Tissue-specific transcriptomics reveal functional differences in floral development

Flowers are produced by floral meristems, groups of stem cells that give rise to floral organs. Grass flowers (florets) are contained in spikelets, which contain one to many florets depending on the species. Members of the Andropogoneae tribe, including maize, produce spikelets containing two florets; the upper and lower florets are usually dimorphic, with the lower floret greatly reduced compared to the upper floret. In maize, early development appears identical in both florets, although development of the lower floret is delayed relative to the upper floret. To gain insight into the functional differences between upper and lower floral meristems, we used laser capture microdissection coupled with RNA-seq to globally examine gene expression in floral meristems. Approximately 700 genes were differentially expressed between upper and lower floral meristems, including genes involved in hormone regulation, cell wall, sugar and energy homeostasis. Furthermore, cell wall modifications and sugar accumulation differed between the upper and lower florets. Finally, we identified a novel boundary domain between upper and lower florets, which we hypothesize is important for floral meristem activity. We propose a model in which growth is repressed in the lower floret by limiting sugar availability and upregulating genes involved in growth repression and suggest similar processes may also regulate meristem activity in other developmental contexts.