Arbuscular mycorrhizal (AM) symbiosis, a ubiquitous symbiotic association established between AM fungi and roots of higher plants in most terrestrial ecosystems, is essentially important for plant adaptation to various environmental stresses, such as nutrient deficiency, environmental pollution and drought, etc. Many studies proved the positive influences of AM on plant drought tolerance and made efforts to uncover the underlying mechanisms: for plant individuals, AM fungi could stimulate plant physiological responses to drought stress; at the ecosystem level, AM fungi could interact with host plant to adapt to an adverse environment. However, systematic study is still necessary to reveal the fundamental role of AM fungi in improving plant drought tolerance.
In this mini-review we summarized recent research progresses in the involvements of AM fungi in regulation of plant drought tolerance related genes, such as LEA encoding late embryogenesis-abundant proteins, P5CS encoding △¹-pyrroline-5-carboxylate synthetase, MIPs encoding major intrinsic proteins, and NCED encoding 9-cis-epoxycarotenoid dioxygenase. As seen in reports, AM fungi could up- or down-regulate these genes under drought stresses, however, there were conflicting results as for the mycorrhizal effects on gene expressions in different experiments. In most cases, this could be attributed to incomparable experimental conditions, considering that 1) not all members in a gene family had been examined in each experiment; 2) different symbiotic associations (plant-AM fungus combinations) might exert different strategies to resist drought stresses, and each fungal species/strain might exhibit different capacity to assist host plant against environmental adversities; 3) gene expression varies in different plant tissues at different plant developmental stages. Obviously, it is still necessary to carry out further research for a better understanding of AM regulation of drought tolerance related genes in host plants, and model plants and AM fungal strains might be ideal choices to make sure of comparable results from different experiments.
In addition to a full discussion on the insufficiency of previous studies, we also introduced the advances of proteomics in AM physiology and proposed perspectives for future research by the end of this review.