Molecular and physiological strategies to increase aluminum resistance in plants

Aluminum (Al) toxicity is a primary limitation to plant growth on acid soils. Root meristems are the first site for toxic Al accumulation, and therefore inhibition of root elongation is the most evident physiological manifestation of Al toxicity. Plants may resist Al toxicity by avoidance (Al exclusion) and/or tolerance mechanisms (detoxification of Al inside the cells). The Al exclusion involves the exudation of organic acid anions from the root apices, whereas tolerance mechanisms comprise internal Al detoxification by organic acid anions and enhanced scavenging of free oxygen radicals. One of the most important advances in understanding the molecular events associated with the Al exclusion mechanism was the identification of the ALMT1 gene (Al-activated malate transporter) in Triticum aestivum root cells, which codes for a plasma membrane anion channel that allows efflux of organic acid anions, such as malate, citrate or oxalate. On the other hand, the scavenging of free radicals is dependent on the expression of genes involved in antioxidant defenses, such as peroxidases (e.g. in Arabidopsis thaliana and Nicotiana tabacum), catalases (e.g. in Capsicum annuum), and the gene WMnSOD1 from T. aestivum. However, other recent findings show that reactive oxygen species (ROS) induced stress may be due to acidic (low pH) conditions rather than to Al stress. In this review, we summarize recent findings regarding molecular and physiological mechanisms of Al toxicity and resistance in higher plants. Advances have been made in understanding some of the underlying strategies that plants use to cope with Al toxicity. Furthermore, we discuss the physiological and molecular responses to Al toxicity, including genes involved in Al resistance that have been identified and characterized in several plant species. The better understanding of these strategies and mechanisms is essential for improving plant performance in acidic, Al-toxic soils.     

Organic acid anions:An effective defensive weapon for plants against aluminum toxicity and phosphorus deficiency in acidic soils

Aluminum(Al) toxicity and phosphorous(P) deficiency are two major limiting factors for plant growth on acidic soils.Thus,the physiological mechanisms for Al tolerance and P acquisition have been intensively studied.A commonly observed trait is that plants have developed the ability to utilize organic acid anions(OAs;mainly malate,citrate and oxalate) to combat Al toxicity and P deficiency.OAs secreted by roots into the rhizosphere can externally chelate Al~(3+) and mobilize phosphate(Pi),while OAs synthesized in the cell can internally sequester Al~(3+) into the vacuole and release free Pi for metabolism.Molecular mechanisms involved in OA synthesis and transport have been described in detail.Ensuing genetic improvement for Al tolerance and P efficiency through increased OA exudation and/or synthesis in crops has been achieved by transgenic and marker-assisted breeding.This review mainly elucidates the crucial roles of OAs in plant Al tolerance and P efficiency through summarizing associated physiological mechanisms,molecular traits and genetic manipulation of crops.     

Aluminum stress signaling in plants

Aluminum (Al) toxicity is a major constraint for crop production in acidic soil worldwide. When the soil pH is lower than 5, Al³⁺ is released to the soil and enters into root tip cell ceases root development of plant. In acid soil with high mineral content, Al is the major cause of phytotoxicity. The target of Al toxicity is the root tip, in which Al exposure causes inhibition of cell elongation and cell division, leading to root stunting accompanied by reduced water and nutrient uptake. A variety of genes have been identified that are induced or repressed upon Al exposure. At tissue level, the distal part of the transition zone is the most sensitive to Al. At cellular and molecular level, many cell components are implicated in the Al toxicity including DNA in nucleus, numerous cytoplastic compounds, mitochondria, the plasma membrane and the cell wall. Although it is difficult to distinguish the primary targets from the secondary effects so far, understanding of the target sites of the Al toxicity is helpful for elucidating the mechanisms by which Al exerts its deleterious effects on root growth. To develop high tolerance against Al stress is the major goal of plant sciences. This review examines our current understanding of the Al signaling with the physiological, genetic and molecular approaches to improve the crop performance under the Al toxicity. New discoveries will open up new avenues of molecular/physiological inquiry that should greatly advance our understanding of Al tolerance mechanisms. Additionally, these breakthroughs will provide new molecular resources for improving the crop Al tolerance via molecular-assisted breeding and biotechnology.Key words: aluminum, toxicity, tolerance, signal transduction, plants     

Genotype-dependent effects of phosphorus supply on physiological and biochemical responses to Al-stress in cultivated and Tibetan wild barley

Aluminium (Al) toxicity and phosphorus (P) deficiency often co-exist in acidic soils and limit plant growth and crop production. To investigate the alleviating effects of different levels of phosphorus on Al stress, greenhouse hydroponic experiments were conducted using two contrasting Tibetan wild barley genotypes XZ16 and XZ61 of Al tolerant and sensitive, respectively, and Al tolerant cv. Dayton. The results showed that Al stress induced reduction in P accumulation in plants; and stem and leaf P concentrations of the three genotypes, except of XZ16 under HP + Al (100 µM Al with high level of 360 µM P) which was close to the control level. XZ16 recorded significantly higher P accumulation in plants, compared with XZ61 and Dayton, and P concentrations in leaves under Al stress, and in stems under NP + Al (100 µM Al with normal level of 180 µM P) and HP + Al. Meanwhile, H⁺-, Ca²⁺Mg²⁺-, and Total- ATPase activities in XZ16 and Dayton under Al stress were markedly higher than in XZ61. Normal or high level of P under Al stress could relieve Al stress as enhanced plant biomass, with increased photosystem II photochemistry (Fv/Fm) and P content, relative to the low level of 90 µM P. Compared with XZ61, addition of high P concentration for XZ16 significantly increased the values of Gs and Tr, with higher root GPX and H⁺-ATPase activities, and such nutrient elements as P, Mg and Ca in stems and leaves, and induced more malate secretion, but less MDA accumulation.     

The Physiology, Genetics and Molecular Biology of Plant Aluminum Resistance and Toxicity

Aluminum (Al) toxicity is the primary factor limiting crop production on acidic soils (pH values of 5 or below), and because 50% of the world’s potentially arable lands are acidic, Al toxicity is a very important limitation to worldwide crop production. This review examines our current understanding of mechanisms of Al toxicity, as well as the physiological, genetic and molecular basis for Al resistance. Al resistance can be achieved by mechanisms that facilitate Al exclusion from the root apex (Al exclusion) and/or by mechanisms that confer the ability of plants to tolerate Al in the plant symplasm (Al tolerance). Compelling evidence has been presented in the literature for a resistance mechanism based on exclusion of Al due to Al-activated carboxylate release from the growing root tip. More recently, researchers have provided support for an additional Al-resistance mechanism involving internal detoxification of Al with carboxylate ligands (deprotonated organic acids) and the sequestration of the Al-carboxylate complexes in the vacuole. This is a field that is entering a phase of new discovery, as researchers are on the verge of identifying some of the genes that contribute to Al resistance in plants. The identification and characterization of Al resistance genes will not only greatly advance our understanding of Al-resistance mechanisms, but more importantly, will be the source of new molecular resources that researchers will use to develop improved crops better suited for cultivation on acid soils.     

植物地上部对铝毒的生理响应及其耐性

全世界50％以上潜在的可耕地属于酸性土壤,铝毒害是酸性土壤上植物生长最有害因素之一。近年来,为了阐明植物铝毒害及其耐性,前人已进行了大量的研究,并有一些综述性文章发表。然而,大多数文章主要综述铝对植物根系的影响及其耐性,因为根生长受抑是最早的铝毒害症状之一和溶液培养时最容易辨认的铝毒害症状。为此,本文综述了铝对植物地上部光合作用、光保护系统、水分利用效率、含水量、碳水化合物含量、矿质营养、有机酸和氮代谢的影响,并对富铝植物的解铝毒机制（铝与小分子有机酸螯合和把铝隔离在对铝不敏感的表皮细胞和液泡内）进行了综述。本文还对植物耐铝遗传学和分子生物学及今后需要研究的问题进行了讨论。     

酸性土壤上植物应对铝胁迫的过程与机制

铝胁迫是酸性土壤上影响作物产量最重要的因素之一.目前,全球土壤酸化程度进一步加剧了铝胁迫.植物可通过将铝离子与有机酸螯合储藏于液泡和从根系中排出铝毒.排出铝毒主要通过苹果酸转运蛋白ALMT和柠檬酸转运蛋白MATE的跨膜运输来实现.编码ABC转运蛋白和锌指转录因子的基因与植物抗铝胁迫有关.这些抗铝毒基因的鉴别使得通过转基因和分子标记辅助育种等生物技术来提高农作物的抗铝毒能力成为可能.最后提出了植物抗铝胁迫研究中需要解决的关键问题及今后的研究方向.     

Polyamines and abiotic stress tolerance in plants

Environmental stresses including climate change, especially global warming, are severely affecting plant growth and productivity worldwide. It has been estimated that two-thirds of the yield potential of major crops are routinely lost due to the unfavorable environmental factors. On the other hand, the world population is estimated to reach about 10 billion by 2050, which will witness serious food shortages. Therefore, crops with enhanced vigour and high tolerance to various environmental factors should be developed to feed the increasing world population. Maintaining crop yields under adverse environmental stresses is probably the major challenge facing modern agriculture where polyamines can play important role. Polyamines (PAs)(putrescine, spermidine and spermine) are group of phytohormone-like aliphatic amine natural compounds with aliphatic nitrogen structure and present in almost all living organisms including plants. Evidences showed that polyamines are involved in many physiological processes, such as cell growth and development and respond to stress tolerance to various environmental factors. In many cases the relationship of plant stress tolerance was noted with the production of conjugated and bound polyamines as well as stimulation of polyamine oxidation. Therefore, genetic manipulation of crop plants with genes encoding enzymes of polyamine biosynthetic pathways may provide better stress tolerance to crop plants. Furthermore, the exogenous application of PAs is also another option for increasing the stress tolerance potential in plants. Here, we have described the synthesis and role of various polyamines in abiotic stress tolerance in plants.Key words: abiotic stress tolerance, putrescine, spermidine, spermine, polyamines     

Gene Expression Analysis Suggests Temporal Differential Response to Aluminum in Coffea arabica Cultivars

Aluminum (Al) is a limiting factor of crop yields on acidic soils. Ion aluminum (Al³⁺) acts primarily in plant root system retarding its growth and development, leading to the reduction of lateral roots number, and consequently the decrease of vegetal production. Most of coffee producing areas are located in acidic soils, which have Al³⁺ contents enough to damage plant development. Despite the advances in the understanding of physiological and genetic mechanisms of Al tolerance/susceptibility, few are known about Al ion action in coffee plants. This report describes the expression analysis of genes related to aluminum stress in germinating seeds of two cultivars of C. arabica (Catuaí Amarelo IAC 62 and Icatu Vermelho IAC 4045) when challenged with Al³⁺. In silico analyses of Brazilian Coffee Genome Project (BCGP) database were used to select genes previously found to be related with Al-stress. The expression profile of these genes in Catuaí and Icatu was evaluated through Quantitative PCR (qPCR). Based on our data, we suggest that both analyzed cultivars displays mechanisms of resistance or exclusion, which occurs outside the cell excluding Al³⁺ assimilation, and mechanisms of tolerance that occurs inside the cell after Al³⁺ absorption. The major difference is the timing of activation of each mechanism. While Catuaí tends to use resistance mechanisms in early stages of stress, Icatu uses tolerance strategies. In late stages, both cultivars seem to display tolerance mechanisms, but Icatu also displays Al-exclusion strategy.     

Overexpression of MsALMT1, from the Aluminum-Sensitive Medicago sativa, Enhances Malate Exudation and Aluminum Resistance in Tobacco

Aluminum (Al) toxicity is a major limiting factor for plant growth and crop production in acidic soils. Al-induced organic acid (OA) exudation plays an important role in plant Al resistance. The exudation of OAs is mediated by membrane-localized OA transporters. In our previous study, a gene encoding the Al-induced malate transporter (MsALMT1) was identified in the roots of the Al-sensitive plant Medicago sativa L. cv. Yumu no. 1 (YM1). To further validate the function of MsALMT1, transgenic plants that overexpressed MsALMT1 under the control of the CaMV 35S (35S) promoter were generated. This transgenic tobacco showed an enhanced capacity for malate efflux and better Al resistance than wild type (WT) plants after exposure to 30 μM Al for 24 h. The Al content in the transgenic plant roots decreased to 40–52 % of that in WT plant roots. These results demonstrate that MsALMT1 is an Al-resistant gene in YM1 and encodes a malate transporter, the overexpression of which effectively enhances the Al resistance of transgenic tobacco plants.     

Functional characterization of plasma membrane-localized organic acid transporter (CsALMT1) involved in aluminum tolerance in <Emphasis Type="Italic">Camelina sativa</Emphasis> L.

Aluminum (Al) toxicity is a major limiting factor that inhibits root elongation and decreases crop production in acidic soils. The symptoms of inhibited root growth include a reduced uptake of nutrients because the roots become stubby and brittle. The release of organic anions from roots can protect a plant from Al toxicity. The mechanism relies on the efflux of organic anions, such as malate or citrate, which protect roots by chelating the Al³⁺. In this study, homologs of TaALMT1, a Camelina gene that encodes an aluminum-activated malate transporter, were investigated. The expression of this gene was induced by Al in the root, but not in the shoots. Using green fluorescent protein (GFP) fusion constructs and Western-blot analysis, we observed that CsALMT1 was localized in the plasma membrane. Also, to determine the degree to which Al tolerance was affected by malate secretion in Camelina root, we generated CsALMT1 overexpressing plants. CsALMT1 overexpressing transgenic plants showed a higher root elongation rate than the wild-type plant. Damaged cell staining analysis by hematoxylin under 25 µM Al treatment for 2, 4, and 6 h showed a pattern of less damage in CsALMT1 transgenic plants than in wild-type plant, especially in the root elongation zone. Furthermore, the rate of increase of secretion of organic acid in overexpressed plants after Al treatment was higher than that in the wild-type plant. In addition, in the Al-specific dye morin staining on root protoplast under 50 µM Al treatment, less Al accumulation was observed in the CsALMT1 transgenic plants than in the wild-type plant. The Al contents in the roots of the transgenic plants were at a lower level than those in the wild-type plant. These results show that the overexpression of CsALMT1 improves Al tolerance by increasing the release of malate from the root to the soil and, thereby, detoxifies the Al³⁺.     

Induction of abiotic stress tolerance in plants by endophytic microbes

Endophytes are micro‐organisms including bacteria and fungi that survive within healthy plant tissues and promote plant growth under stress. This review focuses on the potential of endophytic microbes that induce abiotic stress tolerance in plants. How endophytes promote plant growth under stressful conditions, like drought and heat, high salinity and poor nutrient availability will be discussed. The molecular mechanisms for increasing stress tolerance in plants by endophytes include induction of plant stress genes as well as biomolecules like reactive oxygen species scavengers. This review may help in the development of biotechnological applications of endophytic microbes in plant growth promotion and crop improvement under abiotic stress conditions.

Significance and Impact of the Study

Increasing human populations demand more crop yield for food security while crop production is adversely affected by abiotic stresses like drought, salinity and high temperature. Development of stress tolerance in plants is a strategy to cope with the negative effects of adverse environmental conditions. Endophytes are well recognized for plant growth promotion and production of natural compounds. The property of endophytes to induce stress tolerance in plants can be applied to increase crop yields. With this review, we intend to promote application of endophytes in biotechnology and genetic engineering for the development of stress‐tolerant plants.