Cell biological changes of outer cortical root cells in early determinate nodulation

In the symbiosis of leguminous plants and Rhizobium bacteria, nodule primor dia develop in the root cortex. This can be either in the inner cortex (ind eterminate-type of nodulation) or outer cortex (determinate-type of nodulat ion), depending upon the host plant. We studied and compared early nodulati on stages in common bean (Phaseolus vulgaris) and Lotus japonicus, both kno wn as determinate-type nodulation plants. Special attention was paid to the occurrence of cytoplasmic bridges, the influence of rhizobial Nod factors (lipochitin oligosaccharides [LCOs]) on this phenomenon, and sensitivity of the nodulation process to ethylene. Our results show that i) both plant sp ecies form initially broad, matrix-rich infection threads; ii) cytoplasmic bridges occur in L. japonicus but not in bean; iii) formation of these brid ges is induced by rhizobial LCOs; iv) formation of primordia starts in L. j aponicus in the middle root cortex and in bean in the outer root cortex; an d v) in the presence of the ethylene-biosynthesis inhibitor aminoethoxyviny lglycine (AVG), nodulation of L. japonicus is stimulated when the roots are grown in the light, which is consistent with the role of cytoplasmic bridg es during nodulation of L. japonicus.