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Morphological and ultrastructural characteristics of root nodules and rhizobium of Dalbergia odorifera
DENG Jiazhen, YE Shaoming, LIN Mingye, LAN Yahui, YAN Yu, FAN Rongyuan, PAN Cailing
JOURNAL OF NANJING FORESTRY UNIVERSITY ›› 2023, Vol. 47 ›› Issue (5) : 259-267.
PDF(3618 KB)
PDF(3618 KB)
Morphological and ultrastructural characteristics of root nodules and rhizobium of Dalbergia odorifera
【Objective】This study aims to elucidate the morphological characteristics, histological characteristics, and ultrastructure of Dalbergia odorifera root nodules and clarify the ultrastructure of rhizobium. The results from this study can provide an intuitive and precise primary data set for the classification of leguminous plant nodules.【Method】The paraffin sections, ultrathin sections and field emission scanning electron microscopy (FESEM) samples were all prepared from D. odorifera root nodules. Optical microscopy (OM) was used to observe the morphology and microscopic structure of nodules, while transmission electron microscopy (TEM) and FESEM were used to observe the ultrastructure of root nodule cells and rhizobium. Additionally, the leghemoglobin content of nodules in different periods was determined, and the relationship between root nodules structure and nitrogen fixation function was analyzed.【Result】(1) Morphological characteristics: from day one to day 60, the shape of D. odorifera root nodules gradually differentiated from round into oval, barbell and irregular oval, and the color changed from yellowish white to yellowish brown. The growing part of nodules gradually spread from the lateral root and base to the principal root. During the development of root nodules, the leghemoglobin content initially rose, then declined, with a peak at age of 3-5 d. (2) Histological traits: the root nodules of D. odorifera generally contained two to three infected areas. The vascular bundles were distributed around the infected areas, and the ringed vessels inside were connected by the end wall. (3) Ultrastructures: under TEM observation, the infected cells were significantly different from the uninfected cells. The volume of infected cells was enlarged without vacuoles and contained multiple vesicles with membranous structure. The vesicles were covered by bacteroids, which were differentiated from rhizobium, and organelles such as mitochondria and endoplasmic reticulum were squeezed to the edge of the cell wall. The uninfected cells were almost filled with vacuoles, and the cell wall was thick and uneven. Starch grains and some plastids existed around the nucleus. The morphologically diverse bacteroids have cell membranes and walls and contain polyphosphate (PP) particles and poly-β-hydroxybutyrate (PHB) particles. There were two membrane synthesis states in the infected cells of D. odorifera root nodules. In one, the membrane vesicles participated in the synthesis of peribacteroid membranes; in the other, a flocculent membrane appeared near the peribacteroid membranes and allowed the peribacteroid membrane to dissociate. FESEM showed a large number of long rod-like rhizobium intertwined and multiple peribacteroid membranes bonded together in a continuous shape to form a tight membrane system.【Conclusion】The D. odorifera root nodules are “determinate nodules”. The nodule age can significantly affect the leghemoglobin content in D. odorifera root nodules. The D. odorifera root nodules contain well-developed vascular tissues, and there are a large number of bacteroids that contain PP and PHB particles with an energy storage function as well as a strong peribacteroid membrane system. These structures provide supports to the nitrogen fixation function of D. odorifera root nodules.
Dalbergia odorifera / root nodule / rhizobium / ultrastructure / bacteroid / symbiotic nitrogen fixation
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Legume root nodules are induced by N-fixing rhizobium bacteria that are hosted in an intracellular manner. These nodules are formed by reprogramming differentiated root cells. The model legume Medicago truncatula forms indeterminate nodules with a meristem at their apex. This organ grows by the activity of the meristem that adds cells to the different nodule tissues. In Medicago sativa it has been shown that the nodule meristem is derived from the root middle cortex. During nodule initiation, inner cortical cells and pericycle cells are also mitotically activated. However, whether and how these cells contribute to the mature nodule has not been studied. Here, we produce a nodule fate map that precisely describes the origin of the different nodule tissues based on sequential longitudinal sections and on the use of marker genes that allow the distinction of cells originating from different root tissues. We show that nodule meristem originates from the third cortical layer, while several cell layers of the base of the nodule are directly formed from cells of the inner cortical layers, root endodermis and pericycle. The latter two differentiate into the uninfected tissues that are located at the base of the mature nodule, whereas the cells derived from the inner cortical cell layers form about eight cell layers of infected cells. This nodule fate map has then been used to re-analyse several mutant nodule phenotypes. This showed, among other things, that intracellular release of rhizobia in primordium cells and meristem daughter cells are regulated in a different manner.
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