Possible roles for a cysteine protease and hydrogen peroxide in soybean nodule development and senescence

The expression of a cysteine protease (CP) gene during soybean (Glycine max ) root nodule development has been studied. In 5-wk-old nodules, when N-2 fixation began to decline, the CP gene appeared to be expressed in the periphery of the central infected tissue which, in determinate nodules, is considered as a developmental zone. Concomitantly, events related to programmed cell death (PCD) and accumulation of significant amounts of H-2 O-2 in the apoplasm were observed in this zone. In 7.5-wk-old nodules, the zone that exhibited CP expression, PCD events and H-2 O-2 accumulation enlarged towards the centre of the organ. In 10-wk-old nodules, which are largely senescent, a widespread expression of the CP gene was observed in all of the central infected tissue; this was associated with the presence of large amounts of hydrogen peroxide (H-2 O-2 ) in the cytoplasmic and apoplastic compartments, and the existence of a necrotic zone. It is proposed that the developmental zone of the nodule is undergoing a PCD process, involving both a CP and H-2 O-2 . This can be viewed as the onset of the nodule senescence process and could be under the control of a plant signal.