Developmental Biology Select

Branching is a reoccurring theme in developmental biology. It is observed in individual cells in the case of neu-ronal axons and dendrites and in the patterning of tissues such as the vasculature of animals or the flowering architecture of plants. This issue's Developmental Biology Select reflects on newly discovered regulators of branching that have been discovered in diverse organisms. Inflorescences are the structures in plants that give rise to flowers. Satoh-Nagasawa et al. (2006) have made the intriguing discovery that a sugar may direct inflorescence branching in maize in a manner similar to that of well-established plant hormones, such as auxin and cytokinin. The authors mapped and sequenced the RAMOSA3 gene, which when mutated disrupts branching and patterning of seeds in the ears of corn (the female inflorescences). This effort reveals that RAMOSA3 encodes a phos-phatase that converts the sugar trehalose-6-phosphate into trehalose. They find that RAMOSA3 is expressed at the base of the axillary meristem, a structure in the wild-type plant that gives rise to two spikelet meristems each of which subsequently generates two floral meristems. However, in ramosa3 mutants, the axillary meristem becomes enlarged and the regular pattern of differentiation is disrupted. Satoh-Naga-sawa et al. conclude that a trehalose metabolite may be acting as a short-range signal that controls meristem identity. It will be interesting to determine whether treha-lose itself is a key signaling molecule that is generated by RAMOSA3 or whether RAMOSA3 acts instead to counteract a signal generated by trehalose-6-phosphate. Moreover, the authors show that RAMOSA3 acts in parallel with another inflorescence branching mutant ramosa2 to control the expression level of the transcription factor RAMOSA1. Still to be elucidated is how RAMOSA3 regulates RAMOSA1 expression and which genes are targets of transcriptional regulation by RAMOSA1. A next step will be to understand how the trafficking of these metabolites is regulated to control meristem branching. Migrating epithelial cells communicate with each other to decide who takes the lead and in what order the others will follow, according to Ghabrial and Krasnow (2006). These investigators studied branching in the tracheal network of fruit fly embryos. The tracheal network extends throughout the Drosophila embryo and is essential for the delivery of air to internal tissues. During tracheal development, epithelial cells migrate out from an epithelial sac to form a tube, from which secondary and terminal branches are established in subsequent steps. In a screen of genetically mosaic larvae, the …