Fig. 3.

Speculative interconnection of mobile signals during anther development. a, b 21nt phasiRNAs are generated in the epidermis early in lobe differentiation. Consecutively, they will migrate toward the lobe center, load onto AGO18, and presumably degrade mRNAs for, e.g., a RLKs or b transcription factors in the emerging cell layers. c, d 24nt phasiRNAs emerge later, when all cell layers are formed, and derive from the tapetum. Both 21nt and 24nt phasiRNAs accumulate in PMCs and the surrounding tapetum. In the tapetum, they might be involved in timely initiation of PCD via gene silencing, by, e.g., DNA methylation of the HXK promoter, and via degradation of RNA for, e.g., RLKs involved in PCD initiation. Once phasiRNAs decrease, PCD could then occur. In PMCs, phasiRNAs seem to mediate DNA methylation, and thus/or independently promote early meiosis initiation with DSBs. DSB: double strand break, AR: archesporial, EN: endothecium, SP: secondary parietal, ML: middle layer, TP: tapetum, PMCs: pollen mother cells; ROS: reactive oxygen species, PCD: programmed cell death, HXK: HEXOKINASE