Diversity and characterization of beneficial microbes for rice production in Ghana

概要

Rice (Oryza sativa L.) is an important staple food and cash crop in the world. It is cultivated across Ghana in all the ten regions covering diverse agroecological zones. In Ghana, domestic rice consumption (at 1.3m tonnes) outstrips domestic production (721,610 tonnes), resulting in the deficit of about 580,300 tonnes which threatens food insecurity of the country. The nation relies on imports from other rice-producing countries at a considerable cost (about $1.2bn annually) to bridge the gap. Water-use efficiencies, inadequate soil nutrient resulting from poor soil fertility management and inefficient fertilizer application by farmers are among the major factors causing domestic rice production. In an attempt to boost domestic rice production and meet the growing demand, these challenges must be adequately addressed.

　Biological production strategies to improve the growth and yield of rice are desired for sustainable rice production. Exploring beneficial microbes such as arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) can be alternatives for fertilization (biofertilizers) to help minimize the ecological threat posed by chemical fertilizers. Studies on the utilization of these microbes in the agricultural production system and for their beneficial roles in enhancing crops, especially rice production, have not been reported in Ghana. The main objectives of this study were: i) to gain a better understanding of how environmental factors, soil properties and farm management influence the diversity and community composition of AMF associated with rice in Ghana, ii) to assess the effect of PGPR (Azospirillum sp. strain B510) inoculation on the growth and yield of Ghanaian rice cultivars.

　To explore the AMF community associated with rice cultivated in Ghana, soils and rice plants were collected from six major rice-producing regions (Upper West, Upper East, Northern, Ashanti Brong-Ahafo and Volta) covering three agroecological zones (Forest–Savannah transition, Deciduous Forest zone, and Guinea Savannah zone) in Ghana. A total of 57 soil samples and 57 root samples were obtained. A significant variation was observed in several soil properties among the regions including available phosphorus content (approximately 3 times), cation exchange capacity (approximately 2 times), and total carbon (approximately 2.3 times) in Deciduous Forest compared to Guinea Savannah. The rate of AMF colonization ranged from 1.9% to 22.9% among the regions, confirming the relationship between AMF colonization and rice cultivation practices. Genetic analysis revealed that 15 different AMF genera colonized in rice roots. Rhizophagus and Glomus were observed to be predominant in all regions except the Ashanti region, which was dominated by the genera Scutellospora and Acaulospora. Comparison of AMF diversity among the agroecological zones revealed that Guinea Savannah, which soils contain low available phosphate, had the highest AMF diversity. Permutational Multivariate Analysis of Variance (PERMANOVA) analysis indicated that the available phosphorus in the soil was the principal determining factor for shaping the AMF community structure (P < 0.05). These revealed for the first time, the AMF diversity and community structure associated with rice cultivated in Ghana and how the AMF communities are affected by the chemical properties of soil from different locations.

　To explore alternative beneficial microbes for rice production in Ghana, we investigated the effect of Azospirillum sp. strain B510 inoculation on the growth and yield of Ghanaian rice cultivars. The pot experiment was set up in the greenhouse at Tokyo University of Agricultural and Technology, using four selected rice cultivars (Amantin, Zebila, Boako and Bawku) from Ghana. Overall, the inoculation of B510 showed positive effects on the growth and yields of rice except for the cultivar Zebila. Especially, B510 inoculation promoted the vegetative growth of three rice cultivars (Amantin, Boako and Bawku) and significantly increased their tiller number. The total yield observed among these three rice cultivars increased with the cv. Amantin showing a significant increase. A field experiment in Ghana confirmed the positive effects of B510 inoculation on the cultivar Exbika. The number of panicles, number of seeds per panicle and total yield recorded were significantly higher in B510-inoculated rice plants compared with control plants. Similarly, aboveground biomass with grain or without grain was found to be significantly higher in inoculated plants.

　In conclusion, this study revealed for the first time, the diversity and community structure of AMF associated with rice in Ghana. The available phosphorus in the soil was identified as the key driver shaping the AMF community. This study also revealed that Azospirillum sp. strain B510 has good potential to promote the growth and yield of Ghanaian rice. These findings provide novel insights into the agroecology of rice fields in Ghana and contribute to improving rice production of the country in a more sustainable way.