The effect of methyl jasmonate application on Rhizophagus intraradices symbiosis efficiency in alfalfa plant under water deficit stress

The arbuscular mycorrhizal symbiosis enhances plant tolerance to water deficit. Methyl jasmonate (MeJA) is a phytohormone related to multiple developmental and growth processes, which might play an important role in the mycorrhizal interaction. Hormonal regulation and the symbiotic relationship provide benefits for plants to overcome stress conditions. The aim of this study was to evaluate the effects of MeJA application and mycorrhizal symbiosis on some growth and biochemical properties of alfalfa plant under water deficit stress. A combined factorial design was performed with three factors: (1) plants non-inoculated (NM) or inoculated with the mycorrhizal fungus Rhizophagus intraradices (AM) (2) untreated plants and plants treated with 50 μm MeJA, and (3) soil moisture levels including field soil capacity (FC) and 55% FC. Half of the plants received a MeJA treatment through foliar spray 30 days after growth and the other half of the plants were considered as not- MeJA treated. Water deficit treatment was applied one week after hormone applying for four weeks. After harvest, root colonization percentage, dry weight of shoots and roots, total chlorophyll and carotenoids contents, soluble sugars and proline contents, as well as P and N contents, were assessed. MeJA application significantly increased total chlorophyll content of AM and NM plants at FC moisture by 76.6% and 106.6%, respectively. MeJA caused a significant increase only in chlorophyll content of NM plants under water deficit stress by 116.4%. Water deficit stress had no significant effect on carotenoids content. Obtained results indicated synergistic effect of the co-treatment of mycorrhiza and MeJA on carotenoids content at FC moisture. In addition, the content of total chlorophyll and carotenoids significantly were higher under water deficit stress in AM plants than NM plants. Dry weights of shoot and root of AM plants under all soil moisture and MeJA treatments were significantly higher than NM plants. Moreover, the application of MeJA augmented the positive effect of mycorrhizal colonization on shoot and root dry weights under both moisture levels. Shoot and root dry weights under water deficit stress and co-treatment of mycorrhiza and MeJA increased 3.4 and 2.8-folds, respectively, compared to NM plants. Mycorrhizal growth dependency (MGD) was increased by 86.9% under water deficit stress condition compared with the non-stressed condition. However, MGD was decreased significantly by MeJA application in stressed-plants. MeJA application and water deficit stress did not exhibit a significant effect on mycorrhizal colonization rate while they increased proline and soluble sugar production. Co-treatment of mycorrhiza and MeJA had a significant synergistic effect on proline accumulation in shoots and roots of stressed plants by 77.3% and 62.2% respectively compared with stressed NM plants. Furthermore, MeJA application caused a significant increase in N and P contents, and root to shoot ratio of soluble sugars of AM plants by 77.8%, 64.3% and 35.1% respectively at FC moisture level. MeJA application induced a significant change in carbohydrate allocation to roots in mycorrhizal plants and also decreased MGD of stressed plants. MeJA treatment and mycorrhizal symbiosis improved plant response to water deficit stress, and there was an interactive positive effect between MeJA and mycorrhizal fungi which alleviated growth impairment under water deficit conditions by modifying the physiological and biochemical properties of the host plant.