Isolation and evaluation of endophytic bacteria against plant parasitic nematodes infesting black pepper (Piper nigrum L.)

Abstract

Abundant and diverse populations of bacterial endophytes have been identified in many plants. In the present study, 80 isolates of endophytic bacteria were isolated from different varieties of black pepper (Piper nigrum L.) grown at different locations in India. Another 30, isolates were obtained from tissue cultured black pepper plants. These isolates were tentatively grouped into Bacillus spp. (32 strains), pseudomonads (26 strains), Arthrobacter spp. (20 strains), Micrococcus spp. (10 strains), Curtobacterium sp. (one strain), Serratia (one strain) and twenty unidentified strains based on morphology and biochemical tests. Their nematicidal properties, when tested in an in vitro bioassay using Meloidogyne incognita juveniles, varied from 0 -31.03%. Consortia of these endophytic bacteria were made and evaluated in nurseries for their nematode suppression and growth promotion in black pepper rooted cuttings. All the bacterial consortia were able to suppress nematodes, M. incognita and Radopholus sintilis, significantly. The maximum number of cuttings (243 cuttings / plant) was obtained with phorate treatment followed by treatment with consortia 1 and 4 indicating the potential of these bacteria to be used as nematode biological control agents. Black pepper (Piper nigrum L.), the king of spices, is one of the oldest and the most popular spice in the world. Various nematode pests affect this crop of which the burrowing (Radopholus similis) and root-knot (Meloidogyne spp.) nematodes are the most destructive. However, nematicides being a short-term solution to the problem of nematodes and the time required for the development of resistant cultivars is considerably long, are the impetus behind strong movements in determining the potential of biological management of plant parasitic nematodes. The past two decades investigators are concentrating their efforts on integrating biological control agents in nematode management strategies. Endophytic bacteria are those bacteria that can be isolated from surface-disinfected plant tissue or extract from within the plant, but which do not visibly harm the plant (Hallmann et al., 1997a). Gram-positive and gram-negative bacterial endophytes have been isolated from several tissue types in numerous plant species. Abundant and diverse populations of bacterial endophytes were identified in potato (Garbeva et al., 2001; Sturz et al., 1999), maize (Fisher et al., 1992; McInroy & Kloepper, 1995), rice (Stoltzfus et al., 1998), cotton (McInroy & Kloepper, 1995) and cucumber (Mahafee & Kloepper, 1997). Studies have shown that these endophytes entered the plant tissue primarily through the root zone; however, aerial portions of plants, such as flowers, stems, and cotyledons, are also used for entry (Kobayashi & Palumbo, 2000). Endophytes inside a plant are either localized at the point of entry or spread throughout the plant (Hallmann et al., 1997a) and reside within the cells (Jacobs et al., 1985), in the intercellular spaces, or in the vascular system (Bell et al., 1995). Indigenous endophytic bacteria communities cover a broad spectrum of bacterial species like Pseudomonas fluorescens, Bacillus spp., Herbaspirillum spp., Serratia marcescens, Streptomyces spp. etc. (McInroy & Kloepper, 1995). Endophytic bacteria for biocontrol purpose would eliminate the need to select bacterial types with high level of rhizosphere competence that are often considered necessary for successful seed or root bacterization treatment before or at planting. In the present study, attempts were made to isolate endophytic bacteria from healthy black pepper plants and to evaluate their antagonistic potential against plant parasitic nematodes under laboratory and greenhouse conditions.