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Many disease resistance genes in wheat (Triticum aestivum L.) confer strong resistance to specific pathogen races or strains, and only a small number of genes confer multipathogen resistance. The Leaf rust resistance 67 (Lr67) gene fits into the latter category as it confers partial resistance to multiple biotrophic fungal pathogens in wheat and encodes a Sugar Transport Protein 13 (STP13) family hexose-proton symporter variant. Two mutations (G144R, V387L) in the resistant variant, Lr67res, differentiate it from the susceptible Lr67sus variant. The molecular function of the Lr67res protein is not understood, and this study aimed to broaden our knowledge on this topic. Biophysical analysis of the wheat Lr67sus and Lr67res protein variants was performed using Xenopus laevis oocytes as a heterologous expression system. Oocytes injected with Lr67sus displayed properties typically associated with proton-coupled sugar transport proteins—glucose-dependent inward currents, a Km of 110 ± 10 µM glucose, and a substrate selectivity permitting the transport of pentoses and hexoses. By contrast, Lr67res induced much larger sugar-independent inward currents in oocytes, implicating an alternative function. Since Lr67res is a mutated hexose-proton symporter, the possibility of protons underlying these currents was investigated but rejected. Instead, currents in Lr67res oocytes appeared to be dominated by anions. This conclusion was supported by electrophysiology and 36Cl− uptake studies and the similarities with oocytes expressing the known chloride channel from Torpedo marmorata, TmClC-0. This study provides insights into the function of an important disease resistance gene in wheat, which can be used to determine how this gene variant underpins disease resistance in planta.

Root architecture is key in determining how effective plants are at intercepting and absorbing nutrients and water. Previously, the wheat (Triticum aestivum) cultivars Spica and Maringa were shown to have contrasting root morphologies. These cultivars were crossed to generate an F6:1 population of recombinant inbred lines (RILs) which was genotyped using a 90 K single nucleotide polymorphisms (SNP) chip. A total of 227 recombinant inbred lines (RILs) were grown in soil for 21 days in replicated trials under controlled conditions. At harvest, the plants were scored for seven root traits and two shoot traits. An average of 7.5 quantitative trait loci (QTL) were associated with each trait and, for each of these, physical locations of the flanking markers were identified using the Chinese Spring reference genome. We also compiled a list of genes from wheat and other monocotyledons that have previously been associated with root growth and morphology to determine their physical locations on the Chinese Spring reference genome. This allowed us to determine whether the QTL discovered in our study encompassed genes previously associated with root morphology in wheat or other monocotyledons. Furthermore, it allowed us to establish if the QTL were co-located with the QTL identified from previously published studies. The parental lines together with the genetic markers generated here will enable specific root traits to be introgressed into elite wheat lines. Moreover, the comprehensive list of genes associated with root development, and their physical locations, will be a useful resource for researchers investigating the genetics of root morphology in cereals.

Purpose Durum wheat is sensitive of acid soils because it lacks effective genes for Al3+ tolerance. Previous research showed introgression of the TaMATE1B and TaALMT1 genes individually increased the Al3+ tolerance of durum wheat. Here we aimed to (a) combine the genes into a single durum line, (b) compare the various introgression lines and (c) establish the effectiveness of the introgressions in improving the acid soil tolerance in the field. Methods Durum wheat lines homozygous for Al3+-tolerant alleles of TaMATE1B and TaALMT1 were crossed to develop a line that incorporated both genes. The parental cultivar, lines with the individual genes and the line with both genes introgressed were screened for Al3+ tolerance by hydroponic and soil cultures in a growth cabinet. The lines were also assessed for biomass production and grain yield in the field on acid soils. Results The durum wheat lines with the various Al3+-tolerance genes introgressed performed better based on root growth than Jandaroi, the parental cultivar, in both hydroponic and soil assays when grown in a cabinet. The various introgression lines were tolerant of acid soils compared to Jandaroi when grown in the field as assessed by shoot biomass and grain yield. Conclusion The TaALMT1 and TaMATE1B genes improve the acid soil tolerance of durum wheat with indications that combining both genes is the most effective strategy. The various lines will be valuable to breeders who wish to enhance the acid soil tolerance of durum germplasm.

Certain soil microorganisms can improve plant growth, and practices that encourage their proliferation around the roots can boost production and reduce reliance on agrochemicals. The beneficial effects of the microbial inoculants currently used in agriculture are inconsistent or short-lived because their persistence in soil and on roots is often poor. A complementary approach could use root exudates to recruit beneficial microbes directly from the soil and encourage inoculant proliferation. However, it is unclear whether the release of common organic metabolites can alter the root microbiome in a consistent manner and if so, how those changes vary throughout the whole root system. In this study, we altered the expression of transporters from the ALUMINUM-ACTIVATED MALATE TRANSPORTER and the MULTIDRUG AND TOXIC COMPOUND EXTRUSION families in rice (Oryza sativa L.) and wheat (Triticum aestivum L.) and tested how the subsequent release of their substrates (simple organic anions, including malate, citrate, and γ-amino butyric acid) from root apices affected the root microbiomes. We demonstrate that these exudate compounds, separately and in combination, significantly altered microbiome composition throughout the root system. However, the root type (seminal or nodal), position along the roots (apex or base), and soil type had a greater influence on microbiome structure than the exudates. These results reveal that the root microbiomes of important cereal species can be manipulated by altering the composition of root exudates, and support ongoing attempts to improve plant production by manipulating the root microbiome., One-sentence summary: The root microbiome of rice and wheat can be manipulated by altering the activity of root transporters and exudates.

There is increasing interest in understanding how the microbial communities on roots can be manipulated to improve plant productivity. Root systems are not homogeneous organs but are comprised of different root types of various ages and anatomies that perform different functions. Relatively little is known about how this variation influences the distribution and abundance of microorganisms on roots and in the rhizosphere. Such information is important for understanding how root–microbe interactions might affect root function and prevent diseases. This study tested specific hypotheses related to the spatial variation of bacterial and fungal communities on wheat (Triticum aestivum L.) and rice (Oryza sativa L.) roots grown in contrasting soils. We demonstrate that microbial communities differed significantly between soil type, between host species, between root types, and with position along the root axes. The magnitude of variation between different root types and along individual roots was comparable with the variation detected between different plant species. We discuss the general patterns that emerged in this variation and identify bacterial and fungal taxa that were consistently more abundant on specific regions of the root system. We argue that these patterns should be measured more routinely so that localised root–microbe interactions can be better linked with root system design, plant health and performance.

Acid soils limit yields of many important crops including canola (Brassica napus), Australia’s third largest crop. Aluminium (Al3+) stress is the main cause of this limitation primarily because the toxic Al3+ present inhibits root growth. Breeding programmes do not target acid-soil tolerance in B. napus because genetic variation and convincing quantitative trait loci have not been reported. We conducted a genome-wide association study (GWAS) using the BnASSYST diversity panel of B. napus genotyped with 35 729 high-quality DArTseq markers. We screened 352 B. napus accessions in hydroponics with and without a toxic concentration of AlCl3 (12 μM, pH 4.3) for 12 days and measured shoot biomass, root biomass, and root length. By accounting for both population structure and kinship matrices, five significant quantitative trait loci for different measures of resistance were identified using incremental Al3+ resistance indices. Within these quantitative trait locus regions of B. napus, 40 Arabidopsis thaliana gene orthologues were identified, including some previously linked with Al3+ resistance. GWAS analysis indicated that multiple genes are responsible for the natural variation in Al3+ resistance in B. napus. The results provide new genetic resources and markers to enhance that Al3+ resistance of B. napus germplasm via genomic and marker-assisted selection.

Partial resistance to multiple biotrophic fungal pathogens in wheat (Triticum aestivum L.) is conferred by the Lr67 gene, which encodes a Sugar Transport Protein 13 (STP13) family hexose-proton symporter variant. Two mutations (G144R, V387L) differentiate the resistant and susceptible protein variants (Lr67res and Lr67sus). The molecular function of the Lr67res protein is not well understood. We functionally characterized the wheat Lr67res protein variant using two heterologous expression systems - Xenopus laevis oocytes and Saccharomyces cerevisiae yeast. Wheat and barley (Hordeum vulgare) were used to verify disease resistance capability of Lr67/STP13 variants. The Lr67res allele, but not Lr67sus, induced large sugar-independent, anion-dominated currents in oocytes and an increased sensitivity to ions in yeast, implicating a novel gain-of-function. We demonstrate that the single mutant variant, Lr67susG144R, confers disease resistance in wheat and that transgenic barley (Hordeum vulgare L.) plants expressing the orthologous HvSTP13 gene carrying the equivalent mutations present in Lr67res exhibited increased resistance to Puccinia hordei. NaCl treatment was found to induce leaf tip necrosis in Lr67res wheat. An Lr67res-like gain-of-function can be introduced into orthologous plant hexose transporters via single amino acid mutation, highlighting the possibility of generating disease resistance in other crop species, especially with gene editing.

Six cycles of recurrent selection for early shoot vigour in wheat resulted in significant increases in leaf width and shoot biomass. Here, in replicated controlled-environment studies, the effect of early shoot vigour on root biomass, rhizosheath size, root hair length, and cell size in the roots and leaves was examined across different cycles of selection. Increased shoot vigour was associated with greater root biomass, larger rhizosheath size, and longer root hairs. Our findings demonstrate that rhizosheath size was a reliable surrogate for root hair length in this germplasm. Examination of the root epidermis revealed that the ‘cell body’ of the trichoblasts (hair-forming cells) and the atrichoblasts (non-hair-forming cells) decreased in size as shoot vigour increased. Therefore, in higher vigour germplasm, longer root hairs emerged from smaller trichoblasts so that total trichoblast volume (root hair plus cell body) was generally similar regardless of shoot vigour. Similarly, the sizes of the four main cell types on the leaf epidermis became progressively smaller as shoot vigour increased, which also increased stomatal density. The relationship between shoot vigour and root traits is considered, and the potential contribution of below-ground root traits to performance and competitiveness of high vigour germplasm is discussed.

The yields of cereal crops grown on acidic soils are often reduced by aluminum (Al) toxicity because the prevalence of toxic Al3+ cations increases as pH falls below 5.0. The Al-dependent release of citrate from resistant lines of maize is controlled by ZmMATE1 which encodes a multidrug and toxic compound extrusion (MATE) transporter protein. ZmMATE6 is another member of this family in maize whose expression is also increased by Al treatment. We investigated the function of this gene in more detail to determine whether it also contributes to Al resistance. Quantitative RT-PCR measurements found that ZmMATE6 was expressed in the roots and leaves of Al-resistant and sensitive inbred lines. Treatment with Al induced ZmMATE6 expression in all tissues but several other divalent or trivalent cations tested had no effect on expression. This expression pattern and the induction by Al treatment was confirmed in ZmMATE6 promoter-β-glucuronidase fusion lines. Heterogeneous expression of ZmMATE6 displayed a greater Al-activated release of citrate from the roots and was significantly resistant to Al toxicity than controls. This was associated with reduced accumulation of Al in the root tissues. Our results demonstrated that ZmMATE6 expression is induced by Al and functions as a citrate transporter.

There is increasing interest in understanding how the microbial communities on roots can be manipulated to improve plant productivity. Root systems are not homogeneous organs but are comprised of different root types of various ages and anatomies that perform different functions. Relatively little is known about how this variation influences the distribution and abundance of microorganisms on roots and in the rhizosphere. Such information is important for understanding how root-microbe interactions might affect root function and prevent diseases. This study tested specific hypotheses related to the spatial variation of bacterial and fungal communities on wheat (Triticum aestivum L.) and rice (Oryza sativa L.) roots grown in contrasting soils. We demonstrate that microbial communities differed significantly between soil type, between host species, between root types, and with position along the root axes. The magnitude of variation between different root types and along individual roots was comparable with the variation detected between different plant species. We discuss the general patterns that emerged in this variation and identify bacterial and fungal taxa that were consistently more abundant on specific regions of the root system. We argue that these patterns should be measured more routinely so that localised root-microbe interactions can be better linked with root system design, plant health and performance.

High-throughput sequencing of universal bacterial 16S rRNA gene (16S rDNA) amplicons is a routine method for characterizing bacterial diversity in a range of environments. For eukaryotic host-associated communities, however, plastid and mitochondrial genes are often co-amplified with, and greatly outnumber, bacterial 16S rDNA. This makes it difficult to obtain sufficient numbers of target 16S rDNA sequences to characterize the diversity of endophytic bacterial communities. This chapter describes a method that improves the amplification of bacterial 16S rDNA from plant tissues by using a peptide nucleic acid (PNA) PCR clamp. The PNA clamp selectively binds to a targeted region of the plant genome and inhibits its amplification during PCR. PNA clamps are especially useful for characterizing bacterial communities on plant tissues with lower levels of microbial colonization such as the root tips and leaves.

High-throughput sequencing of universal bacterial 16S rRNA gene (16S rDNA) amplicons is a routine method for characterizing bacterial diversity in a range of environments. For eukaryotic host-associated communities, however, plastid and mitochondrial genes are often co-amplified with, and greatly outnumber, bacterial 16S rDNA. This makes it difficult to obtain sufficient numbers of target 16S rDNA sequences to characterize the diversity of endophytic bacterial communities. This chapter describes a method that improves the amplification of bacterial 16S rDNA from plant tissues by using a peptide nucleic acid (PNA) PCR clamp. The PNA clamp selectively binds to a targeted region of the plant genome and inhibits its amplification during PCR. PNA clamps are especially useful for characterizing bacterial communities on plant tissues with lower levels of microbial colonization such as the root tips and leaves.

Phosphorus (P) is an essential plant nutrient and regular applications are essential in most farming systems to maintain high yields. Yet the P fertilizers applied to crops and pastures are derived from non-renewable resources. It is therefore important to find agronomic and genetic strategies for using this resource efficiently, especially since only a proportion of the applied P is absorbed by crops. The aim of this study was to identify Quantitative Trait Loci (QTL) for P use efficiency (PUE) in wheat using a Multiparent Advanced Generation InterCross (MAGIC) population grown in the field. The 357 genotypes were arranged in paired plots with and without P fertilization. Yield and biomass were measured and PUE was calculated as either the performance of the genotype relative to the average response to fertilization, or the performance of the genotype relative to the average resilience in the absence of fertilization. Five trials were conducted over three years in Australia at three sites with contrasting clay and sandy soil types.Genotypic variation for response and resilience were identified in all trials with moderate to strong correlation with productivity with and without P between trials. Multiparent Whole Genome Average Interval Mapping (WGAIM) QTL analyses were conducted on the four traits (Biomass / Yield × P Response / Resilience) across the five trials and identified 130 QTL in total. QTL within 10 cM of each other were clustered into 56 groups that were likely to represent identical or linked loci. Of the clusters 27 (48%) contained only a single QTL but 17 (31%) contained 3 or more in different trials or traits. This suggests multiple biological mechanisms driving PUE in different environments. Eight of the 56 groups collocated with QTL for seedling root hair length identified in the same MAGIC population in an earlier study.HighlightIdentification of genetic loci for phosphorus use efficiency in a multigenic population of Australian wheats grown on contrasting soils.

Plant root–biota associations involve complex networks between micro- and macroorganisms that interact across diverse temporal and spatial scales and in response to a wide range of climatic and environmental variables. The root–microbiome has a major impact on plant health through interactions on growth and development, facilitation of nutrient uptake and ability to tolerate biotic and abiotic stresses. Renewed interest in plant–microbe interactions has been driven by the availability of more precise molecular and analytical techniques that allow the detailed description and understanding of the dynamics of rhizosphere and endosphere communities. A better understanding of the succession of the root microbiome, i.e. how the microbiome changes in relation to crop growth, provides new opportunities for capturing the potential benefits from positive root–microbiome interactions. Recent evidence has confirmed that root type and localization has a significant influence on the diversity and composition of bacterial, actinobacterial and fungal communities, including specific functional groups such as arbuscular mycorrhizal fungi communities across a wide range of plant species. Such differences are driven by variations in root structure and the quantity and composition of root exudates along with microbe–microbe, plant–microbe and microbe–fauna interactions. Evidence for association between temporal changes in the composition of root exudates and the functionality of microbial substrate preferences has also been shown. The interaction between root and rhizosphere development and community composition, in terms of taxonomic/phylogenetic makeup, however, needs to extend to functional groups associated with key traits related to plant health and nutrition. An improved ability to predict functionality from the dynamic changes in root microbiome composition is an important requirement to develop management interventions for ‘designer’ microbiomes and to achieve improved productivity and sustainability in agricultural systems. For this, a more detailed genomic and functional characterization of plant-associated microorganisms, their genes and plant–microbe interactions for specific beneficial functions is required.

Fungal pathogens are a major constraint to global crop production; hence, plant genes encoding pathogen resistance are important tools for combating disease. A few resistance genes identified to date provide partial, durable resistance to multiple pathogens and the wheat (Triticum aestivum) Lr67 hexose transporter variant (Lr67res) fits into this category. Two amino acids differ between the wild-type and resistant alleles – G144R and V387L. Exome sequence data from 267 barley (Hordeum vulgare) landraces and wild accessions was screened and neither of the Lr67res mutations was detected. The barley ortholog of Lr67, HvSTP13, was functionally characterized in yeast as a high affinity hexose transporter. The G144R mutation was introduced into HvSTP13 and abolished Glc uptake, whereas the V387L mutation reduced Glc uptake by ∼ 50%. Glc transport by HvSTP13 heterologously expressed in yeast was reduced when coexpressed with Lr67res. Stable transgenic Lr67res barley lines exhibited seedling resistance to the barley-specific pathogens Puccinia hordei and Blumeria graminis f. sp. hordei, which cause leaf rust and powdery mildew, respectively. Barley plants expressing Lr67res exhibited early senescence and higher pathogenesis-related (PR) gene expression. Unlike previous observations implicating flavonoids in the resistance of transgenic sorghum (Sorghum bicolor) expressing Lr34res, another wheat multipathogen resistance gene, barley flavonoids are unlikely to have a role in Lr67res-mediated resistance. Similar to observations made in yeast, Lr67res reduced Glc uptake in planta. These results confirm that the pathway by which Lr67res confers resistance to fungal pathogens is conserved between wheat and barley.

Mutants without root hairs show reduced inorganic orthophosphate (Pi) uptake and compromised growth on soils when Pi availability is restricted. What is less clear is whether root hairs that are longer than wild-type provide an additional benefit to phosphorus (P) nutrition. This was tested using transgenic Brachypodium lines with longer root hairs. The lines were transformed with the endogenous BdRSL2 and BdRSL3 genes using either a constitutive promoter or a root hair-specific promoter. Plants were grown for 32 d in soil amended with various Pi concentrations. Plant biomass and P uptake were measured and genotypes were compared on the basis of critical Pi values and P uptake per unit root length. Ectopic expression of RSL2 and RSL3 increased root hair length three-fold but decreased plant biomass. Constitutive expression of BdRSL2, but not expression of BdRSL3, consistently improved P nutrition as measured by lowering the critical Pi values and increasing Pi uptake per unit root length. Increasing root hair length through breeding or biotechnology can improve P uptake efficiency if the pleotropic effects on plant biomass are avoided. Long root hairs, alone, appear to be insufficient to improve Pi uptake and need to be combined with other traits to benefit P nutrition.

Made available in DSpace on 2018-12-11T17:35:24Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-03-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Aim: A major limitation to plant growth in acid soils is the prevalence of toxic Al3+. Most genotypic variation for acid soil-tolerance in wheat is linked with the Al3+-activated efflux of malate anions from roots which is controlled by TaALMT1 on chromosome 4DL. Recent studies have also linked TaALMT1 with tolerance to high pH solutions and alkaline soils. This study tested the hypothesis that an Al3+-resistant allele of TaALMT1 also confers tolerance to alkaline conditions. Methods: The near-isogenic wheat lines, ET8 (Al3+-resistant) and ES8 (Al3+-sensitive), have different alleles of the TaALMT1 gene and contrasting resistance to Al3+ toxicity. Growth of these lines was compared in acid and alkaline soils with contrasting mineralogy and in a range of high pH hydroponic solutions of varying composition. Results: No consistent differences in root or shoot growth were detected between the lines in the alkaline soils or in the high pH hydroponic treatments. Malate efflux was detected from ET8 in acidic solution with Al3+ but no substantial malate efflux was detected at pH 9.0 treatment with added Na2SO4. Conclusion: The results are inconsistent with the hypothesis that the TaALMT1 gene confers an advantage to wheat on alkaline soils. CSIRO Agriculture and Food, GPO Box 1700 Botanics department Sao Paulo State University UNESP Chengdu Institute of Biology Chinese Academy of Sciences Botanics department Sao Paulo State University UNESP

Aluminum-activated malate transporters (ALMTs) form a family of anion channels in plants, but little is known about most of its members. This study examined the function of OsALMT4 from rice (Oryza sativa). We show that OsALMT4 is expressed in roots and shoots and that the OsALMT4 protein localizes to the plasma membrane. Transgenic rice lines overexpressing (OX) OsALMT4 released malate from the roots constitutively and had 2-fold higher malate concentrations in the xylem sap than nulls, indicating greater concentrations of malate in the apoplast. OX lines developed brown necrotic spots on the leaves that did not appear on nulls. These symptoms were not associated with altered concentrations of any mineral element in the leaves, although the OX lines had higher concentrations of Mn and B in their grain compared with nulls. While total leaf Mn concentrations were not different between the OX and null lines, Mn concentrations in the apoplast were greater in the OX plants. The OX lines also displayed increased expression of Mn transporters and were more sensitive to Mn toxicity than null plants. We showed that the growth of wild-type rice was unaffected by 100 µm Mn in hydroponics but, when combined with 1 mm malate, this concentration inhibited growth. We conclude that increasing OsALMT4 expression affected malate efflux and compartmentation within the tissues, which increased Mn concentrations in the apoplast of leaves and induced the toxicity symptoms. This study reveals new links between malate transport and mineral nutrition.

Background The structure and function of plant roots and their interactions with soil are exciting scientific frontiers that will ultimately reveal much about our natural systems, global water and mineral and carbon cycles, and help secure food supplies into the future. This Special Issue presents a collection of papers that address topics at the forefront of our understanding of root biology. Scope These papers investigate how roots cope with drought, nutrient deficiencies, toxicities and soil compaction as well as the interactions that roots have with soil microorganisms. Roots of model plant species, annual crops and perennial species are studied in short-term experiments through to multi-year trials. Spatial scales range from the gene up to farming systems and nutrient cycling. The diverse, integrated approaches described by these studies encompass root genetics as applied to soil management, as well as documenting the signalling processes occurring between roots and shoots and between roots and soil. Conclusions This Special Issue on roots presents invited reviews and research papers covering a span of topics ranging from fundamental aspects of anatomy, growth and water uptake to roots in crop and pasture systems. Understanding root structure and function and adaptation to the abiotic and biotic stresses encountered in field conditions is important for sustainable agricultural production and better management of natural systems.

TaALMT1 from wheat (Triticum aestivum) and AtALMT1 from Arabidopsis thaliana encode aluminum (Al)-activated malate transporters, which confer acid-soil tolerance by releasing malate from roots. Chimeric proteins from TaALMT1 and AtALMT1 (Ta::At, At::Ta) were previously analyzed in Xenopus laevis oocytes. Those studies showed that Al could activate malate efflux from the Ta::At chimera but not from At::Ta. Here, functions of TaALMT1, AtALMT1 and the chimeric protein Ta::At were compared in cultured tobacco BY-2 cells. We focused on the sensitivity and specificity of their activation by trivalent cations. The activation of malate efflux by Al was at least two-fold greater in the chimera than the native proteins. All proteins were also activated by lanthanides (erbium, ytterbium, gadolinium, and lanthanum), but the chimera again released more malate than TaALMT1 or AtALMT1. In Xenopus oocytes, Al, ytterbium, and erbium activated inward currents from the native TaALMT1 and the chimeric protein, but gadolinium only activated currents from the chimera. Lanthanum inhibited currents from both proteins. These results demonstrated that function of the chimera protein was altered compared to the native proteins and was more responsive to a range of trivalent cations when expressed in plant cells.

The mechanisms of aluminum (Al) resistance in wheat and rye involve the release of citrate and malate anions from the root apices. Many of the genes controlling these processes have been identified and their responses to Al treatment described in detail. This study investigated how the major Al resistance traits of wheat and rye are transferred to triticale (x Tritosecale Wittmack) which is a hybrid between wheat and rye. We generated octoploid and hexaploid triticale lines and compared them with the parental lines for their relative resistance to Al, organic anion efflux and expression of some of the genes encoding the transporters involved. We report that the strong Al resistance of rye was incompletely transferred to octoploid and hexaploid triticale. The wheat and rye parents contributed to the Al-resistance of octoploid triticale but the phenotypes were not additive. The Al resistance genes of hexaploid wheat, TaALMT1, and TaMATE1B, were more successfully expressed in octoploid triticale than the Al resistance genes in rye tested, ScALMT1 and ScFRDL2. This study demonstrates that an important stress-tolerance trait derived from hexaploid wheat was expressed in octoploid triticale. Since most commercial triticale lines are largely hexaploid types it would be beneficial to develop techniques to generate genetically-stable octoploid triticale material. This would enable other useful traits that are present in hexaploid but not tetraploid wheat, to be transferred to triticale.

Aluminium (Al) toxicity can severely reduce root growth and consequently affect plant development and yield. A mechanism by which many species resist the toxic effects of Al relies on the efflux of organic anions (OAs) from the root apices via OA transporters. Several of the genes encoding these OA transporters contain transposable elements (TEs) in the coding sequences or in flanking regions. Some of the TE-induced mutations impact Al resistance by modifying the level and/or location of gene expression so that OA efflux from the roots is increased. The importance of genomic modifications for improving the adaptation of plants to acid soils has been raised previously, but the growing number of examples linking TEs with these changes requires highlighting. Here, we review the role of TEs in creating genetic modifications that enhance the adaptation of plants to acid soils by increasing the release of OAs from the root apices. We argue that TEs have been an important source of beneficial mutations that have co-opted OA transporter proteins with other functions to perform this role. These changes have occurred relatively recently in the evolution of many species and likely facilitated their expansion into regions with acidic soils.

The original version of this article unfortunately contained a mistake. The online supplementary material (composed of Table S1, Fig. S1, and Fig. S2) was omitted in the published online version. The supplementary material is available in this version.

The aluminum (Al3+) tolerance of barley cultivars predominately from Brazil was compared to that of cultivars from other countries, wild barley accessions, and a transgenic line (L5) over-expressing TaALMT1, the major Al3+ tolerance gene from wheat. After screening conventional germplasm for Al3+ tolerance in hydroponics, 18 genotypes were further characterized in a short-term soil experiment. Among the Brazilian cultivars, Antarctica 01 and BRS Mariana showed the greatest relative root length (RRL) in acid soil. However, these cultivars were significantly less tolerant than the foreign cultivars Dayton (USA) and Murasakimochi (Japan) and the transgenic line L5 which out-performed all conventional genotypes. In long-term growth trials, the transgenic line produced the greatest relative root and relative shoot dry weight. Relative grain yield was greatest in the transgenic line and Dayton. All genotypes were also scored for two genetic markers linked to HvAACT1, the major Al3+ tolerance gene in barley. One marker detects a 1-kb insertion in the promoter that increases gene expression and leads to increased Al3+-activated citrate efflux from root apices. The other marker detects a 21-bp indel downstream of the coding region. The 1-kb insertion was only detected in Dayton and Murasakimochi that were the best performing cultivars among the non-transgenic germplasm. Interestingly, the Brazilian cultivars with an intermediate level of tolerance, Antarctica 01 and BRS Mariana, lacked the 1-kb insertion but had enhanced HvAACT1 expression compared to an Al3+-sensitive cultivar. No clear correlation was observed between Al3+ tolerance and the 21-bp indel marker in the short-term soil trials. We conclude that improved Al3+ tolerance in barley could be achieved by combining the best allele of HvAACT1 along with TaALMT1 as a transgene.

We examined the function of OsALMT4 in rice (Oryza sativa L.) which is a member of the aluminum-activated malate transporter family. Previous studies showed that OsALMT4 localizes to the plasma membrane and that expression in transgenic rice lines results in a constitutive release of malate from the roots. Here, we show that OsALMT4 is expressed widely in roots, shoots, flowers, and grain but not guard cells. Expression was also affected by ionic and osmotic stress, light and to the hormones ABA, IAA, and salicylic acid. Malate efflux from the transgenic plants over-expressing OsALMT4 was inhibited by niflumate and salicylic acid. Growth of transgenic lines with either increased OsALMT4 expression or reduced expression was measured in different environments. Light intensity caused significant differences in growth between the transgenic lines and controls. When day-time light was reduced from 700 to 300 μmol m-2s-1 independent transgenic lines with either increased or decreased OsALMT4 expression accumulated less biomass compared to their null controls. This response was not associated with differences in photosynthetic capacity, stomatal conductance or sugar concentrations in tissues. We propose that by disrupting malate fluxes across the plasma membrane carbon partitioning and perhaps signaling are affected which compromises growth under low light. We conclude that OsALMT4 is expressed widely in rice and facilitates malate efflux from different cell types. Altering OsALMT4 expression compromises growth in low-light environments.

Highlight: Early vigour in wheat was shown to be a valuable trait for improving biomass accumulation and phosphate uptake from a low-P soil., Quantitative trait loci (QTLs) for shoot biomass were identified in wheat grown on a soil high in total phosphorus (P) but low in plant-available P. The two populations screened included recombinant inbred lines (RILs) from Chuan-Mai 18/Vigour 18 and doubled-haploid lines from Kukri/Janz. Glasshouse-grown plants were harvested at the five-leaf stage. Seven QTLs for shoot biomass were identified in the RILs, with the largest on chromosome 7A accounting for 7.4% of the phenotypic variance. RILs from the upper tail had larger embryos than RILs from the lower tail. Tail lines were then grown in non-limiting P and the results indicated that early vigour and the capacity to access P contributed to the initial distribution. The influence of early vigour on P nutrition was examined further with advanced vigour lines (AVLs). The AVLs accumulated more shoot biomass, maintained lower shoot P concentrations, and showed greater P-acquisition efficiency than Vigour 18. Nine QTLs for shoot biomass were identified in the Kukri/Janz population. Two on chromosomes 4B and 4D accounted for 24.8% of the variance. Candidates underlying these QTLs are the Rht genes. We confirmed the influence of these genes using near-isogenic lines with different Rht alleles. The dwarf and semi-dwarf alleles affected shoot and root biomass at high and low P but not the efficiency of P acquisition. We conclude that early vigour contributed to the distributions in both populations. Early vigour can increase plant growth at suboptimal P and some sources can also improve the efficiency of P acquisition.

Seventy-one cultivars of sweet sorghum (Sorghum bicolor L.) were screened for aluminium (Al) tolerance by measuring relative root growth (RRG). Two contrasting cultivars, ROMA (Al tolerant) and POTCHETSTRM (Al sensitive), were selected to study shorter term responses to Al stress. POTCHETSTRM had higher callose synthase activity, lower β-1,3-glucanase activity and more callose deposition in the root apices during Al treatment compared with ROMA. We monitored the expression of 12 genes involved in callose synthesis and degradation and found that one of these, SbGlu1 (Sb03g045630.1), which encodes a β-1,3-glucanase enzyme, best explained the contrasting deposition of callose in ROMA and POTCHETSTRM during Al treatment. Full-length cDNAs of SbGlu1 was prepared from ROMA and POTCHETSTRM and expressed in Arabidopsis thaliana using the constitutive cauliflower mosaic virus (CaMV) 35S promoter. Independent transgenic lines displayed significantly greater Al tolerance than wild-type plants and vector-only controls. This phenotype was associated with greater total β-1,3-glucanase activity, less Al accumulation and reduced callose deposition in the roots. These results suggest that callose production is not just an early indicator of Al stress in plants but likely to be part of the toxicity pathway that leads to the inhibition of root growth.

Background and Aims: Expected increases in world population will continue to make demands on agricultural productivity and food supply. These challenges will only be met by increasing the land under cultivation and by improving the yields obtained on existing farms. Genetic engineering can target key traits to improve crop yields and to increase production on marginal soils. Soil acidity is a major abiotic stress that limits plant production worldwide. The goal of this study was to enhance the acid soil tolerance of wheat by increasing its resistance to Al3+ toxicity. Methods: Particle bombardment was used to transform wheat with TaALMT1, the Al3+ resistance gene from wheat, using the maize ubiquitin promoter to drive expression. TaALMT1 expression, malate efflux and Al3+ resistance were measured in the T1 and T2 lines and compared with the parental line and an Al3+-resistant reference genotype, ET8. Key Results: Nine T2 lines showed increased TaALMT1 expression, malate efflux and Al3+ resistance when compared with untransformed controls and null segregant lines. Some T2 lines displayed greater Al3+ resistance than ET8 in both hydroponic and soil experiments. Conclusions: The Al3+ resistance of wheat was increased by enhancing TaALMT1 expression with biotechnology. This is the first report of a major food crop being stably transformed for greater Al3+ resistance. Transgenic strategies provide options for increasing food supply on acid soils.

In wheat (Triticum aestivum), it is commonly assumed that Al is detoxified by the release of organic anions into the rhizosphere, but it is also possible that detoxification occurs within the apoplast and symplast of the root itself. Using Al-resistant (ET8) and Al-sensitive (ES8) near-isogenic lines of wheat, we utilized traditional and synchrotron-based approaches to provide in situ analyses of the distribution and speciation of Al within root tissues. Some Al appeared to be complexed external to the root, in agreement with the common assumption. However, root apical tissues of ET8 accumulated 4- to 6-times more Al than ES8 when exposed to Al concentrations that reduce root elongation rate by 50 % (3.5 µM Al for ES8 and 50 µM for ET8). Furthermore, in situ analyses of ET8 root tissues indicated the likely presence of Al-malate and other forms of Al, predominantly within the apoplast. To our knowledge, this is the first time that X-ray absorption near edge structure (XANES) analyses have been used to examine the speciation of Al within plant tissues. The information obtained in the present study is important in developing an understanding of the underlying physiological mode of action for improved root growth in systems with elevated soluble Al.

Do root hairs help roots take up water from the soil? Despite the well-documented role of root hairs in phosphate uptake, their role in water extraction is controversial. We grew barley (Hordeum vulgare cv Pallas) and its root-hairless mutant brb in a root pressure chamber, whereby the transpiration rate could be varied whilst monitoring the suction in the xylem. The method provides accurate measurements of the dynamic relationship between the transpiration rate and xylem suction. The relationship between the transpiration rate and xylem suction was linear in wet soils and did not differ between genotypes. When the soil dried, the xylem suction increased rapidly and non-linearly at high transpiration rates. This response was much greater with the brb mutant, implying a reduced capacity to take up water. We conclude that root hairs facilitate the uptake of water by substantially reducing the drop in matric potential at the interface between root and soil in rapidly transpiring plants. The experiments also reinforce earlier observations that there is a marked hysteresis in the suction in the xylem when the transpiration rate is rising compared with when it is falling, and possible reasons for this behavior are discussed.

Wheat and Arabidopsis plants respond to aluminum (Al) ions by releasing malate from their root apices via Al-activated malate transporter. Malate anions bind with the toxic Al ions and contribute to the Al tolerance of these species. The genes encoding the transporters in wheat and Arabidopsis, TaALMT1 and AtALMT1, respectively, were expressed in Xenopus laevis oocytes and characterized electrophysiologically using the two-electrode voltage clamp system. The Al-activated currents generated by malate efflux were detected for TaALMT1 but not for AtALMT1. Chimeric proteins were generated by swapping the N- and C-terminal halves of TaALMT1 and AtALMT1 (Ta::At and At::Ta). When these chimeras were characterized in oocytes, Al-activated malate efflux was detected for the Ta::At chimera but not for At::Ta, suggesting that the N-terminal half of TaALMT1 is necessary for function in oocytes. An additional chimera, Ta(48)::At, generated by swapping 17 residues from the N-terminus of AtALMT1 with the equivalent 48 residues from TaALMT1, was sufficient to support transport activity. This 48 residue region includes a helical region with a putative transmembrane domain which is absent in AtALMT1. The deletion of this domain from Ta(48)::At led to the complete loss of transport activity. Furthermore, truncations and a deletion at the C-terminal end of TaALMT1 indicated that a putative helical structure in this region was also required for transport function. This study provides insights into the structure-function relationships of Al-activated ALMT proteins by identifying specific domains on the N- and C-termini of TaALMT1 that are critical for basal transport function and Al responsiveness in oocytes.

Phosphorus (P) deficiency in some plant species triggers the release of organic anions such as citrate and malate from roots. These anions are widely suggested to enhance the availability of phosphate for plant uptake by mobilizing sparingly-soluble forms in the soil. Carazinho is an old wheat (Triticum aestivum) cultivar from Brazil, which secretes citrate constitutively from its root apices, and here we show that it also produces relatively more biomass on soils with low P availability than two recent Australian cultivars that lack citrate efflux. To test whether citrate efflux explains this phenotype, we generated two sets of near-isogenic lines that differ in citrate efflux and compared their biomass production in different soil types and with different P treatments in glasshouse experiments and field trials. Citrate efflux improved relative biomass production in two of six glasshouse trials but only at the lowest P treatments where growth was most severely limited by P availability. Furthermore, citrate efflux provided no consistent advantage for biomass production or yield in multiple field trials. Theoretical modeling indicates that the effectiveness of citrate efflux in mobilizing soil P is greater as the volume of soil into which it diffuses increases. As efflux from these wheat plants is restricted to the root apices, the potential for citrate to mobilize sufficient P to increase shoot biomass may be limited. We conclude that Carazinho has other attributes that contribute to its comparatively good performance in low-P soils.

Background and Aims Aluminium is toxic in acid soils because the soluble Al 3+ inhibits root growth. A mechanism of Al 3+ tolerance discovered in many plant species involves the release of organic anions from root apices. The Al 3+ -activated release of citrate from the root apices of Al 3+ -tolerant genotypes of barley is controlled by a MATE gene named HvAACT1 that encodes a citrate transport protein located on the plasma membrane. The aim of this study was to investigate whether expressing HvAACT1 with a constitutive promoter in barley and wheat can increase citrate efflux and Al 3+ tolerance of these important cereal species. Methods: HvAACT1 was over-expressed in wheat ( Triticum aestivum ) and barley ( Hordeum vulgare ) using the maize ubiquitin promoter. Root apices of transgenic and control lines were analysed for HvAACT1 expression and organic acid efflux. The Al 3+ tolerance of transgenic and control lines was assessed in both hydroponic solution and acid soil. Key Results and Conclusions: Increased HvAACT1 expression in both cereal species was associated with increased citrate efflux from root apices and enhanced Al 3+ tolerance, thus demonstrating that biotechnology can complement traditional breeding practices to increase the Al 3+ tolerance of important crop plants.

Background Aluminium (Al) toxicity is the main factor limiting the crop production in acid soils and barley (Hordeum vulgare L.) is one of the most Al-sensitive of the small-grained cereals. The major gene for Al tolerance in barley is HvAACT1 (HvMATE) on chromosome 4H which encodes a multidrug and toxic compound extrusion (MATE) protein. The HvAACT1 protein facilitates the Al-activated release of citrate from root apices which protects the growing cells and enables root elongation to continue. A 1 kb transposable element-like insert in the 5’ untranslated region (UTR) of HvAACT1 is associated with increased gene expression and tolerance and a PCR-based marker is available to score for this insertion. Results We screened a wide range of barley genotypes for Al tolerance and identified a moderately tolerant Chinese genotype named CXHKSL which did not show the typical allele in the 5’ UTR of HvAACT1 associated with tolerance. We investigated the mechanism of Al tolerance in CXHKSL and concluded it also relies on the Al-activated release of citrate from roots. Quantitative trait loci (QTL) analysis of double haploid lines generated with CXHKSL and the Al-sensitive variety Gairdner mapped the tolerance locus to the same region as HvAACT1 on chromosome 4H. Conclusions Our results show that the Chinese barley genotype CXHKSL possesses a novel allele of the major Al tolerance gene HvAACT1. Electronic supplementary material The online version of this article (doi:10.1186/s12864-016-2551-3) contains supplementary material, which is available to authorized users.

Highlight Large rhizosheaths on wheat genotypes grown on acid soils improved the phosphorus acquisition efficiency compared with genotypes with small rhizosheaths. The rhizosheath trait was mapped to five major quantitative trait loci with largely additive genetic effect., Rhizosheaths comprise soil bound to roots, and in wheat (Triticum aestivum L.) rhizosheath size correlates with root hair length. The aims of this study were to determine the effect that a large rhizosheath has on the phosphorus (P) acquisition by wheat and to investigate the genetic control of rhizosheath size in wheat grown on acid soil. Near-isogenic wheat lines differing in rhizosheath size were evaluated on two acid soils. The soils were fertilized with mineral nutrients and included treatments with either low or high P. The same soils were treated with CaCO3 to raise the pH and detoxify Al3+. Genotypic differences in rhizosheath size were apparent only when soil pH was low and Al3+ was present. On acid soils, a large rhizosheath increased shoot biomass compared with a small rhizosheath regardless of P supply. At low P supply, increased shoot biomass could be attributed to a greater uptake of soil P, but at high P supply the increased biomass was due to some other factor. Generation means analysis indicated that rhizosheath size on acid soil was controlled by multiple, additive loci. Subsequently, a quantitative trait loci (QTL) analysis of an F6 population of recombinant inbred lines identified five major loci contributing to the phenotype together accounting for over 60% of the total genetic variance. One locus on chromosome 1D accounted for 34% of the genotypic variation. Genetic control of rhizosheath size appears to be relatively simple and markers based on the QTL provide valuable tools for marker assisted breeding.

The rhizosphere microbiome is regulated by plant genotype, root exudates and environment. There is substantial interest in breeding and managing crops that host root microbial communities that increase productivity. The eudicot model species Arabidopsis has been used to investigate these processes, however a model for monocotyledons is also required. We characterized the rhizosphere microbiome and root exudates of Brachypodium distachyon, to develop it as a rhizosphere model for cereal species like wheat. The Brachypodium rhizosphere microbial community was dominated by Burkholderiales. However, these communities were also dependent on how tightly they were bound to roots, the root type they were associated with (nodal or seminal roots), and their location along the roots. Moreover, the functional gene categories detected in microorganisms isolated from around root tips differed from those isolated from bases of roots. The Brachypodium rhizosphere microbiota and root exudate profiles were similar to those reported for wheat rhizospheres, and different to Arabidopsis. The differences in root system development and cell wall chemistry between monocotyledons and eudicots may also influence the microorganism composition of these major plant types. Brachypodium is a promising model for investigating the microbiome of wheat.

Summary •We found significant genetic variation in the ability of wheat (Triticum aestivum) to form rhizosheaths on acid soil and assessed whether differences in aluminium (Al3+) tolerance of root hairs between genotypes was the physiological basis for this genetic variation. •A method was developed to rapidly screen rhizosheath size in a range of wheat genotypes. Backcrossed populations were generated from cv Fronteira (large rhizosheath) using cv EGA-Burke (small rhizosheath) as the recurrent parent. •A positive correlation existed between rhizosheath size on acid soil and root hair length. In hydroponic experiments, root hairs of the backcrossed lines with large rhizosheaths were more tolerant of Al3+ toxicity than the backcrossed lines with small rhizosheaths. •We conclude that greater Al3+ tolerance of root hairs underlies the larger rhizosheath of wheat grown on acid soil. Tolerance of the root hairs to Al3+ was largely independent of the TaALMT1 gene which suggests that different genes encode the Al3+ tolerance of root hairs. The maintenance of longer root hairs in acid soils is important for the efficient uptake of water and nutrients.

Allelochemicals and other metabolites released by plant roots play important roles in rhizosphere signalling, plant defence and responses to abiotic stresses. Plants use a variety of sequestration and transport mechanisms to move and export bioactive products safely into the rhizosphere. The use of mutants and molecular tools to study gene expression has revealed new information regarding the diverse group of transport proteins and conjugation processes employed by higher plants. Transport systems used for moving secondary products into and out of root cells are similar to those used elsewhere in the plant but are closely linked to soil environmental conditions and local root health. Root cells can rapidly generate and release large quantities of allelochemicals in response to stress or local rhizosphere conditions, so the production and transport of these compounds in cells are often closely linked. Plants need to manage the potentially toxic allelochemicals and metabolites they produce by sequestering them to the vacuole or other membrane-bound vesicles. These compartments provide secure storage areas and systems for safely moving bioactive chemicals throughout the cytosol. Release into the apoplast occurs either by exocytosis or through membrane-bound transport proteins. This review discusses the possible transport mechanisms involved in releasing specific root-produced allelochemicals by combining microscopic observations of the specialized root cells with the physical and chemical properties of the exudates.

Phosphorus (P)-deficiency is a significant challenge for agricultural productivity on many highly P-sorbing weathered and tropical soils throughout the world. On these soils it can be necessary to apply up to five-fold more P as fertiliser than is exported in products. Given the finite nature of global P resources, it is important that such inefficiencies be addressed. For low P-sorbing soils, P-efficient farming systems will also assist attempts to reduce pollution associated with P losses to the environment. P-balance inefficiency of farms is associated with loss of P in erosion, runoff or leaching, uneven dispersal of animal excreta, and accumulation of P as sparingly-available phosphate and organic P in the soil. In many cases it is possible to minimise P losses in runoff or erosion. Uneven dispersal of P in excreta typically amounts to ~5% of P-fertiliser inputs. However, the rate of P accumulation in moderate to highly P-sorbing soils is a major contributor to inefficient P-fertiliser use. We discuss the causal edaphic, plant and microbial factors in the context of soil P management, P cycling and productivity goals of farms. Management interventions that can alter P-use efficiency are explored, including better targeted P-fertiliser use, organic amendments, removing other constraints to yield, zone management, use of plants with low critical-P requirements, and modified farming systems. Higher productivity in low-P soils, or lower P inputs in fertilised agricultural systems can be achieved by various interventions, but it is also critically important to understand the agroecology of plant P nutrition within farming systems for improvements in P-use efficiency to be realised.

Salinity stress is a major limitation in barley production. Substantial genetic variation in tolerance occurs among genotypes of barley, so the development of salt-tolerant cultivars is a potentially effective approach for minimizing yield losses. The lack of economically viable methods for screening salinity tolerance in the field remains an obstacle to breeders, and molecular marker-assisted selection is a promising alternative. In this study, salinity tolerance of 172 doubled-haploid lines generated from YYXT (salinity-tolerant) and Franklin (salinity-sensitive) was assessed in glasshouse trials during the vegetative phase. A high-density genetic linkage map was constructed from 76 pairs of simple sequence repeats and 782 Diversity Arrays Technology markers which spanned a total of 1,147 cM. Five significant quantitative trait loci (QTL) for salinity tolerance were identified on chromosomes 1H, 2H, 5H, 6H and 7H, accounting for more than 50% of the phenotypic variation. The tolerant variety, YYXT, contributed the tolerance to four of these QTL and Franklin contributed the tolerance to one QTL on chromosome 1H. Some of these QTL mapped to genomic regions previously associated with salt tolerance in barley and other cereals. Markers associated with the major QTL identified in this study have potential application for marker-assisted selection in breeding for enhanced salt tolerance in barley.