Ultrastructure and functioning of the transport system of the leguminous root nodule

Abstract

The structure of the vascular tissues of nitrogen-fixing nodules of 27 genera of legumes and some non-legumes has been investigated by light microscopy. Pisum and Trifolium nodules have been examined by electron microscopy.Attention is directed to the presence of a pericycle in the vascular bundles of the nodules. In 7 of the legumes the pericycle cells possess a wall labyrinth consisting of branched filiform protuberances. The ultrastructure of the pericycle cell cytoplasm is described: its most striking feature is its abundant rough endoplasmic reticulum. These cells surround the xylem and phloem of the bundles, and are in turn surrounded by a layer of endodermal cells with Casparian strips. The pericycle cells develop their wall labyrinth in the levels of the nodule at which the bacterial tissue becomes pigmented; in nodule senescence their cytoplasm is disrupted level with the breakdown of the bacterial tissue.A pathway for symplastic lateral transfer of assimilates exists, from the sieve elements through the pericycle, endodermis and cortex to the bacterial tissue. The apoplast within the endodermis consists largely of the pericycle wall labyrinth and the xylem. The ultrastructure of the Casparian strip resembles that of roots.Intact, detached nodules can be induced to bleed a fluid from their severed vascular tissue. This fluid is exceptionally rich in organic nitrogen, particularly amides, but does not appear to contain sugars. Comparison between its amino acid composition and that of other parts of the nodule suggests that an active uptake or secretion of nitrogenous compounds precedes export from the nodule. Special functions are suggested for the nodule endodermis and the pericycle cells in this export process.

Developmentally Regulated Expression of the Gene Family for Cytosolic Glutamine Synthetase in Pisum sativum

“…The remaining ammonia might move out of the bacteroid containing cells to be assimilated elsewhere by the products of the "twin" GS genes which may function in cells specialized for intercellular transport (e.g. nodule "transfer" cells) (18). It is noteworthy that "transfer" cells specialized for metabolite export also occur in cotyledons of germinating seedlings (19), a context where "twin" GS gene expression is also induced in pea.…”

Poly-β-hydroxybutyrate Utilization by Soybean (Glycine max Merr.) Nodules and Assessment of Its Role in Maintenance of Nitrogenase Activity

“…The work of Pate et al (28) has revealed that the bleeding sap of excised nodules from white clover contain an unusually high concentration of asparagine, aspartic acid, glutamine, and glutamic acid. These high concentrations of amides and amino acids were restricted to conditions where fixed nitrogen was being exported from portions of nodules where N2 fixation occurred.…”

The White Clover enod40 Gene Family. Expression Patterns of Two Types of Genes Indicate a Role in Vascular Function

“…The vascular traces of older nodes are involved in the intensive transport of solutes between established nodal roots, developing lateral shoots, and apical and basal parts of the stem. A role in intensive lateral transport has also been proposed for the pericycle of nodule vascular bundles (Pate et al, 1969), where enod40 expression is detectable throughout the onset of nodule maturation, as well as in the mature nodules. Developing lateral organs are strong sinks that require very intensive shortdistance transport due to the lack of established vasculature.…”