Pinwheel inclusions in morphogenesis: a possible alternative to induction by viruses.

Pinwheel inclusions (PWs) were found in cells of callus tissue derived from explants of secondary phloem parenchyma of carrot (Daucus carota) storage root and grown on a basal medium containing zeatin and indoleacetic acid or coconut milk, naphthalene acetic acid, or combinations of these. Preliminary attempts to demonstrate the presence of viruses in the callus tissue failed. The possibility that the tissles were infected by a low titer or unstable conventional virus or by a defective mutant has not been ruled out. However, two lines of evidence suggest that the PWs in these tissues may be a result of culture conditions and not of virus infection. First, no PWs or other cytoplasmic inclusions were found in cells of otherwise similar tissue cultured on basal medium alone, and multifibrillar bundles (MFBs) but not PWs were found when the tissues were cultured on a medium that stimulates differentiation and morphogenesis. Second, culture stimulated to differentiate and containing MFBs only were returned to the supplemented basal medium and subsequently found to contain both PWs and MFBs.