Model for miR165/6 regulation of SPL/NZZ in the formation of the boundary between inner and outer microsporangia in anthers. During anther development, miR165/6 silence PHB by complementary sequences. PHB binds to SPL/NZZ to inhibit the activity of SPL/NZZ. miR165/6 and SPL/NZZ domains move from the two lateral regions of stamens to the four corners of anthers and then to microsporangia, while PHB domains migrate from the middle regions of thecae to the boundary between inner and outer microsporangia and then to the stomium regions. PHB facilitates the formation of the boundary, where it determines the formation of stomium through WUS but inhibits the differentiation of microsporocytes. SPL/NZZ promotes the initiation of microsporocytes but obstructs the formation of the boundary. BD, Boundary between inner and outer microsporangia; IM, inner microsporangia; OM, outer microsporangia; SR, stomium region.