Abstract
Vacuoles play very important physiological roles in plant cells. Pea root nodules, which exhibit distinct zonation (meristematic zone and central tissue zones), may serve as a good experimental model for the investigations of vacuole development and its importance to cell and tissue functioning. Moreover, the nodule central tissue is composed of both infected and uninfected cells which play different physiological roles and differ in the level of vacuolation. Cytological observations revealed that central vacuoles of the infected cells of the effective nodules expand toward cell walls. Thus only thin layers of the cytoplasm separate each central vacuole from plasma membrane and cell wall. This finding is discussed from the viewpoint of improved exchange of solutes and water between the central vacuole and apoplast of the infected cell. Three-dimensional reconstruction of the vacuoles of infected cells within a fragment of effective nodule central tissue, showed their spatial arrangement. Possible advantages coming from the spatial arrangement of vacuoles within the central tissue are discussed. A comparative study of the central tissue (bacteroidal tissue) and meristem vacuolation of the effective and ineffective pea root nodules is also presented. Morphometric measurements revealed that the effective nodule central tissue was more vacuolated than the ineffective one. It was proved that maturation of the infected cells involves dynamic changes in their vacuolation. Having numerous fixing nitrogen bacteroids, the infected cells of effective central tissue were less vacuolated than uninfected cells. On the other hand, both infected and uninfected cells of the effective central tissue showed a much higher level of vacuolation in nitrogen-fixing zone than cells of the same type in ineffective tissue. These results indicate that vacuolation is an important factor in development and functioning of pea root nodule central tissue.
Highlights
The root nodule of leguminous plants is a unique organ, which develops as a result of many interactions between host plant and rizobia
This paper describes for the first time the expansion of the central vacuole of the infected cells toward plasma membrane
Divisions of meristematic cells led to the formation of nodule central tissue
Summary
The root nodule of leguminous plants is a unique organ, which develops as a result of many interactions between host plant and rizobia. The meristematic region of the nodule produces cells, which undergo infection by the bacteria released from infection threads, or become uninfected. Bacteria released from the infection threads differentiate into nitrogen fixing bacteroids which form the so-called symbiosomes together with surrounding them peribacteroidal membranes. Up to 20 thousands symbiosomes within one infected cell can be regarded as a new cytoplasmic compartment (Verma and Stanley 1985). Both infected and uninfected cells form the so-called central tissue (Vasse et al 1990; Hirsch 1992; see Fig. 1) which is named bacteroidal tissue. The infected cells of pea nodules enlarge because of substantial enlargement of their cytoplasm (including symbiosomes) and vacuome volumes (Borucki 1997). The pattern of the infected cell enlargement is not typical as plant cell enlargement is usually a consequence of vacuome enlargement rather than that of cytoplasm (Maeshima 1990; Streeter 1992)
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