Abstract:
Cysteine proteases and cystatins havemany functions that remain poorly characterised, particularly in crop
plants. We therefore investigated the responses of these proteins to nitrogen deficiency in wild-type soybeans
and in two independent transgenic soybean lines (OCI-1 and OCI-2) that express the rice cystatin,
oryzacystatin-I (OCI). Plants were grown for four weeks under either a high (5 mM) nitrate (HN) regime
or in the absence of added nitrate (LN) in the absence or presence of symbiotic rhizobial bacteria. Under
the LN regime all lines showed similar classic symptoms of nitrogen deficiency including lower shoot biomass
and leaf chlorophyll. However, the LN-induced decreases in leaf protein and increases in root protein
tended to be smaller in the OCI-1 and OCI-2 lines than in the wild type. When LN-plants were grown with
rhizobia, OCI-1 and OCI-2 roots had significantly more crown nodules than wild-type plants. The growth
nitrogen regime had a significant effect on the abundance of transcripts encoding vacuolar processing
enzymes (VPEs), LN-dependent increases in VPE2 and VPE3 transcripts in all lines. However, the LN-dependent
increases of VPE2 and VPE3 transcripts were significantly lower in the leaves of OCI-1 and OCI-2 plants
than in the wild type. These results show that nitrogen availability regulates the leaf and root cysteine protease,
VPE and cystatin transcript profiles in a manner that is in some cases influenced by ectopic OCI
expression. Moreover, the OCI-dependent inhibition of papain-like cysteine proteases favours increased
nodulation and enhanced tolerance to nitrogen limitation, as shown by the smaller LN-dependent
decreases in leaf protein observed in the OCI-1 and OCI-2 plants relative to the wild type.
