Drought stress has a negative impact on plant growth and production. Arbuscular mycorrhizal (AM) fungi, which establish symbioses with most terrestrial vascular plant species, play important roles in improving host plant mineral nutrient acquisition and resistance to drought. However, the physiological and molecular regulation mechanisms occurring in mycorrhizal Eucalyptus grandis coping with drought stress remain unclear. Here, we studied the physiological changes and mitogen-activated protein kinase (MAPK) cascade gene expression profiles of E. grandis associated with AM fungi under drought stress. The results showed that colonization by AM fungi significantly enhanced plant growth, with higher plant biomass, shoot height, root length, and relative water content (RWC) under drought conditions. Mycorrhizal plants had lower levels of accumulation of proline, malondialdehyde (MDA), H2O2, and O2·- than seedlings not colonized with AM fungi. In addition, mycorrhizal E. grandis also had higher peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) activities under drought conditions, improving the antioxidant system response. Eighteen MAPK cascade genes were isolated from E. grandis, and the expression levels of the MAPK cascade genes were positively induced by symbiosis with AM fungi, which was correlated with changes in the proline, MDA, H2O2, and O2·- contents and POD, SOD, and CAT activities. In summary, our results showed that AM symbiosis enhances E. grandis drought tolerance by regulating plant antioxidation abilities and MAPK cascade gene expression. IMPORTANCE Arbuscular mycorrhizal (AM) fungi play an important role in improving plant growth and development under drought stress. The MAPK cascade may regulate many physiological and biochemical processes in plants in response to drought stress. Previous studies have shown that there is a complex regulatory network between the plant MAPK cascade and drought stress. However, the relationship between the E. grandis MAPK cascade and AM symbiosis in coping with drought remains to be investigated. Our results suggest that AM fungi could improve plant drought tolerance mainly by improving the antioxidant ability to protect plants from reactive oxygen species (ROS) and alleviate oxidative stress damage. The expression of the MAPK cascade genes was induced in mycorrhizal E. grandis seedlings under drought stress. This study revealed that MAPK cascade regulation is of special significance for improving the drought tolerance of E. grandis. This study provides a reference for improving mycorrhizal seedling cultivation under stress.
Keywords: Eucalyptus grandis; MAPK cascade genes; antioxidation system; arbuscular mycorrhizal fungi; drought resistance genes; drought stress; osmotic regulation.