No abstract

Salt stress is widely recognized as one of the important abiotic stressors results the severe reduction of medicinal plants growth and its herb yield. It also critically reduces the production of medicinally important active chemical content in the plants. There are three main stages on the growth of medicinal plants where the high salinity stress severely affects the plant growth. Recent studies showed that there is reduction of herb yield around 60% and simultaneous active chemical content reduction of around 50% under the exposure of high salinity of 300 mM of NaCl concentration. There are many reasons behind the reduction of growth of medicinal plants due to salt stress. There are also morphological, physiological and biochemical changes observed on the medicinal plants but there is still mystery exists whether any ultra-structural changes occur on the medicinal plants for salinity stress. The strategies employed in recent years to reduce the effect of high salinity stress on medicinal plant production mainly focused on a selection of salinity tolerant host genotypes. However, such efforts need high cost and inputs, there is an immediate urgency to build simple, low cost, sustainable and short term methods for salinity stress management. Hence the use of plant growth promoting microbes inhabiting rhizosphere, phyllosphere or endosphere might play a significant role in this aspect, if we exploit them as a multi-trait PGPR like stress-tolerant, PGPR activities, manipulation of signalling between both partners as plant and microbes with specific compatible solutes.

Salt stress is widely recognized as one of the important abiotic stressors results the severe reduction of medicinal plants growth and its herb yield. It also critically reduces the production of medicinally important active chemical content in the plants. There are three main stages on the growth of medicinal plants where the high salinity stress severely affects the plant growth. Recent studies showed that there is reduction of herb yield around 60% and simultaneous active chemical content reduction of around 50% under the exposure of high salinity of 300 mM of NaCl concentration. There are many reasons behind the reduction of growth of medicinal plants due to salt stress. There are also morphological, physiological and biochemical changes observed on the medicinal plants but there is still mystery exists whether any ultra-structural changes occur on the medicinal plants for salinity stress. The strategies employed in recent years to reduce the effect of high salinity stress on medicinal plant production mainly focused on a selection of salinity tolerant host genotypes. However, such efforts need high cost and inputs, there is an immediate urgency to build simple, low cost, sustainable and short term methods for salinity stress management. Hence the use of plant growth promoting microbes inhabiting rhizosphere, phyllosphere or endosphere might play a significant role in this aspect, if we exploit them as a multi-trait PGPR like stress-tolerant, PGPR activities, manipulation of signalling between both partners as plant and microbes with specific compatible solutes.

The present investigation was focused on isolation and in vitro characterization of high abiotic stress tolerance, PGPR traits and dinitrogen fixation efficiency among cultivable rhizobia nodulating mothbean, clusterbean and mungbean. A total of 201 rhizobial isolates were retrieved from 101 different nodule samples collected from various locations of four districts i.e. Churu, Bikaner, Jaisalmer and Barmer situated under hyper-arid zone of Rajasthan, India. Around 70% of rhizobial isolates showed steady growth at 30% of polyethylene glycol 6000 (PEG 6000) and 30% of isolates showed tolerance at 40% PEG when drought tolerance of different rhizobia were assessed. Similarly 70% of the rhizobial isolates showed luxurious growth at temperature of 40 °C and only 30% of isolates showed survival ability at 45 °C. Combined abiotic stress tolerance experiment revealed that 54 rhizobial isolates were tolerant to 30% PEG and 40 °C while only 16 isolates showed high drought tolerance of 40% PEG 6000 and high temperature tolerance at 45 °C. In vitro assessment of dinitrogen fixation efficiency showed all 54 stress tolerant isolates possessed dinitrogen fixation ability with different degrees of efficacy. Stress tolerant rhizobial isolates also harbored multiple PGPR traits like phosphate solubilization, IAA production, ammonia excretion, bacteriocin production and ACC deaminase activity. Hence, our result showed a promising percentage (30%) of different rhizobial isolates can be used further in situ trials in pot house and field conditions as high abiotic stress tolerant isolate accompanying with multiple PGPR traits for future bioinoculant production.

A total of 81 rhizobia were retrieved from nodules of clusterbean grown in hyper-arid zone of Rajasthan. Twenty one rhizobial isolates showed combined drought tolerance of 40% concentration of polyethylene glycol 6000 and temperature tolerance at 45°C. All the stress-tolerant rhizobia were authenticated by plant infectivity test and further showed the presence of nitrogen fixation nifH gene. Most of the stress-tolerant rhizobia harbour multiple PGPR traits. The molecular diversity among stress-tolerant rhizobia was accomplished through RFLP of 16S rDNA using restriction enzymes MspI and HaeIII. Dendrogram data showed that all 21 isolates were distributed into two major clusters. Total of 20 genotypes were formed but 13 biotypes were constituted at 80% level of similarity. Out of these, biotype 10 was found to be the most prevalent biotype of hyper-arid zone. Moreover, isolates from same nodule were not 100% similar. It indicated that vast diversity was present among stress-tolerant clusterbean rhizobial isolates.