Cytokinin and abscisic acid control plastid gene transcription during barley seedling de-etiolation

The regulatory role of cytokinins and abscisic acid (ABA) during de-etiolation of monocotyledonous plants is not sufficiently studied. The effects of cytokinin—6-benzyladenine (BA) and ABA on leaf greening and chloroplast gene transcription were studied on leaves detached from etiolated barley ( Hordeum vulgare L.) seedlings. The influence of both phytohormones was age-dependent. Cytokinin practically had no effect on the youngest (3-day-old) leaves, increased chlorophyll accumulation in 6-day-old leaves, and this effect decreased in 9-day-old leaves. ABA reduced chlorophyll accumulation in the youngest leaves, and this effect was blocked by cytokinin. ABA alone had practically no effect on greening of leaves detached from 6- and 9-day-old seedlings, but fully inhibited activation of this process by cytokinin. Thus, antagonism between ABA and cytokinin was observed for seedlings of all ages. The rate of transcription of chloroplast genes was studied by run-on transcription in isolated chloroplasts in the course of etiolated seedling growth in darkness, and age-dependent decrease was demonstrated. It was established that cytokinin activated transcription of 11 chloroplast genes among 30 studied, and ABA suppressed transcription of 12 plastid genes. Such differential effect of cytokinin and ABA on chloroplast gene transcription in monocot seedlings during etiolated leaf greening was not known before.