Ravi Tiwari

Exopolysaccharides (EPS) represent an energy expensive composition of high molecular weight sugar polymers. Many physiological roles have been associated with EPS including pathogenesis, biofilm maturation, stress tolerance and symbiotic performance. Coincidentally, many of these functions are paralleled by the stringent response, a phase defined by a metabolic downshift in response to nutrient-limiting conditions. In many rhizobia, the synthesis of EPS and specific stationary phase regulators are required for the successful symbiosis with their legume host. Considerable research has examined the genetic and physical basis by which EPS is produced to initiate plant root infection, however, the physiological conditions that affect and coordinate these genes is not dearly understood. This work aimed to define the roles of EPS and the relationship to stationary phase in Sinorhizobiurn medicae whilst determining specific nutritional elements that affect the level of EPS synthesis.

Arid and semi arid regions of the Indian Thar Desert possess several nodulating native legumes important to the local inhabitants for food, fodder, shelter and medicines. These legumes are useful in dry land Cultivation and soil improvement for rain-fed crops in arid and semi arid areas, The present study was aimed to characterize the genetic diversity of thirty one root nodule bacterial endophytes strains from Tephrosia purpurea and Tephrosia villosa growing in Jodhpur and surrounding districts of Rajasthan (India), These two plants are widely distributed in degraded and waste land in the arid and semi arid regions of many states of India.

Exopolysaccharides (EPS's) play an important role in forming and extending infection threads in the symbiosis between Medicago and Sinorhizobium. S. meliloti 1021 (Sm1021) is able to produce one type of EPS (succinoglycan or EPS I) but not another (galactoglucan or EPS II), due to the presence of an insertion sequence (ISRm2011-1) in expR. ExpR forms part of the SinlR quorum sensing system and is a regulator of galactoglucan synthesis. Previous work in our laboratory determined that under N-limited conditions, Sm1021 was poorly effective at fixing N with the model indeterminate legume Medicago truncatula, while two other mucilaginous strains, S. medicae WSM419 and S. melitoti WSM1022, were significantly more effective on this host. While the expR status of WSM1022 is unknown, WSM419 possesses an intact expR gene. These data indicate that the interrupted expR gene in Sm1021 might account for the reduced effectiveness of this strain on the model legume.

The acid tolerance response of Sinorhizobium medicae WSM419 enables cell adaptation to lethal acid after cell exposure to mild acidity. The expression of the LPIA (low pH induced gene A) gene is critical for this response and is add-activated at least 20-fold in the mild acidic conditions. The expression of this gene is specifically induced by acidity and not by any other stress. We have previously shown that full acid-induction of lpiA requires functional FsrR (fused sensor-regulator) (Reeve et al 2006). However, even in the absence of FsrR, there is still a residual6-fold acid-induction of tpiA implicating other proteins in the regulation process. Other genes have been suspected to be involved including tcsA (two component sensor) tcrA (two component regulator), acvB (acid virulence protein B) and rpoN (RNA polymerase N-metabolism).

The poor availability of nitrogen is one of the principal factors limiting global biomass. Legumes are vital components of agricultural systems because of their ability to associate symbiotically with root nodule bacteria (RNB) and subsequently fix atmospheric nitrogen to a form that can be utilised by the plant partner. Furthermore, this symbiotic relationship provides available soil nitrogen for subsequent non-leguminous crops. This RNB-legume interaction is affected by a number of environmental factors. Progressive acidification of agricultural soils is one of the big challenges in agriculture as soil acidity negatively impacts legume productivity. One genus of RNB, Sinorhizobium, is particularly acid-sensitive causing a major reduction in Medicago productivity in acidic soils. Due to the importance of Medic pasture production, alternative strains have been captured, and are still being captured, from the genetic pool that display superior acid tolerance characteristics. This presentation will focus on the acid-tolerant species S. medicae (previously known as S. meliloti) and in particular on the previously used commercial inoculant WSM419.

The coming avalanche of genomic data is bringing opportunities for new insights into the genome architecture and evolution of root nodule bacteria. Comparative genomic analyses of root nodule bacteria reveal that mobile genetic elements ranging from transposons, integrons, genomic islands and plasmids are widespread in these genomes. Genomic islands are plasmid-like DNA regions that are integrated into prokaryotic chromosomes and confer a variety of functions (resistance, degradation, metabolism, pathogenicity, secretion and symbiosis) to the host genome. Acquisition can extend the capacity of the host bacterium to adapt to new environments. Genomic islands that confer nitrogen fixation capacity to non symbiotic bacteria are termed 'symbiosis islands' because they carry nodulation and nitrogen fixation genes required for the legume symbiosis as well as genes required for the excision, insertion and transfer of the island.

Plants growing in inhospitable environment may have association with useful microbes that help them to withstand harsh conditions like poor and degraded soil, high temperature and high salinity. Isolation and screening of isolates from plants of such habitats showing plant growth promoting traits (PGP) would be useful to enrich the bank of agriculturally important microbes. In present investigation more than one hundred isolates were obtained from root nodules of ten native legumes growing in arid regions of Indian Thar desert.