MicroRNA166 Monitors for Building of the Anther Internal Boundary.

The internal boundary between inner and outer microsporangia within anthers is essential for male fertility of vascular plants. Dehiscence zones embedded in the boundary release pollen for fertilization. However, the molecular mechanism underlying boundary formation in anthers remains poorly understood. Here, we report that microRNA166 (miR166) and its target () regulate (), which controls microsporogenesis. In developing anthers of Arabidopsis (), the expression domains of miR165/6 and are overlapped and rearranged synchronously. Dominant mutation of suppresses expression on the adaxial sides of stamens, resulting in a thickened boundary, whereas activation of up-regulates expression, leading to ectopic microsporogenesis in the boundary. PHB limits the expression domains of to facilitate construction of the boundary, while miR166 preserves the expression domains of by inhibiting to allow the inner microsporangia to take shape. Subsequently, PHB activates the key stem cell maintainer in anthers to restrict the stomium cells to the boundary so that dehiscence zones develop and release pollen properly. These findings link adaxial/abaxial polarity to microsporogenesis in building of the internal boundary of anthers and thus advance the concepts underlying the establishment of the internal structure of male organs.