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

Summary • The expression of a cysteine protease (CP) gene during soybean (Glycine max) root nodule development has been studied. In 5-wk-old nodules, when N2 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 H2O2 in the apoplasm were observed in this zone. In 7.5-wk-old nodules, the zone that exhibited CP expression, PCD events and H2O2 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 (H2O2) 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 H2O2. This can be viewed as the onset of the nodule senescence process and could be under the control of a plant signal.