Ultrastructural localization of a diprenylated isoflavone inRhizobium lupini-Lupinus albussymbiotic association

Abstract

Polyclonal IgGs raised against the diprenylated isoflavone, 2'-hydroxylupalbigenin were used in an immunocytochemical study of the root and nodule tissues of white lupin (lupinus albus L.). In the roots, the antigen was detected in the secondary walls of both xylem vessels and pericycle cells. Examination of nodules revealed the presence of the antigen in the inner cortex, associated with a neoparietal material which is deposited as globules on the cell wall and occluding the intercellular spaces. The discrete location of the diprenylated isoflavone in specialized cytoplasmic organelles suggests that its compartmentation within the wall is mediated by membrane vesicles. In the infected cells, more than half of the bacteroids exhibited a specific labelling of their inner core associated with a central fibrillar system, and sometimes surrounding an electron-dense nucleoid region. These results suggest that 2'-hydroxylupalbigenin, and possibly other prenylated isoflavones, may play an important role as a biochemical factor in early symbiotic events other than nod gene induction or inhibition. Its putative biological functions in the post-infectional phases of symbiosis are discussed