Dynamics and functions of small interfering RNAs in germline cells

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Small interfering RNAs (siRNAs) play essential roles in regulating germline functions. In flowering plants, 21-22 nt siRNAs have been implicated in the movement from sperm companion cells (called vegetative cells) to silence sperm transposons. However, the nature and silencing mechanism of the non-cell autonomous signal have remained controversial. Recently, my host lab discovered that 24-nt siRNAs produced by meiocyte nurse cells (called tapetum) drive DNA methylation reprogramming in the Arabidopsis male germline, thereby regulating genes and suppressing transposons. These tapetum siRNAs are sufficient to shape the entire de novo methylome in sperm, however, it is unclear how this is achieved given the absence of cytoplasmic connections between the tapetum and sperm.My preliminary work indicates that there are two waves of siRNA transport during germline development: the first wave from tapetal cells to meiocytes, and the second from vegetative cells to sperm. 21-22and 24 nt siRNAs are both suggested to be transported, playing important roles in post-transcriptional and transcriptional gene silencing, respectively, via interacting with cell-specific Argonaute proteins. Using a combination of cell biology, molecular genetics, single-cell-type genomics, epigenetics and biochemistry approaches, I will test these innovative hypotheses and elucidate the underlying molecular mechanisms by investigating: Obj.1) how siRNA biogenesis and transport mediate DNA methylation in the male germline, Obj.2) the origin and function of 21-22 nt siRNAs during male germline development, and Obj.3) the central role of cell-specific Argonauts in germline siRNA pathways. My investigations will open a new horizon in our understanding of plant germlines and heredity by revealing the intricate dynamics of siRNA biogenesis, transport, and silencing functions in the germ cells. This in-depth knowledge will also allow cell- and site-specific engineering of methylation for crop improvement.