Lotus japonicus has been shown to be a very suitable model system to study nodulation and nitrogen fixation (Handberg, Stougaard 1992). We are currently using this model system to study in detail the infection and nodulation process induced by Mesorhizobium loti (Pacios Bras et al. 2000). We have shown recently that L. japonicus responds to the addition of Nod factors by the formation of cytoplasmic bridges in the outer cortex (van Spronsen et al. 2001). In this respect it is identical in responsiveness to Nod factors as Vicia sativa and Pisum sativum (van Brussel et al. 1992). This response, which underlies the formation of the infection thread, is also dependent on the presence of ethylene, again similar to the situation in V. sativa and P. sativum. However, whereas the latter legumes form indeterminate nodules (of the Galegae form: in the inner cortex), L. japonicus forms determinate nodules. Interestingly, Phaseolus vulgaris, which also forms determinate nodules, does not respond to Nod factor by the formation of cytoplasmic bridges (van Spronsen et al. 2001). In conclusion, L. japonicus presents a sort of intermediate mechanism of nodulation in between that found in the P. vulgaris nodulation, where cells just below the epidermis form the primordia for nodulation, and the Galegae nodulation, where cells in the inner cortex form the nodule primordia. As a consequence, L. japonicus is a very useful model system to study the formation of cytoplasmic bridge since they are formed close to the outer surface of the root and therefore are more easily accessible for microscopic studies. Cytoplasmic bridge formation is interesting to study because it represents an early response to Nod factors which in many ways is similar to the responses of root hairs, e.g. both represent changes in cell polarity based on reorganization of the cytoskeleton.
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