Index >> Rhizobium and Legume Root Nodulation >>Infection

	Infection Studies on clover and lucerne have revealed that the first reaction of the root system to the presence of rhizobia is the curling and deformation of root hairs. The formation of a typical "Shepherd's crook" on the root hair is generally considered as a necessary prelude to the formation of a thread­like structure visible inside the root hair called "infection thread". The curling effect has been attributed to indole acetic acid (IAA) produced in the root region by rhizobia. lAA is also known to be produced by seveal microorganisms other than rhizobia.
In view of this, a specific root hair curling factor, believed to be a water soluble polysaccharide produced by rhizobia has been implicated in the typical curling of root hairs in which infection threads are formed.There appears to be an intense interaction between the nucleus of root hair cell and the infection thread originating at the tip of the curled portion of the root hair. The nucleus guides the path of the infection thread in the hair as is evident from the fact that in the event the nucleus gets disorganised, the growth of the thread also ceases. If the nucleus moves to the distal end of the hair and then wanders towards the proximal end near the cortex, the infection thread also traverses up and down before entering the cortex. Obviously, some kind of a message or impulse is transferred by the nucleus of the host to the contents of the infection thread. Diagrams showing curling of root haris due to the presence of Rhizobium in the root region and the formation of infection thread in root hairs of lucerne (Medicago Sativa).
Rhizobia are incapable of producing pectinase or cellulase in culture medium amended with pectin or cellulose. Recently, evidences have been provided to show that legume roots liberate pectic enzymes, in response to the presence of homologous rhizobia in the root region. The validity of such induced production of pectinase by plant/homologous rhizobia associations has been discounted by other workers since results have not always been reproducible.
However, detailed investigation on the role of enzymes in the mechanism of infection of legume roots by Rhizobium may throw more light on the problem. A-Light Microscope view of a root hair of Trifolium glomeratum showing the typical 'shepherd's crook' in which the infection thread could be seen in the early stage of development B-Light Microscopic view of root hairs of T. glomeratum. Note the infection thread in one of them in an advanced stage of development C-the same as in B where the thread is seen entering the cortex D-Electron Micrograph of Longitudinal section through an infected root hair of Trifolium Parviflorum. The section passes through a typical 'shepherd's crook' showing invagination. The infection thread appears a s a rather thick lump which contains bacteria. Fragments of the thread can be seen inside the hairs. The arrows indicate the location of infection threads. Research work with clover seedlings has demonstrated the following important points with regard to root hair infection: (1) the infection of root hairs does not take place at random but takes place at a few well separated points, (2) these primary infection sites give rise to zones of infection by later infections of root hairs, (3) the number of infected root hairs increases exponentially until the first nodule is formed followed by a reduction in the number of infections thereafter, and (4) not all infections result in nodule formation.
	Two modes of entry of rhizobia into the root hair have been suggested: (1) entry of small coccoid swarmers through the gaps in cellulose microfibrils, and (2) direct invagination of the root hair cell. The invagination hypothesis rests on the ground that auxins and pectic enzymes on the root surface interact to produce localized soft regions on the root hair facilitating the inward growth of the root hair cell wall.

This inward growth has to take place against the hydrostatic pressure of the cell contents which appears to be an obvious argument against the hypothesis. A combination of the two hypotheses viz., the entry of swarmers through microfibrils of the root hair cell wall and the invagination of the cell wall may explain the entry of Rhizobium into root hairs. The coccoid swarmers may be able to penetrate between the gaps in the microfibrils of the root hair cell wall. At this juncture, overlaying of more primary or secondary wall material might result in the incorporation of the bacteria into the cell wall. Following this event, the outer wall of the root hair gets strengthened at a specified point to allow invagination of the inner layers. In spite of these explanations and conjectures, the modus operandi of infection is still not quite clear. Fine structure studies of infected root hairs showing the continuation of the wall of the infection thread with the cell wall of the root hair lend support to the invagination hypothesis.

As explained above, the infection thread appears to be of host origin. More than one infection thread is often seen in the same root hair. Upon it sentry into the cortical cell of the root, the thread branches and then traverse intracellularly. It so happens that the contents of an infection thread (bacteria) are liberated into a tetraploid cell of the root cortex stimulating the cell to intense meristematic activity. A meristem is constituted in the differentiating cellular mass or the initial nodular tissue. Sooner or later, well differentiated areas are demarcated showing a diploid nodule cortex and a central tetraploid bacteroid zone, having vascular connections with the parent root system.

Not much work has been done on the mechanism of infection in plants where infection takes place directly through the cells of the root cortex. It is presumed that in such instances, rhizobia enter roots by mechanical injury caused to the roots or by some unexplained enzymatic process. However, based on the work done with clovers and lucerne, the physiological events leading to infection can be summarized as follows: