BACKGROUND: Drought stress is currently the primary abiotic stress factor for crop loss worldwide. Although drought stress reduces the crop yield significantly, species and genotypes differ in their stress response; some tolerate the stress effect while others not. In several systems, it has been shown that, some of the beneficial soil microbes ameliorate the stress effect and thereby, minimizing yield losses under stress conditions. Realizing the importance of beneficial soil microbes, a field experiment was conducted to study the effect of selected microbial inoculants namely, N-fixing bacteria, Bradyrhizobium liaoningense and P-supplying arbuscular mycorrhizal fungus, Ambispora leptoticha on growth and performance of a drought susceptible and high yielding soybean cultivar, MAUS 2 under drought condition.
RESULTS: Drought stress imposed during flowering and pod filling stages showed that, dual inoculation consisting of B. liaoningense and A. leptoticha improved the physiological and biometric characteristics including nutrient uptake and yield under drought conditions. Inoculated plants showed an increased number of pods and pod weight per plant by 19% and 34% respectively, while the number of seeds and seed weight per plant increased by 17% and 32% respectively over un-inoculated plants under drought stress condition. Further, the inoculated plants showed higher chlorophyll and osmolyte content, higher detoxifying enzyme activity, and higher cell viability because of less membrane damage compared to un-inoculated plants under stress condition. In addition, they also showed higher water use efficiency coupled with more nutrients accumulation besides exhibiting higher load of beneficial microbes.
CONCLUSIONS: Dual inoculation of soybean plants with beneficial microbes would alleviate the drought stress effects, thereby allowing normal plants\' growth under stress condition. The study therefore, infers that AM fungal and rhizobia inoculation seems to be necessary when soybean is to be cultivated under drought or water limiting conditions.

Vegetable soybean (Glycine max [L.]) is mainly consumed in Asian countries, but has recently attracted attention worldwide due to its high nutritional value. We aimed to identify the indigenous rhizobia of vegetable soybean in Yao City, Osaka Prefecture, Japan, and to clarify the relationships between the rhizobial community and soil environmental factors. Soil samples were collected from 12 vegetable soybean cultivation fields under two different conditions (six greenhouses and six open fields) in Yao City with different varieties of vegetable soybean. A total of 217 isolates were obtained from the nodules and clustered into nine operational taxonomic units (OTUs) with 97% homology based on the 16S-23S rRNA internal transcribed spacer (ITS) region. A phylogenetic ana-lysis showed that OTUs were closely related to Bradyrhizobium liaoningense, B. ottawaense, B. elkanii, and other Bradyrhizobium species and were dominant in this order. B. liaoningense was widely found in sampled sites and accounted for 50.7% of all isolates, while B. ottawaense was mostly limited to open fields. This rhizobial community differed from Japanese soybean rhizobia, in which B. diazoefficiens, B. japonicum, and B. elkanii were dominant. These results imply the characteristic differences among host plants or regional specialties. A non-metric multidimensional scaling (NMDS) ana-lysis revealed the significant impact of soil pH and the contents of Ca, Mg, Mn, total nitrogen (TN), and total carbon (TC) on the distribution of rhizobia. B. liaoningense was detected in soils with a neutral pH, and high TN and low Mn contents increased its abundance. The present study provides novel insights into Japanese rhizobia and potentially novel resources for sustainable agriculture.