Rupam Kapoor

Arbuscular mycorrhizal fungi in alleviation of salt stress: a review

BACKGROUND Salt stress has become a major threat to plant growth and productivity. Arbuscular mycorrhizal fungi colonize plant root systems and modulate plant growth in various ways. SCOPE This review addresses the significance of arbuscular mycorrhiza in alleviation of salt stress and their beneficial effects on plant growth and productivity. It also focuses on recent progress in unravelling biochemical, physiological and molecular mechanisms in mycorrhizal plants to alleviate salt stress. CONCLUSIONS The role of arbuscular mycorrhizal fungi in alleviating salt stress is well documented. This paper reviews the mechanisms arbuscular mycorrhizal fungi employ to enhance the salt tolerance of host plants such as enhanced nutrient acquisition (P, N, Mg and Ca), maintenance of the K(+) : Na(+) ratio, biochemical changes (accumulation of proline, betaines, polyamines, carbohydrates and antioxidants), physiological changes (photosynthetic efficiency, relative permeability, water status, abscissic acid accumulation, nodulation and nitrogen fixation), molecular changes (the expression of genes: PIP, Na(+)/H(+) antiporters, Lsnced, Lslea and LsP5CS) and ultra-structural changes. Theis review identifies certain lesser explored areas such as molecular and ultra-structural changes where further research is needed for better understanding of symbiosis with reference to salt stress for optimum usage of this technology in the field on a large scale. This review paper gives useful benchmark information for the development and prioritization of future research programmes.

Effects of arbuscular mycorrhiza and phosphorus application on artemisinin concentration in Artemisia annua L.

Annual wormwood (Artemisia annua L.) produces an array of complex terpenoids including artemisinin, a compound of current interest in the treatment of drug-resistant malaria. However, this promising antimalarial compound remains expensive and is hardly available on the global scale. Synthesis of artemisinin has not been proved to be feasible commercially. Therefore, increase in yield of naturally occurring artemisinin is an important area of investigation. The effects of inoculation by two arbuscular mycorrhizal (AM) fungi, Glomus macrocarpum and Glomus fasciculatum, either alone or supplemented with P-fertilizer, on artemisinin concentration in A. annua were studied. The concentration of artemisinin was determined by reverse-phase high-performance liquid chromatography with UV detection. The two fungi significantly increased concentration of artemisinin in the herb. Although there was significant increase in concentration of artemisinin in nonmycorrhizal P-fertilized plants as compared to control, the extent of the increase was less compared to mycorrhizal plants grown with or without P-fertilization. This suggests that the increase in artemisinin concentration may not be entirely attributed to enhanced P-nutrition and improved growth. A strong positive linear correlation was observed between glandular trichome density on leaves and artemisinin concentration. Mycorrhizal plants possessed higher foliar glandular trichome (site for artemisinin biosynthesis and sequestration) density compared to nonmycorrhizal plants. Glandular trichome density was not influenced by P-fertilizer application. The study suggests a potential role of AM fungi in improving the concentration of artemisinin in A. annua.

Glomus macrocarpum: a potential bioinoculant to improve essential oil quality and concentration in Dill (Anethum graveolens L.) and Carum (Trachyspermum ammi (Linn.) Sprague)

The effects of application of two arbuscular mycorrhizal (AM) fungi Glomus macrocarpum and G. fasciculatum on shoot biomass and concentration of essential oil in Anethum graveolens L. and Trachyspermum ammi (Linn.) Sprague fruits were evaluated. Results revealed significant variation in effectiveness of the two AM fungal species. AM fungal inoculation in general improved the growth of the plants. On mycorrhization, the concentration of essential oil increased up to 90% in dill and 72% in carum over their respective controls. Glomus macrocarpum was more effective than G. fasciculatum in enhancing the oil concentration. The constituents of the essential oils were characterized by gas liquid chromatography. The levels of limonene and carvone were enhanced in essential oil obtained from G. macrocarpum-inoculated dill plants, while G. fasciculatum inoculation resulted in a higher level of thymol in carum.

Vesicular arbuscular mycorrhiza — an overview

Living organisms in the biosphere exhibit a number of interactions which either alter their environment and/or the size and composition of each other’s populations. Of these, perhaps the most striking relationship is ‘symbiosis’ in which the partners live in a state of physical and physiological equilibrium and derive benifit from each other. There exist a number of plant-fungus relationships which are beneficial to both. They are called mycorrhiza. These are of various types but this chapter will give emphasis on the Endomycorrhiza also referred to as Vesicular-Arbuscular Mycorrhiza (VAM) or Arbuscular Mycorrhiza (AM). Vesicular Arbuscular Mycorrhiza (VAM) is one such association where fungal members of order Glomales colonize roots of higher plants. The fungal symbiont gets shelter and food from the plant which in turn aquires an array of benefits ranging from better uptake of phosphorus and relatively immobile micronurients like zinc and copper, increase in Nitrogen fixing capacity of leguminous plant species, salinity and drought tolerance, maintainence of water balance, increased rate of photosynthesis to overall increase in plant growth and development. Mycorrhizal plants show higher tolerance to high soil temperatures and various soil and root borne pathogens. In Eutrophic soil these plants can take up nitrogen in the form of ammonia. Seedlings which are colonized by these fungi perform better during transplantation. The mycorrhizal plants are also more tolerant towards heavy metal toxicity. A general lack of host-fungus specificity is evident by their widespread geographical distribution and also by the fact that almost eighty percent of the plant species show such association. A lot of work done in the past few decades has enabled these fungi to emerge as a potential biofertilizer; a cheap and environment friendly alternative to expensive, petroleum based chemical fertilizers. This aspect especially gains significance for a developing country like India where judicious and large scale utilization of this technology can prove very useful for getting maximum and long term gains in various wasteland reclamation, reforestation and afforestation programmes apart from giving a much needed thurst in the production of important agricultural crops on which the Economy of the country is dependent.

CO2 utilizing microbes — A comprehensive review

CO₂ fixing microbes are the species primarily engaged in complexing the inorganic carbon dioxide to organic carbon compounds. There are many microorganisms from archaeal and bacterial domain that can fix carbon dioxide through six known CO₂ fixing pathways. These organisms are ubiquitous and can survive in wide range of aerobic and anaerobic habitats. This review focuses on the prior research, that has been conducted in this field and presents a summarized overview of all the mechanisms (along with their genes and enzymes) used by these microbes for CO₂ incorporation. In addition, this review provides a better understanding of diversity and taxonomy of CO₂ fixing microorganisms. The information presented here will motivate researchers to further explore the diversity of CO₂ fixing microorganisms as well as to decipher the underlying mechanisms of CO₂ utilization.

Arbuscular mycorrhiza increase artemisinin accumulation in Artemisia annua by higher expression of key biosynthesis genes via enhanced jasmonic acid levels

It is becoming increasingly evident that the formation of arbuscular mycorrhiza (AM) enhances secondary metabolite production in shoots. Despite mounting evidence, relatively little is known about the underlying mechanisms. This study suggests that increase in artemisinin concentration in Artemisia annua colonized by Rhizophagus intraradices is due to altered trichome density as well as transcriptional patterns that are mediated via enhanced jasmonic acid (JA) levels. Mycorrhizal (M) plants had higher JA levels in leaf tissue that may be due to induction of an allene oxidase synthase gene (AOS), encoding one of the key enzymes for JA production. Non-mycorrhizal (NM) plants were exogenously supplied with a range of methyl jasmonic acid concentrations. When leaves of NM and M plants with similar levels of endogenous JA were compared, these matched closely in terms of shoot trichome density, artemisinin concentration, and transcript profile of artemisinin biosynthesis genes. Mycorrhization increased artemisinin levels by increasing glandular trichome density and transcriptional activation of artemisinin biosynthesis genes. Transcriptional analysis of some rate-limiting enzymes of mevalonate and methyl erythritol phosphate (MEP) pathways revealed that AM increases isoprenoids by induction of the MEP pathway. A decline in artemisinin concentration in shoots of NM and M plants treated with ibuprofen (an inhibitor of JA biosynthesis) further confirmed the implication of JA in the mechanism of artemisinin production.

Arbuscular Mycorrhiza: Approaches for Abiotic Stress Tolerance in Crop Plants for Sustainable Agriculture

Soil stresses such as salinity, drought, and heavy metals are major hindrances to crop productivity as these can significantly decrease plant growth and production. Colonization with arbuscular mycorrhizal fungi (AMF) can enhance plant tolerance to these stresses. Arbuscular mycorrhizal associates are integral, functioning parts of plant roots and can significantly increase plant growth and production due to the formation of extensive hyphal network and production of biochemicals like glomalin. This chapter provides an overview of mechanisms evolved by AMF to help plants survive in these stressful conditions. These mechanisms include enhanced growth, prevention of nutrient deficiency and ion toxicity, osmotic adjustment, enhancing the activities of antioxidants and prevention of oxidative damage, improving photosynthesis, and water status. In addition, stress avoidance strategies include exclusion of toxic ions, restriction of entry of toxic ions and immobilization of heavy metals in soil or plant roots.

Preinoculation with Arbuscular Mycorrhizae Helps Acacia auriculiformis Grow in Degraded Indian Wasteland Soil

Abstract The effect of inoculation of two arbuscular mycorrhizal fungi, Glomus fasciculatum, and G. macrocarpum, alone and in combination, on establishment and growth of Acacia auriculiformis in a wasteland soil was studied under nursery and field conditions. Under nursery conditions, mycorrhiza-colonized seedlings showed significantly higher root shoot dry weights and higher concentrations of phosphorus (P), potassium (K), zinc (Zn), copper (Cu), iron (Fe), and sodium (Na) in shoots of mycorrhizal than nonmycorrhizal seedlings. However, concentration of calcium (Ca) was unchanged. Acacia auriculiformis exhibited a maximal mycorrhizal dependency of 79.6% on dual inoculation. Mycorrhizal dependency differed with AM fungal isolates and age of the plant. Under field conditions, AM colonization of A. auriculiformis enhanced tree survival rates (85%) after transplantation. Arbuscular mycorrhiza-colonized plants showed significant increase in height, biomass production, and girth as compared to nonmycorrhizal plants. In general, all growth parameters were higher on dual inoculation of G. fasciculatum and G. macrocarpum as compared to uninoculated plants under both nursery and field conditions.

Enhanced production of steviol glycosides in mycorrhizal plants: A concerted effect of arbuscular mycorrhizal symbiosis on transcription of biosynthetic genes

Stevia rebaudiana (Bertoni) produces steviol glycosides (SGs)--stevioside (stev) and rebaudioside-A (reb-A) that are valued as low calorie sweeteners. Inoculation with arbuscular mycorrhizal fungi (AMF) augments SGs production, though the effect of this interaction on SGs biosynthesis has not been studied at molecular level. In this study transcription profiles of eleven key genes grouped under three stages of the SGs biosynthesis pathway were compared. The transcript analysis showed upregulation of genes encoding 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway enzymes viz.,1-deoxy-D-xylulose 5-phospate synthase (DXS), 1-deoxy-D-xylulose 5-phospate reductoisomerase (DXR) and 2-C-methyl-D-erytrithol 2,4-cyclodiphosphate synthase (MDS) in mycorrhizal (M) plants. Zn and Mn are imperative for the expression of MDS and their enhanced uptake in M plants could be responsible for the increased transcription of MDS. Furthermore, in the second stage of SGs biosynthesis pathway, mycorrhization enhanced the transcription of copalyl diphosphate synthase (CPPS) and kaurenoic acid hydroxylase (KAH). Their expression is decisive for SGs biosynthesis as CPPS regulates flow of metabolites towards synthesis of kaurenoid precursors and KAH directs these towards steviol synthesis instead of gibberellins. In the third stage glucosylation of steviol to reb-A by four specific uridine diphosphate (UDP)-dependent glycosyltransferases (UGTs) occurs. While higher transcription of all the three characterized UGTs in M plants explains augmented production of SGs; higher transcript levels of UGT76G1, specifically improved reb-A to stev ratio implying increased sweetness. The work signifies that AM symbiosis upregulates the transcription of all eleven SGs biosynthesis genes as a result of improved nutrition and enhanced sugar concentration due to increased photosynthesis in M plants.

Analyses of genetic and pathogenic variability among Botrytis cinerea isolates

Seventy nine isolates of Botrytis cinerea were collected from different host plants and different locations of India and Nepal. All the isolates were identified as B. cinerea based on morphological features and were confirmed using B. cinerea specific primers. Differentiation among the isolates was assessed using morphological, genetic and biochemical approaches. To analyze morphological variability, differences in conidial size, presence or absence of sclerotia and their arrangement were observed. Genetic variability was characterized using RAPD analysis, presence or absence of transposons and mating type genes. Cluster analysis based on RAPD markers was used for defining groups on the basis of geographical region and host. The biochemical approach included determining differences in concentration of oxalic acid and activity of lytic enzymes. All the isolates were categorized into different pathogenic groups on the basis their variable reaction towards chickpea plants. Isolates with higher concentration of oxalic acid and greater activity of lytic enzymes were generally more pathogenic. Pathogenicity was also correlated to transposons. Isolates containing transposa group showed some degree of correlation with pathogenic behavior. However, isolates could not be grouped on the basis of a single approach which provides evidence of their wide diversity and high evolution potential. Sensitivity of sampled isolates was also tested against five botryticides. Most of the isolates from same region were inhibited by a particular fungicide. This feature provided interesting cues and would assist in devising novel and more effective measures for managing the disease.