Legume Root Symbioses

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Besides the absorption of water and nutrients, virtually all roots have symbiotic associations with soil microorganisms. The term "Rhizosphere" is used to indicate the interface of the root Epidermis and its immediate zone of soil contact. It has been estimated that 90% of land plants have mycorrhizal associations with soil fungi. This is probably an underestimate. Those who want to raise endangered species for out-planting to nature, achieve good results when the propagules have been grown in medium containing the soil microflora of the out-planting sites. This is very pertinent for Hawaiian species which are hard to reestablish in nature. Dr. Wong has detailed information concerning this & he teaches a Mycology course! Dr. Daehler studies Mycorrhizae!

There are a few famous symbiotic relationships. The most well known is that between legumes and soil bacteria in the genus Rhizobium. The Rhizobia can fix nitrogen gas when the symbiosis is achieved. They can't fix nitrogen in the free-living state, however!

Fixed nitrogen is obtained by the legume and the Rhizobium gets a nice place to live with all of the amenities!. This includes photosynthate, water and minerals. This is an extremely important relationship because Nitrogen is usually the most limiting element in terrestrial ecosystems. Furthermore, many legumes, like soybean, form the basis for agriculture on a world-scale. Some legume seeds, like soybeans, contain high levels of protein. These are the most important agricultural sources of protein in the world. 

Nodule Types

Two basic types of root nodules are produced by legumes. One type is ephemeral and lasts days or a few weeks. This is called a determinate structure. It has a short, predestined life-span. Consequently, new nodules are being formed as the root grows in the soil and others are being lost on older parts of the root system. Soybean nodules are like this. The nodule is a spherical elaboration of the ground tissue system in the root cortex and has a specialized anatomy.

The outer part of the nodule becomes sclerotic as parenchyma cells are converted into sclereids. This tissue blocks gas exchange to some extent. More internal Vascular Bundles surround a large central mass of parenchyma which contains cells infected with bacteroids that fix nitrogen. The term Bacteroid refers to the growth form of the Bacteria in N-fixing Cells. These are structurally modified compared to free-living bacteria. Uninfected Parenchyma cells are also present.

The second nodule type is illustrated by Clover. In this case the nodule has an Apical Meristem which functions for many months. It is called Indeterminate in that meristematic activity is theoretically unlimited.

These are elongate compared to the determinate nodules. They are tumescent, (swollen). The apical meristem continuously produces new cells which become infected with bacteria from older cells.

These nodules have a much more extensive vascular system which surrounds the nitrogen-fixing parenchyma that occupy the center of the nodule. This central location is not a coincidence.

The enzyme which fixes nitrogen (Nitrogenase) needs an anaerobic environment. Consequently, the location of the bacteroids, inside living, non-photosynthetic cells favors N-fixation. Lignified external layers may also limit gas exchange.

Cork, which can be impervious to air can develop especially on Indeterminate Nodules. This also promotes anoxia.

Furthermore, the bacteroids stimulate the production of Leghaemoglobin which acts like animal hemoglobin and binds oxygen, thus reducing oxygen levels.

Acacia koa (koa) is a legume tree. It produces Indeterminate nodules which are similar to clover. Dr. Borthakur and one of his students James Leary study these. James recently discovered that koa produces adventitious nodules from aerial parts of koa stems. These nodules appear to have a similar anatomy compared to root nodules.

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