Vol 17, No 1 (2019)

The modern stage of development of symbiogenetics, a biological discipline that addresses the formation of super-species genetic systems, is associated with the study of molecular mechanisms and environmental consequences of combining the hereditary factors of prokaryotes and eukaryotes into functionally integrated symbiogenomes, which, as partners lose their ability to autonomous existence, are transformed into structurally integrated hologenomes. The loss by intracellular symbionts of eukaryotes of their genetic individuality, determined by the ability to independently maintain and express the genome, representing a key step in symbiogenesis which results in the transformation of bacteria into cellular organelles. Genetic reconstruction of symbiogenesis provides the broad prospects for its artificial reproduction aimed at the synthesis of new organisms and biosystems possessing the predetermined sets of practically significant features.

The ubiquity of endophytic microorganisms is an accepted fact nowadays and the possibility of using it in agriculture keeps attracting attention of scientific community. In contrast to rhizospheric (living on root surface) and phyllospheric (colonizing aerial parts of plants) members of plant-microbial interactions endophytes are able to establish closer relationships with host-plant, in some cases strongly influencing its phenotype, bringing benefits. However, these microorganisms do not form any specific structures like nodules in case of symbiosis between legumes and rhizobium bacteria. Having a great amount of functions including phytohormone level modulation, vitamins production and nutrient supply improving, endophytes could serve as a basis for biofertilizer, which could potentially minimize the necessity of mineral fertilizers, thus reducing the negative impact of the latter on soil fertility, biodiversity and human health. Our main aim here is to highlight the question of functional significance of endophytes and endophytic bacteria in particular, as well as the way of its application in agriculture and to identify key points in understanding biology of these organisms. In this review we will consider such aspects of plant-endophytic symbiosis as biodiversity of legume and non-legume endophytes, ecology of endophytes and some ways which are commonly in use by studying these microorganisms.

For the first time, bacteria were isolated and identified from the root nodules of a wild-growing medicinal legume plant Thermopsis lanceolata, originated from Mongolia. The taxonomic position of 14 isolates obtained was determined using of sequencing of the 16S rRNA (rrs) and atpD genes. It was shown a significant biodiversity of the isolates from T. lanceolata, which belonged to three genera of the order Rhizobiales: Phyllobacterium (family Phyllobacteriaceae), Rhizobium (family Rhizobiaceae) and Bosea (family Bradyrhizobiaceae). Six isolates belonged to the species Phyllobacterium zundukense and Phyllobacterium trifolii (100 и 99,9% rrs similarity with the type strains P. zundukense Tri-48^T and P. trifolii PETP02^T, respectivelly), three isolates were identified as Rhizobium anhuiense (99,8% rrs similarity with the type strain R. anhuiense CCBAU 23252^T). Two slow-growing isolates of the genus Bosea Tla-534 and Tla-545 may potentially belong to new species, since their rrs-similarity to the closest type strains B. massiliensis LMG 26221^T, B. lathyri LMG 26379^T and B. vaviloviae Vaf18T was 98,5-99,0%. Non-rhizobial strains were not isolated. The isolation and future investigation of the rhizobial microsymbionts of the valuable medicinal legume Thermopsis lanceolata is one of the necessary prerequisites for its industrial cultivation.

Background. The transcription factor CYCLOPS/IPD3 is a key activator of the organogenesis of symbiotic nodules. Its participation in the development of infection threads and symbiosomes is also shown. In pea, three mutant alleles were identified for this gene (sym33-1 – sym33-3). The phenotypic manifestations of the sym33-3 allele of the SGEFix¯-2 mutant, characterized by a “leaky” phenotype (the formation of two types of nodules: white and pinkish) were the most studied. The sym33-2 allele in the mutant SGEFix¯-5 was described as a strong allele, however, its phenotypic manifestations have not been studied in detail.

Materials and methods. In this study, the histological and ultrastructural nodule organization of the SGEFix¯-5 mutant was analyzed using confocal laser scanning microscopy and transmission electron microscopy.

Results. In the nodules “locked” infection threads were observed, from which no bacteria release into the cytoplasm of the plant cell occurs. In this case, in some infection threads, bacteria were degraded, which may indicate the activation of strong defense reactions in the nodules of the SGEFix¯-5 mutant.

Conclusions. The sym33-2 allele in the mutant SGEFix¯-5 is a strong allele, which triggers the severe defense reactions, when rhizobia are already perceived as pathogens in infection threads.

The review is aimed to analyze molecular mechanisms of carbohydrate transport during the formation of arbuscular mycorrhiza (AM), a widespread symbiosis of plants with Glomeromycotina subdivision fungi. Due to AM-symbiosis, plants receive microelements, mainly phosphorus, and fungi are supplied by products of carbon assimilation. The study of sugar transport mechanisms in plants as well as between plants and symbiont is methodologically difficult because of the obligatory status of AM fungi. The mechanisms of carbohydrate transport in leaf and root cells are concerned, particular interest is paid to transporters, specific to AM structures. Several resumptive schemes are designed. SWEET family of transporters (Sugars Will Eventually be Exported Transporters), including AM-specific uniporters are reviewed. We summarize results on expression of genes encoding transporter in cells of plants without AM, in AM-plant cells with arbuscules and AM-plant cells without arbuscules. The data on genes of MST proteins family (Monosaccharide Transporters) participating in direct transport of sugars from the soil to the foliar mycelium of AM fungi are considered.