ساز و کار جذب و انتقال فسفر در گیاهان همزیست با قارچ‌های میکوریزا آربسکولار (شناخته‌ها و ناشناخته‌ها)

Background and Objectives: The many advantages of establishing symbiosis between arbuscular mycorrhizal (AM) fungi and most plants, including agricultural and horticultural plants, especially in terms of the growth and nutritional components of the host plant, have given special importance to this symbiosis. Increasing CO² fixation by the host plants, they participate in the global carbon cycle, and as a result, they increase the organic carbon stock of the soil and in a way participate in the control of global temperature increase. It is now well documented that AM fungi improve mineral nutrition, particularly phosphorus (P) nutrition of the host plant. This beneficial effect of AM symbiosis is due primarily to enhanced P uptake by mycorrhizal roots. Mycorrhizal colonization can also improve the phosphorus nutrition of the host plant under abiotic and biotic stresses such as drought, salinity, oil pollutants, heavy metals and plant diseases. The synergistic effects between mycorrhizal fungi and a special group of useful soil microorganisms such as nitrogen fixing bacteria and phosphate solubilizing bacteria will lead to the improvement of plant phosphorus nutrition and ultimately lead to an increase in plant growth components. The basis of this symbiosis is the two-way exchange of nutrients that takes place in a unit consisting of the arbuscule and the host plant cell as "the central unit of symbiosis or the heart of symbiosis". Although the results of studies show the beneficial role of these fungi, due to the specific complexities of symbiotic relationships, there is still no complete understanding of the mechanisms of this symbiosis. Thus, extensive studies have been conducted with physiological, biochemical and molecular approaches. However, our understanding of the mechanism of this transfer, the exact route of this transfer and the transporters involved in transferring P to the host plant are not well known. The objective of this article is to review the new findings of P uptake and transport mechanisms in AM plants. It is focused particularly on the routes of P transfer, the contribution of each of these two symbionts in P uptake and transfer it and the mechanisms involved. With the advancement of this knowledge and a complete understanding of the hidden concepts of the fungus-plant relationship, it is possible to develop strategies for plant products by increasing the productivity of this symbiosis, especially in areas facing abiotic and biotic stresses.




Materials and Methods: The materials and methods used in the studies of mycorrhizal symbiosis including the mechanisms of phosphorus uptake and transfer and other related topics, are very diverse, and only a few of them are mentioned here. The use of labelled phosphorus and a compartment culture system has been used to determine the contribution of each of the two symbionts in the uptake and transfer of P. In genomic studies (Gene expression analysis), quantitative PCR have been used to determine DNA and RNA. Fluorescence microscope and 4,6-diamidino-2-phenylindole (DAPI) staining method are used to visible polyphosphates in arbuscules (young arbuscules, mature arbuscules and degenerated arbuscules) and intraradical hyphae of AM fungi.




Results: The findings of this review show that the P uptake and transport in AM plants takes place through two different pathways, one directly through the plant root and the other indirectly through the external hyphae of the fungus. These two paths interact with each other in a complex and sometimes unknown way. Physiological approaches using labelled phosphorus to trace the relative contribution of direct and fungal pathways in plant P nutrition show that the contribution of the fungal pathway varies from negligible to almost all plant phosphorus. The reported results indicate that not only the percentage of root colonization, but also the total length of external hyphae and phosphorus concentration are very important in determining the contribution of direct and indirect pathways. Transporters involved in P uptake and transport belong to several families. The PHT family with 5 subfamilies (PHT1, PHT2, PHT3, PHT4, and PHT5) based on their sequence and location has a vital role in P transport. Among these transporters, the PHT1 transporters play an important role in P uptake from the rhizosphere, its distribution and homeostasis. The results of molecular studies show that when Arabidopsis thaliana receives enough P, at least 4 transporters (PHT1,1-4) are involved in P uptake. Among these four transporters, PHT1,1 is the transporter that contributes the most to the uptake and transfer of P from the roots to the leaves of the plant. Most of the P absorbed in the tonoplast is polymerized and a linear chain of polyphosphate with high-energy bonds that can reach the number of 3 to thousands of units is formed inside the vacuole. Polyphosphates are transported mainly through a cytoplasmic stream along a tubular vacuole system towards the intraradical hyphae. How and by what molecular mechanism P is delivered to the host plant is not well known. However, three hypothetical pathways for P delivery to the plant have been proposed.




Conclusion: A set of plant and fungal transporters are responsible for absorbing, translocation, distributing, accumulating and redistributing P (P homeostasis) in a mycorrhizal plant in an interconnected, precise, complex and sometimes unknown process. However, the mechanism of this transfer, the interaction of plant and fungal transporters in the absorption and transfer of P and its delivery to the host plant not well known. Much work conducted focusing on biomolecular and physiological studies to better understand these mechanisms. Although many advances have been made to elucidate the complex mechanisms for the integrated roles of nutrient transport in AM symbiosis, much research work needs to be done to improve our understanding of these mechanisms and to answer the key questions. With the progress of this knowledge and a complete understanding of the complex relationships between fungi and plants, it is possible to develop plant product strategies by increasing the efficiency of this symbiosis, especially in areas facing biotic and abiotic stresses.