THE PLANT CELL, Vol 3, Issue 3 259-270, Copyright © 1991 by American Society of Plant Biologists
A Nodulin Specifically Expressed in Senescent Nodules of Winged Bean Is a Protease Inhibitor
J. F. Manen, P. Simon, J. C. Van Slooten, M. Osteras, S. Frutiger and G. J. Hughes
Jardin Botanique, 1 ch. de I'Imperatrice, 1292-Chambesy, Geneve, Switzerland
Nodule senescence is one aspect of nitrogen fixation that is important to
study from the perspective of improving the host-bacteroid interaction. In
winged bean nodules, a 21-kilodalton protein is specifically expressed when
senescence begins. Using subcellular fractionation, we observed that this
plant protein interacts with the bacteroids. Microsequencing of the protein
allowed us to obtain a specific oligonucleotide that was used to isolate
the corresponding nodule cDNA. Sequence analysis of this cDNA revealed that
the 21-kilodalton protein has all of the features of a legume Kunitz
protease inhibitor. Subsequent analysis confirmed that this nodulin is
indeed a protease inhibitor. Immunocytochemical study showed that the
protease inhibitor is exclusively localized in infected senescent cells of
the nodule, particularly in disorganized bacteroids, the peribacteroid
membrane, vacuole membranes, and in the vacuole fluid. The specific
expression of a protease inhibitor at senescence may be of particular
interest if the targeted proteolytic activity is important for the
symbiotic relationship. This point is discussed in relation to the known
nodule proteases.
