Strigolactones as Regulators of Symbiotrophy of Plants and Microorganisms

Strigolactones (SLs) are signal molecules of a butenolide nature from the apocarotenoids family that are synthesized by plant organisms. They are widely propagated in nature and control different aspects of plant development, which allows us to consider them as phytohormones. It is known that SLs production and release into the rhizosphere are enhanced under a deficit of basic elements of plant nutrition. In addition, these compounds carry out the signaling function in the course of establishing supraorganismal relationships upon parasitism and symbiosis. In this review, the role of SLs in the formation of arbuscular mycorrhiza (AM), which provides a symbiotic pathway of P_i uptake by plants and partly nitrogen-fixing legume nodules, is examined. The data on SLs impact on AM fungi (AMFs) development at presymbiotic and symbiotic stages of plant growth were generalized. The main peculiarities of SLs structure providing their efficiency as “branching factors” of AMF hyphae and basic mechanisms of the possible influence of SLs on AMFs, including stimulation of mitochondria biogenesis, were described. Analysis of these data and phenotypes of plant mutants with disturbances in SLs biosynthesis and its regulation and also SLs perception and transport leads to the conclusion that their role in AM development is predominantly revealed in induction of AMF hyphae branching at the presymbiotic plant growth stage and is associated with plant response to P_i deficit. Analysis of the role these components of common symbiotic signaling cascade play in the regulation of SLs biosynthesis in the course of AM development and nitrogen-fixing legumes nodules was carried out. An important role of SLs in nodule development likely caused by their endogenous influence on its organogenesis was demonstrated. A possibility of the presence of different pathways usable by a plant upon AM and nodule development for activation of common symbiotic transcriptional factors, NSP1 and NSP2, participating in the regulation of SLs biosynthesis is discussed. The data concerning structural specificity of SLs and the results of phylogenetic analysis of genes encoding different components of SLs biosynthesis and signaling pathways and also symbiotic signaling cascade in plants provide evidence for the possible transformation of SLs signaling function from the hormonal one manifested inside the plant into the communicative one providing the establishment of interorganismal relations.