Selected Bioactive Principles of Neem and Their Specific Activity
Anti-inflammatory, analgesic, and antipyretic activities
Kaempferol, reported from neem as well as from a different, unrelated plant Rhamnus procumbens, was found to have anti-inflammatory and anti-ulcer activities [83], [207]. Anti-inflammatory and immunomodulatory activity was observed in 2 flavonoids, catechin and epi-catechin, reported from the bark of the neem tree [177]. Nimbidin, a major active component of the Azadirachta indica seed oil, was found to significantly inhibit some of the functions of macrophages and neutrophils relevant to the inflammatory responses both in vitro as well as in vivo, signifying anti-inflammatory and anti-arthritic potential of the compound [209]. In another study, the effect of aqueous extract of neem leaves (400 mg/kg body weight) was compared with that of dexamethasone (0.75 mg, intraperitoneal) in rats, which demonstrated statistically
significant reduction by extract, albeit less pronounced compared to dexamethasone [210]. Crude ethanol extract of neem leaves yielded anti-inflammatory responses by suppression of paw edema induced by carrageenan and reduced cotton pellet-induced granuloma formation in chronic model rats [211]. Furthermore, the ethanol extract obtained from A. indica leaves (1 g/kg w/w) showed significant antinociceptive effect by inhibition of abdominal writhes produced by acetic acid. A. indica extract also showed statistically significant antipyretic effect (p < 0.05) at 1 g/kg and 500 mg/kg dose level on yeast-induced pyrexia in rats [211].
Immunostimulant activities
Neem oil is shown to selectively activate the cell-mediated immune mechanisms that elicit an enhanced response to subsequent mitogenic or antigenic challenges by acting as a non-specific immunostimulant [212]. Pre-treatment of rats with an odorous and volatile fraction of neem oil, coded NIM-76, was found to increase polymorphonuclear leukocytes, with a decrease in lymphocyte count displaying immunomodulatory efficacy [213].
Radiosensitizing effects
Neem oil was found to increase the radiosensitivity of the Balbc/3 T3 cells and severe combined immunodeficiency (SCID) cells during x-irradiation under aerobic conditions [214]. Application of neem oil reduced the G2 + M phase of the cell cycle, thereby inhibiting the repair of cells from lethal damage [214].
Hypoglycemic activities
Neem kernel powder, in combination with glibenclamide, yielded significant antidiabetic and antihyperlipemic effects in alloxan diabetic rats [215]. Antihyperglycemic effect of aqueous neem leaf extract was also observed in insulin-dependent diabetes mellitus and non-insulin-dependent diabetes mellitus animal models [216]. Mixed water extracts of Abroma augusta roots, when combined with the leaves of A. indica and given orally to alloxan diabetic rats, showed hypoglycemic action with decreased formation of lipid peroxides estimated as thiobarbituric acid reactive substance along with increased antioxidants in erythrocytes [217]. Extracts of A. indica combined with extracts of Vernonia amygdalina (Del.) was found to have enhanced anti-diabetic effect in albino Wistar rats [218]. Ethanolic extracts of A. indica in
streptozotocin-induced hyperglycemia normalized the glucose level and reversed dyslipidemia [219]. Hypoglycemic action of ethanolic neem leaf extract was evaluated in diabetic rats, which demonstrated that after treatment for 24 h with a single dose of 250 mg/kg extract reduced glucose (18%), cholesterol (15%), triglycerides (32%), urea (13%), creatinine (23%), and lipids (15%) [220]. Further, in a multiple-dose study that lasted for 15 days, reduction of creatinine, urea, lipids, triglycerides, and glucose were observed [221]. In the year 2012, a new tetranortriterpenoid named meliacinolin was isolated from chloroform extract of dried neem leaves, which demonstrated in vivo inhibition of α-glucosidase and α-amylase enzyme activities in streptozotocin-nicotinamide-induced type 2 diabetes in mice [55]. Inhibition of both these enzymes
offers an effective strategy of lowering the levels of postprandial hyperglycemia that prevents the digestion of carbohydrates, offering promising potential of meliacinolin as an antidiabetic agent [55].
Anti-ulcer effects
The aqueous leaf extract of neem showed anti-ulcer properties in stressed rats by preventing mast cell deregulation and increasing the amount of adherent gastric mucus [221]. Neem leaf extract exhibited anti-ulcer activity on gastric lesions in rats by blocking acid secretion through inhibition of H+-K+-ATPase and by preventing oxidative damage and apoptosis [222].
Antifertility effects
Prolonged antifertility effects were observed by a single intrauterine administration of neem oil in female Wistar rats [223]. In another study, a single dose of 50 µl of neem oil on each side of the lumen of the vas deferens of male Wistar rats acted as a long-term male contraception [224]. The neem oil fraction NIM-76 was shown to have spermicidal activity in vivo not only in rats but also in rabbits and rhesus monkeys; NIM-76 was further found to affect the motility of sperm leading to the observed infertility [225], [226]. Aqueous extract of old and tender neem leaf was found to immobilize and kill 100% human spermatozoa within 20 s [227].
Antimalarial activities
Gedunin, a tetranortriterpenoid isolated from neem, was reported to be active against Plasmodium falciparum, the causative organism of malaria [228]. The antimalarial activity of the limonoids (meldenin, isomeldenin, nimocinol, and nimbandiol) isolated from the ethanolic extract of fresh neem tree was reported to be active against chloroquine-resistant P. falciparum strain K1 [229]. Schwikkard and van Heerden (2002) discussed the antimalarial activity of neem compounds such as the limonoid gedunin, meldenin, and azadirachtin [230]. NeemAzal, a commercial neem seed extract containing the limonoid azadirachtin as the main component, was found to block the activity of rodent malarial parasite, Plasmodium berghei, in its vector Anopheles stephensi
[231].
Antiretroviral activities
An acetone-water extract of neem leaves was found to prevent the invasion of human lymphocytes by human immunodeficiency virus (HIV), thereby protecting the target cells without any adverse effects [232]. The acetone-water extract significantly increased CD4 cell count in HIV I or HIV II patients that also led to a substantial increase in mean body weight, hemoglobin concentration, lymphocyte differential count with no adverse effects, and abnormalities in kidney and liver function parameters [233].
Antifungal activities
Khan and Shah (1992) tested leaf extracts of A. indica on wheat seed mycoflora and noted considerable reduction in seed mycoflora vis-à-vis better seed germination [234]. Suresh et al. (1997) studied the antifungal activity of polar extract and the impure HPLC fractions of green leaves of A. indica against groundnut rust disease (causal agent Puccinia arachidis Speg.) [235]. Govindachari et al. (1998) also showed the synergistic effect of various neem terpenoids on different fungal pathogens [236]. Minimum inhibitory concentration (MIC) of neem seed extract was found to be 31 µg/ml against clinical isolates of dermatophytes (Trichophyton rubrum, Trichophyton mentagrophytes, and Microsporum nanum) [237]. Wang et al. (2010) reported a significant reduction in the growth of the pathogens Monilinia fructicola,
Penicillium expansum, Trichothecium roseum, and Alternaria alternata by neem seed kernel extracts [238].
Antibacterial activities
Mahmoodin, a novel limonoid, isolated from neem oil, showed significant antibacterial activity against various Gram-positive and Gram-negative bacteria [71]. Aquaneem, an emulsified product prepared from the neem kernel, exhibited antibacterial activity against Aeromonas hydrophila and Pseudomonas fluorescens as well as Myxobacteria sp., which are pathogenic to fish [239]. Moreover, SaiRam and co-workers (2000) studied the antimicrobial activity of the extract NIM-76 against certain bacteria, fungi, and Poliovirus and compared the same with neem oil [240]. The results revealed that NIM-76 inhibited the growth of various bacterial pathogens tested including Escherichia coli and Kleibsiella pneumoniae. The extract also showed antifungal activity against Candida albicans and antiviral activity against Poliovirus replication in Vero cell lines.
Overall, NIM-76 showed stronger anti-microbial activity as compared to the neem oil. Neem seed kernel extract was found to be active against Bacillus mycoides, B. thuringiensis, B. subtilis, Nocardia sp., and Corynebacterium fascians in in vitro assays [241]. In another study, neem mouthwash was found to show antibacterial activity against salivary levels of Streptococcus mutans and Lactobacillus
[242]. Neem leaf extract gel also showed antiplaque activity [243]. Polyester/cotton blend fabric treated with neem extract was reported to have antibacterial activity against both Gram-positive (Bacillus subtilis) and Gram-negative bacteria (Proteus vulgaris) [244]. Neem oil was also found to be active against Staphylococcus aureus, Salmonella typhi, Pseudomonas aeruginosa, and Escherichia coli
[245], [246]. The tetranortriterpenoid, nimolicinol, isolated from neem, was reported to be moderately antibacterial against several human pathogenic bacteria [93], [95]. The antibacterial activity of neem leaf extract and various phytoconstituents of neem such as alkaloids, steroids, tannins, glycosides, flavonoids, and saponins were evaluated and confirmed to have antibacterial efficacies, with crude flavonoids revealing maximum antibacterial activities [247]. 9-Octadecanoic acid-hexadecanoic acid-tetrahydrofuran-3,4-diyl ester obtained from neem oil was found active against Staphylococcus aureus, Escherichia coli, and Salmonella sp. in in vitro assays [113]. M-Octadecanoic acid-3,4-tetrahydrofuran diester, isolated from the petroleum ether extract of neem oil, also showed
potent antibacterial activity [248]. Alcoholic extracts of neem leaves were found to be active against the human bacterial pathogens Bacillus pumillus, Pseudomonas aeruginosa, and Staphylococcus aureus
[249].
Antiviral activities
Foliar application of neem seed oil, when compared with neem seed oil-free extract, inhibited transmission of potato virus Y to sweet pepper by the green peach aphid, Myzus persicae (Sulzer) suggesting that the oil interferes with virus transmission [250]. A methanolic extract fraction of leaves of neem showed antiviral activity against the Coxsackie B group of viruses [251]. Crude aqueous extract of neem leaves was reported both in vitro and in vivo to display antiviral activity against the replication of Dengue virus type-2 [252]. Aqueous neem bark extract, at concentrations ranging from 50 to 100 µg/ml, when pre-incubated with herpes simplex virus type 1 (HSV-1), considerably blocked its entry into cells; additionally, virions treated with the extract failed to bind to the cells, suggesting role of the extract either as an attachment-blocker or having direct
anti-HSV-1 property. Furthermore, cells treated with extract also inhibited herpes simplex virus type 1 glycoprotein-mediated cell-cell fusion and polykaryocytes formation, signifying an additional role of the bark extract at the viral fusion step [253]. The crude acidic extract of leaves and seeds and alkaline extract of seeds were found to show high antiviral activity against HSV-1 when compared with the well-known antiviral drug acyclovir [254].
Anticarcinogenic activities
Azadirone, a limonoidal constituent isolated from methanolic extract of neem flowers, was found to be a potent cytotoxic agent with good in vitro and in vivo activities [255]. The studies also revealed that the α,β-unsaturated enone moiety, or its equivalent conjugated system of A-ring, C-7 acetyloxy/chloroacetyloxy or keto group of B-ring and the furan moiety, are the structural requirements for the potent activity of azadirone and its analogs [255]. Four prenylated flavanones, 5,7,4′-trihydroxy-8-prenylflavanone, 5,4′-dihydroxy-7-methoxy-8-prenylflavanone, 5,7,4′-trihydroxy-3′,8-diprenylflavanone, and 5,7,4′-trihydroxy-3′,5′-diprenylflavanone, were isolated by activity-guided fractionation from the methanolic extract of the flowers of neem, which acted as potent antimutagens against Trp-P-1 (3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole) in the Salmonella typhimurium TA98 assay [61]. Aqueous extract of neem was found to show chemopreventive potential when given to Syrian male hamsters having 7,12-dimethylbenz[a]anthracene (DMBA) induced buccal pouch carcinogenesis by modulation of lipid peroxidation, antioxidants, and detoxification systems [256]. Pre-treatment with ethanolic neem leaf extract significantly lowered the concentration of lipid peroxides and increased antioxidant levels on induced oxidative stress by the potent gastric carcinogen N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) in male Wistar rats suggesting its chemoprotective effects [257]. Significant anticarcinogenic potential was also found in leaf extracts of A. indica in a tumor model system (p < 0.005 to p < 0.001) [258]. Subapriya et al. (2005) suggested that the chemopreventive effects of ethanolic neem leaf extract might be mediated by the induction
of apoptosis [259]. Treatment of rats by aqueous neem extracts significantly decreased the proliferating cell nuclear antigen labeling indices of colon epithelium and aberrant crypt foci, suggesting a chemopreventive role in the short-term colon carcinogenesis bioassay [260]. Nimbolide, a triterpenoid extracted from the flowers of the neem, was found to have antiproliferative activity and apoptosis-inducing property against U937, HL-60, THP1, and B16 cancer cell lines [261]. The acidic extract of leaves and neutral extract of seeds possessed anticancer activity, inhibiting Ehrlich ascites carcinoma cell line growth and IC50 values were 669.43 and 724.63 µg/ml, respectively [254]. 7-Deacetyl-7-benzoylepoxyazadiradione, 7-deacetyl-7-benzoylgeduin, and 28-deoxonimbolide exhibited potent cytotoxic activity against HL60 leukemia cells while
4 other compounds (7-benzoylnimbocinol, epoxyazadiradione, gedunin, and ohchinin acetate) exhibited cytotoxic activity against 1 or more cell lines [114]. Cytotoxic activities of nimbolide isolated from branches and leaves against HL-60 have also been reported [121]. Sulfonoquinovosyldiacylglyceride, a water-soluble constituent of dried neem leaves, showed anti-cancerous activity in human leukemic cell lines U937 and K562 with IC50 of 9 µg/ml [49]. Nimbolide was shown to exert apoptotic activity in estrogen-dependent (MCF-7) and estrogen-independent (MDA-MB-231) human breast cancer cell lines activating caspase-8, caspase-9, caspase-3, and cleavage of PARP [122]. Induction of apoptosis in human breast cancer cells by nimbolide ratifies its future in cancer treatment as a chemotherapeutic agent [122]. NIM-76, a
volatile fraction of neem oil, was reported to have no mutagenic effects and regarded as safe concerning genotoxic potential in humans [262]. In vitro inhibition of growth of mouse sarcoma was found on treatment with neem leaf glycoprotein (25 µg/mice/wk subcutaneously for 4 wks) [263]. This anti-tumor immunity inhibiting the growth of mouse sarcoma was reported to be associated with increased expression of CD69, CD44, and Ki67 on CD8+ T cells [263]. Neem leaf glycoprotein showed no toxicity to various physiological functions of Swiss mice and Sprague-Dawley rats even though type 1 cytokines increased in serum with a decrease in type 2 cytokines and total IgG content in leaf glycoprotein-treated mice [264]. Change in type 1 cytokines were associated with increased anti-tumor immunity [264]. Neem oil limonoids were
found to induce caspase-dependent and apoptosis-inducing factor-mediated apoptosis, as well as autophagy in cancer cells [265].
Hepatoprotective activities
Aqueous leaf extracts of neem significantly prevented changes in the serum levels of bilirubin, protein, alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase, and prevented the histological changes, thus having an antihepatotoxic activity against the damage induced by antitubercular drugs in rats [266]. Chattopadhyay and Bandyopadhyay (2005) discussed the possible mechanism of hepatoprotective activity of neem leaf extracts against paracetamol-induced hepatic damage in rats and concluded that hepatoprotective activity was possibly due to its potent antioxidant activity [267]. Mercury-induced oxidative damage in hepatic tissues was improved with neem leaf extract through its antioxidant effects [268].
Antioxidant activities
Sithisarn et al. (2006) compared free radical scavenging activity of Siamese neem tree leaf extracts against the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical and reported that most active extract was obtained with the leaf decoction method showing antioxidant activity with half-maximal effective concentration (EC50) of 31.4 µg/ml [269]. In another study, significant antioxidant properties were observed in leaf and bark extracts/fractions of neem, while bark was found to possess higher phenolic content than the leaves [270].
Effects on CNS
Anxiolytic activity of leaf extracts of A. indica was studied in rats [271]. Neem extracts could attenuate anxiogenic and appetite-suppressant effects of stress by decreasing the brainʼs 5-hydroxytryptamine and 5-hydroxyindolacetic acid concentration in albino Wistar rats [272]. The pharmacotherapeutic value of neem leaves was also seen in anxiety disorders of albino Wistar rats [273].
Molluscicidal activities
Singh and co-workers (1996) showed the effect of leaf, bark, cake, neem oil, and the neem-based pesticides, achook and nimbecidine of neem, against the snails Lymnaea acuminata and Indoplanorbis exustus and found that pure azadirachtin was more toxic compared to synthetic molluscicides [274]. In another study, crude extracts of bark, roots, and leaves of neem at 500 mg/kg and 700 mg/kg were found lethal to edible tropical land snails Archachatina marginata and Limicolaria aurora (Jay) after exposure for 72 h and 48 h, respectively [275].
Insecticidal activities
The neem tree is well known for its insecticidal properties, which has been documented in a large assortment of studies. Insect growth-regulating properties were found in 23-O-methylnimocinolide and 1, 7-O-deacetyl-23-O-methyl-7α-O-senecioylnimocinolide [39] belonging to γ-hydroxybutenolides group of compounds. Siddiqui et al. (2002) reported desfurano-6-α-hydroxyazadiradione and 22, 23-dihydronimocinol as having insecticidal activity against the fourth instar larvae of mosquito (Anopheles stephensi) [36]. Two nitrogen-containing limonoids, salannolactam-21 and salannolactam-23, have been reported from neem seed kernels, which possess antifeedant activities [151]. Various tetranortriterpenoids–meliatetraolenone, zafaral, 6a-O-acetyl-7-deacetylnimocinol, meliacinol, 17-β-hydroxyazadiradione, azadironic acid, limocin-A, limocin-B, epoxyazadiradione, mahmoodin,
gedunin, 7-decacetylgedunin, 1, 3-di-O-acetylvilasinin, 1-O-tigloyl-3-O-acetylvilasinin, nimbin, azadiradione, and 7-deacetylazadirone–isolated from different tissues of neem have been reported to either have insecticidal activities or insect anti-feeding activities (see [Table 1]). Additionally, azadirachtin and related compounds, such as 6-deactylnimbin, nimbolin B, salannin, 3-deacetylsalannin, salannol, and salannol acetate, have been isolated from various tissues of neem and also have insecticidal or insect anti-feeding properties. Insect growth-regulating activity was observed in desfuranoazadiradione, an octanortriterpenoid isolated from fresh fruit coats [77], [82]. Meliacinin, a dinortriterpenoid isolated from fruit coats, was found to be toxic against mosquito (Anopheles stephensi) [77], [78].
β-sitosterol, a steroid, has also shown insecticidal potential [276], [277]. Odoratone (protolimonoid), isolated from methanolic extract of fresh leaves, has demonstrated a lethal effect on the fourth instar larvae of mosquitoes (A. stephensi) [56]. Larvicidal properties of neem oil were also reported against A. stephensi, Culex quinquefasciatus, and Aedes aegypti
[278].
Neem-based shampoos, amended with neem seed extract, are effective against all stages of head lice [279]. Extracts of neem oil [280], petroleum ether extracts of neem oil, and its 4 fractions separated by column chromatography [281] were reported to be lethal in in vitro assays against rabbit mite Sarcoptes scabiei var. cuniculi larvae. Further, octadecanoic acid-tetrahydrofuran-3,4-diyl ester isolated from an active fraction of the chloroform extract of neem oil was reported to have acaricidal in vitro activity against S. scabiei larvae [282]. Neem oil microemulsion was very effective against Sarcoptes scabie var. cuniculi larvae in vitro
[283].
Antifilarial activities
Alcohol and aqueous extracts of flowers of A. indica showed inhibition of cattle filarial parasite Setaria cervi
[284].
Synthesis and biological sources of azadirachtin
In addition to its remarkable insecticidal activity, azadirachtin also exhibits a range of other biological properties. The first complete structure of azadirachtin was elucidated by Nakanishi and coworkers in 1975 [285] using extensive NMR spectroscopy, which was further revised by Kraus in 1985 [87], who proposed a C13 – C14 epoxide. However, it took 22 y for azadirachtin to be produced by total synthesis [286], [287]. Initially, Veitch and co-workers discussed the probable route leading to the successful synthesis of azadirachtin [288]. This was followed by Jauch (2008) [286] and Ley et al. (2008) [287] who reported the full mechanism of complete chemical synthesis of azadirachtin. Meanwhile, another group focused on the biotechnological approaches for the
production of azadirachtin; its production was reported using A. indica cell suspension cultures [289]. Further, azadirachtin biosynthesis could be induced in hairy root cultures of A. Indica
[290], which was enhanced in hairy root cultures of A. indica by Satdive et al. in 2007 [291]. Production of azadirachtin in neem callus and suspension cultures has also been reported [292]. Another method of androgenic culture of A. indica showed increased azadirachtin production [293]. In 2012, Kusari and coworkers reported the biosynthesis of azadirachtin by an endophytic fungus, Eupenicillium parvum, isolated from neem [294].
Endophytic microorganisms (endophytes)
Endophytes are one of the predominant classes of microorganisms, which reside inside healthy tissues of host plants; endophytes include bacteria, fungi, nematodes, and viruses. Fungal endophytes (or endophytic fungi) are a dynamic and multitrophic group of microorganisms that are ubiquitous in plants thriving in every ecological niche ([Fig. 3]). Fungal endophytes have been found associated with algae [295], lichens [296], mosses [297], ferns [298], conifers [299], large trees [300], small trees [301], palms [302], [303], mangroves [304], halophytes [305], grasses [306], marine sponges
[307], and seagrasses [308] to name a few. Endophytic fungi were isolated from every plant tissue including bark, flower, leaves, petioles, root, seed, and twigs [7], [309], [310], [311]. Further, endophytic fungi are well-established producers of a plethora of bioactive compounds and extracellular enzymes such as amylase, cellulase, chitinase, chitosonase, laccase, lipase, pectinase, and protease [312], [313], [314], [315]. Being colonizers of host tissues, the endobiome plays a crucial role in creating an extra layer of protection to their host during several adverse conditions [316], [317], [373]. They also modulate host metabolism for enhanced production of high-value secondary metabolites in medicinal plants like Withania, Coleus, Papaver; this positive modulation is a result of significant-high expression of genes and transcription factors of biosynthetic pathways [318], [319], [320]. Occasionally, few endophytic species mimic host metabolic pathways and produce host signature metabolites independently [294]. Therefore, unmatched beneficial traits of the endophytes were well recognized by research communities, and several of these endophytes have been utilized for several industrial and agricultural purposes.
Fig. 3 General representation of coexistence of and interaction between neem plants and associated endophytic microorganisms.
Fungal endophytic diversity of A. indica
Following-up the cues on ethnobotanical history of neem, Rajagopal and Suryanarayanan (2000) investigated and isolated endophytic fungi from green and senescent leaves of A. indica from Chennai, India, continuously for 2 y on a monthly basis. They reported 5 selected endophytes, 4 of which were sterile forms and the fifth was identified as Fusarium avenaceum
[321]. They proposed that the restricted number of endophytic fungal genera and the absence of common endophytic fungi in the neem leaves could be due to the antifungal metabolites present in the leaves. The frequency of colonization of green leaves by endophytes was maximal during the rainy season although no new endophyte species could be discovered. It was found that the occurrence of foliar endophytes was influenced by seasonal changes [311], [322]. Since this was also found to be the case with the foliar
endophytes of neem, it was suggested that the occurrence of foliar endophytes in tropical trees was influenced by the environment, soil type, and chemistry of the host tissue [321]. Mahesh and co-workers (2005) studied endophytic mycoflora harboring the inner bark of A. indica and reported 77 endophytic fungal isolates belonging to 15 genera [323]. Among them, 71.4% were hyphomycetes, followed by 18.2% coelomycetes, 6.5% ascomycetes, and 3.9% sterile mycelia. The colonization frequency was found to be 38.5%. Although Rajagopal and Suryanarayanan (2000) recovered only Fusarium avenaceum and 4 sterile forms of endophytes [321], Mahesh and co-workers (2005) were able to recover endophytic genera such as Curvularia, Cochlonema, Gliomastix, and Verticillium sp. [323]. Later, the same group identified endophyte diversity in bark
segments of A. indica, which exposed high species richness with an average of 20 species, and prevalent colonization of Trichoderma and Chaetomium globosum was observed [324]. Verma and co-workers (2007) studied the fungal endophytes of A. indica in several of its natural habitats in India and recovered a total of 233 isolates of endophytic fungi, representing 18 fungal taxa from segments of bark, stem, and leaves [310]. Interestingly, in the whole study, the authors observed that hyphomycetes were the most prevalent group (62.2%), followed by the coelomycetes (27.4%) and a minor percentage by mycelia-sterilia (7.7%). The leaf samples from all locations were nearly constant in their endophytic composition, whereas the bark samples showed maximum diversity at different locations. This study also revealed, for the first time, that endophytes of genera Periconia, Stenella, and
Drechslera were associated with A. indica. Not only was the endophytic fungal colonization frequency higher in leaves (45.5%) than bark (31.5%), but the maximum species richness and frequency of colonization also were as well [310]. Shao and coworkers (2008) have studied the distribution of endophytic fungi in A. Indica from Yuanjiang county of Yunnan Province, PR China [325]. They isolated a total of 372 endophytic fungal strains from the stem, leaves, and fruits. Colletotrichum was found to be the most dominant genera, followed by Alternaria and Xylaria. Another group characterized 85 endophytic fungi belonging to 10 genera, which were isolated from 200 segments of fresh A. indica leaves collected from the Panchmarhi biosphere reserve [326]. Here, the most dominant endophytes observed were Trichoderma, Pestalotiopsis, and
Penicillium sp.
Rajagopal and Suryanarayanan (2000) found that even though the endophytic genera Phomopsis, Phyllosticta, and Xylaria are ubiquitous and commonly isolated from many hosts, these were absent from the leaves of the neem plants under their study [321]. However, these genera were found to be endophytic in neem leaves by other studies [310], [323], [325]. Dominant endophytes fungi isolated from the inner bark of A. indica from South India were Trichoderma, Penicillium, and Pestalotiopsis spp. [323], while those from North India were typically Phomopsis oblonga, Cladosporium cladosporioides, Pestalotiopsis sp., Trichoderma sp., and Aspergillus sp. [310]. Further, isolated species had exhibited inhibitory properties against Trichophyton,
Microsporum
[327]. In China, Colletotrichum was reported as the most dominant genus, followed by Alternaria and Xylaria
[325]. This clearly indicates that endophyte diversity and species richness are not only dependent on specific hosts but also are location and niche specific. This further illustrates the importance of sampling different tissues of a given plant at several locations to obtain an enormous species diversity of endophytes. Taken together, endophytic fungal diversity in neem has been found to be highest in stems (42%), followed by bark (20%), while leaves and fruits harbor a similar percentage of endophytic fungi (19%) ([Fig. 4]). With the isolation of endophytic fungi from roots and fruits of neem, in addition to previous isolation and characterization from leaves, stems and bark, Verma and colleagues completed sampling of all organs of
selected neem trees for their endophytic microflora [7], [300], [303]. Overall, a unique diversity pattern emerges from these studies: endophytic fungi isolated from A. indica belong mostly to the hyphomycetes, followed by coelomycetes and finally, ascomycetes [310], [313], [321], [323], [325].
Fig. 4 Percentage of the endophytic fungi isolated from different parts of neem.
Endophytic actinomycetes of A. indica
In addition to endophytic fungi, neem plants have been studied for the presence of associated endophytic actinomycetes. Kharwar and coworkers characterized 55 endophytic actinomycetes from 20 different samples, 60% of which showed in vitro inhibitory activity against 1 or more pathogenic fungi or bacteria [313]. Actinomycetes were most commonly recovered from roots (54.5% of all isolates), followed by stems (23.6%), and finally, leaves (21.8%). The dominant genus was Streptomyces (49.09% of all isolates), while Streptosporangium (14.5%), Microbispora (10.9%), Streptoverticillium (5.5%), Sacchromonospora sp. (5.5%), and Nocardia (3.6%) were also isolated. In another study, Gohain and coworkers identified the actinomycetes diversity of 6 medicinal plants collected from Gibbon wildlife sanctuary, Assam, and revealed that A. indica possesses the high Shannon diversity index (1.49) with
predominance of Streptomyces species and Streptomyces significantly expressed Polyketide synthase-II (PKS) gene [328]. Endophytic actinomycetes species isolated from A. indica improved plant growth of tomato through the production of siderophores and Indole acetic acid, and inhibited the growth of the pathogen Alternaria alternata that causes blight disease in tomato [324]. Further, an actinomycete Micromonospora costi has been isolated from A. indica from Thailand. The unique characteristics of this species include the presence of meso-diaminopimelic acid in peptidoglycan and the presence of phospholipids like diphosphatidylglycerol, phosphatidylethanolamine, and phosphatidylinositol in the plasma membrane [329].
Metabolomics of Endophytes
In 1993, the landmark discovery of biosynthesis of the anticancer compound paclitaxel (Taxol) by endophytic Taxomyces andreanae
[330] captured the attention of the scientific community towards endophytes as a treasure trove of novel, unique, bioactive natural products. A considerable number of discoveries followed the remarkable work, which cemented the virtually inexhaustible biosynthetic capabilities of endophytic fungi. Some important compounds produced by endophytic fungi are antifungal compounds such as cryptocandin A [331], cryptocin [332], ambuic acid [333], [334], pestaloside [335] and jesterone [336]; antibacterial compounds such as cytosporone A [337], [338] and javanicin [314]; anticancer compounds such as torreyanic acid [339], vincristine [340], chaetoglobosin A [341], penicillenols A1 and B1 [342], and camptothecin [343]; antioxidants like pestacin [344] and isopestacin [345]; and immunosuppressant subglutinols A and B [346] and HIV-1 integrase inhibitors [347]. Several reviews exemplify the vast chemical diversity of compounds produced by endophytes isolated from various plants prospected from different parts of the world [313], [348], [349]. Recently, Chutulo et al. (2018) briefly reported the metetabolites produced by endophytes isolated from neem plant and
their activities [350]. The bioactive compounds produced by endophytes not only have an ecological significance but also provide a scientific handle to study the biochemical and molecular blueprints associated with their production [351]. Herein, we present detailed elaboration on the recent developments in compounds identified from the endophytic fungi of neem plant.
Bioactive natural compounds of endophytic fungi isolated from A. indica
Over 30 compounds have already been reported to be produced by neem-associated fungal endophytes. For instance, chlorinated oxazinane derivatives, 10-membered lactones, solanapyrone analogues, naphthaqinones, anthraquinones, napthodianthrone derivatives, and ring-C-seco-tetranortriterpenoids are some of the essential compound classes reported to be biosynthesized by endophytes associated with neem ([Fig. 5] and [Table 2]).
Fig. 5 Bioactive natural compounds isolated from endophytic fungi of A. indica.
Table 2 Major bioactive compounds derived from endophytic fungi of Azadirachta indica.
Sl. No.
|
Compound
|
Derivative
|
Activity
|
Endophytic fungi
|
Reference
|
n. a.: not available
|
1
|
1-[(2R*,4S*,5S*)-2-chloro-4-methyl-1,3-oxazinan-5-yl]ethanone (1)
|
Chlorinated oxazinane derivate
|
Nematicidal
|
Geotrichum sp.
|
[352]
|
2
|
1-[(2R*,4S*,5R*)-2-chloro-4-methyl-1,3-oxazinan-5-yl]ethanone (2)
|
Chlorinated oxazinane derivate
|
Nematicidal
|
Geotrichum sp.
|
[352]
|
3
|
[2,3-dihydro-2-(1-methylethenyl)-1-benzofuran-5-yl]methanol (3)
|
Benzofuran derivative
|
n. a.
|
Geotrichum sp.
|
[352]
|
4
|
1-(2,4-dihydroxyphenyl)-ethanone (4)
|
Polyphenol
|
Nematicidal
|
Geotrichum sp.
|
[352]
|
5
|
8α-Acetoxy-5α-hydroxy-7-oxodecan-9-olide (5)
|
10-membered lactone ring
|
Antifungal
|
Phomopsis sp.
|
[353]
|
6
|
7α, α-Dihydroxy-3,5-decadien-10-olide (6)
|
10-membered lactone ring
|
Antifungal
|
Phomopsis sp.
|
[353]
|
7
|
7α-Acetoxymultiplolide A (7)
|
10-membered lactone ring
|
Antifungal
|
Phomopsis sp.
|
[353]
|
8
|
8α-Acetoxymultiplolide A (8)
|
10-membered lactone ring
|
Antifungal
|
Phomopsis sp.
|
[353]
|
9
|
Multiplolide A (9)
|
10-membered lactone ring
|
Antifungal
|
Phomopsis sp.
|
[353]
|
10
|
Solanapyrone N (Methyl 4-Amino-6-[(1R,2S,4aR,8aR)-1,2,4a,5,6,7,8,8a-octahydro-2-methylnaphthalen-1-yl]-2-oxo-2H-pyran-3-carboxylate) (10)
|
Solanapyrone analogues
|
Antifungal
|
Nigrospora sp.
|
[354]
|
11
|
Solanapyrone O (11)
|
Solanapyrone analogues
|
Antifungal
|
Nigrospora sp.
|
[354]
|
12
|
Solanapyrone C (12)
|
Solanapyrone analogues
|
Antifungal
|
Nigrospora sp.
|
[354]
|
13
|
Nigrosporalactone (13)
|
Lactones
|
Antifungal
|
Nigrospora sp.
|
[354]
|
14
|
Phomalactone (14)
|
Lactones
|
Antifungal
|
Nigrospora sp.
|
[354]
|
15
|
Javanicin (15)
|
Naphthaqinone
|
Antibacterial
|
Chloridium sp.
|
[314]
|
16
|
Chrysophanol (16)
|
Anthraquinone
|
Antibacterial
|
Aspergillus aculeatus
|
[355], [356]
|
17
|
Emodin (17)
|
Napthodianthrone derivative
|
Antibacterial, anticancerous
|
Aspergillus aculeatus
|
[355], [357], [358]
|
18
|
Succinic acid (18)
|
Dicarboxylic acid
|
Weak insecticidal activity against Plutella xylostella
|
Aspergillus aculeatus and Xylaria sp.
|
[355], [359]
|
19
|
1,5-Dimethyl citrate (19)
|
Oxobutanoate
|
n. a.
|
Aspergillus aculeatus
|
[352]
|
20
|
5-hydroxymellein (20)
|
Isocoumarin
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
21
|
5-methylmellein (21)
|
Isocoumarin
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
22
|
5-carboxymellein (22)
|
Isocoumarin
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
23
|
Hymatoxin C (23)
|
Diterpene
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
24
|
Hymatoxin D (24)
|
Diterpene
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
25
|
Halorosellinic acid (25)
|
Ophiobolane sesterterpene
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
26
|
Cerebroside C (26)
|
Sphingolipids
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
27
|
(2S,3S,4R,2′R)-2-(2′-Hydroxytetracosanoylamino)-octadecane-1,3,4-triol (27)
|
Ceramides (Lipids)
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
28
|
Cerevisterol (28)
|
Steroids
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
29
|
Adenosine (29)
|
Purine nucleoside
|
Weak insecticidal activity against Plutella xylostella
|
Xylaria sp.
|
[359]
|
30
|
Azadirachtin A (30)
|
Ring-C-seco-tetranotriterpenoids
|
Insecticidal activity
|
Eupenicillium parvum
|
[294]
|
31
|
Azadirachtin B (31)
|
Ring-C-seco-tetranotriterpenoids
|
Insecticidal activity
|
Eupenicillium parvum
|
[294]
|
Two new chlorinated, epimeric 1,3-oxazinane derivatives possessing nematicidal activity were characterized from Geotrichum sp. residing endophytically in leaves of neem [352], namely 1-ethanone (1) and 1-[(2R*,4S*,5R*)-2-chloro-4-methyl-1,3-oxazinan-5-yl]ethanone (2), an epimer of the first. Another nematicide active against the nematodes Bursaphelenchus xylophilus and Panagrellus redivivus, identified as [2,3-dihydro-2-(1-methylethenyl)-1-benzofuran-5-yl]methanol (3), was also reported from Geotrichum sp. in addition to 1-(2,4-dihydroxyphenyl)-ethanone (4) [352]. Ten-membered lactones viz. 8a-acetoxy-5a-hydroxy-7-oxodecan-9-olide (5), 7a,8α-dihydroxy-3,5-decadien-10-olide (6), 7α-acetoxymultiplolide A (7), 8α-acetoxymultiplolide A (8), and multiplolide A (9) have been reported
from Phomopsis sp. isolated from stems of A. indica. These compounds exhibited antifungal activities against Aspergillus niger, Botrytis cinerea, Fusarium avenaceum, Fusarium moniliforme, Helminthosporium maydis, Penicillium islandicum, and Ophiostoma minus
[353]. Multiplolide A (9), previously isolated from the fungus Xylaria multiplex
[360], was also isolated from endophytic Phomopsis sp. associated with neem [353]. The main difference between multiplolide A (9) and 7α,8a-dihydroxy-3,5-decadien-10-olide (6) is that the epoxide moiety at C-3 and C-4 in the former is substituted by a double bond in the latter [360]. Solanapyrones have been previously reported as phytotoxins from Ascochyta rabiei
[361], [362], [363] and Alternaria solani
[364], [365]. Interestingly, 2 analogs solanapyrone N (10) and solanapyrone O (11) were isolated from Nigrospora sp. recovered from stems of A. indica, with both being structurally different in the substitution pattern of the a-pyrone unit compared to other solanapyrones [354]. Solanapyrone N (10), solanapyrone O (11), solanapyrone C (12), nigrosporalactone (13), and phomalactone (14) were shown to possess antifungal activities [354]. Structurally-related analogs of solanapyrones have also been isolated from an unidentified marine fungus associated with the surface of the green alga Halimeda monile, which demonstrated anti-algal activity [366]. Wu et al. [367] isolated guanine sesquiterpenes and isopimarane diterpenes from Xylaria sp. isolated from A. indica, and these compounds have shown inhibitory activities against Candida albicans, Hormodendrum compactum, and Pyricularia oryzae with MIC values ranging between 16 µg to 256 µg/ml. Similarly, 5 new guanine sesquiterpenes were further isolated from Xylaria sp. which also possesses antipathogenic activities [368]. Recently, Chatterjee et al. [369] identified the metabolites produced by Alternaria alternata isolated from A. indica showing inhibitory activities against Gram-negative and Gram-positive bacteria.
The highly functionalized antibacterial naphthaquinone, javanicin (15), has been reported from an endophytic fungus Chloridium sp. isolated from roots of A. indica
[314], which displayed strong inhibition of Pseudomonas aeruginosa and P. fluorescens. Chrysophanol (16), emodin (17), succinic acid (18), and 1,5-dimethyl citrate (19) were obtained from the broth extract of an endophytic fungus Aspergillus aculeatus, a resident of A. indica
[355]. Chrysophanol (1,8-dihydroxy-3-methylanthracenedione) (16), an anthraquinone responsible for antimicrobial efficacy against Bacillus subtilis and Staphylococcus aureus, was detected in the extract of Colubrina greggii
[356]. Emodin (17) and related compounds were previously described as having significant inhibitory activities against
P-388 leukemia in mice [357]. Emodin (17), postulated as the primary precursor in the endophytic biochemical pathway to the naphthodianthrone derivative hypericin, also showed antimicrobial activity against the Gram-positive bacterium Staphylococcus aureus, Gram-negative bacteria Klebsiella pneumoniae f. sp. ozaenae, Pseudomonas aeruginosa, Salmonella enterica f. sp. enterica, and Escherichia coli, and fungal organisms Aspergillus niger and Candida albicans
[358]. Eleven compounds, namely 5-hydroxymellein (20), 5-methylmellein (21), 5-carboxymellein (22), hymatoxin C (23), hymatoxin D (24), halorosellinic acid (25), cerebroside C (26), (2S,3S,4R,2′R)-2-(2′-hydroxytetracosanoylamino)-octadecane-1,3,4-triol (27), cerevisterol (28), adenosine (29), and succinic acid (18) have been
reported to be produced by endophytic Xylaria sp. YC-10 isolated from the stems of A. indica collected in Yuanjiang County, Yunnan Province, P. R. China [359]. Although all the compounds exhibited weak insecticidal activity against Plutella xylostella, 9 of these compounds were reported from Xylaria for the first time [359]. Further, Verma et al. [370] attempted to synthesize silver nanoparticles from the extracts of endophytic fungus Aspergillus clavatus and tested against human pathogens such as Candida albicans, Pseudomonas fluorescens, and Escherichia coli, and they were effective against pathogens at 9.7 µg/ml (minimum fungicidal concentration) and 5.83 µg/ml (minimum inhibitory concentration). Kusari et al. (2012) identified and quantified azadirachtin A (30) and B (31) as biosynthetic products of a novel
neem-associated endophytic fungus, Eupenicillium parvum
[294]. This study highlighted an interesting plant-endophyte association where plant “mimetic” compounds are produced by endophytes to render similar functional traits in their ecological habitats.