Localization of ubiquitin to differentiating vascular tissues

The selective degradation of proteins, an essential process of any developmental program, may entail conjugation of the protein to be destroyed to the polypeptide ubiquitin. Experiments were designed to localize ubiquitin as a first step in determining whether this molecule is crucial for certain developmental processes in plant tissues and cells. Antibodies to ubiquitinated protein were detected on tissue prints of cross sections of bean petioles (Phaseolus vulgaris, Fabaceae), cotton hypocotyls (Gossypium hirsutum, Malvaceae), and Coleus stems (Coleus x hybridus, Lamiaceae). For most of the material investigated, there appears to be an accumulation of ubiquitin antibodies in vascular tissues, but not preferentially in the abscission zone of bean petioles. Vascular localization was confirmed using immunohistochemical methods on fixed and sectioned internodal tissues of Coleus. Antibodies to ubiquitin are detected in parenchyma cells of the cortex and pith, but are most concentrated in the xylem, especially secondary xylem, and in the cambial region, and in the phloem. Thus, ubiquitin accumulates in certain vascular tissues, some of which may be undergoing programmed cell death. Ubiquitin can also be detected in nondifferentiating cells, and its level is elevated in rapidly dividing cambial cells.