Abstract
Plant–microbe interactions, including those of arbuscular mycorrhiza (AM), have been investigated for a wide spectrum of model plants. The present study focuses on an analysis of gene expression that encodes phosphate and sugar transporters and carbohydrate metabolic enzymes in a new model plant, the highly mycotrophic Medicago lupulina MLS-1 line under conditions of phosphorus deficiency and inoculation with Rhizophagus irregularis. Expression profiles were detected by RT-PCR at six plant stages of development (second leaf, third leaf, shooting, axillary shoot branching initiation, axillary shoot branching, flowering initiation). In comparison to control (without AM), the variant with AM inoculation exhibited a significant elevation of transcription levels of carbohydrate metabolic enzymes (MlSUS, MlHXK1) and sucrose transporters (MlSUC4) in M. lupulina leaves at the shooting stage. We suggest that this leads to a significant increase in the frequency of AM infection, an abundance of mycelium in roots and an increase in AM efficiency (which is calculated by the fresh weight of aerial parts and roots at the axillary shoot branching initiation stage). In roots, the specificity of MlPT4 and MlATP1 gene expressions were revealed for effective AM symbiosis. The level of MlPT4 transcripts in AM roots increased more than tenfold in comparison to that of non-specific MlPT1 and MlPT2. For the first time, MlPT1 expression was shown to increase sharply against MlPT2 in M. lupulina roots without AM at the shooting initiation stage. A significant increase in MlRUB expression was revealed at late stages in the host plant’s development, during axillary shoot branching and flowering initiation. The opposite changes characterized MlHXK1 expression. Alteration in MlHXK1 gene transcription was the same, but was more pronounced in roots. The obtained results indicate the importance of genes that encode phosphate transporters and the enzymes of carbohydrate metabolism for effective AM development at the shooting stage in the host plant.
Highlights
More than 80% of plant species can form arbuscular mycorrhiza (AM) with the Glomeromycotina subdivision of the Mucoromycota division in native ecosystems [1]
The analysis of mycorrhization parameters of the M. lupulina MLS-1 plant line with a highly intensive response to AM inoculation showed that the frequency of mycorrhizal infection (F) even at the second true leaf stage of plant development (2L)
This study was the first to analyze the expression of phosphate transport and carbohydrate transport, as well as carbohydrate metabolism in genes in M. lupulina AM plants that formed highly effective symbiosis with R. irregularis under conditions of Pi deficiency
Summary
More than 80% of plant species can form arbuscular mycorrhiza (AM) with the Glomeromycotina subdivision of the Mucoromycota division in native ecosystems [1]. The investigation of the mechanisms responsible for the symbiotic efficiency of plant interaction with AM fungi has both theoretical and practical importance. In AM symbiosis, the host plant supplies AM fungus with photosynthates in the form of hexose [2]. The plant, in turn, receives water and a number of macro- and microelements, especially phosphorus [5]. AM efficiency depends on the intensity of phosphate and carbohydrate transport processes between the partners. Alteration in the carbohydrate/mineral nutrient exchange might convert the plant–microbial interaction from a mutualistic to a parasitic one, which might affect the intensity of host plant development [5,6,7]
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