Abstract
The tubulin cytoskeleton plays an important role in establishing legume–rhizobial symbiosis at all stages of its development. Previously, tubulin cytoskeleton organization was studied in detail in the indeterminate nodules of two legume species, Pisum sativum and Medicago truncatula. General as well as species-specific patterns were revealed. To further the understanding of the formation of general and species-specific microtubule patterns in indeterminate nodules, the tubulin cytoskeleton organization was studied in three legume species (Vicia sativa, Galega orientalis, and Cicer arietinum). It is shown that these species differ in the shape and size of rhizobial cells (bacteroids). Immunolocalization of microtubules revealed the universality of cortical and endoplasmic microtubule organization in the meristematic cells, infected cells of the infection zone, and uninfected cells in nodules of the three species. However, there are differences in the endoplasmic microtubule organization in nitrogen-fixing cells among the species, as confirmed by quantitative analysis. It appears that the differences are linked to bacteroid morphology (both shape and size).
Highlights
The symbiotic interactions between legumes and rhizobia culminate in the formation of nitrogen-fixing root nodules [1]
Analysis of the ultrastructural organization of the nodules and confocal images of isolated bacteroids demonstrated that bacteroids of G. orientalis and V. sativa belong to the E morphotype (Figure 1B–D,F–H), whereas bacteroids of C. arietinum belong to the
The smallest was observed in C. arietinum, V. sativa bacteroids were of intermediate size, and the largest was G. orientalis bacteroids, reaching 6–7 μm in
Summary
The symbiotic interactions between legumes and rhizobia culminate in the formation of nitrogen-fixing root nodules [1]. In response to flavonoids produced by legume roots, rhizobia secrete specific lipochitooligosaccharides, known as Nod factors, which trigger nodule development [2]. When an infection thread reaches a primordium, an infection droplet lacking a cell wall is formed. Indeterminate nodule development is accompanied by the cell differentiation of both symbiotic partners [7]. The significant increase in the volume of a nodule cell allows it to host up to 50,000 differentiated rhizobial cells (bacteroids) [9]. The bacteroids in legumes in the inverted repeat-lacking clade (IRLC) undergo terminal differentiation, which is triggered by nodule-specific cysteine-rich (NCR) peptides [10]
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