Azospirillum and arbuscular mycorrhizal colonization enhance rice growth and physiological traits under well-watered and drought conditions.

The response of rice plants to inoculation with an arbuscular mycorrhizal (AM) fungus, Azospirillum brasilense, or combination of both microorganisms, was assayed under well-watered or drought stress conditions. Water deficit treatment was imposed by reducing the amount of water added, but AM plants, with a significantly higher biomass, received the same amount of water as non-AM plants, with a poor biomass. Thus, the water stress treatment was more severe for AM plants than for non-AM plants. The results showed that AM colonization significantly enhanced rice growth under both water conditions, although the greatest rice development was reached in plants dually inoculated under well-watered conditions. Water level did not affect the efficiency of photosystem II, but both AM and A. brasilense inoculations increased this value. AM colonization increased stomatal conductance, particularly when associated with A. brasilense, which enhanced this parameter by 80% under drought conditions and by 35% under well-watered conditions as compared to single AM plants. Exposure of AM rice to drought stress decreased the high levels of glutathione that AM plants exhibited under well-watered conditions, while drought had no effect on the ascorbate content. The decrease of glutathione content in AM plants under drought stress conditions led to enhance lipid peroxidation. On the other hand, inoculation with the AM fungus itself increased ascorbate and proline as protective compounds to cope with the harmful effects of water limitation. Inoculation with A. brasilense also enhanced ascorbate accumulation, reaching a similar level as in AM plants. These results showed that, in spite of the fact that drought stress imposed by AM treatments was considerably more severe than non-AM treatments, rice plants benefited not only from the AM symbiosis but also from A. brasilense root colonization, regardless of the watering level. However, the beneficial effects of A. brasilense on most of the physiological and biochemical traits of rice plants were only clearly visible when the plants were mycorrhized. This microbial consortium was effective for rice plants as an acceptable and ecofriendly technology to improve plant performance and development.

The arbuscular mycorrhizal symbiosis enhances the photosynthetic efficiency and the antioxidative response of rice plants subjected to drought stress.

Rice (Oryza sativa) is the most important crop for human consumption, providing staple food for more than half of the world's population. Rice is conventionally grown under flooded conditions for most of its growing cycle. However, about half of the rice area in the world does not have sufficient water to maintain optimal growing conditions and yield is reduced by drought. One possible way to increase rice production in order to meet the rice demand is to improve its drought tolerance by means of the arbuscular mycorrhizal (AM) symbiosis. Thus, AM and non-AM rice plants were maintained under well-watered conditions or were subjected to moderate and severe drought stress for 15d. After that, half of the plants from each treatment were harvested, while the other half were allowed to recover from drought for additional 25d. The results showed that rice can benefit from the AM symbiosis and improve their long-term development after a drought stress period. In fact, at each watering level, AM plants showed about 50% enhanced shoot fresh weight as compared to non-AM plants. The AM symbiosis enhanced the plant photosynthetic efficiency under stress over 40%, induced the accumulation of the antioxidant molecule glutathione and reduced the accumulation of hydrogen peroxide and the oxidative damage to lipids in these plants. Thus, these combined effects enhanced the plant performance after a drought stress period.