INDUCTION OF CYSTEINE AND SERINE PROTEASES DURING XYLOGENESIS IN ZINNIA-ELEGANS

The terminal process of xylogenesis, autolysis, is essential for the f ormation of a tubular system for conduction of water and solutes throu ghout the whole plant. Several hydrolase types are implicated in autol ysis responsible for the breakdown of cytoplasm. Here, we characterize p48h-17 cDNA from in vitro tracheary elements (TEs) of Zinnia elegans which encodes a preproprotein similar to papain. The putative mature protein, a cysteine protease, has a molecular mass of 22,699 Da with a pI of 5.7. DNA gel blot analysis indicated that p48h-17 is likely enc oded by one or two genes. The p48h-17 mRNA accumulated markedly in in vitro differentiating TEs, whereas it appeared not to be induced in re sponse to senescence and wounding in the leaves or H2O2 challenge in t he cultured mesophyll cells. In stems, the expression of the p48h-17 g ene was preferentially associated with differentiating xylem. Activity gel assays demonstrated that a cysteine and a serine protease, which had apparent molecular masses of 20 kDa and 60 kDa, respectively, were markedly induced during in vitro TE differentiation. The cysteine pro tease activity was also preferentially present in the xylem of Zinnia stems. Transient expression of the p48h-17 cDNA in tobacco protoplasts resulted in the production of a 20 kDa cysteine protease. Taken toget her, the results indicate that the p48h-17 gene appears to be preferen tially associated with xylogenesis, and both the cysteine and serine p roteases might be involved in autolysis during xylogenesis.