either sterile agar (pH 5.0 or 5.5) or sand-vermiculite media (pH 5.0). The total P content of shoots from IHP-supplied plants grown in agar was between 20% and 34% of that for seedlings supplied with an equivalent amount of P as inorganic phosphate (Pi), while in sand-vermiculite, the total P content of IHP-grown plants was between 5 and 10% of control plants. The poor ability of plants to utilize P from IHP resulted in significantly lower tissue P concentrations and, in general, reduced plant dry weight accumulation. In contrast, the P nutrition of plants supplied with IHP was significantly improved by inoculating media with either a cultured population of total soil micro-organisms or with a specific isolate of Pseudomonas sp., selected for its ability to release phosphate from IHP (strain CCAR59; Richardson and Hadobas, 1997 Can. J. Micro. 43, 509-516). In agar and sand-vermiculite media, respectively, the P content of IHP-grown plants increased with inoculation by up to 3.9- and 6.8-fold, such that the dry weight and P content of the plant material were equivalent to those observed for control plants supplied with Pi. However, the response to inoculation was dependent on the growth medium and the source of micro-organisms used. In sand-vermiculite, the cultured population of soil micro-organisms was effective when IHP was supplied at an equivalent level of Pi required for maximum plant growth. By comparison, inoculation of plants with the Pseudomonas strain was only effective at very high levels of IHP supply (×36), whereas in agar a response to inoculation occurred at all levels of IHP. The ability of pasture plants to acquire P from phytate was, therefore, influenced by the availability of IHP substrate, which was further affected by the presence of soil micro-organisms. Our results show that in addition to having an effect on the sorption characteristics of the growth media, soil micro-organisms also provided a source of phytase for the dephosphorylation of phytate for subsequent utilization of Pi by plants.
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Genetic and molecular characterization of the maize rp3 rust resistance locus
(Genetics, 2002) Webb, C.; Richter, T.; Collins, N.; Nicolas, M.; Trick,, H.; Pryor, A.; Hulbert, S.
In maize, the Rp3 gene confers resistance to common rust caused by Puccinia sorghi. Flanking marker analysis of rust-susceptible rp3 variants suggested that most of them arose via unequal crossing over, indicating that rp3 is a complex locus like rp1. The PIC13 probe identifies a nucleotide binding site-leucine-rich repeat (NBS-LRR) gene family that maps to the complex. Rp3 variants show losses of PIC13 family members relative to the resistant parents when probed with PIC13, indicating that the Rp3 gene is a member of this family. Gel blots and sequence analysis suggest that at least 9 family members are at the locus in most Rp3-carrying lines and that at least 5 of these are transcribed in the Rp3-A haplotype. The coding regions of 14 family members, isolated from three different Rp3-carrying haplotypes, had DNA sequence identities from 93 to 99%. Partial sequencing of clones of a BAC contig spanning the rp3 locus in the maize inbred line B73 identified five different PIC13 paralogues in a region of ~140 kb.
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Short term transport of iron and copper to various parts of wheat grains
(2002) Garnett, Trevor Paul; Graham, Robin David; Jenner, Colin F.; Australian Centre for Plant Functional Genomics (ACPFG)
Increasing the micronutrient content of staple foods such as cereal grains is a way of addressing micronutrient deficiencies in humans. This strategy will only be successful if the micronutrients are distributed in parts of the grain that are generally consumed (i.e. the endosperm). To date there are limited reports of micronutrient distribution within wheat grains with most simply reporting content of the whole grain. We investigated the transport of radioactively labelled iron and copper into the grain of cultured wheat ears 18–22 days post anthesis. Ears were loaded for 24 hours and then the grains dissected into various components. The grains were dissected into: epidermis; percarp-testa (combined hypodermis/cross cells/tube cells/testa/nucellar layer); outer endosperm (with aleurone layer attached); central endosperm; embryo; combined tissues of the crease. The majority of the iron was transported into the endosperm (61%), followed by the crease (15%). The majority of the copper was found in the crease (51%) followed by the endosperm (24%). Samples taken from the inner part of the endosperm contained no detectable iron or copper suggesting that the iron and copper present in the endosperm was found in the aleurone layer. If these grains were milled industrially for white flour then most of the iron and copper in the grain would be lost. Although these measurements were short term, they reinforce the point that strategies to increase iron and copper transport into wheat grains must keep in view the destinations of iron and copper within the grain.
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The barley MLO modulator of defense and cell death Is responsive to biotic and abiotic stress stimuli
(Amer Soc Plant Physiologists, 2002) Piffanelli, P.; Zhou, F.; Casais, C.; Orme, J.; Jarosch, B.; Schaffrath, U.; Collins, N.; Panstruga, R.; Schulze-Lefert, P.
Lack of the barley (Hordeum vulgare) seven-transmembrane domain MLO protein confers resistance against the fungal pathogen Blumeria graminis f. sp. hordei (Bgh). To broaden the basis for MLO structure/function studies, we sequenced additional mlo resistance alleles, two of which confer only partial resistance. Wild-type MLO dampens the cell wall-restricted hydrogen peroxide burst at points of attempted fungal penetration of the epidermal cell wall, and in subtending mesophyll cells, it suppresses a second oxidative burst and cell death. Although the Bgh-induced cell death in mlo plants is spatially and temporally separated from resistance, we show that the two processes are linked. Uninoculated mutant mlo plants exhibit spontaneous mesophyll cell death that appears to be part of accelerated leaf senescence. Mlo transcript abundance increases in response to Bgh, rice (Oryza sativa) blast, wounding, paraquat treatment, a wheat powdery mildew-derived carbohydrate elicitor, and during leaf senescence. This suggests a broad involvement of Mlo in cell death protection and in responses to biotic and abiotic stresses.
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Plant thioredoxins: the multiplicity conundrum
(Birkhauser Verlag Ag, 2002) Baumann, U.; Juttner, J.; Australian Centre for Plant Functional Genomics (ACPFG)
Thioredoxins are small proteins distinguished by the presence of a conserved dicysteine active site. In oxidized thioredoxin, the two cysteines form a disulfide bond that is targeted by the enzyme thioredoxin reductase. Together with an electron donor, thioredoxin and thioredoxin reductase form the 'thioredoxin system' that is present in all organisms. Thioredoxins participate in dithiol/disulfide exchange reactions with a large range of cellular substrates. Higher plants possess a very complex thioredoxin profile consisting of at least two different thioredoxin systems that contain distinct, multigenic thioredoxin classes which have different intracellular localizations. In this review we summarise the current state of knowledge regarding the function of plant thioredoxins representing all systems and classes.
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Integration of transgenes into sexual polyploidization schemes for potato (Solanum tuberosum L.)
(Kluwer Academic Publ, 2003) Johnson, A.; Veilleux, R.; Australian Centre for Plant Functional Genomics (ACPFG)
Most potato transgenic research has focused on development of resistance to pathogens and modification of potato physiology. Many transgenes, particularly those conferring pathogen resistance, could substantially lower potato production costs in developing countries. However, transgenes have not been reported in sexually propagated 4x-2x potato hybrids commonly grown in developing countries. Two transgenes,the Bacillus thuringiensis cry3Aa endotoxin protein gene and the PVY°coat protein gene, were engineered intodiploid and tetraploid potato using Agrobacterium tumefaciens-mediated transformation. Cry3Aa was produced at high levels in several lines while the PVY° coat protein was not expressed. Diplandroid and tetraploid genotypes were crossed to produce transgenic 4x-2x hybrids. Genetic transformation had no discernable effect on fertility ofthe primary transformants, germination of4x-2x seed derived from the transformants and agronomic performance(tuber set, average tuber weight and total tuber yield) of the 4x-2xhybrids. The transgenic 4x-2xhybrids produced non-viable pollen and could only be crossed as female parents. Results suggest that transgenes, such ascry3Aa, could be expressed in 4x-2x hybrids to lower costs of production with no significant effect on plant phenotype.
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Transmission of a Bacillus thuringiensis cry3Aa transgene from diploid to tetraploid potato using 4x-2x hybridization: effect of ploidy increase on transgene expression and implications for TPS hybrid production
(Blackwell Verlag GMBH, 2003) Johnson, A.; Nault, B.; Veilleux, R.; Australian Centre for Plant Functional Genomics (ACPFG)
The study examined the effect of ploidy elevation through unreduced gametes on transgene expression in potato. Tetraploid transgenic progenies were obtained from one tetraploid potato cultivar crossed with 2n pollen producing diploid clones harbouring an exogenous transgene (cry3Aa). Both single- and multiple-insert diploid transgenic lines that were regenerated by Agrobacterium tumefaciens leaf disc inoculation were used in crosses. A DAS–ELISA system and no-choice feeding bioassay enabled characterization of the parental lines as either 'high' or 'low' expressers of the Cry3Aa protein. High Cry3Aa expression was observed for both single-insert transgenic diploids and their 4x–2x progeny. On the contrary, 68% of 4x–2x progeny derived from a multiple-insert, diploid transgenic had significantly reduced Cry3Aa expression compared with the parent, with 32% demonstrating nearly complete silencing of the transgene. Multiple copies of a transgene, like homologous native genes, may be susceptible to transgene silencing following polyploidization. Therefore, incorporation of exogenous transgenes into a true potato seed (TPS) production system is feasible if a single-insert diploid parent is used. Gene-centromere mapping of the cry3Aa transgene demonstrated that a non-transgenic refuge might be naturally created in a TPS hybrid system through genetic recombination.
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SNARE-protein-mediated disease resistance at the plant cell wall
(Nature Publishing Group, 2003) Collins, N.; Thordal-Christensen, H.; Lipka, V.; Bau, S.; Kombrink, E.; Qiu, J.; Huckelhoven, R.; Stein, M.; Freialdenhoven, A.; Somerville, S.; Schulze-Lefert, P.
Failure of pathogenic fungi to breach the plant cell wall constitutes a major component of immunity of non-host plant species—species outside the pathogen host range—and accounts for a proportion of aborted infection attempts on 'susceptible' host plants (basal resistance).Neither form of penetration resistance is understood at the molecular level. We developed a screen for penetration (pen) mutants of Arabidopsis, which are disabled in non-host penetration resistance against barley powdery mildew, Blumeria graminis f. sp. hordei, and we isolated the PEN1 gene. We also isolated barley ROR2 , which is required for basal penetration resistance against B. g. hordei. The genes encode functionally homologous syntaxins, demonstrating a mechanistic link between non-host resistance and basal penetration resistance in monocotyledons and dicotyledons. We show that resistance in barley requires a SNAP-25 (synaptosome-associated protein, molecular mass 25 kDa) homologue capable of forming a binary SNAP receptor (SNARE) complex with ROR2. Genetic control of vesicle behaviour at penetration sites, and plasma membrane location of PEN1/ROR2, is consistent with a proposed involvement of SNARE-complex-mediated exocytosis and/or homotypic vesicle fusion events in resistance. Functions associated with SNARE-dependent penetration resistance are dispensable for immunity mediated by race-specific resistance (R) genes, highlighting fundamental differences between these two resistance forms.
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Al-induced efflux of organic acid anions is poorly associated with internal organic acid metabolism in triticale roots
(Oxford Univ Press, 2003) Hayes, J.; Ma, J.; Australian Centre for Plant Functional Genomics (ACPFG)
The secretion of organic acid anions from roots has been identi®ed as a mechanism of resistance to Al. However, the process leading to the secretion of organic acid anions is poorly understood. The effect of Al on organic acid metabolism was investigated in two lines of triticale (X Triticosecale Wittmark) differing in Al-induced secretion of malate and citrate and in Al resistance. The site of Al-induced secretion of citrate and malate from a resistant line was localized to the root apices (terminal 5 mm). The levels of citrate (root apices and mature root segments) and malate (mature segments only) in roots increased during exposure to Al, but similar changes were observed in both triticale genotypes. The in vitro activities of four enzymes involved in malate and citrate metabolism (citrate synthase, phosphoenolpyruvate carboxylase, malate dehydrogenase, and NADP-isocitrate dehydrogenase) were similar for sensitive and resistant lines in both root apices and mature root segments. The response of these enzymes to pH did not differ between tolerant and sensitive lines or in the presence and absence of Al. Moreover, cytoplasmic and vacuolar pH were not affected by exposure to Al in either line. Together, these results indicate that the Aldependent ef¯ux of organic acid anions from the roots of triticale is not regulated by their internal levels in the roots or by the capacity of the root cells to synthesize malate and citrate.

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