MBD3 Regulates Male Germ Cell Division and Sperm Fertility in Arabidopsis thaliana

DNA methylation plays important roles through the methyl-CpG-binding domain (MBD) to realize epigenetic modifications. Thirteen AtMBD proteins have been identified from the Arabidopsis thaliana genome, but the functions of some members are unclear. AtMBD3 was found to be highly expressed in pollen and seeds and it preferably binds methylated CG, CHG, and unmethylated DNA sequences. Then, two mutant alleles at the AtMBD3 locus were obtained in order to further explore its function using CRISPR/Cas9. When compared with 92.17% mature pollen production in the wild type, significantly lower percentages of 84.31% and 78.91% were observed in the mbd3-1 and mbd3-2 mutants, respectively. About 16-21% of pollen from the mbd3 mutants suffered a collapse in reproductive transmission, whereas the other pollen was found to be normal. After pollination, about 16% and 24% of mbd3-1 and mbd3-2 mutant seeds underwent early or late abortion, respectively. Among all the late abortion seeds in mbd3-2 plants, 25% of the abnormal seeds were at the globular stage, 31.25% were at the transition stage, and 43.75% were at the heart stage. A transcriptome analysis of the seeds found 950 upregulated genes and 1128 downregulated genes between wild type and mbd3-2 mutants. Some transcriptional factors involved in embryo development were selected to be expressed, and we found significant differences between wild type and mbd3 mutants, such as WOXs, CUC1, AIB4, and RGL3. Furthermore, we found a gene that is specifically expressed in pollen, named PBL6. PBL6 was found to directly interact with AtMBD3. Our results provide insights into the function of AtMBD3 in plants, especially in sperm fertility.

The methyl-CpG-binding domain family member PEM1 is essential for Ubisch body formation and pollen exine development in rice

Pollen exine is composed of finely-organized nexine, bacula and tectum, and is crucial for pollen viability and function. Pollen exine development involves a complicated molecular network that coordinates the interaction between pollen and tapetal cells, as well as the biosynthesis, transport and assembly of sporopollenin precursors; however, our understanding of this network is very limited. Here, we report the roles of PEM1, a member of methyl-CpG-binding domain family, in rice pollen development. PEM1 expressed constitutively and, in anthers, its expression was detectable in tapetal cells and pollen. This predicted PEM1 protein of 240 kDa had multiple epigenetic-related domains. pem1 mutants exhibited abnormal Ubisch bodies, delayed exine occurrence and, finally, defective exine, including invisible bacula, amorphous and thickened nexine and tectum layer structures, and also had the phenotype of increased anther cuticle. The mutation in PEM1 did not affect the timely degradation of tapetum. Lipidomics revealed much higher wax and cutin contents in mutant anthers than in wild-type. Accordingly, this mutation up-regulated the expression of a set of genes implicated in transcriptional repression, signaling and diverse metabolic pathways. These results indicate that PEM1 mediates Ubisch body formation and pollen exine development mainly by negatively modulating the expression of genes. Thus, the PEM1-mediated molecular network represents a route for insights into mechanisms underlying pollen development. PEM1 may be a master regulator of pollen exine development.