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. 2023 Jan 18;35(6):1787–1800. doi: 10.1093/plcell/koad010

Figure 1.

Figure 1

Male reproductive development, siRNA biogenesis, and proposed intercellular movement. A, Male germ line development. A microspore mother cell undergoes meiosis to generate four haploid microspores (shown in tetrad stage). Each microspore has an asymmetric mitotic division to create the vegetative cell and the generative cell. The generative cell later divides to create the two sperm cells. Together, the vegetative cell and sperm cells form the male gametophyte or mature pollen grain. Microspore mother cells, microspores, and immature pollen are surrounded by a layer of sporophytic cells called the tapetum or tapetal nurse cells. B, The Arabidopsis tapetum produces 24-nt nurse cell siRNAs through the canonical RdDM pathway. Pol IV and CLSY3 produce RNA precursors which are converted into dsRNA by RDR2 before processing by DCL3 to generate 24-nt nurse cell siRNAs. Nurse-cell siRNAs move into microspore mother cells, inducing DNA methylation with the aid of DRM1 and/or DRM2 (DRM). C, In the tapetum of many other angiosperms, reproductive phasiRNAs are produced from PHAS loci. Pol II transcribes PHAS loci to generate precursor transcripts, which are targeted by miRNAs for cleavage. The cleaved transcripts are converted into dsRNA by RDR6, and those targeted by miR2275 are further processed by DCL5 to generate 24-nt phasiRNAs. These phasiRNAs move intercellularly into germ cells, and might induce DNA methylation (left arrow) or post-transcriptional gene silencing through transcript cleavage (right arrow). D, Epigenetically-activated (ea)siRNAs are produced from reactivated transposons in the vegetative nucleus, where DME and ROS1 actively demethylate transposons and some protein coding genes. The demethylated loci are transcribed by either Pol II or Pol IV and the resulting RNAs are cleaved by miRNA/AGO1 complexes, triggering their conversion into dsRNA by RDR2 or/and RDR6. DCL2 and DCL4 then produce 22- and 21-nt easiRNAs from the dsRNA, respectively. These vegetative cell-derived easiRNAs can induce gene silencing in the sperm cell, either through transcriptional gene silencing (left arrow) or post-transcriptional gene silencing (right arrow).