Molecular Soil Biology 2024, Vol.15, No.3, 129-139 http://bioscipublisher.com/index.php/msb 131 Figure 1 Stages and regulatory networks of indeterminate nodule organogenesis in Medicago truncatula (Adopted from Yang et al., 2021) Image caption: (A) The structure of the M. truncatula root tip. The SHORT ROOT – SCARECROW (SHR–SCR) module is constitutively present in the pericycle, cortex, and epidermis, where it predisposes legume root cells, especially cortical cells, to divide. (B) Priming. The earliest anticlinal cell divisions are induced by NIN and downstream genes, which are activated by a transcriptional complex consisting of IPD3, DELLA, NSPs, and NF-Ys. These divisions are observed in pericycle cells in the epidermis in response to infection. The SHR–SCR module is strongly upregulated by symbiosis signals and NIN. IPT and LOG cytokinin (CK) biosynthesis genes are upregulated by the induction of KNOX genes. CK levels are sufficient to promote NIN expression via the CK receptor CRE1 and establish a negative feedback loop with CK oxidase. (C) Initiation. The inner cortex (C4/C5) cells undergo anticlinal and periclinal divisions, followed by anticlinal divisions in the middle cortex (C3). The expression of NIN and downstream genes in the nodule primordium and nodule apical meristem (NAM) is critical for these cell divisions. (D) Outgrowth. The epidermis cells fall off, and numerous cell divisions in the cortex result in the formation of a nodule primordium on the root, with an activated NAM that persistently divides and differentiates to enlarge the nodule. (E) Maturation. The organ forms a peripheral nodule vascular bundle (NVB) system and an infection zone containing bacteroids. Although cell divisions begin in the pericycle and endodermis, most cells in mature nodules are derived from cortical cells (Adopted from Yang et al., 2021) 3 Ecological and Agricultural Benefits 3.1 Enhancement of soil nitrogen levels and fertility Rhizobiumbacteria play a crucial role in enhancing soil nitrogen levels through the process of biological nitrogen fixation. This symbiotic relationship between Rhizobium and leguminous plants allows for the conversion of atmospheric nitrogen into a form that plants can utilize, significantly improving soil fertility. For instance, studies have shown that inoculation with effective Rhizobiumstrains can lead to substantial increases in nitrogen fixation and nutrient uptake in crops like Vicia faba, resulting in improved soil nitrogen balance (Allito et al., 2020). Additionally, the presence of Rhizobiumin the rhizosphere can enhance the availability of other essential nutrients, such as phosphorus, further contributing to soil fertility (Mabrouk et al., 2018). 3.2 Reduction in the need for synthetic nitrogen fertilizers The use of Rhizobium in legume cultivation can significantly reduce the dependency on synthetic nitrogen fertilizers. This is particularly important given the environmental and economic costs associated with the production and application of these fertilizers. By enhancing biological nitrogen fixation, Rhizobium bacteria
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