Symbiotic relationships and root nodule ultrastructure of the pasture legume Biserrula pelecinus L.—a new legume in agriculture K.G. Nandasena * , G.W. O’Hara, R.P. Tiwari, R.J. Yates, B.D. Kishinevsky, J.G. Howieson Centre for Rhizobium Studies, Murdoch University, South St, Murdoch, WA 6150, Australia Abstract Biserrula pelecinus is a pasture legume species new to Australian agriculture. The potential N benefit from B. pelecinus pastures in agricultural systems may not be realised if its symbiotic interactions with Mesorhizobium spp. are not well understood. This study evaluated the symbiotic interactions of four strains of Biserrula root-nodule bacteria (WSM1271, WSM1283, WSM1284, WSM1497) with four genotypes of B. pelecinus (cv. Casbah, 93GRC4, 93ITA33, IFBI1) and with a range of related legumes, including species known to be nodulated by strains of Mesorhizobium loti and other Mesorhizobium spp. Structures of root nodules were studied using light and electron microscopy enabling the ultrastructure of effective and ineffective nodules to be compared. B. pelecinus always formed typical indeterminate, finger-like nodules. The number of bacteroids inside symbiosomes varied between host £ strain combinations, however, nodules formed by ineffective associations had well developed peribacteroid membranes and abundant bacteroids. Considerable variation was found in N 2 -fixing effectiveness of strains isolated from B. pelecinus on the four B. pelecinus genotypes. Strains WSM1271, WSM1284 and WSM1497 nodulated Astragalus membranaceus, only strains WSM1284 and WSM1497 nodulated Astragalus adsurgens. Strain WSM1284 also nodulated Dorycnium rectum, Dorycnium hirsutum, Glycyrrhiza uralensis, Leucaena leucocephala, Lotus edulis, Lotus glaber, Lotus maroccanus, Lotus ornithopodioides, Lotus pedunculatus, Lotus peregrinus, Lotus subbiflorus and Ornithopus sativus. The four strains from B. pelecinus did not nodulate Amorpha fruticosa, Astragalus sinicus, Cicer arietinum, Hedysarum spinosissimum, Lotus parviflorus, Macroptilium atropurpureum or Trifolium lupinaster. M. loti strain SU343 nodulated all four genotypes of B. pelecinus. However, M. loti strain CC829 only nodulated B. pelecinus genotypes 93ITA33 and IFBI1 and the nodules were ineffective. The root nodule isolates from H. spinosissimum (E13 and H4) nodulated B. pelecinus cv. Casbah whereas the commercial inoculant strain for Cicer (CC1192) could not nodulate any genotype of B. pelecinus. These results indicate that strains WSM1271, WSM1283 and WSM1497 isolated originally from B. pelecinus have a specific host range while strain WSM1284 is promiscuous in its capacity to nodulate with a broad range of related species. As B. pelecinus can be nodulated by Mesorhizobium spp. from other agricultural legumes, particularly Lotus, there is an opportunity to utilise this trait in cultivar development. q 2004 Elsevier Ltd. All rights reserved. Keywords: Biserrula; Mesorhizobium; Pasture legumes; Rhizobia; Root nodule structure; Symbiotic relationships; Nitrogen fixation 1. Introduction Pasture legumes have significant roles in agriculture in producing high protein feed for grazing animals, controlling cereal diseases in ley farming systems and fixing atmos- pheric N (O’Hara, 1998; Sessitsch et al., 2002). Recently, new pasture legume species have been examined for their ability to control herbicide resistant weeds, to transpire water from deep in the soil profile, to resist insect attack and to be flexible in cropping rotations (Howieson et al., 2000). One new legume to emerge in Mediterranean-type agricul- ture is Biserrula pelecinus L. (Howieson and Loi, 1994; Howieson et al., 1995; Loi et al., 1997; 1999). This monospecific genus can provide high quality feed for cattle and sheep (Howieson et al., 2000) and has many agronomic attributes that make it an ideal legume for use in agricultural systems in southern Australia (Francis, 1999). These attributes include tolerance of soil acidity and an ability to grow well on the sandy and duplex soils of these regions (Loi et al., 1997, 1999). Soil bacteria belonging to the genera Azorhizobium, Bradyrhizobium, Mesorhizobium, Rhizobium and Sinorhi- zobium, collectively known as rhizobia, have the ability to elicit root nodules on leguminous plants and convert atmospheric N 2 into forms of N usable by the plant (Van Berkum and Eardly, 1998). However, there can be a high degree of specificity involved in the interaction 0038-0717/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2004.04.012 Soil Biology & Biochemistry 36 (2004) 1309–1317 www.elsevier.com/locate/soilbio * Corresponding author. Tel.: þ 61-8-9360-2360; fax: þ61-8-9360-6486. E-mail address: kemanthi@central.murdoch.edu.au (K.G. Nandasena).