Photoreceptor control of shoot meristem activity and leaf initiation

When Arabidopsis seedlings are grown in the dark, their shoot meristem remains in a repressed state and does not develop leaves. The tight control of leaf development by light provides the opportunity to analyse a fundamental plant developmental process in its very early stages. We have observed that activation of photoreceptors in dark-grown seedlings leads to a rapid and dramatic increase in mitotic activity in and around the shoot meristem. Cotyledons also exhibit an induction of cell cycle activity by light, but this consists of endoreduplication and occurs later. Many other processes take place upon induction by light, including the development of the photosynthetic apparatus and the synthesis of sunscreens, but these processes are expected to take place in both the leaf primordia and the cotyledons. We have used this information to carry out a whole transcriptome analysis of shoot meristem activation and early stages of leaf development by light. For this we dissected the shoot apical regions from seedlings grown in the dark or 1, 2, 6, 24, 48 or 72 h after transfer to light. We also dissected cotyledons from equivalent seedlings grown in the dark or 1 or 6 h after transfer to light. From these samples we obtained RNA used for hybridisations against the ATH1 GenChip. 16 samples were used in this experiment.