MicroRNA166 Monitors SPOROCYTELESS/NOZZLE for Building of the Anther Internal Boundary

Abstract

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 PHABULOSA (PHB) regulate SPOROCYTELESS/NOZZLE (SPL/NZZ), which controls microsporogenesis. In developing anthers of Arabidopsis (Arabidopsis thaliana), the expression domains of miR165/6 and SPL/NZZ are overlapped and rearranged synchronously. Dominant mutation of PHB suppresses SPL/NZZ expression on the adaxial sides of stamens, resulting in a thickened boundary, whereas activation of MIR166g up-regulates SPL/NZZ expression, leading to ectopic microsporogenesis in the boundary. PHB limits the expression domains of SPL/NZZ to facilitate construction of the boundary, while miR166 preserves the expression domains of SPL/NZZ by inhibiting PHB to allow the inner microsporangia to take shape. Subsequently, PHB activates the key stem cell maintainer WUSCHEL 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.

Irregular adaxial–abaxial polarity rearrangement contributes to the monosymmetric-to-asymmetric transformation of Canna indica stamen

“…AtPHB and AtFIL , which are the homologues of CiPHB1 and CiFIL , were used as marker genes. As the expression patterns of these two genes have been reported before ( Siegfried et al 1999 ; Li et al 2019 ), only some typical data is shown here. In the young stamen of arabidopsis, when the four-microsporangia pattern was established, AtPHB expression in the anther was restricted in the stomium regions between the inner and outer microsporangia ( Fig.…”

“…In contrast to flat organs such as leaves, the stamen development process undergoes substantial morphological changes. Correspondingly, the expression patterns of adaxial–abaxial polarity marker genes also change dynamically in this process ( Sessions et al 1997 ; Toriba et al 2010 ; Li et al 2019 ). In the spherical stamen primordium of rice, the adaxial marker gene is initially expressed in the domain adjacent to the meristem, and the abaxial marker is expressed on the opposite side, as in the primordia of other lateral organs.…”

“…In general, stamens are monosymmetric with the adaxial–abaxial axis as the axis of symmetry. However, asymmetrically patterned stamens exist in some mutants ( Toriba et al 2010 ; Li et al 2019 ). In the rod-like lemma ( rol ) mutant of rice, three types of aberrant stamen develop.…”

“…In our previous research, ectopic expression of PHB was shown to inhibit the development of microsporangium in the stamen of arabidopsis, whereas PHB normally promotes the formation of the internal boundary between inner and outer microsporangia within the anthers ( Li et al 2019 ). We have undertaken a preliminary analysis of the adaxial–abaxial polarity in the asymmetric stamen of C. indica ( Tian et al 2018 ), but the detailed structural patterning process of the asymmetric stamen which, significantly, controls the symmetry of the whole flower, is not well understood.…”

The roles of rice microRNAs in rice-Magnaporthe oryzae interaction

“…The miR166 family has 13 members in the rice genome, forming six mature isoforms ( Table 1). The function of miR166 has been reported in plant development, targeting transcription factor genes of the class III homeodomain-leucine zipper family, such as PHABU-LOSA and PHABOLUTA, to specify the fate of the shoot apical meristem (Zhu et al 2011;Li et al 2019b). Its function in plant immunity has been reported only in rice.…”