Effect of inoculation with Arbuscular mycorrhizal fungus Rhizophagus irregularis on auxin content in highly mycotrophic black medick under low phosphorus in soil

The investigation is focused on the elucidation of symbiotic effectiveness mechanisms of arbuscular mycorrhiza (AM) which is one of the most widespread symbiosis, developed between Glomeromycota fungi and 92 % of land plants. The role of auxin in regulation of plant development with AM fungi symbiosis might be considered as one of such mechanisms, since this plant hormone plays a key role in root development and accelerates spore germination, intensifies the infection process and subsequent growth of the hyphae after exogenous addition. This study is aimed to identify for the first time the dynamics of endogenous indole acetic acid (IAA) concentration in roots and leaves of strongly mycotrophic black medick line ( Medicago lupulina ) under conditions of low-phosphorus available for plants nutrition in the soil. Analyses were carried out in the course of plant development. On day 14 after sowing (DAS), at the stage of the first true leaf development, we observed primary infection, the formation of the first arbuscules (a, %) and vesicles in roots, and the beginning of lateral root formation and primary growth response to mycorrhization manifested in the development of above-ground plant parts. On DAS 21 (the second leaf stage) there was a significant growth response to mycorrhization and arbuscule development in roots. On DAS 35, at shooting stage, the arbuscule development and active development of vesicles in roots occurred. Finally, on DAS 50 (at flowering start) the vesicles development was more active. Analysis of mycorrhization level showed that arbuscules played a key role in the formation of effective symbiosis in black medick at the stages of the first and second leaf, and the a/(100 % - a) value increased to 8.0±0.7 and 18.5±1.3, respectively. At later stages of shooting and flowering, their role diminished as compared to the mycelium development in the roots. The efficiency of the inoculation with AM Rhizophagus irregularis symbiotic fungi tested by weight of aboveground parts significantly increased starting with the phase of the second leaf, and remained high until the end of the experiment (above 120 %). At the same time, the weight response was absent in roots until the phase of flowering. A 2.2-fold increase in IAA level in roots at the phase of the first leaf, i.e. at the earliest stages of AM symbiosis establishing, led to inhibition of the root growth. On the other hand, the 1.8-fold increase in endogenous IAA in leaves at the second leaf phase was preceded by the elevation of AM efficiency by weight for aboveground parts, and, therefore, a shift in IAA concentration significantly intensified development of the aboveground parts at the shooting and flowering.