Effect of Piriformospora indica on reducing adverse effects of salt stress on corn plant (Zea mays L.)

Salt stress is the most prevalent environmental stresses limiting growth and yield of plants which has threatened crop production especially in arid and semi-arid regions of the world. Therefore, improving salinity tolerance in crop plants to grow in soils containing high amounts of soluble salts is very important regards to decrease in crop production. Soil beneficial microorganisms including bacteria and endophytic fungi have important role in improving host plants adaption to abiotic (drought, salinity, heavy metals …) and biotic (pests and plant pathogens environmental stresses. Piriformospora indica is one of the important endophytic fungi which not only promotes plant growth, but also increases plant tolerance to environmental stresses including salinity. The main purpose of this research is to investigate the effect of P. indica on increasing salinity tolerance of corn. Materials and methods A greenhouse experiment in a completely randomized design with two factors including salinity (0, 100, 300 mM NaCl) and fungal inoculation (inoculation and non-inoculation) was conducted in a 2/1 (v/v) mixture of sterile sand and perlite. Salt stress was treated after 10 days of planting and continued for 8 weeks. After the plants finish their vegetative stage, root sampling was performed and percentage root colonization by the fungus and total biomass yield, that is, root dry weight and shoot fresh and dry weights were measured. Also, the concentration of some nutrients such as sodium (Na), potassium (K) and phosphorus (P) was determined. Results and discussion Microscopic investigations of inoculated roots with P. indica indicating high potential of this fungus to colonize roots of the studied plant, so that large amount of external hyphae from germinating spore was seen at the outer surface of the roots and the root cortex. The obtained results indicate that in sever salinity stress (300 mM NaCl), the shoot dry weight of inoculated plants with P. indica was approximately 90% higher than the non-inoculated control plants. Phosphorous content of the shoot part of plants with fungal symbiosis at the levels of 0 and 100 mM of NaCl was 8.5% and 12% greater than the control plants, respectively. Sodium content in the leaf of inoculated plants with P. indica was lower than the controls. Unlike sodium, increasing salinity resulted in decrease potassium content in the root.  Calculating K+/Na+ shows a decrease of this ratio in all treatments except shoot part of inoculated plants with the fungus.   P. indica significantly by increasing the absorption of water by plant cells which ultimately results in increased plant water potential, significantly inhibits deleterious effects of salinity on plant. In addition, through inducing the uptake of the necessary nutrients for plant and synthesizing plant growth promoting compounds, this fungus improves the morphological characteristics of plant such as fresh and dry weights of root and shoots as well as plant yield. Excess absorption of sodium ion by plant in saline environment increases the amount of this ion in plant root and shoots which ultimately disturb cell enzymatic and plasma membrane systems. So, low sodium concentration in the leaves of inoculated plants with P. indica and high amount of this ion in the roots in compare to the control plants shows that prevention of excess sodium movement to the leaves and its accumulation in the root is probably one of the possible mechanisms for plant growth promotion of this fungus. Also, high K+/Na+ ratio in the shoot of inoculated plants with P. indica in compare to the controls at different levels of stress demonstrate the tolerance of these plants to salinity. Conclusions This research revealed the positive effects of Piriformospora indica as an endophytic fungus on promoting plant growth. In addition to beneficial effects of P. indica as a growth-promoting fungus (GPF), P. indica inoculations ultimately lead to increase barley resistance to salt stress.