RCN1/OsABCG5, an ATP-binding cassette (ABC) transporter, is required for hypodermal suberization of roots in rice (Oryza sativa)

OsABCG9 Is an Important ABC Transporter of Cuticular Wax Deposition in Rice

The importance of the cuticular layer in regulating a plant's water status and providing protection from environmental challenges has been recognized for a long time. The cuticular layer in plants restricts non-stomatal water loss and protects plants against damage from pathogen infection and UV radiation. Much genetic and biochemical research has been done about cutin and wax transportation in Arabidopsis thaliana, but little is known about it in rice. Here, we report that a rice ATP-binding cassette (ABC) transporter, OsABCG9, is essential for normal development during vegetative growth and could play a critical role in the transportation of epicuticular wax in rice. Rice phenotypes with mutated OsABCG9 exhibited growth retardation and sensitivity to low humidity. The total amount of cuticular wax on the leaves of the osabcg9-1 mutant diminished by 53% compared with the wild type, and wax crystals disappeared completely in osabcg9-2 mutant leaves. However, OsABCG9 does not seem to be involved in cutin transportation, even though its ortholog in Arabidopsis, AtABCG11, transports both wax and cutin. Furthermore, the osabcg9-1 mutant had increased leaf chlorophyll leaching and more severe drought susceptibility. This study provides new insights about differences between rice and A. thaliana in wax and cutin transportation associated with the ABCG family during evolution.

Energy-crises in well-aerated and anoxic tissue: does tolerance require the same specific proteins and energy-efficient transport?

Many of the profound changes in metabolism that are caused by O2 deficiency also occur in well-aerated tissues when oxidative phosphorylation is partially or wholly inhibited. For these well-aerated tissues, reduction in energy formation occurs during exposure to inhibitors of oxidative phosphorylation, cold/chilling and wounding, so we prefer the term 'energy crisis' metabolism over 'anaerobic' metabolism. In this review, we note that the overwhelming body of data on energy crises has been obtained by exposure to hypoxia-anoxia, which we will indicate when discussing the particular experiments. We suggest that even transient survival of an energy crisis requires a network of changes common to a large number of conditions, ranging from changes in development to various adverse conditions such as high salinity, drought and nutrient deficiency, all of which reduce growth. During an energy crisis this general network needs to be complemented by energy specific proteins, including the so called 'anaerobic proteins' and the group of ERFVII transcription factors, which induces the synthesis of these proteins. Crucially, the difference between anoxia-intolerant and -tolerant tissues in the event of a severe energy crisis would mainly depend on changes in some 'key' energy crisis proteins: we suggest these proteins would include phytoglobin, the V-H+PPiase and pyruvate decarboxylase. A second characteristic of a high tolerance to an energy crisis is engagement of energy efficient transport. This feature includes a sharp reduction in rates of solute transport and use of energy-efficient modifications of transport systems by primary H+ transport and secondary H+-solute transport systems. Here we also discuss the best choice of species to study an energy crisis. Further, we consider confounding of the acclimative response by responses to injury, be it due to the use of tissues intolerant to an energy crisis, or to faulty techniques.

Suberized transport barriers in Arabidopsis, barley and rice roots: From the model plant to crop species

Water is the most important prerequisite for life and plays a major role during uptake and transport of nutrients. Roots are the plant organs that take up the major part of water, from the surrounding soil. Water uptake is related to the root system architecture, root growth, age and species dependent complex developmental changes in the anatomical structures. The latter is mainly attributed to the deposition of suberized barriers in certain layers of cell walls, such as endo- and exodermis. With respect to water permeability, changes in the suberization of roots are most relevant. Water transport or hydraulic conductivity of roots (Lp_r) can be described by the composite transport model and is known to be very variable between plant species and growth conditions and root developmental states. In this review, we summarize how anatomical structures and apoplastic barriers of roots can diversely affect water transport, comparing the model plant Arabidopsis with crop plants, such as barley and rice. Results comparing the suberin amounts and water transport properties indicate that the common assumption that suberin amount negatively correlates with water and solute transport through roots may not always be true. The composition, microstructure and localization of suberin may also have a great impact on the formation of efficient barriers to water and solutes.

Transcriptome analysis of Petunia axillaris flowers reveals genes involved in morphological differentiation and metabolite transport

The biosynthesis of plant secondary metabolites is associated with morphological and metabolic differentiation. As a consequence, gene expression profiles can change drastically, and primary and secondary metabolites, including intermediate and end-products, move dynamically within and between cells. However, little is known about the molecular mechanisms underlying differentiation and transport mechanisms. In this study, we performed a transcriptome analysis of Petunia axillaris subsp. parodii, which produces various volatiles in its corolla limbs and emits metabolites to attract pollinators. RNA-sequencing from leaves, buds, and limbs identified 53,243 unigenes. Analysis of differentially expressed genes, combined with gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses, showed that many biological processes were highly enriched in limbs. These included catabolic processes and signaling pathways of hormones, such as gibberellins, and metabolic pathways, including phenylpropanoids and fatty acids. Moreover, we identified five transporter genes that showed high expression in limbs, and we performed spatiotemporal expression analyses and homology searches to infer their putative functions. Our systematic analysis provides comprehensive transcriptomic information regarding morphological differentiation and metabolite transport in the Petunia flower and lays the foundation for establishing the specific mechanisms that control secondary metabolite biosynthesis in plants.

Characterization and expression of the ABC family (G group) in 'Dangshansuli' pear (Pyrus bretschneideri Rehd.) and its russet mutant

The plant genes encoding ABCGs that have been identified to date play a role in suberin formation in response to abiotic and biotic stress. In the present study, 80 ABCG genes were identified in ‘Dangshansuli’ Chinese white pear and designated as PbABCGs. Based on the structural characteristics and phylogenetic analysis, the PbABCG family genes could be classified into seven main groups: classes A-G. Segmental and dispersed duplications were the primary forces underlying the PbABCG gene family expansion in ‘Dangshansuli’ pear. Most of the PbABCG duplicated gene pairs date to the recent whole-genome duplication that occurred 30~45 million years ago. Purifying selection has also played a critical role in the evolution of the ABCG genes. Ten PbABCG genes screened in the transcriptome of ‘Dangshansuli’ pear and its russet mutant ‘Xiusu’ were validated, and the expression levels of the PbABCG genes exhibited significant differences at different stages. The results presented here will undoubtedly be useful for better understanding of the complexity of the PbABCG gene family and will facilitate the functional characterization of suberin formation in the russet mutant.

Functions of ABC transporters in plant growth and development

ABC transporters are essential for plant development, playing roles in processes such as gametogenesis, seed development, seed germination, organ formation, and secondary growth. ABC transporters are directly energized by ATP and can transport complex organic materials against concentration gradients; thus, they are uniquely suited to provide the complex building blocks required for the development of specialized plant cells. We review recent progress in our understanding of the contribution ABC transporters make to the growth and development of plants, including their roles in protective layer formation and in transporting phytohormones.

In vivo cytological and chemical analysis of Casparian strips using stimulated Raman scattering microscopy

The Casparian strip, a barrier to the apoplastic movement of solutes from the cortex to the stele, is essential for the exclusion of salts, selective nutrient uptake, and many other processes. To date, extensive studies have focused on the physiological functions of endodermal Casparian strips. However, the chemical deposition nature of Casparian strips, as well as its relevance with respect to diffusion barrier functions, remains to be further elucidated. Here, we revealed three developmental stages of Casparian strips in maize primary roots using a traditional fluorescent staining method. Apoplastic permeability tests demonstrated that the barrier function of Casparian strips is largely related to their developmental stage and the pattern of lignin and suberin deposits. Fourier transform infrared (FTIR) analysis showed that the Casparian strips from the roots exhibited significant absorption bands characteristic of lignin and suberin, implying that the Casparian strips in maize primary roots consist largely of lignin and suberin. Furthermore, we developed a new method for label-free, in vivo structural, and biochemical analysis of Casparian strips based on stimulated Raman scattering (SRS) microscopy. Using SRS microscopy, we found that lignin and suberin accumulate simultaneously during the Casparian strip formation process. Based on these results, we propose a potential application of SRS for the chemical composition analysis of plant Casparian strips in situ.

The endodermis, a tightly controlled barrier for nutrients

Plant roots acquire nutrients from the soil and transport them upwards to the aerial parts. To reach the central vasculature of the root, water and nutrients radially cross all external cell layers. The endodermis surrounds the vascular tissues and forms diffusion barriers. It thereby compartmentalizes the root and allows control of nutrient transport from the soil to the vasculature, as well as preventing backflow of nutrients from the stele. To achieve this role, endodermal cells undergo two specialized differentiations states consisting of deposition of two impermeable polymers in the cell wall: lignin, forming the Casparian strips, and suberin lamellae. Recent publications showed that endodermal barrier formation is not a hard-wired, irreversible process. Synthesis and degradation of suberin lamellae is highly regulated by plant hormones in response to nutrient stresses. Moreover, Casparian strip continuity seems to be constantly checked by two small peptides produced in the vasculature that diffuse into the apoplastic space in order to test endodermal barrier integrity. This review discusses the recent understanding of endodermal barrier surveillance and plasticity and its role in plant nutrition.

Diet of Arbuscular Mycorrhizal Fungi: Bread and Butter?

Most plants entertain mutualistic interactions known as arbuscular mycorrhiza (AM) with soil fungi (Glomeromycota) which provide them with mineral nutrients in exchange for reduced carbon from the plant. Mycorrhizal roots represent strong carbon sinks in which hexoses are transferred from the plant host to the fungus. However, most of the carbon in AM fungi is stored in the form of lipids. The absence of the type I fatty acid synthase (FAS-I) complex from the AM fungal model species Rhizophagus irregularis suggests that lipids may also have a role in nutrition of the fungal partner. This hypothesis is supported by the concerted induction of host genes involved in lipid metabolism. We explore the possible roles of lipids in the light of recent literature on AM symbiosis.

The endodermis as a checkpoint for nutrients

Contents 1604 I. 1604 II. 1604 III. 1605 IV. 1608 V. 1609 1609 References 1609 SUMMARY: Plant roots forage the soil for nutrients and transport them upwards to the aerial parts. Nutrients entering the plant are transported through the concentric layers of epidermis, cortex and endodermis before reaching the central vasculature. The endodermis is the innermost cortical cell layer that surrounds the vasculature. The endodermis forms barriers, the Casparian strips and suberin lamellae, which have been assumed to play a major role in controlling nutrient acquisition. However, the molecular network controlling its differentiation has started to be investigated only recently, giving an unprecedented opportunity to address the role of these barriers in plant nutrition. This insight aims to present recent advances regarding endodermis differentiation, its function as a barrier for nutrients and its developmental plasticity, all pointing to a pivotal role of the endodermis as a checkpoint for nutrients.

A major locus involved in the formation of the radial oxygen loss barrier in adventitious roots of teosinte Zea nicaraguensis is located on the short-arm of chromosome 3

A radial oxygen loss (ROL) barrier in roots of waterlogging-tolerant plants promotes oxygen movement via aerenchyma to the root tip, and impedes soil phytotoxin entry. The molecular mechanism and genetic regulation of ROL barrier formation are largely unknown. Zea nicaraguensis, a waterlogging-tolerant wild relative of maize (Zea mays ssp. mays), forms a tight ROL barrier in its roots when waterlogged. We used Z. nicaraguensis chromosome segment introgression lines (ILs) in maize (inbred line Mi29) to elucidate the chromosomal region involved in regulating root ROL barrier formation. A segment of the short-arm of chromosome 3 of Z. nicaraguensis conferred ROL barrier formation in the genetic background of maize. This chromosome segment also decreased apoplastic solute permeability across the hypodermis/exodermis. However, the IL and maize were similar for suberin staining in the hypodermis/exodermis at 40 mm and further behind the root tip. Z. nicaraguensis contained suberin in the hypodermis/exodermis at 20 mm and lignin at the epidermis. The IL with ROL barrier, however, did not contain lignin in the epidermis. Discovery of the Z. nicaraguensis chromosomal region responsible for root ROL barrier formation has improved knowledge of this trait and is an important step towards improvement of waterlogging tolerance in maize.

An ABC Transporter Is Involved in the Silicon-Induced Formation of Casparian Bands in the Exodermis of Rice

Silicon (Si) promotes the formation of Casparian bands (CB) in rice and reduces radial oxygen loss (ROL). Further transcriptomic approaches revealed several candidate genes involved in the Si-induced formation of CB such as ATP binding cassette (ABC) transporter, Class III peroxidases, ligases and transferases. Investigation of these genes by means of overexpression (OE) and knockout (KO) mutants revealed the contribution of the ABC transporter (OsABCG25) to CB formation in the exodermis, which was also reflected in the expression of other OsABCG25 in the Si-promoted formation of CB genes related to the phenylpropanoid pathway, such as phenylalanine-ammonia-lyase, diacylglycerol O-acyltransferase and 4-coumarate-CoA ligase. Differential CB development in mutants and Si supply also affected the barrier function of the exodermis. OE of the ABC transporter and Si supply reduced the ROL from roots and Fe uptake. No effect on ROL and Fe uptake could be observed for the KO mutant. The presented research confirms the impact of the OsABCG25 in the Si-promoted formation of CB and its barrier functions.

Ethylene-dependent aerenchyma formation in adventitious roots is regulated differently in rice and maize

In roots of gramineous plants, lysigenous aerenchyma is created by the death and lysis of cortical cells. Rice (Oryza sativa) constitutively forms aerenchyma under aerobic conditions, and its formation is further induced under oxygen-deficient conditions. However, maize (Zea mays) develops aerenchyma only under oxygen-deficient conditions. Ethylene is involved in lysigenous aerenchyma formation. Here, we investigated how ethylene-dependent aerenchyma formation is differently regulated between rice and maize. For this purpose, in rice, we used the reduced culm number1 (rcn1) mutant, in which ethylene biosynthesis is suppressed. Ethylene is converted from 1-aminocyclopropane-1-carboxylic acid (ACC) by the action of ACC oxidase (ACO). We found that OsACO5 was highly expressed in the wild type, but not in rcn1, under aerobic conditions, suggesting that OsACO5 contributes to aerenchyma formation in aerated rice roots. By contrast, the ACO genes in maize roots were weakly expressed under aerobic conditions, and thus ACC treatment did not effectively induce ethylene production or aerenchyma formation, unlike in rice. Aerenchyma formation in rice roots after the initiation of oxygen-deficient conditions was faster and greater than that in maize. These results suggest that the difference in aerenchyma formation in rice and maize is due to their different mechanisms for regulating ethylene biosynthesis.

Rice Stomatal Closure Requires Guard Cell Plasma Membrane ATP-Binding Cassette Transporter RCN1/OsABCG5

Water stress is one of the major environmental stresses that affect agricultural production worldwide. Water loss from plants occurs primarily through stomatal pores. Here, we report that an Oryza sativa half-size ATP-binding cassette (ABC) subfamily G protein, RCN1/OsABCG5, is involved in stomatal closure mediated by phytohormone abscisic acid (ABA) accumulation in guard cells. We found that the GFP-RCN1/OsABCG5-fusion protein was localized at the plasma membrane in guard cells. The percentage of guard cell pairs containing both ABA and GFP-RCN1/OsABCG5 increased after exogenous ABA treatment, whereas they were co-localized in guard cell pairs regardless of whether exogenous ABA was applied. ABA application resulted in a smaller increase in the percentage of guard cell pairs containing ABA in rcn1 mutant (A684P) and RCN1-RNAi than in wild-type plants. Furthermore, polyethylene glycol (drought stress)-inducible ABA accumulation in guard cells did not occur in rcn1 mutants. Stomata closure mediated by exogenous ABA application was strongly reduced in rcn1 mutants. Finally, rcn1 mutant plants had more rapid water loss from detached leaves than the wild-type plants. These results indicate that in response to drought stress, RCN1/OsABCG5 is involved in accumulation of ABA in guard cells, which is indispensable for stomatal closure.

Flexible Ion Barrier

The central vasculature of plant roots is protected by a hydrophobic ring of endodermal cells that are enclosed by lamellae of suberin. Barberon et al. demonstrate that endodermal suberization plasticity facilitates ion homeostasis, with antithetical regulation of suberin deposition and degradation by the phytohormones abscisic acid and ethylene.

Adaptation of Root Function by Nutrient-Induced Plasticity of Endodermal Differentiation

Plant roots forage the soil for minerals whose concentrations can be orders of magnitude away from those required for plant cell function. Selective uptake in multicellular organisms critically requires epithelia with extracellular diffusion barriers. In plants, such a barrier is provided by the endodermis and its Casparian strips--cell wall impregnations analogous to animal tight and adherens junctions. Interestingly, the endodermis undergoes secondary differentiation, becoming coated with hydrophobic suberin, presumably switching from an actively absorbing to a protective epithelium. Here, we show that suberization responds to a wide range of nutrient stresses, mediated by the stress hormones abscisic acid and ethylene. We reveal a striking ability of the root to not only regulate synthesis of suberin, but also selectively degrade it in response to ethylene. Finally, we demonstrate that changes in suberization constitute physiologically relevant, adaptive responses, pointing to a pivotal role of the endodermal membrane in nutrient homeostasis.

Ammonium-induced architectural and anatomical changes with altered suberin and lignin levels significantly change water and solute permeabilities of rice (Oryza sativa L.) roots

Non-optimal ammonium levels significantly alter root architecture, anatomy and root permeabilities for water and nutrient ions. Higher ammonium levels induced strong apoplastic barriers whereas it was opposite for lower levels. Application of nitrogen fertilizer increases crop productivity. However, non-optimal applications can have negative effects on plant growth and development. In this study, we investigated how different levels of ammonium (NH4 (+)) [low (30 or 100 μM) or optimum (300 μM) or high (1000 or 3000 μM)] affect physio-chemical properties of 1-month-old, hydroponically grown rice roots. Different NH4 (+) treatments markedly altered the root architecture and anatomy. Plants grown in low NH4 (+) had the longest roots with a weak deposition of suberised and lignified apoplastic barriers, and it was opposite for plants grown in high NH4 (+). The relative expression levels of selected suberin and lignin biosynthesis candidate genes, determined using qRT-PCR, were lowest in the roots from low NH4 (+), whereas, they were highest for those grown in high NH4 (+). This was reflected by the suberin and lignin contents, and was significantly lower in roots from low NH4 (+) resulting in greater hydraulic conductivity (Lp r) and solute permeability (P sr) than roots from optimum NH4 (+). In contrast, roots grown at high NH4 (+) had markedly greater suberin and lignin contents, which were reflected by strong barriers. These barriers significantly decreased the P sr of roots but failed to reduce the Lp r below those of roots grown in optimum NH4 (+), which can be explained in terms of the physical properties of the molecules used and the size of pores in the apoplast. It is concluded that, in rice, non-optimal NH4 (+) levels differentially affected root properties including Lp r and P sr to successfully adapt to the changing root environment.

Orchestration of three transporters and distinct vascular structures in node for intervascular transfer of silicon in rice

Requirement of mineral elements in different plant tissues is not often consistent with their transpiration rate; therefore, plants have developed systems for preferential distribution of mineral elements to the developing tissues with low transpiration. Here we took silicon (Si) as an example and revealed an efficient system for preferential distribution of Si in the node of rice (Oryza sativa). Rice is able to accumulate more than 10% Si of the dry weight in the husk, which is required for protecting the grains from water loss and pathogen infection. However, it has been unknown for a long time how this hyperaccumulation is achieved. We found that three transporters (Lsi2, Lsi3, and Lsi6) located at the node are involved in the intervascular transfer, which is required for the preferential distribution of Si. Lsi2 was polarly localized to the bundle sheath cell layer around the enlarged vascular bundles, which is next to the xylem transfer cell layer where Lsi6 is localized. Lsi3 was located in the parenchyma tissues between enlarged vascular bundles and diffuse vascular bundles. Similar to Lsi6, knockout of Lsi2 and Lsi3 also resulted in decreased distribution of Si to the panicles but increased Si to the flag leaf. Furthermore, we constructed a mathematical model for Si distribution and revealed that in addition to cooperation of three transporters, an apoplastic barrier localized at the bundle sheath cells and development of the enlarged vascular bundles in node are also required for the hyperaccumulation of Si in rice husk.

Flood adaptive traits and processes: an overview

Unanticipated flooding challenges plant growth and fitness in natural and agricultural ecosystems. Here we describe mechanisms of developmental plasticity and metabolic modulation that underpin adaptive traits and acclimation responses to waterlogging of root systems and submergence of aerial tissues. This includes insights into processes that enhance ventilation of submerged organs. At the intersection between metabolism and growth, submergence survival strategies have evolved involving an ethylene-driven and gibberellin-enhanced module that regulates growth of submerged organs. Opposing regulation of this pathway is facilitated by a subgroup of ethylene-response transcription factors (ERFs), which include members that require low O₂ or low nitric oxide (NO) conditions for their stabilization. These transcription factors control genes encoding enzymes required for anaerobic metabolism as well as proteins that fine-tune their function in transcription and turnover. Other mechanisms that control metabolism and growth at seed, seedling and mature stages under flooding conditions are reviewed, as well as findings demonstrating that true endurance of submergence includes an ability to restore growth following the deluge. Finally, we highlight molecular insights obtained from natural variation of domesticated and wild species that occupy different hydrological niches, emphasizing the value of understanding natural flooding survival strategies in efforts to stabilize crop yields in flood-prone environments.

Suberin: biosynthesis, regulation, and polymer assembly of a protective extracellular barrier

Suberin is a lipid-phenolic biopolyester deposited in the cell walls of certain boundary tissue layers of plants, such as root endodermis, root and tuber peridermis, and seed coats. Suberin serves as a protective barrier in these tissue layers, controlling, for example, water and ion transport. It is also a stress-induced anti-microbial barrier. The suberin polymer contains a variety of C16-C24 chain-length aliphatics, such as ω-hydroxy fatty acids, α,ω-dicarboxylic fatty acids, and primary fatty alcohols. Suberin also contains high amounts of glycerol and phenolics, especially ferulic acid. In addition, non-covalently linked waxes are likely associated with the suberin polymer. This review focusses on the suberin biosynthetic enzymes identified to date, which include β-ketoacyl-CoA synthases, fatty acyl reductases, long-chain acyl-CoA synthetases, cytochrome P450 monooxygenases, glycerol 3-phosphate acyltransferases, and phenolic acyltransferases. We also discuss recent advances in our understanding of the transport of suberin components intracellularly and to the cell wall, polymer assembly, and the regulation of suberin deposition.

ABCG transporters are required for suberin and pollen wall extracellular barriers in Arabidopsis

Effective regulation of water balance in plants requires localized extracellular barriers that control water and solute movement. We describe a clade of five Arabidopsis thaliana ABCG half-transporters that are required for synthesis of an effective suberin barrier in roots and seed coats (ABCG2, ABCG6, and ABCG20) and for synthesis of an intact pollen wall (ABCG1 and ABCG16). Seed coats of abcg2 abcg6 abcg20 triple mutant plants had increased permeability to tetrazolium red and decreased suberin content. The root system of triple mutant plants was more permeable to water and salts in a zone complementary to that affected by the Casparian strip. Suberin of mutant roots and seed coats had distorted lamellar structure and reduced proportions of aliphatic components. Root wax from the mutant was deficient in alkylhydroxycinnamate esters. These mutant plants also had few lateral roots and precocious secondary growth in primary roots. abcg1 abcg16 double mutants defective in the other two members of the clade had pollen with defects in the nexine layer of the tapetum-derived exine pollen wall and in the pollen-derived intine layer. Mutant pollen collapsed at the time of anther desiccation. These mutants reveal transport requirements for barrier synthesis as well as physiological and developmental consequences of barrier deficiency.