Clerodendrum – A Novel Herb Having Broad Spectrum Antimicrobial Properties

Viral diseases are of immense importance considering the extensive damage and severe losses they cause to the crops. Because of their peculiar nature and characteristic association with hosts and vectors, no therapeutic method to completely control them has been found successful. However, certain preventive measures, if adopted suitably can be of great help in avoiding viral diseases. Antiviral substances of plant origin may be used as a component for disease management. Infection of several viral diseases could be prevented by the application of extracts/antiviral compounds from Clerodendrum spp. Many higher plants are known to contain endogenous proteins that act as virus inhibitors (Hansen, 1989; Chessin et al., 1995). All these belong to a class of proteins called ribosome-inactivating proteins. These proteins have been studied in Phytolacca americana (Irvin, 1975), Mirabilis jalapa (Kubo et al., 1995), and Trichosanthes kirilowii (Yeung et al., 1988). These proteins show antiviral activity when mixed with virus inoculum and are found to be localized extracellularly in the plants (Loebenstein, 1972; Kumar et al., 1997). On the other hand some virus inhibitors of plant origin have been reported to induce systemic resistance in non-treated parts of plants also and thereby preventing infection of viruses (Verma et al., 1979c; 1995, 1996). In Ayurvedic system of medicine, certain plants and their extracts have been used to control viral diseases of human beings and a number of plant products have been identifi ed through phytochemistry (Sukhdev, 2006). One such antiviral substance isolated from Clerodendrum spp. has prevented virus infection and multiplication in plants. It has shown very high antiviral activity when mixed with viruses in-vitro and provoked the plant system to produce new protein(s) in the treated plants which is the actual virus inhibitory agent (VIA). This substance thus induces antiviral state in the plants through formation of de novo synthesized protein and perhaps is active in signaling the activation of defense mechanism in susceptible hosts. Characteristics of viruses Viruses are very small (submicroscopic) infectious particles (virions) composed of a protein coat and a nucleic acid core. They 34 Clerodendrum: antimicrobial properties carry genetic information encoded in their nucleic acid, which typically specifi es two or more proteins. Translation of the genome (to produce proteins) or transcription and replication (to produce more nucleic acid) takes place within the host cell and uses some of the host’s biochemical “machinery”. Viruses do not capture or store free energy and are not functionally active outside their host. They are therefore parasites (and usually pathogens) but are not usually regarded as genuine microorganisms. A large number of phytopathogenic viruses infect a wide range of crops and cause great economic losses every year throughout the world. Yield losses caused by viruses Disease is an alteration in one or more of the ordered sequential series of physiological processes culminating in a loss of coordination of energy utilization in a plant as a result of the continuous irritation from the presence or absence of some factor or agent. All types of living organisms including animals, plants, fungi, and bacteria are hosts for viruses, but most viruses infect only one type of host. Viruses cause many important plant diseases and are responsible for losses in crop yield and quality in all parts of the world. Virus and vector control In recent years the most active areas of research for the control of viral diseases are: breeding of resistant or immune cultivars by classic genetic procedures; production of virus-free stocks of seed and vegetative propagules; production of transgenic plants containing viral genes that confer resistance to the virus; and control strategies by removal of infected host, breeding for resistance and interruption of the disease cycle by measures such as vector control, and screening of partially infected seed lots. Direct control measures including chemicals have been tried by many workers but none of the chemicals could prevent or control the infection and spread of the viruses in fi elds. Little success has been achieved for the control of viruses by managing their vectors through insecticides. Insecticides may kill the insect vectors and prevent the spread of vector-borne viruses up to some extent. Many friendly insects which serve as pollinators/benefi cial insects like honeybees, silkworm, etc. are also killed. Majority of vectors have developed resistance against insecticides. The agrochemicals are very costly. Besides, they cause various human health hazards, including soil, water, and environmental pollution. After indiscriminate use for a long time, most of the agrochemicals leave various types of residues in crop produce, soil, water, and environment. Alternative approach Considering all these facts an eco-friendly approach by using extracts from different plant parts from a number of phenerogamic plants has been followed. These plants, viz., Boerhaavia diffusa (root extract), Clerodendrum aculeatum (CA), Datura metel, Solanum melongena, Euphorbia hirta, Mirabilis jalapa, Phytolacca americana, Azadirachta indica, Terminalia arjuna, Aloe vera, Ipomoea fi stulosa (leaf extract), Asian Agri-History Vol. 19, No. 1, 2015 35 Tinospora cordifolia, and Cuscuta refl exa (fi laments extract) were found effective in preventing the infection and spread of many plant viruses. Of these, the leaf extract from CA plants was found very effective. The antiviral substance isolated from CA has shown broad spectrum and very high antiviral activity against isometric as well as anisometric viruses, both in hypersensitive and systemic hosts. This induced systemic resistance against viruses when sprayed onto the host plants before virus infection. The systemic resistance was induced in whole plant (at treated as well as at remote site), if the CA inhibitor was applied only onto the two basal or upper leaves of the hosts, and thus protects the plants against virus infection. We have referred these antiviral substances of plant origin as virus interfering agents. History of antiviral substances Duggar and Armstrong (1925) reported for the fi rst time that the crude sap extract of pokeweed (Phytolacca decandra) markedly inhibited the infectivity of tobacco mosaic virus (TMV). Kuntz and Walker (1947) made the fi rst attempt to investigate the nature and property of spinach extract. A variety of plants belonging to different taxonomic families were used for viral disease management. Loebenstein and Ross (1963) demonstrated the formation of virus interfering substances in sap extracted from resistant apical uninoculated halves of datura leaves, whose basal halves had been inoculated ten days earlier with TMV. The sap from resistant halves of leaves when mixed with virus reduces infectivity of TMV, as compared to control sap. The average reduction in lesion number is about 50%. Verma and Awasthi (1979a, 1979b, 1979c) and Verma et al. (1979a, 1979b, 1979c) conducted experiments with antivirus substance of plant origin and found considerable reduction in infection by the viruses. Awasthi and Mukerjee (1980) found protection of potato virus infection by extract from some medicinal plants. The control of viral diseases of some cucurbitaceous crops was also reported by Verma et al. (1980). Awasthi et al. (1984) observed that preinoculation sprays of Boerhaavia diffusa root extract were effective against TMV in tobacco (Nicotiana tabacum), cucumber mosaic virus (CMV) and TMV in tomato (Lycopersicon esculentum), cucumber green mottle mosaic virus in melon, sunnhemp rosette virus in Crotalaria juncea and in Gomphrena globosa. Verma et al. (1985) suggested possible control of natural infection of mungbean yellow mosaic virus in mungbean (Vigna radiata) and black gram (Vigna mungo) by plant extracts. Zaidi et al. (1988) reported the inhibitory effect of the extract of Azadirachta indica against spinach mosaic virus in In Ayurvedic system of medicine, certain plants and their extracts have been used to control viral diseases of human beings and a number of plant products have been identifi ed through phytochemistry. One such antiviral substance isolated from Clerodendrum spp. has prevented virus infection and multiplication in plants. 36 Clerodendrum: antimicrobial properties Chenopodium amaranticolor. The effi cacy decreased by gradually spraying with neem leaf extract on upper surface of the test plant leaf and was effective up to 4 hours by increasing the interval between treatment and inoculation. Verma et al. (1984) observed the effi cacy of leaf extract of different species of Clerodendrum, which increased the resistance of the host plants. Bharathi (1999) reported that extract of Mirabilis jalapa completely inhibited CMV in brinjal or eggplant (Solanum melongena) while the inhibition of CMV by the plant extract of Prosopis chilensis, Bougainvillea spectabilis, and Eucalyptus citriodora was 83%, 75%, and 58% respectively. In pre-inoculation treatments with Mirabilis jalapa, infection of CMV on brinjal ranged from 0 to 56% as compared to 94% in control and also the prevention of tomato yellow leaf curl vector-borne virus was checked signifi cantly by the application of Boerhaavia diffusa root extract (Awasthi and Rizvi, 1999). Jayashree et al. (1999) studied the effi cacy of 10 plant extracts against pumpkin yellow vein mosaic virus in pumpkin and showed maximum inhibition of virus transmission by Bemisia tabaci by Bougainvillea spectabilis followed by Boerhaavia diffusa. Surendran et al. (1999) observed the antiviral activity of plant extracts (Azadirachta indica, Clerodendrum infortunatum, Ocimum sanctum, and Vitex negundo) against brinjal mosaic virus on local lesion host Datura stramonium. The pre-inoculation sprays of 10% leaf extract or oil formulations of Azadirachta indica was found effective in reducing the number of local lesions and their effi cacy against the virus under fi eld conditions. Singh (2002) and Singh and Awasthi (2002) reported that aqueous root extract of Boerhaavia diffusa effectively reduced mungbean yellow mosaic and bean common mosaic in mungbean and black gram and thus increased grain yield in fi eld condition. Later Awasthi and Kumar (2003a, 2003b) and Kumar and Awasthi (2003a, 2003b) revealed that weekly sprays of aqueous root extract of Boerhaavia diffusa signifi cantly prevented infection, multiplication, and spread of CMV, bottle gourd mosaic virus, cucumber green mottle mosaic virus, and pumpkin mosaic virus in cucurbitaceous crops. Kumar and Awasthi (2008) were able to prevent infection and spread of cucumber mosaic disease in cucumber through plant proteins. Singh and Awasthi (2009) tested various medicinal plants for the management of yellow mosaic in mungbean. Awasthi and Yadav (2009) and Yadav et al. (2009) worked on the management of viral diseases of tomato by seed treatment and foliar sprays of Boerhaavia diffusa root extract and CA leaf extract. Virus inhibitors and their characteristics Proteins are important constituents of living bodies. Proteins constitute about 75% in dead cells and 12–14% in living cells. Antiviral proteins act on any step of virus synthesis, i.e., from the un-coating of viral proteins to the appearance of symptoms. Proteins inhibit virus infection or multiplication when applied before or after virus multiplication. Virus stimulating and depressing property of virus inhibitors depend on their concentration and time of application. Functions of proteins are also affected by temperature and pH. Asian Agri-History Vol. 19, No. 1, 2015 37 Nomenclature and systematic position of Clerodendrum The genus Clerodendrum belongs to family Lamiaceae (Verbenaceae), order Lamiales, and phylum Angiosperm. It is very widely distributed in tropical and subtropical regions of the world and is comprised of small trees, shrubs, and herbs. The fi rst description of the genus was given by Linnaeus in 1753, with identifi cation of C. infortunatum. A decade later in 1763, Adanson changed the Latin name “Clerodendrum” to its Greek form “Clerodendron”. Clerodendrum is a very large and diverse genus and till now 580 species of the genus have been identifi ed and are widely distributed in Asia, Australia, Africa, and America (Table 1). Clerodendrum aculeatum is a constituent in Ayurvedic tonic ‘Dashammola’. Chemical composition of Clerodendrum Clerodendrum is reported in various indigenous systems of medicine throughout the world for the treatment of various diseases. Efforts have been made by various researchers to isolate and identify biologically active principle and other major chemical constituents from various species of the genus. Research reports on the genus denote that the major class of chemical constituents present in the Clerodendrum genus are steroids such as β-sitosterol, γ-sitosterol octacosanol, clerosterol, bungein A, acteoside, betulinic acid, clerosterol 3-O-β-D-glucopyranoside, colebrin A-E, campesterol, 4α-methyl sterol, cholesta-5-22-25-trien-3-β-ol, 24β-cholesta-5-22-25-trine, cholestanol, 24-methyl-22-dihydrocholestanol, 24-β-2225-bis-dehydrocholesterol, 24-α-methylTable 1. Distribution of some species of Clerodendrum in the world. Species Distribution C. inerme India, Sri Lanka, Southeast Asia C. aculeatum India, Sri Lanka, Southeast Asia C. phlomidis (syn. C. multifl orum) India