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Physiology and Biochemistry

Suppression of Binding between Rhizobia and Soybean Roots by Heterodera glycines. Jeng- sheng Huang, Associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; K. R. Barker(2), and C. G. Van Dyke(3). (2)(3)Professor, and associate professor, respectively, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 74:1381-1384. Accepted for publication 20 July 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1381.

Soybean seedlings inoculated with juveniles of race 1 of Heterodera glycines, the soybean cyst nematode (SCN), were incubated with a suspension of Rhizobium japonicum prepared from cultures grown in a synthetic medium containing d-(1-3H) glucose. After washing to remove unbound rhizobia, the roots were oxidized, and the radioactivity of the resulting tritiated water was measured. Roots from SCN-inoculated seedlings had lower radioactivity on a per root or per unit weight basis than those from controls. Binding of R. japonicum to control soybean roots also was inhibited by pretreatment of roots with N-acetyl-d-galactosamine or d-galactose, the haptens of the 120,000-dalton soybean lectin, but not glucose. These results suggest that the soybean lectin is involved in binding R. japonicum to soybean roots, and that SCN infection suppresses the binding between roots and rhizobia. Scanning electron microscopy revealed that abundant rhizobia were on the surfaces of control soybean roots. Very few rhizobia were observed on root surfaces of SCN-infected plants. Nematode infection caused an increase in numbers of root hairs and therefore the surface area of total root system. In contrast, SCN infection caused a reduction in hemagglutination activity in root homogenates. We conclude that reduction in binding of rhizobia to SCN-infected soybean roots apparently was not due to the reduction in surface areas of infected roots but resulted from interference of the nematode with soybean lectin metabolism.

Additional keywords: glycine max, nitrogen fixation, lectins of Rhizobium japonicum.