Taxonomic Appraisal of Nodulation in the Leguminosae of Pakistan

ABSTRACT: Rhizobia are gram negative bacteria that infect roots of leguminous plants and form root nodules. Legume-rhizobia symbioses are of practical importance in providing sustainable food supply and increased agricultural productivity. Existing lists published on the nodulating ability of Pakistani legumes were merged to compile a comprehensive list. The list contained 225 species distributed in 75 genera that were native to Pakistan. Legumes were arranged according to the recommendations of International Legume Database and Information Center (ILDIS) for legumes of south Asia. Out of 225 species examined, 28 belonged to Caesalpinioideae, 29 belonged to Mimosoideae and 168 belonged to Papilionoideae. The percentage of nodulation in Caesalpinioideae, Mimosoideae and Papilionoideae was 0%, 96%, 99% respectively. Conflicting reports on the nodulation status of some Caesalpiniod legumes have been discussed. Doubtful reports on nodulation may arise from inaccurate identification of root nodules. This is particularly due to the fact that structures like galls, tumors, knots, hypertrophies and mycorrhizae that grow on the roots bear superficial resemblance with nodules. It is interesting to note that major cases of doubtful nodulation reported in the past for Leguminosae concern Caesalpinioideae. The matter needs investigation at the molecular level. Present results confirm early findings that nodule formation is more commonly present than absent in Mimosoideae and Papilionoideae, the reverse is true for Caesalpinoideae.

Influence of chemical and biological properties of field grown pea and lentil rhizospheres on nodulation status and resident rhizobial population in acidic soils of Assam, India

The influence of rhizospheric properties on nodulation status and resident rhizobial population of pea and lentil grown at twenty three sites of Assam were assessed. Besides the plant infectivity of rhizobia isolated from pea and lentil nodules were tested for symbiotic effectiveness. The survey established significant variation of nodule number ((35.-223.plant)), nodule dry weight (6.6 to 271. mgplant) and most probable number count ( to 147g soil) across the twenty three diverse sites. Rhizospheric properties <italic>viz</italic> pH, organic carbon, available P₂O₅ and K₂O could establish significant correlation with nodule number (r=0.892*, 0.771*, 0.753* and 0.669*) and most probable number(r=0.784*, 0.580*, 0.615* and 0.649*) counts, correspondingly. Microbial biomass carbon showed significant correlation with nodule number(r=0.672*). Dehydrogenase and phosphomonoesterase activities too showed significant correlation with nodule number(r=0.695* and 0.634*) and most probable number (r=.575* and 0.534*) count, respectively. The symbiotic effectiveness of isolated rhizobia demonstrated significant variation of nodule score (1.33-3.67). Only 6 isolates could attain nodule score of ≥3.0, irrespective of resident rhizobial population.

Transcription profiling of the isoflavone phenylpropanoid pathway in soybean in response to Bradyrhizobium japonicum inoculation

Isoflavones are legume-specific secondary metabolites that function as defence compounds, signal molecules and regulators of gene expression during both pathogen attack and beneficial plant–microbe interactions. They are synthesised by a branch of the core phenylpropanoid pathway, using several isoenzymes within each enzymatic step. Gene-specific quantitative real-time reverse transcriptase PCR (qRT-PCR) was used to quantify expression of isoflavone synthesis genes in soybean (Glycine max L). Genes encoding chalcone synthase 7 (CHS7), chalcone synthase 8 (CHS8) and isoflavone synthase 1 (IFS1) displayed high basal expression levels in roots compared with hypocotyls, suggesting they could be the gene family members encoding the isoenzyme that contributes the most to the principal substrate flux towards specific isoflavone synthesis in roots. The genes encoding phenylalanine ammonia lyase 1 (PAL1) and IFS1 showed induction in root tissue after inoculation with Bradyrhizobium japonicum (Kirchner) Jordan, suggesting a control point. The absence of a functional nodulation regulator, GmNARK (G. max nodulation autoregulation receptor kinase), in the soybean mutant nts1007 resulted in significantly increased basal expression of PAL1 compared with levels induced by B. japonicum, suggesting that GmNARK is a negative regulator for isoflavone phenylpropanoid pathway genes during nodulation and that distinct genes, as opposed to the complete pathway, are coordinately regulated by the nodulation status of the mutant.

In Situ Phylogenetic Structure and Diversity of Wild Bradyrhizobium Communities

ABSTRACTBacteria often infect their hosts from environmental sources, but little is known about how environmental and host-infecting populations are related. Here, phylogenetic clustering and diversity were investigated in a natural community of rhizobial bacteria from the genusBradyrhizobium. These bacteria live in the soil and also form beneficial root nodule symbioses with legumes, including those in the genusLotus. Two hundred eighty pure cultures ofBradyrhizobiumbacteria were isolated and genotyped from wild hosts, includingLotus angustissimus,Lotus heermannii,Lotus micranthus, andLotus strigosus. Bacteria were cultured directly from symbiotic nodules and from two microenvironments on the soil-root interface: root tips and mature (old) root surfaces. Bayesian phylogenies ofBradyrhizobiumisolates were reconstructed using the internal transcribed spacer (ITS), and the structure of phylogenetic relatedness among bacteria was examined by host species and microenvironment. Inoculation assays were performed to confirm the nodulation status of a subset of isolates. Most recovered rhizobial genotypes were unique and found only in root surface communities, where little bacterial population genetic structure was detected among hosts. Conversely, most nodule isolates could be classified into several related, hyper-abundant genotypes that were phylogenetically clustered within host species. This pattern suggests that host infection provides ample rewards to symbiotic bacteria but that host specificity can strongly structure only a small subset of the rhizobial community.

Do Rhizobia Infect Roots of American Yellowwood and Japanese Pagodatree?

Knowing whether leguminous trees have the potential to nodulate after infection by rhizobial bacteria is important for managing nitrogen (N) applications during tree production and for culture in the landscape. Although 98% of studied species in the Papilionoideae nodulate, the nodulation status of two tree species in this subfamily is uncertain. Cladrastis kentukea (Dum.-Cours.) Rudd (American yellowwood) did not form nodules during inoculation studies in 1939 and 1992. Nodules were observed on mature Sophora japonica L. (Japanese pagodatree) in Japan and Hawaii in the 1940s, but compatible rhizobia reportedly isolated in Japan are no longer held in bacterial collections. Our objective was to verify further that American yellowwood does not nodulate and to confirm reports that Japanese pagodatree does nodulate. Rhizobia that infect many plant hosts, soil samples and rhizobial isolates from other Sophora spp., and soil samples from mature American yellowwood and Japanese pagodatree were used to inoculate 5-day-old seedlings of American yellowwood, Japanese pagodatree, and control species. Soil from indigenous and introduced trees in the continental United States, Hawaii, Japan, and China was used. Inoculated and uninoculated plants were grown for 7 weeks in sterile Leonard jars or clay pots containing perlite and irrigated with sterile, N-free Hoagland's solution. No inoculation treatment elicited nodulation of American yellowwood or Japanese pagodatree. Our results provide additional evidence that American yellowwood lacks that capacity to nodulate and cast further doubt on nodulation of Japanese pagodatree.