NUCLEAR RNA POLYMERASE D1 is essential for tomato embryogenesis and desiccation tolerance in seeds

Plant-unique RNA polymerase IV (RNA Pol IV) governs the establishment of small RNA (sRNA)-directed DNA methylation (RdDM). In dicotyledon, elevated RdDM activity is often associated with embryogenesis; however, the loss of RdDM frequently results in indiscernible phenotypes. Here, we report that the absence of SlNRPD1, encoding the largest subunit of RNA Pol IV, leads to diminished RdDM and abnormal embryogenesis in tomato (Solanum lycopersicum). Hypermethylation at pericentric transposable elements (TEs) and a burst of 21/22-nt siRNA from the distal and pericentric genes are observed in slnrpd1 embryos. The specific activation of endoribonuclease Dicer-like 2 (SlDCL2b/c/d) is correlated with 21/22-nt sRNA accumulation. Auxin and WUSCHEL-related homeobox (WOX) signaling gene expression is altered by mCHH hypomethylation, which may lead to defective embryos. Due to improper maturation, the slnrpd1 embryos cannot withstand post-harvest desiccation. This study provides insights into how DNA methylation regulates plant embryogenesis.