Functional divergence and characterization of the HOTHEAD gene family in rice

The HOTHEAD (HTH) gene family, belonging to the Glucose-Methanol-Choline oxidase superfamily, regulates long-chain fatty acid redox metabolism and influences plant male reproductive development. However, a systematic characterization of HTH genes in rice (Oryza sativa) remains unexplored. Here, we identified seven rice HTH genes (OsHTH1-OsHTH7) and classified them into two subfamilies via phylogenetic analysis. RT-PCR demonstrated anther-preferential expression of OsHTH1, OsHTH3, and OsHTH7, while subcellular localization indicated their distribution in the cytoplasm and endoplasmic reticulum. CRISPR-Cas9-mediated knockout experiments revealed distinct phenotypes: oshth1 mutants exhibited semi-sterility, oshth5 mutation caused seedling lethality, and oshth3 and oshth7 mutants showed complete male sterility. These findings demonstrate the functional divergence of OsHTH genes in anther development and lipid metabolism, laying a foundation for understanding their roles in rice reproduction.

Rice (Oryza sativa L.) is a globally important food crop, and its yield is crucial for food security. As in higher flowering plants, pollen development in rice occurs within the anthers and is critical for yield. Lipid metabolism plays a key role in this process The HOTHEAD (HTH) family of genes encodes a class of Glucose-Methanol-Choline (GMC) oxidoreductases. Substantial evidence suggests that, as oxidases, HTH enzymes are involved in dehydrogenating the ω-hydroxy group of fatty alcohols and play a key role in long-chain and very-long-chain fatty acid metabolism within anthers. However, a systematic study of the HTH gene family in rice is currently lacking. Here, we investigated the HTH genes in rice using bioinformatics, cytology, gene editing, and other approaches.

In this study, seven HTH genes (OsHTH1-HTH7) were identified in rice. Based on evolutionary relationships, they were classified into two subfamilies: Clade I (OsHTH1, OsHTH3, OsHTH5, OsHTH6) and Clade II (OsHTH2, OsHTH4, OsHTH7). RT-PCR analysis showed that most HTH genes were highly expressed in anthers, with OsHTH1, OsHTH3, and OsHTH7 exhibiting an anther-preferential expression pattern. Subcellular localization indicated that OsHTH1 showed a Cytoplasmic localization pattern, while OsHTH3, 6, 7 were localized in the Endoplasmic Reticulum. In addition, OsHTH4 had localization signals detected in both the Cytoplasm and the Endoplasmic Reticulum. Furthermore, these seven HTH genes were knocked out by the CRISPR-Cas9 technology and five Homozygous mutants were finally obtained. Phenotypic identification showed that oshth1 exhibited a semi-sterile phenotype, while oshth3 and oshth7 showed complete male sterility, and oshth2 and oshth4 did not show obvious phenotypic changes. This laid the foundation for further research on the HTH family of genes.

In summary, seven HTH genes were classified into two subfamilies. RT-PCR revealed that several genes (OsHTH1, OsHTH3, OsHTH7) exhibited anther-specific expression. Subcellular localization differed among the seven genes, but they were primarily localized to the endoplasmic reticulum (ER) and cytoplasm. Using CRISPR-Cas9 knockout, we obtained mutants for five genes. Notably, oshth1 mutation resulted in a semi-sterile phenotype, and oshth5 mutation caused seedling lethality, while oshth3 and oshth7 mutants were completely male sterile. The oshth2 and oshth4 mutants showed no obvious phenotypic alterations. These results lay the groundwork for further study of the HTH gene family.