THE SECRETORY NATURE OF THE LESION OF CARROT CELL VARIANT TS11, RESCUABLE BY ENDOCHITINASE

The carrot cell variant ts11 is unable to form somatic embryos at the non-permissive temperature of 32 degrees C, but the block can be overc ome by the addition of a 32-kDa acidic endochitinase to the medium. In this work we conducted a cyto-histological analysis of the blocked em bryo forms. The morphology of the endomembrane system is altered; in p articular, the ER is dilated and may show electron-dense precipitates and continuity with the plasma membrane. These morphological alteratio ns do not occur in the presence of externally-added endochitinase. We also noticed modifications of the culture medium that are probably rel ated to the morphological observations: the total amount of secreted p roteins is reduced and pulse-chase experiments revealed that, compared with wild-type cells, the secretion of major polypeptides is reduced while new minor polypeptides are secreted. Western blot analysis revea led the presence of the binding protein BiP, a resident of the ER and of glutamine synthase, a cytosolic protein, in the medium of ts11 but not wild-type cells. These results indicate that ts11 is altered in th e secretory pathway but do not clarify the role of endochitinase.