A PECTATE LYASE FROM ZINNIA-ELEGANS IS AUXIN INDUCIBLE

The Zinnia mesophyll cell system consists of isolated leaf mesophyll c ells in culture that can be induced, by auxin and cytokinin, to reprod ucibly trans-differentiate into tracheary elements (TE) after 96 h, wh ile in the presence of auxin alone the cells simply elongate. In a sea rch for genes involved in modifications to cell-wall architecture befo re any overt signs of cell differentiation, a differential hybridizati on of a 72-h cDNA library with probes from mRNA at time-points of 24 h and 72 h was done revealing a number of transcripts up-regulated betw een these times. One of these cDNAs shows homology to pectate lyase, a pectin-degrading enzyme. The complete cDNA sequence (ZePel) correspon ds to a translated protein of 44 kDa with an N-terminal signal peptide of about 2 kDa, and one potential N-glycosylation site. Northern anal ysis confirms that the strong expression of this gene during TE induct ion occurs at a very early stage of the process and is due solely to t he presence of auxin in the induction medium. In situ hybridization st udies in young Zinnia stems show that ZePel expression is associated w ith vascular bundles and shoot primordia. Recombinant protein made in Escherichia coli possesses calcium-dependent pectate lyase activity. P ectate lyase activity is detected in elongating and differentiating in vitro cell populations. The role of this enzyme in remodelling the ce ll wall during cell elongation and differentiation is discussed.