A Thiol Protease and an Anionic Peroxidase Are Induced by Lowering Cytokinins during Callus Growth in Petunia

We previously identified a group of proteins that increase early in Petunia hybrida calli subcultured on a low-cytokinin medium, unlike the calli subcultured on a high-cytokinin medium. The calli on the low-cytokinin medium do not regenerate (J.-P. Renaudin, C. Tournaire, B, Teyssendier de la Serve [1991] Physiol Plant 82: 48-56). Two of these proteins, P21 and P17, have been identified by peptide sequencing and cloned. P21 is highly homologous to a group of thiol proteases, including barely aleurain, rice oryzain gamma, Arabidopsis SAG2, and mammalian cathepsin H. P17 is highly homologous to a group of anionic peroxidases from potato and tomato. A study of their expression in two P. hybrida lines, PC6 and St40 which differ in their ability to regenerate, showed that the genes for P21 and P17 are differentially expressed depending on the type and the age of the organ, with the highest expression in senescing leaves and in aged calli. The data are in favor of these genes being associated with an early step of senescence, which may be due, in part, to a reduction in total cytokinin. The two Petunia lines are, thus, functionally different concerning the action of cytokinin in two developmental phenomena: in vitro organogenesis and senescence.