Regional and cell-specific gene expression patterns during petal development.

We investigated gene expression patterns that occur during tobacco petal development. Two petal mRNA classes were identified that are present at elevated levels relative to other organs. One class is represented equally in the unpigmented tube and pigmented limb regions of the corolla. The other class accumulates preferentially within the limb region. Limb-specific mRNAs accumulate at different times during corolla development, peak in prevalence prior to flower opening, and are localized in either the epidermal cell layers or the mesophyll. The epidermal- and mesophyll-specific mRNAs change abruptly in concentration within a narrow zone of the limb/tube border. Preferential accumulation of at least one limb-specific mRNA occurs within the corolla upper region early in development prior to limb maturation and pigment accumulation. Limb-specific mRNAs also accumulate preferentially within the unpigmented corolla limb region of Nicotiana sylvestris, a diploid progenitor of tobacco. Runoff transcription studies and experiments with chimeric beta-glucuronidase genes showed that petal gene organ, cell, and region specificities are controlled primarily at the transcriptional level. We conclude that during corolla development transcriptional processes act coordinately on limb-specific genes to regulate their regional expression patterns, but act individually on these genes to define their cell specificities.