Enhanced growth and nodulation of pigeon pea by co-inoculation of Bacillus strains with Rhizobium spp

Endophytic bacteria which are known to reside in plant tissues have often been shown to promote plant growth. Present study deals with the isolation of putative endophytes from the surface sterilized root nodules of pigeon pea (Cajanus cajan) designated as non-rhizobial (NR) isolates. Three of these non-rhizobial isolates called NR2, NR4 and NR6 showed plant growth promotion with respect to increase in plant fresh weight, chlorophyll content, nodule number and nodule fresh weight when co-inoculated with the rhizobial bioinoculant strain IC3123. The three isolates were neither able to nodulate C. cajan nor did they show significant plant growth promotion when inoculated alone without Rhizobium spp. IC3123. All the three isolates were gram positive rods with NR2 and NR4 showing endospore formation and formed one single cluster in Amplified Ribosomal DNA Restriction Analysis (ARDRA). Partial sequences of 16S rRNA genes of NR4 and NR6 showed 97% similarity to Bacillus megaterium. The Bacillus strains NR4 and NR6 were able to produce siderophores which the rhizobial bioinoculant IC3123 was able to cross-utilize. Under iron starved conditions IC3123 showed enhanced growth in the presence of the Bacillus isolates indicating that siderophore mediated interactions may be underlying mechanism of beneficial effect of the NR isolates on nodulation by IC3123.

The first set of EST resource for gene discovery and marker development in pigeonpea (Cajanus cajan L.)

BACKGROUND

Pigeonpea (Cajanus cajan (L.) Millsp) is one of the major grain legume crops of the tropics and subtropics, but biotic stresses [Fusarium wilt (FW), sterility mosaic disease (SMD), etc.] are serious challenges for sustainable crop production. Modern genomic tools such as molecular markers and candidate genes associated with resistance to these stresses offer the possibility of facilitating pigeonpea breeding for improving biotic stress resistance. Availability of limited genomic resources, however, is a serious bottleneck to undertake molecular breeding in pigeonpea to develop superior genotypes with enhanced resistance to above mentioned biotic stresses. With an objective of enhancing genomic resources in pigeonpea, this study reports generation and analysis of comprehensive resource of FW- and SMD- responsive expressed sequence tags (ESTs).

RESULTS

A total of 16 cDNA libraries were constructed from four pigeonpea genotypes that are resistant and susceptible to FW ('ICPL 20102' and 'ICP 2376') and SMD ('ICP 7035' and 'TTB 7') and a total of 9,888 (9,468 high quality) ESTs were generated and deposited in dbEST of GenBank under accession numbers GR463974 to GR473857 and GR958228 to GR958231. Clustering and assembly analyses of these ESTs resulted into 4,557 unique sequences (unigenes) including 697 contigs and 3,860 singletons. BLASTN analysis of 4,557 unigenes showed a significant identity with ESTs of different legumes (23.2-60.3%), rice (28.3%), Arabidopsis (33.7%) and poplar (35.4%). As expected, pigeonpea ESTs are more closely related to soybean (60.3%) and cowpea ESTs (43.6%) than other plant ESTs. Similarly, BLASTX similarity results showed that only 1,603 (35.1%) out of 4,557 total unigenes correspond to known proteins in the UniProt database (or= 5 sequences detected 102 single nucleotide polymorphisms (SNPs) in 37 contigs. As an example, a set of 10 contigs were used for confirming in silico predicted SNPs in a set of four genotypes using wet lab experiments. Occurrence of SNPs were confirmed for all the 6 contigs for which scorable and sequenceable amplicons were generated. PCR amplicons were not obtained in case of 4 contigs. Recognition sites for restriction enzymes were identified for 102 SNPs in 37 contigs that indicates possibility of assaying SNPs in 37 genes using cleaved amplified polymorphic sequences (CAPS) assay.

CONCLUSION

The pigeonpea EST dataset generated here provides a transcriptomic resource for gene discovery and development of functional markers associated with biotic stress resistance. Sequence analyses of this dataset have showed conservation of a considerable number of pigeonpea transcripts across legume and model plant species analysed as well as some putative pigeonpea specific genes. Validation of identified biotic stress responsive genes should provide candidate genes for allele mining as well as candidate markers for molecular breeding.

Effect of genotype and root colonization in biological control of fusarium wilts in pigeonpea and chickpea by Pseudomonas aeruginosa PNA1

Pseudomonas aeruginosa PNA1, an isolate from chickpea rhizosphere in India, protected pigeonpea and chickpea plants from fusarium wilt disease, which is caused by Fusarium oxysporum f.sp. ciceris and Fusarium udum. Inoculation with strain PNA1 significantly reduced the incidence of fusarium wilt in pigeonpea and chickpea on both susceptible and moderately tolerant genotypes. However, strain PNA1 protected the plants from fusarium wilt until maturity only in moderately tolerant genotypes of pigeonpea and chickpea. Root colonization of pigeonpea and chickpea, which was measured using a lacZ-marked strain of PNA1, showed tenfold lower root colonization of susceptible genotypes than that of moderately tolerant genotypes, indicating that this plant-bacteria interaction could be important for disease suppression in this plant. Strain PNA1 produced two phenazine antibiotics, phenazine-1-carboxylic acid and oxychlororaphin, in vitro. Its Tn5 mutants (FM29 and FM13), which were deficient in phenazine production, caused a reduction or loss of wilt disease suppression in vivo. Hence, phenazine production by PNA1 also contributed to the biocontrol of fusarium wilt diseases in pigeonpea and chickpea.

Endophytic fungi from pigeon pea [Cajanus cajan (L.) Millsp.] produce antioxidant Cajaninstilbene acid

In this study, novel endophytic fungi producing cajaninstilbene acid (CSA) from pigeon pea [ Cajanus cajan (L.) Millsp.] were investigated and screened. CSA has prominent pharmacological activities. A total of 110 endophytic fungi isolates were grouped into 8 genera on the basis of morphological characteristics, and CSA-producing fungi were screened by liquid chromatography-tandem mass spectrometry (LC-MS/MS). According to ITS-rDNA sequences analysis, the CSA-producing fungi were identified as Fusarium solani (ERP-07), Fusarium oxysporum (ERP-10), and Fusarium proliferatum (ERP-13), respectively. The amount of CSA produced by the ERP-13 reached 504.8 ± 20.1 μg/L or 100.5 ± 9.4 μg/g dry weight of mycelium. In a DPPH radical-scavenging assay, when the concentration of fungal CSA was 500 μg/mL, inhibition percentage could reach 80%, which was almost the same as that of standard CSA. This study first reported the natural antioxidant CSA from endophytic fungi F. solani and F. proliferatum isolated from pigeon pea.

Bioformulation of Burkholderia sp. MSSP with a multispecies consortium for growth promotion of Cajanus cajan

The present work was undertaken to formulate an effective bioformulation using Burkholderia sp. strain MSSP, a known plant-growth-promoting rhizobacterium. MSSP was tagged with the reporter gene of green fluorescent protein (gfp) to monitor its population in cost-effective solid carriers, including sugarcane-bagasse, sawdust, cocoa peat, rice husk, wheat bran, charcoal, and rock phosphate, and paneer-whey as liquid carrier. Physical and chemical properties of different low-cost carrier materials were studied. The viability of the green fluorescent tagged variant of MSSP was estimated in different sterile carrier materials. Whey and wheat bran proved to be efficient carrier materials for the bioformulation. Sawdust, rock phosphate, rice husk, and cocoa peat were average, while charcoal and sugarcane-bagasse proved to be inferior carriers. The viability of strain MSSP was also assessed in wheat bran and whey-based consortium, having three other bacterial strains, namely Sinorhizobium meliloti PP3, Rhizobium leguminosarum Pcc, and Bacillus sp. strain B1. Presence of other plant-growth-promoting bacteria did not have any detrimental effect on the viability of MSSP. Efficiency of the wheat-bran-based multispecies consortium was studied on the growth of pigeonpea in field conditions. A considerable increase in plant biomass, nodule number and weight, and number of pods was recorded as compared with individual trials and with the control.

Effectiveness of native and exotic arbuscular mycorrhizal fungi on nutrient uptake and ion homeostasis in salt-stressed Cajanus cajan L. (Millsp.) genotypes

Soil salinity is an increasing problem worldwide, restricting plant growth and production. Research findings show that arbuscular mycorrhizal (AM) fungi have the potential to reduce negative effects of salinity. However, plant growth responses to AM fungi vary as a result of genetic variation in mycorrhizal colonization and plant growth responsiveness. Thus, profitable use of AM requires selection of a suitable combination of host plant and fungal partner. A greenhouse experiment was conducted to compare effectiveness of a native AM fungal inoculum sourced from saline soil and two single exotic isolates, Funneliformis mossseae and Rhizophagus irregularis (single or dual mix), on Cajanus cajan (L.) Millsp. genotypes (Paras and Pusa 2002) under salt stress (0-100 mM NaCl). While salinity reduced plant biomass and disturbed ionic status in both genotypes, Pusa 2002 was more salt tolerant and ensured higher AM fungal colonization, plant biomass and nutrient content with favourable ion status under salinity. Although all AM fungi reduced negative effects of salt stress, R. irregularis (alone or in combination with F. mosseae) displayed highest efficiency under salinity, resulting in highest biomass, yield, nutrient uptake and improved membrane stability with favourable K(+)/Na(+) and Ca(2+)/Na(+) ratios in the host plant. Higher effectiveness of R. irregularis correlated with higher root colonization, indicating that the symbiosis formed by R. irregularis had more stable viability and efficiency under salt stress. These findings enhance understanding of the functional diversity of AM fungi in ameliorating plant salt stress tolerance and suggest the potential use of R. irregularis for increasing Cajanus cajan productivity in saline soils.

Sinorhizobium fredii HH103 mutants affected in capsular polysaccharide (KPS) are impaired for nodulation with soybean and Cajanus cajan

The Sinorhizobium fredii HH103 rkp-1 region, which is involved in capsular polysaccharides (KPS) production, was isolated and sequenced. The organization of the S. fredii genes identified, rkpUAGHIJ and kpsF3, was identical to that described for S. meliloti 1021 but different from that of S. meliloti AK631. The long rkpA gene (7.5 kb) of S. fredii HH103 and S. meliloti 1021 appears as a fusion of six clustered AK631 genes, rkpABCDEF. S. fredii HH103-Rif(r) mutants affected in rkpH or rkpG were constructed. An exoA mutant unable to produce exopolysaccharide (EPS) and a double mutant exoA rkpH also were obtained. Glycine max (soybean) and Cajanus cajan (pigeon pea) plants inoculated with the rkpH, rkpG, and rkpH exoA derivatives of S. fredii HH103 showed reduced nodulation and severe symptoms of nitrogen starvation. The symbiotic capacity of the exoA mutant was not significantly altered. All these results indicate that KPS, but not EPS, is of crucial importance for the symbiotic capacity of S. fredii HH103-Rif(r). S. meliloti strains that produce only EPS or KPS are still effective with alfalfa. In S. fredii HH103, however, EPS and KPS are not equivalent, because mutants in rkp genes are symbiotically impaired regardless of whether or not EPS is produced.

Indel-seq: a fast-forward genetics approach for identification of trait-associated putative candidate genomic regions and its application in pigeonpea (Cajanus cajan)

Identification of candidate genomic regions associated with target traits using conventional mapping methods is challenging and time-consuming. In recent years, a number of single nucleotide polymorphism (SNP)-based mapping approaches have been developed and used for identification of candidate/putative genomic regions. However, in the majority of these studies, insertion-deletion (Indel) were largely ignored. For efficient use of Indels in mapping target traits, we propose Indel-seq approach, which is a combination of whole-genome resequencing (WGRS) and bulked segregant analysis (BSA) and relies on the Indel frequencies in extreme bulks. Deployment of Indel-seq approach for identification of candidate genomic regions associated with fusarium wilt (FW) and sterility mosaic disease (SMD) resistance in pigeonpea has identified 16 Indels affecting 26 putative candidate genes. Of these 26 affected putative candidate genes, 24 genes showed effect in the upstream/downstream of the genic region and two genes showed effect in the genes. Validation of these 16 candidate Indels in other FW- and SMD-resistant and FW- and SMD-susceptible genotypes revealed a significant association of five Indels (three for FW and two for SMD resistance). Comparative analysis of Indel-seq with other genetic mapping approaches highlighted the importance of the approach in identification of significant genomic regions associated with target traits. Therefore, the Indel-seq approach can be used for quick and precise identification of candidate genomic regions for any target traits in any crop species.

Rhizodegradation of Petroleum Oily Sludge-contaminated Soil Using Cajanus cajan Increases the Diversity of Soil Microbial Community

Most components of petroleum oily sludge (POS) are toxic, mutagenic and cancer-causing. Often bioremediation using microorganisms is hindered by the toxicity of POS. Under this circumstance, phytoremediation is the main option as it can overcome the toxicity of POS. Cajanus cajan a legume plant, was evaluated as a phyto-remediating agent for petroleum oily sludge-spiked soil. Culture dependent and independent methods were used to determine the rhizosphere microorganisms' composition. Degradation rates were estimated gravimetrically. The population of total heterotrophic bacteria (THRB) was significantly higher in the uncontaminated soil compared to the contaminated rhizosphere soil with C. cajan, but the population of hydrocarbon-utilizing bacteria (HUB) was higher in the contaminated rhizosphere soil. The results show that for 1 to 3% oily sludge concentrations, an increase in microbial counts for all treatments from day 0 to 90 d was observed with the contaminated rhizosphere CR showing the highest significant increase (p  < 0.05) in microbial counts compared to other treatments. The metagenomic study focused on the POS of 3% (w/w) and based on the calculated bacterial community abundance indices showed an increase in the values for Ace, Cho, Shannon (Shannon-Weaver) and the Simpson's (measured as InvSimpson) indices in CR3 compared to CN3. Both the Simpson's and the Shannon values for CR3 were higher than CN3 indicating an increase in diversity upon the introduction of C. cajan into the contaminated soil. The PCoA plot revealed community-level differences between the contaminated non-rhizosphere control and contaminated rhizosphere microbiota. The PCoA differentiated the two treatments based on the presence or absence of plant. The composition and taxonomic analysis of microbiota-amplified sequences were categorized into eight phyla for the contaminated non-rhizosphere and ten phyla for the contaminated rhizosphere. The overall bacterial composition of the two treatments varied, as the distribution shows a similar variation between the two treatments in the phylum distribution. The percentage removal of total petroleum hydrocarbon (TPH) after 90 days of treatments with 1, 2, 3, 4, and 5% (w/w) of POS were 92, 90, 89, 68.3 and 47.3%, respectively, indicating removal inhibition at higher POS concentrations. As the search for more eco-friendly and sustainable remediating green plant continues, C. cajan shows great potential in reclaiming POS contaminated soil. Our findings will provide solutions to POS polluted soils and subsequent re-vegetation.

Production of Laccase by a New Myrothecium verrucaria MD-R-16 Isolated from Pigeon Pea [Cajanus cajan (L.) Millsp.] and its Application on Dye Decolorization

The present study was conducted to screen a laccase-producing fungal endophyte, optimize fermentation conditions, and evaluate the decolorization ability of the laccase. A new fungal endophyte capable of laccase-producing was firstly isolated from pigeon pea and identified as Myrothecium verrucaria based on a ITS-rRNA sequences analysis. Meanwhile, various fermentation parameters on the laccase production were optimized via response surface methodology (RSM). The optimal fermentation conditions were a fermentation time of five days, temperature 30 °C and pH 6.22. Laccase activity reached 16.52 ± 0.18 U/mL under the above conditions. Furthermore, the laccase showed effective decolorization capability toward synthetic dyes (Congo red, Methyl orange, Methyl red, and Crystal violet) in the presence of the redox mediator ABTS, with more than 70% of dyes decolorizing after 24 h of incubation. Additionally, the activity of laccase was relatively stable with pH (4.5-6.5) and a temperature range of 35-55 °C. Therefore, the high laccase production of the strain and the new fungal laccase could provide a promising alterative approach for industrial and environmental applications.

Genetic Analysis Reveals the Essential Role of Nitrogen Phosphotransferase System Components in Sinorhizobium fredii CCBAU 45436 Symbioses with Soybean and Pigeonpea Plants

The nitrogen phosphotransferase system (PTS(Ntr)) consists of EI(Ntr), NPr, and EIIA(Ntr). The active phosphate moiety derived from phosphoenolpyruvate is transferred through EI(Ntr) and NPr to EIIA(Ntr). Sinorhizobium fredii can establish a nitrogen-fixing symbiosis with the legume crops soybean (as determinate nodules) and pigeonpea (as indeterminate nodules). In this study, S. fredii strains with mutations in ptsP and ptsO (encoding EI(Ntr) and NPr, respectively) formed ineffective nodules on soybeans, while a strain with a ptsN mutation (encoding EIIA(Ntr)) was not defective in symbiosis with soybeans. Notable reductions in the numbers of bacteroids within each symbiosome and of poly-β-hydroxybutyrate granules in bacteroids were observed in nodules infected by the ptsP or ptsO mutant strains but not in those infected with the ptsN mutant strain. However, these defects of the ptsP and ptsO mutant strains were recovered in ptsP ptsN and ptsO ptsN double-mutant strains, implying a negative role of unphosphorylated EIIA(Ntr) in symbiosis. Moreover, the symbiotic defect of the ptsP mutant was also recovered by expressing EI(Ntr) with or without the GAF domain, indicating that the putative glutamine-sensing domain GAF is dispensable in symbiotic interactions. The critical role of PTS(Ntr) in symbiosis was also observed when related PTS(Ntr) mutant strains of S. fredii were inoculated on pigeonpea plants. Furthermore, nodule occupancy and carbon utilization tests suggested that multiple outputs could be derived from components of PTS(Ntr) in addition to the negative role of unphosphorylated EIIA(Ntr).

Bradyrhizobium sacchari sp. nov., a legume nodulating bacterium isolated from sugarcane roots

Members of the genus Bradyrhizobium are well-known as nitrogen-fixing microsymbionts of a wide variety of leguminous species, but they have also been found in different environments, notably as endophytes in non-legumes such as sugarcane. This study presents a detailed polyphasic characterization of four Bradyrhizobium strains (type strain BR 10280T), previously isolated from roots of sugarcane in Brazil. 16S rRNA sequence analysis, multilocus sequence analysis (MLSA) and analysis of the 16S-23S rRNA internal transcribed spacer showed that these strains form a novel clade close to, but different from B. huanghuaihaiense strain CCBAU 23303T. Average nucleotide identity (ANI) analyses confirmed that BR 10280T represents a novel species. Phylogenetic analysis based on nodC gene sequences also placed the strains close to CCBAU 23303T, but different from this latter strain, the sugarcane strains did not nodulate soybean, although they effectively nodulated Vigna unguiculata, Cajanus cajan and Macroptilium atropurpureum. Physiological traits are in agreement with the placement of the strains in the genus Bradyrhizobium as a novel species for which the name Bradyrhizobium sacchari sp. nov. is proposed.

Bradyrhizobium cajani sp. nov. isolated from nodules of Cajanus cajan

Two slow-growing strains, AMBPC1010T and AMBPC1011, were isolated from nodules of Cajanus cajan in the Dominican Republic. 16S rRNA gene analysis placed these strains within the genus Bradyrhizobium, being phylogenetically equidistant to several species of this genus. Analysis of the recA and atpD genes showed that the strains isolated belong to a cluster containing the strains Bradyrhizobium ottawaense OO99T, 'Bradyrhizobium americanum' CMVU44 and Bradyrhizobium daqingense CCBAU 15774T, and presented similarity values lower than 96 % for both genes with respect to the strains nodulating C. cajan. DNA-DNA hybridization analysis showed averages of 36, 40 and 39 % relatedness with respect to the representative strains of Bradyrhizobium ottawaense, 'Bradyrhizobium americanum' and Bradyrhizobium daqingense, respectively. Phenotypic characteristics also differed from those of the most closely related species of the genus Bradyrhizobium. Therefore, based on the data obtained in this study, we propose to classify the strains AMBPC1010T (=LMG 29967T=CECT 9227T) and AMBPC1011 into a novel species named Bradyrhizobium cajani sp. nov.

Functional expression of Escherichia coli fhuA gene in Rhizobium spp. of Cajanus cajan provides growth advantage in presence of Fe3+: ferrichrome as iron source

Cajanus cajan rhizobial isolates were found to be unable to utilize iron bound to ferrichrome, desferrioxamine B or rhodotorulic acid, all being hydroxamate type siderophores. A broad host range expression vector containing the Escherichia coli fhuA gene, encoding the outer membrane receptor for Fe-ferrichrome, was constructed. The plasmid construct (pGR1), designed to express fhuA under the lac promoter of E. coli, complemented E. coli MB97 DeltafhuA mutant for ferri-ferrichrome utilization and also allowed Rhizobium spp. ST1 and Rhizobium spp. IC3123 to grow using iron bound to ferrichrome. Sensitivity to the antibiotic albomycin, transported via the FhuA receptor, was found in case of MB97 as well as rhizobial transformants harboring pGR1. The rhizobial transformants expressing fhuA showed growth stimulation when co-inoculated with Ustilago maydis, a fungal species known to produce ferrichrome under iron starved conditions. Growth stimulation was also observed in the presence of externally supplied ferrichrome. The significance of these findings in terms of the potential for improving the survivability of rhizobial bioinoculant strains in natural soils is discussed.

High effectiveness of Rhizophagus irregularis is linked to superior modulation of antioxidant defence mechanisms in Cajanus cajan (L.) Millsp. genotypes grown under salinity stress

Salinity stress leads to the production of reactive oxygen species (ROS) which can cause oxidative damage in plants. A correlation between antioxidant capacity and salt tolerance has been demonstrated in several plant species, which may be enhanced by inoculation with arbuscular mycorrhizal fungi (AMF). However, plant responses to mycorrhization may differ depending on the host plant as well as AMF isolate. It has been proposed that AMF sourced from stressed environments may be better suited as stress ameliorators than non-native/exotic ones. The present study compared the effectiveness of a native inoculum from saline soil and two exotic single isolates, Funneliformis mossseae and Rhizophagus irregularis (single or dual mix), and associated their effectiveness with modulation of antioxidant defence, in two Cajanus cajan (pigeonpea) genotypes (salt sensitive-Paras, salt tolerant-Pusa 2002) under NaCl stress. Plants subjected to NaCl (0-100 mM) recorded a substantial build-up of ROS, more in Paras than Pusa 2002. Although mycorrhization with all AMF improved plant biomass and reduced oxidative burst by strengthening antioxidant enzymatic activities, inoculation with R. irregularis (alone or in combination with F. mosseae) resulted in higher biomass accumulation which correlated with its higher root colonization and improved redox stability through rapid recycling of reduced ascorbate and glutathione. The study thus suggested that mitigation of salt-induced oxidative burden by increased activation of scavenging antioxidants is an important mechanism that determined the higher effectiveness of R. irregularis over the native saline mix in pigeonpea plants.

Root colonization by heavy metal resistant Enterobacter and its influence on metal induced oxidative stress on Cajanus cajan

BACKGROUND

Heavy metal resistant bacterium Enterobacter sp. C1D was evaluated for cadmium (Cd) mediated exopolysaccharide production, biofilm formation and legume root colonization ability under Cd stress to alleviate metal induced stress.

RESULTS

The plant was sensitive to Cd (IC₅₀ 3-4 μg mL^-1 ), whereas the bacterium showed high Cd tolerance (MIC₉₉ 120 μg mL^-1 ). Confocal laser scanning microscopy of the Cajanus cajan roots showed heavy loads of green fluorescence protein labelled Enterobacter sp. C1D on the surface of plant root, specifically at the point of root hair/lateral root formation along with cortex, even under metal stress. The root colonizing ability of Enterobacter sp. C1D was not affected by the presence of Rhizobium and the bacteria could be observed after 30 days of incubation in soil. Various plant growth parameters, antioxidant metabolites and oxidative stress indicator were significantly influenced by bacterial treatment, which, overall, reduced the adverse effect of Cd.

CONCLUSION

Heavy metal tolerant bacteria may be a good choice for the development of biofertilizers and may work well with the native soil microbes such as Rhizobium under the metal polluted soil. © 2019 Society of Chemical Industry.

Construction and pilot screening of a signature-tagged mutant library of Sinorhizobium fredii

Sinorhizobium fredii is well known for its ability to establish symbiosis with diverse legumes such as Glycine max (soybean, determinate nodules) and Cajanus cajan (pigeon pea, indeterminate nodules). In order to make screening of S. fredii genes related to symbiosis cost-effective, we constructed a large Tn5 insertion mutant library of S. fredii CCBAU45436 using the signature-tagged mutagenesis (STM) technique. This STM library contains a total of 25,500 independent mutants distributed in 17 sublibraries tagged by corresponding distinct DNA bar-code sequences. After the pilot screening of 255 mutants in 15 batches, Tag85-4, Tag4-17, Tag4-11 and Tag10-13 were found to have attenuated competitiveness (0-30 % in nodule occupation) compared to the wild-type strain when inoculated on soybean. Further characterization of these mutants suggests that Tag4-11 (a pyrC mutant) and Tag10-13 (a nrdJ mutant) are defective in establishing symbiosis with soybean. The pyrC mutant induced uninfected pseudonodules while the nrdJ mutant formed significantly more nodules containing bacteroids with poor persistence ability. When these two mutants were tested on pigeon pea, host-specific symbiotic defects were found. These results demonstrated the STM library as a valuable resource for identifying S. fredii genes relevant to symbiosis.

Removal of toluene vapour using agro-waste as biofilter media

Biodegradation of toluene vapour was investigated in a laboratory scale biofilter packed with cylindrical pieces of yellow-gram (Cajanus cajan) stalk. Inlet concentrations and volumetric flow rates of toluene were varied from 2.56 to 34.73 g/m3 and 0.18 to 0.24 m3/h, respectively. The steady state was achieved within seven days and the degradation of toluene followed an exponential behaviour with time. Elimination capacity increased and tended towards a constant value but removal efficiency decreased with increase in inlet toluene loading. Depending upon loading rate, the process was either mass transfer or reaction-controlled.

An efficient method for zoospore production, infection and real-time quantification of Phytophthora cajani causing Phytophthora blight disease in pigeonpea under elevated atmospheric CO₂

BACKGROUND

Phytophthora blight caused by Phytophthora cajani is an emerging disease of pigeonpea (Cajanus cajan L.) affecting the crop irrespective of cropping system, cultivar grown and soil types. Current detection and identification methods for Phytophthora species rely primarily on cultural and morphological characteristics, the assessment of which is time-consuming and not always suitable. Sensitive and reliable methods for isolation, identification, zoospore production and estimating infection severity are therefore desirable in case of Phytophthora blight of pigeonpea.

RESULTS

In this study, protocols for isolation and identification of Phytophthora blight of pigeonpea were standardized. Also the method for zoospore production and in planta infection of P. cajani was developed. Quantification of fungal colonization by P. cajani using real-time PCR was further standardized. Phytophthora species infecting pigeonpea was identified based on mycological characters such as growth pattern, mycelium structure and sporangial morphology of the isolates and confirmed through molecular characterization (sequence deposited in GenBank). For Phytophthora disease development, zoospore suspension of 1 × 10(5) zoospores per ml was found optimum. Phytophthora specific real-time PCR assay was developed using specific primers based on internal transcribed spacer (ITS) 1 and 2. Use of real-time PCR allowed the quantitative estimation of fungal biomass in plant tissues. Detection sensitivities were within the range of 0.001 pg fungal DNA. A study to see the effect of elevated CO₂ on Phytophthora blight incidence was also conducted which indicated no significant difference in disease incidence, but incubation period delayed under elevated CO₂ as compared to ambient level.

CONCLUSION

The zoospore infection method for Phytophthora blight of pigeonpea will facilitate the small and large scale inoculation experiments and thus devise a platform for rapid and reliable screening against Phytophthora blight disease of pigeonpea. qPCR allowed a reliable detection and quantification of P. cajani in samples with low pathogen densities. This can be useful in early warning systems prior to potential devastating outbreak of the disease.

Protease inhibitor (PI) mediated defense in leaves and flowers of pigeonpea (protease inhibitor mediated defense in pigeonpea)

More than 200 insect pests are found growing on pigeonpea. Insects lay eggs, attack and feed on leaves, flowers and developing pods. Plants have developed elaborate defenses against these insect pests. The present work evaluates protease inhibitor (PI) based defense of pigeonpea in leaves and flowers. PIs in the extracts of these tender tissues were detected by using gel X-ray film contact print method. Up to three PIs (PI-3, PI-4 and PI-5) were detected in these tissues as against nine (PI-1-PI-9) in mature seeds. PI-3 is the major component of these tissues. Mechanical wounding, insect chewing, fungal pathogenesis and application of salicylic acid induced PIs in pigeonpea in these tissues. Induction was found to be local as well as systemic but local response was stronger than systemic response. During both local and systemic induction, PI-3 appeared first. In spite of the presence and induction of PIs in these tender tissues and seeds farmers continue to suffer yield loses. This is due to the weak expression of PIs. However the ability of the plant to respond to external stimuli by producing defense proteins does not seem to be compromised. This study therefore indicates that PIs are components of both constitutive and inducible defense and provide a ground for designing stronger inducible defense (PIs or other insect toxin based) in pigeonpea.

Induction of phytoalexins in pigeonpea (Cajanus cajan) in response to inoculation with Fusarium udum and other treatments

Cajanol accumulation was monitored in four cultivars of pigeonpea, Cajanus cajan (L) Millsp, after inoculation with 2 x 10(6) conidia ml-1 of Fusarium udum Butler. Rapid accumulation was observed only in wilt-resistant cultivar ICP 9145. Another wilt-resistant cultivar, ICP 8863, had low levels of cajanol similar to those in wilt-susceptible cultivars ICP 2376 and Malawi local, indicating that rapid cajanol accumulation is not positively correlated with resistance to wilt in pigeonpea. A comparison of various inducing agents showed live conidia of the pathogen to cause more rapid accumulation than the other agents.

Pigeonpea (Cajanus cajan L. Millsp.)

Pigeonpea [Cajanus cajan (L.) Millsp.], also known as redgram, is one of the major grain legume (pulses) crops grown in the semiarid tropics (SAT) extending between 30 degrees N and 30 degrees S; it is the second most important food legume of India. It is cultivated in about 50 countries of Asia, Africa, and the Americas for a variety of uses (food, fodder, fuel wood, rearing lac insects, hedges, wind breaks, soil conservation, green manure, roofing, and so on). The constraints of enhancing its productivity include the damage caused by various fungi, bacteria, viruses, and insect pests. Conventional plant breeding methods have not been successful for the improvement of pigeonpea because of genetic variation and incompatibility among the wild varieties. Genetic engineering technology can therefore be used as an additional tool for the introduction of agronomically useful traits into established varieties. The development of plant transformation techniques has been a major breakthrough in overcoming constraints to achieve precision in genetic manipulation. The development of efficient plant regeneration protocols is a prerequisite for recombinant technology to carry out genetic transformation. This chapter describes an Agrobacterium-mediated transformation protocol for pigeonpea, a simple, efficient, and reproducible method that is applicable across diverse genotypes of pigeonpea.

Classical Soybean (Glycine max (L.) Merr) Symbionts, Sinorhizobium fredii USDA191 and Bradyrhizobium diazoefficiens USDA110, Reveal Contrasting Symbiotic Phenotype on Pigeon Pea (Cajanus cajan (L.) Millsp)

Pigeon pea (Cajanus cajan (L.) Millspaugh) is cultivated widely in semiarid agricultural regions in over 90 countries around the world. This important legume can enter into symbiotic associations with a wide range of rhizobia including Bradyrhizobium and fast-growing rhizobia. In comparison with other major legumes such as soybean and common bean, only limited information is available on the symbiotic interaction of pigeon pea with rhizobia. In this study, we investigated the ability of two classical soybean symbionts-S. fredii USDA191 and B. diazoefficiens USDA110-and their type 3 secretion system (T3SS) mutants, to nodulate pigeon pea. Both S. fredii USDA191 and a T3SS mutant S. fredii RCB26 formed nitrogen-fixing nodules on pigeon pea. Inoculation of pigeon pea roots with B. diazoefficiens USDA110 and B. diazoefficiens Δ136 (a T3SS mutant) resulted in the formation of Fix- and Fix+ nodules, respectively. Light and transmission electron microscopy of Fix- nodules initiated by B. diazoefficiens USDA110 revealed the complete absence of rhizobia within these nodules. In contrast, Fix+ nodules formed by B. diazoefficiens Δ136 revealed a central region that was completely filled with rhizobia. Ultrastructural investigation revealed the presence of numerous bacteroids surrounded by peribacteroid membranes in the infected cells. Analysis of nodule proteins by one- and two-dimensional gel electrophoresis revealed that leghemoglobin was absent in B. diazoefficiens USDA110 nodules, while it was abundantly present in B. diazoefficiens Δ136 nodules. Results of competitive nodulation assays indicated that B. diazoefficiens Δ136 had greater competitiveness for nodulation on pigeon pea than did the wild type strain. Our results suggest that this T3SS mutant of B. diazoefficiens, due to its greater competitiveness and ability to form Fix+ nodules, could be exploited as a potential inoculant to boost pigeon pea productivity.

Changes and induction of aminopeptidase activities in response to pathogen infection during germination of pigeonpea (Cajanas cajan) seeds

Aminopeptidases play important role in the mobilization of storage proteins at the cotyledon during seed germination. It is often referred as inducible component of defense against herbivore attack. However the role of aminopeptidase in response to pathogen attack in germinating seeds is remained to be unknown. An attempt was made to analyze change in the aminopeptidase (EC 3.4.11.1) activity during germination of pigeonpea (Cajanus cajan L.) seeds by infecting the seeds with fungi. Two aminopeptidase activity bands (AP1 and AP2) were detected in control as well as infected pigeonpea seeds. During latter stages of germination in control seeds, AP1 activity was replaced by AP2 activity. However AP1 activity was significantly induced in germinating seeds infected with Fusarium oxysporum f.sp. ciceri and Aspergillus niger var. niger. The estimated molecular weights of AP1 and AP2 were ∼97 and 42.8kDa respectively. The induced enzyme was purified up to 30 fold by gel filtration chromatography. The purified enzyme was preferentially cleaved leucine p-nitroanilide than alanine p-nitroanilide. The enzyme was strongly inhibited by bestatin and 1,10-phenanthroline. Almost 50% of enzyme activity was inhibited by ethylene diamine tetra acetate. The purified enzyme showed broad pH optima ranging from pH 6.0 to 9.0 and optimum at pH 8.5. The induction of aminopeptidase activity during pigeonpea seed germination and in response to pathogen attack indicates significant involvement of these enzymes in primary as well as secondary metabolism of the seeds. These findings could be helpful to further dissect defensive role of aminopeptidases in seed germination which is an important event in plant's life.

Genome sequence and comparative genomics of Rhizobium sp. Td3, a novel plant growth promoting phosphate solubilizing Cajanus cajan symbiont

Rhizobium sp. Td3 is a Sesbania plant growth promoting, Cajanus cajan nodulating rhizobia. Studying its whole genome was important as it is a potent phosphate solubilizer with constitutive gluconic acid production ability through operation of the periplasmic glucose oxidation pathway even under conditions of catabolite repression. This is in contrast to the other explored phosphate solubilizers. Rhizobial isolates sequenced so far are known to lack components of the direct glucose oxidation pathway and cannot produce gluconic acid on its own. Here, we present the genome sequence of Rhizobium sp. Td3. Genome comprises of a single chromosome of size 5,606,547 bp (5.6 Mb) with no symbiotic plasmid. Rhizobium leguminosarum bv. viciae USDA2370 was the closest whole genome known. 109 genes responsible for diverse plant growth promoting activities like P solubilization, synthesis of acetoin, nitric oxide, indole-3 acetic acid, exopolysaccharide, siderophore and trehalose have been identified. Flagellar proteins, genes encoding antibiotic and metal resistance, enzymes required for combating oxidative stress as well as attachment and colonization in the plant rhizosphere are also present. Availability of genome sequence of such a versatile plant growth promoting agent will help in exploiting all the phyto-beneficial traits of Td3 for its use as a biofertilizer.