ISOLATION AND CHARACTERIZATION OF CDNAS ENCODING XYLOGENESIS-ASSOCIATED AND WOUNDING-INDUCED RIBONUCLEASES IN ZINNIA-ELEGANS

The study of plant ribonuclease (RNase) functions is complicated by a complex profile of RNase activities detected in tissues. Thus, isolati on of individual RNase genes will be desirable for the further underst anding of function of each RNase. Here, we describe the isolation of c DNAs encoding two RNases, ZRNaseI and ZRNaseII, in differentiating tra cheary elements (TEs) induced from isolated mesophyll cells of Zinnia elegans. Both the ZRNaseI and ZRNaseII exhibit putative secretion sign al sequences at the amino-terminal ends with predicted molecular masse s of 24 247 Da and 22 448 Da as mature proteins, respectively. DNA gel blot analysis showed that both RNases in Zinnia appear to be encoded by a small gene family. RNA gel blot analysis showed that the expressi on of the ZRNaseI gene was associated with the late stage of in vitro TE differentiation, whereas the ZRNaseII gene was mainly induced in re sponse to stress. Neither RNase gene was induced in response to phosph ate starvation, or to H2O2 challenge in the cultured mesophyll cells, or to senescence in the leaves. In young leaves, the ZRNaseI gene was not induced in response to wounding. But the ZRNaseII gene was markedl y induced by 6 h after wounding. Tissue print hybridization showed tha t the expression of the ZRNaseI gene was preferentially associated wit h the differentiating TEs in Zinnia stems, while the ZRNaseII mRNA was not detected in unwounded Zinnia organs. Taken together, the results indicate that the ZRNaseI gene is expressed during the process of xylo genesis both in vitro and in the plant, whereas the ZRNaseII gene is p redominantly induced in response to wounding. The identification of th ese RNase genes provides molecular tools for the dissection of the pro cess of autolysis during xylogenesis, and for the dissection of the ro le of RNase in wounding response.