On the cover:Plant axial organs such as root, hypocotyl and petiole are typically composed of cylindrically organized layers of different tissue types. In a cross-section these tissue layers appear as concentric rings consisting of the central vascular tissue surrounded by pericycle, enododermis, cortex and epidermis. In the root of Arabidopsis thaliana, cells in a given tissue type can be easily recognized by their characteristic size and shape (bottom panel), as well as the expression of tissue-specific marker genes. In this issue Miyashima et al. (pp. 626-634) report that loss of ARGONAUTE1 (AGO1), a central component of microRNA-dependent gene regulation, affects both concentric organization and the number of root tissue layers. Such effects are independent of the expression of SHORT-ROOT (SHR) and SCARECROW (SCR), the plant-specific GRAS-type transcription factors previously characterized in root tissue patterning. Interestingly, simultaneous loss of the AGO1 and SCR functions results in dramatic reduction in cell identities and such roots are now filled exclusively with uniform cells (upper panel). Such effects are not observed when only one of the two genes is lost, indicating that root tissue organization is controlled by at least two independent pathways, one using the SHR/SCR transcriptional regulators and the other using AGO1-mediated miRNA regulation.
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