Arbuscular mycorrhizae reduced arsenic induced oxidative stress by coordinating nutrient uptake and proline-glutathione levels in Cicer arietinum L. (chickpea).

Accumulation of Arsenic (As) generates oxidative stress by reducing nutrients availability in plants. Arbuscular mycorrhizal (AM) symbiosis can impart metalloid tolerance in plants by enhancing the synthesis of sulfur (S)-rich peptides (glutathione- GSH) and low-molecular-weight nitrogenous (N) osmolytes (proline- Pro). The present study, therefore investigated the efficiency of 3 AM fungal species (Rhizoglomus intraradices-Ri, Funneliformis mosseae -Fm and Claroideoglomus claroideum- Cc) in imparting As (arsenate-AsV -40 at 60 mg kg-1 and arsenite- AsIII at 5 and 10 mg kg-1) tolerance in two Cicer arietinum (chickpea) genotypes (HC 3 and C 235). As induced significantly higher negative impacts in roots than shoots, which was in accordance with proportionately higher reactive oxygen species (ROS) in the former, with AsIII more toxic than AsV. Mycorrhizal symbiosis overcame oxidative stress by providing the host plants with necessary nutrients (P, N, and S) through enhanced microbial enzyme activities (MEAs) in soil, which increased the synthesis of Pro and GSH and established a redox balance in the two genotypes. This coordination between nutrient status, Pro-GSH levels, and antioxidant defense was stronger in HC 3 than C 235 due to its higher responsiveness to the three AM species. However, Ri was most beneficial in inducing redox homeostasis, followed by Fm and Cc, since the Cicer arietinum-Ri combination displayed the maximum ability to boost antioxidant defense mechanisms and establish a coordination with Pro synthesis. Thus, the results highlighted the importance of selecting specific chickpea genotypes having an ability to establish effective mycorrhizal symbiosis for imparting As stress tolerance.

Nitric oxide and AMF-mediated regulation of soil enzymes activities, cysteine-H2S system and thiol metabolites in mitigating chromium (Cr (VI)) toxicity in pigeonpea genotypes.

Cr (VI) hampers plant growth and yield by reducing essential nutrient uptake as it competes for phosphate and sulfate transporters. Nitric oxide (NO) and mycorrhization play important roles in mitigating Cr (VI) toxicity. Present study aimed to compare the potential of AMF (Arbuscular mycorrhizal fungi)-Rhizoglomus intraradices and NO (0.25 mM) in alleviating Cr (VI) stress (0, 10 and 20 mg/kg) in two differentially tolerant pigeonpea genotypes (Pusa 2001 and AL 201). Cr (VI) toxicity reduced growth, mycorrhizal colonization, nutrient uptake, and overall productivity by inducing reactive oxygen species (ROS) generation, with AL 201 more sensitive than Pusa 2001. NO and AM enhanced activities of soil enzymes, thereby increasing nutrients availability as well as their uptake, with AM more effective than NO. Both amendments reduced oxidative stress and restricted Cr (VI) uptake by increasing the activities of antioxidant and S- assimilatory enzymes, with Pusa 2001 more responsive than AL 201. NO was relatively more efficient in regulating cysteine-H2S system by increasing the activities of biosynthetic enzymes (ATP-sulfurylase (ATPS), O-acetylserine thiol lyase (OASTL), D-cysteine desulfhydrase (DCD) and L-cysteine desulfhydrase (LCD), while AM significantly increased glutathione reductase (GR), γ-glutamylcysteine synthetase (γ-ECS) enzymes activities and resultant glutathione (GSH), phytochelatins (PCs), and non-protein thiols (NP-SH) synthesis. Moreover, co-application of NO and AM proved to be highly beneficial in negating the toxic effects of Cr (VI) due to functional complementarity between them. Study suggested the combined use of NO and AM as a useful strategy in re-establishing pigeonpea plants growing in Cr (VI)-stressed environments.

Arbuscular mycorrhizae and silicon alleviate arsenic toxicity by enhancing soil nutrient availability, starch degradation and productivity in Cajanus cajan (L.) Millsp.

Arsenic (As) pollution of soil reduces the growth and reproductive potential of plants. Silicon (Si) and arbuscular mycorrhizal (AM) fungi play significant roles in alleviating adverse effects of As stress. However, studies are scant regarding alleviative effects of Si in pigeonpea (Cajanus cajan L. Millsp.) because legumes are considered low Si-accumulators. We investigated the individual as well as synergistic potential of Si with two AM species (M1-Claroideoglomus etunicatum and M2-Rhizoglomus intraradices) in modulating soil properties, thereby improving growth and productivity of pigeonpea genotype Pusa 2001 grown in AsV and AsIII challenged soils. Both As species hampered the establishment of AM symbiosis, thus, reducing nutrient uptake, growth and yield, with AsIII more toxic than AsV. Exogenously applied Si and AM species enhanced soil glomalin and phosphatases activity, hence decreased metal bioavailability in soil, increased plant nutrient acquisition, biomass and chlorophylls; with maximum benefits provided by M2, closely followed by Si and least by M1. These amendments boosted the activities of starch hydrolytic enzymes (α-, ß-amylase, starch phosphorylase) in plants, along with a simultaneous increase in total soluble sugars (TSS). This enhanced sugar accumulation directly led to improved reproductive attributes, more efficiently by M2 and Si than by M1. Moreover, there was a substantial increase in proline biosynthesis due to significantly enhanced activities of its biosynthetic enzymes. Additionally, combined applications of Si and AM, especially +Si+M2, complemented each other where AM enhanced Si uptake, while Si induced mycorrhization, suggesting their mutual and beneficial roles in ameliorating metal(loid) toxicity and achieving sustainability in pigeonpea production under As stress.

Zinc toxicity in plants: a review.

MAIN CONCLUSION: This review highlights the most recent updated information available about Zn phytotoxicity at physiological, biochemical and molecular levels, uptake mechanisms as well as excess Zn homeostasis in plants. Zinc (Zn) is a natural component of soil in terrestrial environments and is a vital element for plant growth, as it performs imperative functions in numerous metabolic pathways. However, potentially noxious levels of Zn in soils can result in various alterations in plants like reduced growth, photosynthetic and respiratory rate, imbalanced mineral nutrition and enhanced generation of reactive oxygen species. Zn enters into soils through various sources, such as weathering of rocks, forest fires, volcanoes, mining and smelting activities, manure, sewage sludge and phosphatic fertilizers. The rising alarm in environmental facet, as well as, the narrow gap between Zn essentiality and toxicity in plants has drawn the attention of the scientific community to its effects on plants and crucial role in agricultural sustainability. Hence, this review focuses on the most recent updates about various physiological and biochemical functions perturbed by high levels of Zn, its mechanisms of uptake and transport as well as molecular aspects of surplus Zn homeostasis in plants. Moreover, this review attempts to understand the mechanisms of Zn toxicity in plants and to present novel perspectives intended to drive future investigations on the topic. The findings will further throw light on various mechanisms adopted by plants to cope with Zn stress which will be of great significance to breeders for enhancing tolerance to Zn contamination.

Relative effectiveness of arbuscular mycorrhiza and polyamines in modulating ROS generation and ascorbate-glutathione cycle in Cajanus cajan under nickel stress.

Nickel (Ni) is a fundamental micronutrient in plants but hampers plant growth and metabolism at elevated levels in the soil by inducing oxidative stress. In the recent years, use of polyamines (PAs) and arbuscular mycorrhiza (AM) have gained importance for their roles in enabling plants to withstand Ni toxicity. However, information about their comparative effectiveness in alleviating Ni stress is scanty. Therefore, the current study was designed to evaluate relative impacts of three PAs (Put, Spd, and Spm) and AM (Rhizoglomus intraradices) in reducing Ni uptake, ROS generation, and modulating antioxidant defense machinery in two pigeonpea genotypes (Pusa 2001-tolerant and AL 201-sensitive). Roots of Ni supplied plants accumulated significantly more Ni than the leaves, more in AL 201 than Pusa 2001, which was proportionate to reduced dry weights and enhanced oxidative burst. Although all the three PAs as well as AM inoculations upsurge plant growth by remarkably lowering Ni transport as well as the sequential oxidative burden, AM was most effective, followed by Put, Spd with least positive impact of Spm. The combined applications of AM and Put were able to strengthen antioxidant defense mechanisms, including those of ascorbate-glutathione cycle, most strongly when compared with + Spd + AM and + Spm + AM. Pusa 2001 was more responsive to PAs priming because of its proficiency to develop better effective mycorrhizal symbiosis with R. intraradices when compared with AL201. Hence, the results suggest use of combined applications of PAs (mainly Put) and R. intraradices as an effective strategy for mitigating Ni toxicity in pigeonpea genotypes.

Gene expression analysis for selection and validation of suitable housekeeping gene(s) in cadmium exposed pigeonpea plants inoculated with arbuscular mycorrhizae.

The expression stability of six commonly used housekeeping genes (18S rRNA-18S ribosomal RNA, EF1α-elongation factor 1α, ACT1-Actin 1, GAPDH-Glyceraldehyde-3-phosphate dehydrogenase, TUB6-Tubulin/FtsZ family and UBC-Ubiquitin-conjugating enzyme) were scrutinized in leaves and roots of Cd stressed pigeonpea plants inoculated with arbuscular mycorrhizal (AM) species- Rhizoglomus intraradices (Ri), Funneliformis mosseae (Fm), Claroideoglomus etunicatum (Ce), C. claroideum (Cc). The stability profile of each gene was assessed using ΔCt, BestKeeper, NormFinder, RefFinder and geNorm algorithmic programs, which ranked different genes as most and least stable according to the tissues analysed. All the statistical algorithms ranked TUB6 as most stable and EF1α least stable housekeeping (HK) genes in both the plant tissues. The selected HK genes were verified using metallothionein (CcMT1) i.e. a stress responsive gene, whose expression altered under conditions of metal stress and AM inoculation. The expression pattern of CcMT1 varied highly when least stable reference gene was used for normalization as compared to most stable gene, under different treatments. Thus, there is a need of selecting suitable reference gene to achieve reliable results in gene expression studies using quantitative real time PCR (qRT-PCR). The study conducted will help future gene expression analysis in pigeonpea under specific stress.

Relative roles of Arbuscular Mycorrhizae in establishing a correlation between soil properties, carbohydrate utilization and yield in Cicer arietinum L. under As stress.

Accumulation of As (metalloid) degrades soil by negatively affecting the activities of soil enzymes, which in turn reduce growth and yield of the inhabiting plant. Arbuscular mycorrhizal (AM) symbiosis can impart metalloid tolerance in plants by secreting glomalin-related soil protein (GRSP) which binds with As or inertly adsorb in the extraradical mycelial surface. However, profitable use of AM requires selection of the most efficient combination of host plant and fungal species. The current study, therefore designed to study the efficacy of 3 a.m. fungal species: Rhizoglomus intraradices (Ri), Funneliformis mosseae (Fm) and Claroideoglomus claroideum (Cc) in imparting arsenate As(V) and arsenite As(III) stress tolerance in Cicer arietinum (chickpea) genotypes (G) - relatively metalloid tolerant- HC 3 and sensitive- C 235. Roots were found to be more severly affected as compared to shoots which resulted into a major decline in uptake of nutrients, chlorophyll concentrations and yield with As(III) inducing more toxic effects than As(V). HC 3 established more effective mycorrhizal symbiosis and was able to extract higher nutrients from the soil than C 235. Ri was most beneficial in improving plant biomass, carbohydrate utilization and productivity followed by Fm and Cc which could be due to its capability to initiate highest percent colonization and least metalloid uptake in roots through higher glomalin production in the soil. Moreover, Ri was highly efficient in improving soil enzymes activities-phosphatases (PHAs), ß-glucosidase (BGA) and invertase (INV), thereby, imparting metalloid tolerance in chickpea genotypes. The results suggested use of Ri-chickpea symbiosis as a promising strategy for ameliorating As stress in chickpea.

Interactive effects of polyamines and arbuscular mycorrhiza in modulating plant biomass, N2 fixation, ureide, and trehalose metabolism in Cajanus cajan (L.) Millsp. genotypes under nickel stress.

Nickel (Ni) is an essential micronutrient but considered toxic for plant growth when present in excess in the soil. Polyamines (PAs) and arbuscular mycorrhiza (AM) play key roles in alleviating metal toxicity in plants. Present study compared the roles of AM and PAs in improving rhizobial symbiosis, ureide, and trehalose (Tre) metabolism under Ni stress in Cajanus cajan (pigeon pea) genotypes (Pusa 2001, AL 201). The results documented significant negative impacts of Ni on plant biomass, especially roots, more in AL 201 than Pusa 2001. Symbiotic efficiency with Rhizobium and AM declined under Ni stress, resulting in reduced AM colonization, N2 fixation, and ureide biosynthesis. This decline was proportionate to increased Ni uptake in roots and nodules. Put-reduced Ni uptake improved plant growth and functional efficiency of nodules and ureides synthesis, with higher positive effects than other PAs. However, AM inoculations were most effective in enhancing nodulation, nitrogen fixing potential, and Tre synthesis under Ni toxicity. Combined applications of AM with respective PAs, especially +Put+AM, were highly beneficial in alleviating Ni-induced nodule senescence by arresting leghemoglobin degradation and improving functional efficiency of nodules by boosting Tre metabolism, especially in Pusa 2001. The study suggested use of Put along with AM as a promising approach in imparting Ni tolerance to pigeon pea plants.

SA and AM symbiosis modulate antioxidant defense mechanisms and asada pathway in chickpea genotypes under salt stress.

Salt stress disturbs redox homeostasis by perturbing equilibrium between generation and removal of reactive oxygen species (ROS), which alters the normal metabolism of plants through membrane damage, lipid peroxidation and denaturation of proteins. Salicylic acid (SA) seed priming and arbuscular mycorrhizal (AM) fungi impart salt tolerance in legumes by maintaining redox balance. The present investigation focused on the relative and combined applications of SA and Rhizoglomus intraradices in scavenging ROS in Cicer arietinum L. (chickpea) genotypes (salt tolerant-PBG 5, relatively sensitive-BG 256) subjected to salt stress. Despite the enhanced antioxidant mechanisms under salt stress, ROS (superoxide, O2- and hydrogen peroxide, H2O2) accumulation increased significantly and induced lipid peroxidation and lipoxygenase (LOX) activities, which disrupted membrane stability, more in BG 256 than PBG 5. Salt stress also caused redox imbalance by lowering ascorbate/dehydroascorbate (ASA/DHA) and reduced glutathione/oxidized glutathione (GSH/GSSG) ratios, indicating that redox-homeostasis was crucial for salt-tolerance. Exogenous SA was more promising in reducing ROS-generation and lipid-peroxidation, which provided higher membrane stability as compared to AM inoculation. Although, the enzymatic antioxidants were more active in SA treated plants, yet, AM inoculation outperformed in increasing reformative enzyme activities of Foyer-Halliwell-Asada cycle, which resulted in higher plant biomass in a genotype-dependent manner. SA increased AM root colonization and provided functional complementarity to R. intraradices and thereby strengthening antioxidant defense mechanisms through their cumulative contribution. The study suggested the use of +SA+AM as an eco-friendly tool in imparting salt tolerance in chickpea genotypes subjected to long-term salinity.

Joint effects of Si and mycorrhiza on the antioxidant metabolism of two pigeonpea genotypes under As (III) and (V) stress.

Arsenic (As) is the most hazardous soil contaminant, which inactivates metabolic enzymes and restrains plant growth. To withstand As stress conditions, use of some alleviative tools, such as arbuscular mycorrhizal (AM) fungi and silicon (Si), has gained importance. Therefore, the present study evaluated comparative and interactive effects of Si and arbuscular mycorrhiza-Rhizophagus irregularis on phytotoxicity of arsenate (As V) and arsenite (As III) on plant growth, ROS generation, and antioxidant defense responses in pigeonpea genotypes (Tolerant-Pusa 2002; Sensitive-Pusa 991). Roots of As III treated plants accumulated significantly higher total As than As V supplemented plants, more in Pusa 991 than Pusa 2002, which corresponded to proportionately decreased plant growth, root to biomass ratio, and oxidative burst. Although Si nutrition and AM inoculations improved plant growth by significantly reducing As uptake and the resultant oxidative burst, AM was relatively more efficient in upregulating enzymatic and non-enzymatic antioxidant defense responses as well as ascorbate-glutathione pathway when compared with Si. Pusa 2002 was more receptive to Si nourishment due to its ability to establish more efficient mycorrhizal symbiosis, which led to higher Si uptake and lower As concentrations. Moreover, +Si+AM bestowed better metalloid resistance by further reducing ROS and strengthening antioxidants. Results demonstrated that the genotype with more efficient AM symbiosis in As-contaminated soils could accrue higher benefits of Si fertilization in terms of metalloid tolerance in pigeonpea.

Salicylic acid improves arbuscular mycorrhizal symbiosis, and chickpea growth and yield by modulating carbohydrate metabolism under salt stress.

Salt stress is a major abiotic stress restricting plant growth and reproductive yield. Salicylic acid (SA) and arbuscular mycorrhizal (AM) symbioses play key roles in eliminating adverse effects of salt stress by modulating ion homeostasis and carbohydrate metabolism in crop plants. Sugars synthesized via carbohydrate metabolism act as osmotic adjustors and signaling molecules in activation of various defense responses against salt stress. The present study investigated the role of SA (0.5 mM) seed priming in establishment of AM symbiosis with Rhizoglomus intraradices and the impact on growth, ion-homeostasis, nutrient uptake, and sugar metabolism in Cicer arietinum L. (chickpea) genotypes under salt stress. Salinity had a negative correlation with plant growth and AM symbiosis in both genotypes with more negative effects in relatively salt-sensitive genotype than tolerant. SA enhanced the percent root colonization by significantly increasing the number of arbuscules and vesicles under salt stress. AM symbiosis was more effective in improving root biomass, root to shoot ratio, and nutrient acquisition than SA, while SA was more effective in maintaining ion equilibrium and modulating carbohydrate metabolism and reproductive yield when compared with AM inoculation. SA priming directed the utilization of total soluble sugars (TSS) towards reproductive attributes more efficiently than did AM inoculation by activating TSS metabolic consumption. In AM plants, TSS concentrations were more directed towards sink demand by the fungus itself rather than developing reproductive structures. SA priming further increased sugar export to roots of AM plants, thus favored AM symbiosis. Hence, SA seed priming-induced improvement in AM symbiosis can be a promising strategy in achieving sustainable production of chickpea genotypes under salt stress.

High effectiveness of Rhizophagus irregularis is linked to superior modulation of antioxidant defence mechanisms in Cajanus cajan (L.) Millsp. genotypes grown under salinity stress.

Salinity stress leads to the production of reactive oxygen species (ROS) which can cause oxidative damage in plants. A correlation between antioxidant capacity and salt tolerance has been demonstrated in several plant species, which may be enhanced by inoculation with arbuscular mycorrhizal fungi (AMF). However, plant responses to mycorrhization may differ depending on the host plant as well as AMF isolate. It has been proposed that AMF sourced from stressed environments may be better suited as stress ameliorators than non-native/exotic ones. The present study compared the effectiveness of a native inoculum from saline soil and two exotic single isolates, Funneliformis mossseae and Rhizophagus irregularis (single or dual mix), and associated their effectiveness with modulation of antioxidant defence, in two Cajanus cajan (pigeonpea) genotypes (salt sensitive-Paras, salt tolerant-Pusa 2002) under NaCl stress. Plants subjected to NaCl (0-100 mM) recorded a substantial build-up of ROS, more in Paras than Pusa 2002. Although mycorrhization with all AMF improved plant biomass and reduced oxidative burst by strengthening antioxidant enzymatic activities, inoculation with R. irregularis (alone or in combination with F. mosseae) resulted in higher biomass accumulation which correlated with its higher root colonization and improved redox stability through rapid recycling of reduced ascorbate and glutathione. The study thus suggested that mitigation of salt-induced oxidative burden by increased activation of scavenging antioxidants is an important mechanism that determined the higher effectiveness of R. irregularis over the native saline mix in pigeonpea plants.

Silicon and Rhizophagus irregularis: potential candidates for ameliorating negative impacts of arsenate and arsenite stress on growth, nutrient acquisition and productivity in Cajanus cajan (L.) Millsp. genotypes.

Arsenic (As) gets accumulated in plants via phosphorous transporters and water channels and interferes with nutrient and water uptake, adversely affecting growth and productivity. Although, Si and AM have been reported to combat arsenic stress, their comparative and interactive roles in ameliorating As V and As III toxicities have not been reported. Study evaluated effects of Si and Rhizophagus irregularis on growth, As uptake and yield under arsenate and arsenite stress in two pigeonpea genotypes (metal tolerant-Pusa 2002 and metal sensitive-Pusa 991). Higher As accumulation and translocation was observed in As III treated roots of Pusa 991 than those of Pusa 2002 when compared with As V. Roots were more negatively affected than shoots which led to a significant decline in nutrient uptake, leaf chlorophylls, and yield, with As III inducing more negative effects. Pusa 2002 established more effective mycorrhizal symbiosis and had higher biomass than Pusa 991. Si was more effective in inducing shoot biomass while AM inoculation significantly improved root biomass. AM enhanced Si uptake in roots and leaves in a genotype dependent manner. Combined application of Si and AM were highly beneficial in improving leaf water status, chlorophyll pigments, biomass, and productivity. Complete amelioration of negative impacts of both concentrations of As V and lower concentration of As III were recorded under +Si +AM in Pusa 2002. Results highlighted great potential of Si in improving growth and productivity of pigeonpea through R. irregularis under As V and As III stresses.

Interactive effects of silicon and arbuscular mycorrhiza in modulating ascorbate-glutathione cycle and antioxidant scavenging capacity in differentially salt-tolerant Cicer arietinum L. genotypes subjected to long-term salinity.

Salinity is the major environmental constraint that affects legume productivity by inducing oxidative stress. Individually, both silicon (Si) nutrition and mycorrhization have been reported to alleviate salt stress. However, the mechanisms adopted by both in mediating stress responses are poorly understood. Thus, pot trials were undertaken to evaluate comparative as well as interactive effects of Si and/or arbuscular mycorrhiza (AM) in alleviating NaCl toxicity in modulating oxidative stress and antioxidant defence mechanisms in two Cicer arietinum L. (chickpea) genotypes-HC 3 (salt-tolerant) and CSG 9505 (salt-sensitive). Plants subjected to different NaCl concentrations (0-100 mM) recorded a substantial increase in the rate of superoxide radical (O2 (·-)), H2O2, lipoxygenase (LOX) activity and malondialdehyde (MDA) content, which induced leakage of ions and disturbed Ca(2+)/Na(+) ratio in roots and leaves. Individually, Si and AM reduced oxidative burst by strengthening antioxidant enzymatic activities (superoxide dismutase (SOD), catalase (CAT) and guaiacol peroxidase (GPOX)). Si was relatively more efficient in reducing accumulation of stress metabolites, while mycorrhization significantly up-regulated antioxidant machinery and modulated ascorbate-glutathione (ASA-GSH) cycle. Combined applications of Si and AM complemented each other in reducing reactive oxygen species (ROS) build-up by further enhancing the antioxidant defence responses. Magnitude of ROS-mediated oxidative burden was lower in HC 3 which correlated strongly with more effective AM symbiosis, better capacity to accumulate Si and stronger defence response when compared with CSG 9505. Study indicated that Si and/or AM fungal amendments upgraded salt tolerance through a dynamic shift from oxidative destruction towards favourable antioxidant defence system in stressed chickpea plants.

Effectiveness of native and exotic arbuscular mycorrhizal fungi on nutrient uptake and ion homeostasis in salt-stressed Cajanus cajan L. (Millsp.) genotypes.

Soil salinity is an increasing problem worldwide, restricting plant growth and production. Research findings show that arbuscular mycorrhizal (AM) fungi have the potential to reduce negative effects of salinity. However, plant growth responses to AM fungi vary as a result of genetic variation in mycorrhizal colonization and plant growth responsiveness. Thus, profitable use of AM requires selection of a suitable combination of host plant and fungal partner. A greenhouse experiment was conducted to compare effectiveness of a native AM fungal inoculum sourced from saline soil and two single exotic isolates, Funneliformis mossseae and Rhizophagus irregularis (single or dual mix), on Cajanus cajan (L.) Millsp. genotypes (Paras and Pusa 2002) under salt stress (0-100 mM NaCl). While salinity reduced plant biomass and disturbed ionic status in both genotypes, Pusa 2002 was more salt tolerant and ensured higher AM fungal colonization, plant biomass and nutrient content with favourable ion status under salinity. Although all AM fungi reduced negative effects of salt stress, R. irregularis (alone or in combination with F. mosseae) displayed highest efficiency under salinity, resulting in highest biomass, yield, nutrient uptake and improved membrane stability with favourable K(+)/Na(+) and Ca(2+)/Na(+) ratios in the host plant. Higher effectiveness of R. irregularis correlated with higher root colonization, indicating that the symbiosis formed by R. irregularis had more stable viability and efficiency under salt stress. These findings enhance understanding of the functional diversity of AM fungi in ameliorating plant salt stress tolerance and suggest the potential use of R. irregularis for increasing Cajanus cajan productivity in saline soils.

Bacterial quorum sensing: circuits and applications.

Bacterial quorum sensing (QS) systems are cell density-dependent regulatory networks that coordinate bacterial behavioural changes from single cellular organisms at low cell densities to multicellular types when their population density reaches a threshold level. At this stage, bacteria produce and perceive small diffusible signal molecules, termed autoinducers in order to mediate gene expression. This often results in phenotypic shifts, like planktonic to biofilm or non-virulent to virulent. In this way, they regulate varied physiological processes by adjusting gene expression in concert with their population size. In this review we give a synopsis of QS mediated cell-cell communication in bacteria. The first part focuses on QS circuits of some Gram-negative and Gram-positive bacteria. Thereafter, attention is drawn on the recent applications of QS in development of synthetic biology modules, for studying the principles of pattern formation, engineering bi-directional communication system and building artificial communication networks. Further, the role of QS in solving the problem of biofouling is also discussed.

Influence of cadmium stress and arbuscular mycorrhizal fungi on nodule senescence in Cajanus cajan (L.) Millsp.

Cadmium (Cd) causes oxidative damage and affects nodulation and nitrogen fixation process of legumes. Arbuscular mycorrhizal (AM) fungi have been demonstrated to alleviate heavy metal stress of plants. The present study was conducted to assess role of AM in alleviating negative effects of Cd on nodule senescence in Cajanus cajan genotypes differing in their metal tolerance. Fifteen day-old plants were subjected to Cd treatments--25 mg and 50 mg Cd per kg dry soil and were grown with and without Glomus mosseae. Cd treatments led to a decline in mycorrhizal infection (MI), nodule number and dry weights which was accompanied by reductions in leghemoglobin content, nitrogenase activity, organic acid contents. Cd supply caused a marked decrease in nitrogen (N), phosphorus (P), and iron (Fe) contents. Conversely, Cd increased membrane permeability, thiobarbituric acid reactive substances (TBARS), hydrogen peroxide (H2O2), and Cd contents in nodules. AM inoculations were beneficial in reducing the above mentioned harmful effects of Cd and significantly improved nodule functioning. Activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) increased markedly in nodules of mycorrhizal-stressed plants. The negative effects of Cd were genotype and concentration dependent.

Nitrogen fixation and carbon metabolism in legume nodules.

A large amount of energy is utilized by legume nodules for the fixation of nitrogen and assimilation of fixed nitrogen (ammonia) into organic compounds. The source of energy is provided in the form of photosynthates by the host plant. Phosphoenol pyruvate carboxylase (PEPC) enzyme, which is responsible for carbon dioxide fixation in C4 and crassulacean acid metabolism plants, has also been found to play an important role in carbon metabolism in legume root nodule. PEPC-mediated CO2 fixation in nodules results in the synthesis of C4 dicarboxylic acids, viz. aspartate, malate, fumarate etc. which can be transported into bacteroids with the intervention of dicarboxylate transporter (DCT) protein. PEPC has been purified from the root nodules of few legume species. Information on the relationship between nitrogen fixation and carbon metabolism through PEPC in leguminous plants is scanty and incoherent. This review summarizes the various aspects of carbon and nitrogen metabolism in legume root nodules.