Transglutaminase activity during greening and growth of Helianthus tuberosus explants in vitro

Explants of dormant tubers of Helianthus tuberosus were grown in vitro, with or without 10 μM 2,4-D, for 3 weeks. The 2,4-D-treated explants grew by cell enlargement and division and formed a non-photosynthetic friable callus composed of thin-walled cells. However, untreated explants, whose cells did not divide, differentiated chloroplasts and contained intercellular spaces filled with opaque material; chloroplasts were derived from non-photosynthetic plastids with tubular complexes and secondary starch grains: both disappeared when the thylakoids began to organize and form small grana. Nuclei also changed their morphology and became invaginated. Treated and untreated explants showed differences in their protein electrophoretic patterns and transglutaminase activity. This enzyme activity, low in dormant tubers, increased in both explants; considerably in untreated greening explants but much less in 2,4-D-treated growing ones. SDS-PAGE analysis of labelled conjugates, formed by in vitro incubation with labelled putrescine, indicated that, in addition to some apparently common substrates with Mr more than 36 kDa, proteins of lower mass were also labelled in the untreated greening explants. These data are discussed in the light of the possible role of transglutaminase in plants. © 1993 Springer-Verlag.