Differential expression of saporin genes upon wounding, ABA treatment and leaf development.

Ribosome-inactivating proteins (RIPs) are potent inhibitors of protein synthesis that accumulate in different tissues of many plant species. Saporins are type1 ribosome-inactivating proteins (RIPs: EC 3.2.2.22) produced in various organs of Saponaria officinalis L. Two distinct saporin types, saporin-L and saporin-S isoforms, were respectively purified from the intra- and extra-cellular fraction of soapwort leaves. The saporin-L were lowly identical, differed for toxicity, molecular mass and amino acid composition from saporin-S proteins forming a new monophyletic group. Genes encoding both L and S type isoforms were cloned from leaf specific cDNA library; the encoded products included the N-terminal diversity observed by protein sequencing and showed compatible weights with those from mass spectra. These genes were intron-less belonging to small gene families. RT-PCR/qRT-PCR experiments evidenced their differential expression during leaf development, wounding and ABA treatment. The expression of saporin-L genes augmented from young to adult leaves, whereas that of the saporin-S genes maintained a constant rate during leaf growth. In leaf wound assays, the transcription of saporin-L significantly augmented at six hours. On the contrary, saporin-S gene types were intensely triggered at one hour. The saporin-L and -S genes responded differentially to ABA treatment in short time (one hour) experiments, but showed overlapping responses in the long run. The results suggest that the saporin-L and -S proteins may play diversified roles during stress responses