Transglutaminases and their substrates in kinetin-stimulated etioplast-to-chloroplast transformation in cucumber cotyledons

In the light of our previous work, we know that there is a relationship between bound polyamines and the chloroplast differentiation process. This relationship may represent an important component of the process and be part of the mechanism of kinetin action, which stimulates chloroplast differentiation. To clarify the nature of the binding of polyamines to chloroplast structures, the possible involvement of transglutaminases in kinetin-stimulated chloroplast photodevelopment was investigated. Immunodetection of transglutaminases revealed bands at 77, 50 and 30 kDa both in etioplasts and chloroplasts. The data indicated a positive correlation between enzyme level and activity. It also demonstrated the regulation of transglutaminase protein expression by kinetin. The suborganellar location of transglutaminases by electron microscopy showed that the enzyme is peculiarly localised, mainly in pro-thylakoids and appressed grana thylakoids. The data corroborated that spermidine post-translational modification of certain plastid proteins of 58, 29, 26 and 12 kDa occurred. The results we obtained suggest that transglutaminases take part in the formation of the chloroplast structure via a mechanism whereby polyamines bind to their protein substrates. These findings about the effect of kinetin on conjugation provide a new contribution to the understanding of the mechanism of kinetin action on etioplast-to chloroplast transformation.