Expression of cysteine protease genes in pea nodule development and senescence

Coding sequences for two cysteine proteases were amplified from cDNA derived from pea nodule mRNA using primers based on conserved regions of plant cysteine proteases. One of the amplified cDNA sequences corresponded to a previously described cysteine protease gene, Cyp15a, expressed in pea shoots in response to dehydration (J.T. Jones and J.E. Mullet, Plant Mol. Biol. 28:1055-1065, 1995). Inside the pea root nodule, in situ hybridization revealed that this gene is expressed strongly in the apical region and more weakly in the uninfected cortex and in the central infected tissue where nitrogen fixation takes place. The complete sequence of the cDNA corresponding to the other gene, PsCyp1, was obtained. Expression of this gene, which was studied both on RNA blots and in situ, showed good correlation with the onset of nodule senescence. In situ hybridization studies revealed that PsCyp1 was expressed in senescent infected tissue at the base of the nodule. This signal was just detectable in normal symbiotically wild-type nodules but was much stronger in the early senescing nodules formed by a symbiotically defective mutant of Rhizobium leguminosarum.