The occurrence of unusual laminated structures rich in β-1,4-glucans in plastids of Phaseolus vulgaris root-nodule cells infected by an ineffective C4-dicarboxylic-acid mutant of Rhizobium leguminosarum bv. phaseoli.

Root nodules induced in Phaseolus vulgaris L. by the wild-type (WT) and a C4-dicarboxylic-acid mutant strain of Rhizobium leguminosarum biovar. phaseoli were compared on the basis of ultrastructure and cytochemistry of cellulose subunits. The mutant bacteroids failed to colonize infected host cells in a normal manner, and presented a premature degenerative appearance. Starch granules, rough endoplasmic reticulum and mitochondria were found to accumulate in the ineffective nodules. The most striking difference between effective and ineffective nodules was the presence of unusual spherical, laminated structures in plastids of mutant-infected host cells only. Cytochemical observations showed that these structures contain β-1,4-glucans. The presence of β-1,4-glucans within such structures may be caused by the activity of a cellulase which is produced by either the bacteroids or the host cell and is locally hydrolyzing the host cell-wall, thus releasing cellulose subunits into the cytoplasm. Another possibility is denovo synthesis of β-1,4-glucans in the host cell.