CC BY
0NODULE FORMATION ON SOYBEAN ROOTS
Umarov B.R.
Head of the Biotechnology Lab. of Tashkent research institute of vaccines and serums https://doi org/10.5281/zenodo. 13834943
Abstract. The article is devoted to the formation of nodules in legume plants, their effect on the ability to absorb nitrogen from the atmosphere. Nodule nitrogen-fixing bacteria penetrate through the root hairs into the young roots, cause them to form a new organ (nodules) and complex biochemical processes occur in the nodules - nitrification - the transfer of nitrogen contained in the air into a mineral form accessible to plants. Plants, in turn, provide bacteria with a special habitat in which there is no competition with other types of soil bacteria.
Keywords: protein, symbiosis, bacteria, nitrogen fixation, rhizobia, morphogenesis, nodules.
Root nodules are a new organ formed on the roots of some plants caused by the activity of nitrogen-fixing bacteria. Nodule bacteria absorb atmospheric nitrogen and mineralize it, saturating the soil with it. That is why, in soils with a lack of nitrogen, plants often enter into symbiosis with certain strains of bacteria to maintain mineral balance and activate plant growth. The most studied at the moment are nodules formed on the roots of leguminous plants. During the vital activity of bacteria, free nitrogen from the atmosphere is converted into nodules to produce ammonium. And that, in turn, in the process of further synthesis becomes part of other organic compounds, such as vitamins, nucleotides, flavonoids, amino acids, etc. [1]. Due to the symbiotic bonds with nitrogen-fixing bacteria, legumes are the best partners. At the same time, the plant itself controls the development of nodule bacteria. If the soil is sufficiently saturated with nitrogen, the development of nodules is suspended.
Nodule bacteria, along with other genera, include the genera Rhizobium, Bradyrhizobium, Azorhizobium and are referred to by the common name rhizobia. Rhizobia are strictly aerobic gram-negative rods that live in soil and grow heterotrophic in the presence of organic compounds [2]. Such nodules are found mainly in the root system of legumes that form symbiotic bonds with nodule bacteria. Some species are also able to grow autotrophic in the presence of H2, although at a lower rate [3]. Stimulation of nodule development also does not cause difficulties. The energy necessary for the vital activity of nitrogen fixers is produced as a result of the oxidation of photosynthetic products (sugars), which are abundantly present in the foliage. The carbon obtained as a result of the decomposition of sugars is a nutrient medium for bacteria. Another important element required for nitrogen conversion is oxygen. To attract and retain oxygen, bacteria contain a legoglobin-binding protein. A similar protein (myoglobin) is found in animal tissues and is used for cellular respiration [4].
The introduction of rhizobia into the host plant is a controlled infection. The molecular basis of specificity and recognition is currently being actively studied. Rhizobium is formed by species-specific factors of nodule formation. These are lipochitooligosaccharides that acquire high structural specificity, for example, during acylation, acetylation, and sulfation. Nod factors bind to specific receptor kinases of the host, which are part of signal transduction chains. Acylation, acetylation, and sulfation induce twisting of the root hair and cell division of the root cortex of the host, which leads to the formation of a nodule primordium. After the rhizobia penetrate into the
root hair, an infectious thread is formed, which spreads into the root bark, branches there and infects the cells of the nodule primordium [5].
The nodule develops from an infectious thread. The morphogenesis of the nodule is characterized by the same high degree of complexity as the root or shoots. Nodules connect to the root through vascular tissues, which supply them with substances formed during photosynthesis. Bacteria that are incorporated into a plant cell are surrounded by a peribacteroid membrane formed by the plant. Thus, the bacteria are included in the cytoplasm of the plant cell in the so-called symbiosome. In the symbiosome, rhizobia differentiate into bacteroids. The volume of these bacteroids can be 10 times the volume of individual bacteria. The peribacteriodic membrane may surround not one, but several bacteroids. Rhizobia has a respiratory chain, the complexes of which correspond to the main protein complexes of the mitochondrial respiratory steppe. In the Bradyrhizobium species, an additional electron transport pathway is formed during the differentiation of free-moving bacteria into bacteroids. This pathway branches off at the point of the cytochrome-b/c1 complex of the respiratory chain and transfers electrons to another terminal oxidase, which helps to increase the intensity of respiration. The corresponding gene is located in the pSym plasmid, which controls the formation of symbiosis.
Rhizobia capable of entering into symbiosis contain a large number of genes that are "disabled" in free-living bacteria and are activated only after interacting with the host to contribute to the formation of a nitrogen-fixing nodule. Bacterial genes that encode proteins necessary for the fixation of N2 are called nif genes, and genes that induce the formation of a nitrogen-fixing nodule are called nod- genes. The signal of the host plant's readiness to form nodules is the release of certain flavonoids as signaling compounds. These flavonoids bind to a bacterial protein that is encoded by the constructive nod gene. The protein to which the flavonoid binds activates the transcription of other nod genes. The proteins encoded by these nod genes are included in the synthesis of the above-mentioned Nod factors. Four so-called "common" nod genes are present in all ribosias [6]. In addition, more than 20 other nod genes responsible for host recognition specificity are known. Proteins that are especially needed for nodule formation and that are synthesized by the host plant during nodule formation are called nodulins. Nodulins include leghemoglobin, enzymes that decompose carbohydrates, enzymes of the citric acid cycle and the synthesis of glutamine and asparagine, as well as the synthesis of ureides in certain cases. These include aquaporin of the peribacteroid membrane. The plant genes encoding these proteins are called nodulin genes. They are divided into "early" and "late" nodulins. Early nodulins are activated during infection and nodule formation, where the expression of the corresponding genes is induced in part by signaling compounds secreted by rhizobia. Late nodulins are synthesized only after nodule formation. In many cases, nodulins are isoforms of proteins found in other plant tissues. The main substrate that the cells of the host plant supply to the bacteroids is malate, formed from sucrose supplied to the nodule by phloem. Sucrose is metabolized by sucrose synthase, decomposes during glycolysis to phosphoenolpyruvate, which is carboxylated into oxaloacetate, and the latter is reduced to malate. Nodule cells have high phosphoenolpyruvate carboxylase activity. NH4+ as a nitrogen fixation product is supplied to the host cell, where it is then converted mainly into glutamine and asparagine, and then transported through xylem vessels to other parts of the plant. It has recently been shown that alanine can also be exported from bacteria. From the nodules of some legumes, fixed nitrogen is exported in the form of ureides, mainly allantoin and allantoic acid. The nitrogen/carbon ratio is particularly high in these compounds. The formation
of ureides in the cells of the host plant requires a complex synthesis pathway. First, inosine monophosphate is synthesized during the synthesis of purine, which is available in all cells for the synthesis of AMP and GMF, and then it decomposes through the formation of xanthine and uric acid to the above-mentioned ureides. Malate entering the bacteroids is oxidized in the Krebs cycle. The reducing equivalents generated in this way serve as "fuel" for nitrogen fixation [7].
Discussion.
Previously, it was believed that the formation of nodules in legumes is caused by plant disease. In 1888, Hermann Hellriegel and G. Wilfart Wilfahrt H showed that legumes with nodules were able to grow without nitrogen fertilizers [8]. Root nodules in representatives of the legume family were formed at least three times during evolution and are rarely found outside this taxon. The tendency of these plants to develop root nodules is most likely related to the structure of the root. In particular, the tendency to develop lateral roots in response to abscisic acid may contribute to the later evolution of root nodules. The roots of legumes secrete flavonoids, which induce the production of nod factors in bacteria.
When this factor is recognized by the root, a number of morphological and biochemical changes occur: cell divisions are initiated in the root to create a nodule, and the growth trajectory of the root hair changes so that it envelops the bacterium until it is completely encapsulated. Encapsulated bacteria divide several times to form a micro colony. From this colony, bacterial cells enter the developing nodule using a structure called an infectious thread. It grows through the root hair up to the basal part of the epidermis cell, and then to the center of the root. Then the bacterial cells are surrounded by the membrane of the plant root cells and differentiate into bacteroids capable of fixing nitrogen. Normal nodule formation takes approximately four weeks after planting the plant. The size and shape of the nodules depends on the type of plant that was planted. So, soybeans or peanuts will have larger nodules than forage legumes (red clover, alfalfa). By visually analyzing the number of nodules, as well as their color, scientists can determine the effectiveness of nitrogen fixation by the plant. Nodule formation is controlled by both external processes (heat, soil pH, drought, nitrate levels) and internal processes (autoregulation of nodule formation, ethylene).
The autoregulation of nodule formation controls the number of nodules in the plant through processes in which the leaves participate. The leaf tissue senses the early stages of nodulation through an unknown chemical signal, and then restricts the further development of the nodule in the developing root tissue. Leucine-rich Repeat Kinases (Leucine Rich Repeat Kinases LR) of receptor kinases (NARK in soybeans (Glycine max); HAR1 in Lotus japonicas, SUNN in Medicago truncatula) are involved in the autoregulation of nodulation. [9]. Mutations leading to loss of function of these receptor kinases lead to an increased level of nodulation. Root growth abnormalities are often accompanied by a loss of activity of the discussed receptor kinases, which indicates a functional relationship between nodule growth and root growth.
Nodule formation and nitrogen fixation processes have long been known for their agricultural benefits and ability to improve soil health. Recent work on the identification of genes and signals involved in nodule formation has significantly expanded our understanding of these processes and may provide targets for plant breeding and engineering programs aimed at developing legume varieties and possibly even non-legume crops with improved nitrogen absorption capacity.
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