Getting around the legume nodule: II. Molecular biology of its peripheral zone and approaches to study its vasculature

Abstract

All legume nodules exhibit a complex cortex composed of many types of cells and tissues. This zone, which surrounds the central infected zone, plays a critical role in regulating the exchange of oxygen between the rhizosphere and the bacteroids. Not often mentioned, but of importance, are the vascular traces that develop in the nodule inner cortex. Information on their ontogeny is scarce, although their existence is critical to the symbiosis because both the nitrogenous compounds formed as a result of nitrogenase activity and the energy-rich molecules obtained from plant photosynthesis use these conduits. Here, I focus on these tissues, reviewing what we know of their appearance in pseudonodules, boron-treated legumes, and in the vasculature mutants described to date. I also examine the genes known to be expressed in the peripheral tissues and attempt to place them in functional clusters. Finally, I propose that specific vasculature markers known from the Arabidopsis literature be applied to the study of the development of nodule vascular tissues. It is of interest that most of these markers are linked to specific growth regulators, especially auxins and cytokinins but to a lesser extent gibberellins and brassinosteroids.