Cytological Evidence for a Host Defense Response That Reduces Cell and Tissue Invasion in Pea Nodules by Lipopolysaccharide-Defective Mutants of Rhizobium leguminosarum Strain 3841. S. Perotto. Universita di Torino, Dipartimento di Biologia Vegetale, Viale P. A. Mattioli 25, 1-10125 Torino, Italy. N. J. Brewin(2), and E. L. Kannenberg(3). (2) John Innes Institute, Colney Lane, Norwich NR4 7UH, Great Britain, and (3) Universitat Tubingen, Mikrobiologie Botanik, Auf der Morgenstelle 1, D-7400 Tubingen, Germany. MPMI 7:99-112. Accepted 12 October 1993. Copyright 1994 The American Phytopathological Society.

Mutants of Rhizobium leguminosarum bv. viciae with modifications in the structure of their lipopolysaccharide (LPS) macromolecules induce ineffective pea root nodules showing little or no capacity for symbiotic nitrogen fixation. The development of these nodules was investigated by the use of cytochemical techniques and monoclonal antibodies to monitor the progress of plant-microbial cell surface interactions. LPS-defective mutants were only partially successful in colonizing the nodule tissue and were released into only a minority of the host cells. Tissue and cell invasion by Rhizobium was often associated with a host plant defense reaction characterized by the accumulation of intercellular matrix material and possibly cal-lose, secondary cell wall modification, and sporadic cell death. The severity of the host defense response was correlated with the degree of LPS structural modification shown by the various mutants examined. Furthermore, LPS-defective mutants released into host plant cells differentiated into abnormally swollen and elongated bacteroids, apparently defective in bacteroid cell division and in synchronized division of the plant-derived peribacteroid membrane. These observations suggest an essential role for Rhizobium LPS in the avoidance of host cell defense reactions during the invasive phase of nodule development. The molecular structure of LPS also affects the nature of bacteroid development during the endosymbiotic phase of nitrogen-fixing bacteroids.