In darkness, shoot apex growth is repressed, but it becomes rapidly activated by light. We show that phytochromes and cryptochromes play largely redundant roles in this derepression in Arabidopsis thaliana. We examined the light activation of transcriptional changes in a finely resolved time course, comparing the shoot apex (meristem and leaf primordia) and the cotyledon and found >5700 differentially expressed genes. Early events specific to the shoot apices included the repression of genes for Really Interesting New Gene finger proteins and basic domain/leucine zipper and basic helix-loop-helix transcription factors. The downregulation of auxin and ethylene and the upregulation of cytokinin and gibberellin hormonal responses were also characteristic of shoot apices. In the apex, genes involved in ribosome biogenesis and protein translation were rapidly and synchronously induced, simultaneously with cell proliferation genes, preceding visible organ growth. Subsequently, the activation of signaling genes and transcriptional signatures of cell wall expansion, turgor generation, and plastid biogenesis were apparent. Furthermore, light regulates the forms and protein levels of two transcription factors with opposing functions in cell proliferation, E2FB and E2FC, through the Constitutively Photomorphogenic1 (COP1), COP9-Signalosome5, and Deetiolated1 light signaling molecules. These data provide the basis for reconstruction of the regulatory networks for lightregulated meristem, leaf, and cotyledon development.

Linear displacement transducers were used to study the effect on elasticity of exogenous indole acetic acid (IAA) applied to aged sections of parenchyma storage tissues of Helianthus tuberosus. The treated sections were found to have a decreased cell volume elastic modulus, by about 25% relative to controls after 1 day in auxin, and by about 80% relative to controls for periods of 2 days to more than 9 days in auxin. The theory and some possible difficulties associated with the measurements are discussed.

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