A novel embryo phenotype associated with interspecific hybrid weakness in rice is controlled by the MADS-domain transcription factor OsMADS8

Authors: KIM, SUN HA - Chungnam National University; JI, SHI-DONG - Chungnam National University; LEE, HYUN-SOOK - Chungnam National University; JEON, YUN-A - Chungnam National University; SHIM, KYU-CHAN - Chungnam National University; ADEVA, CHERYL - Chungnam National University; LUONG, NGOC HA - Chungnam National University; YUAN, PINGRONG - Chungnam National University; KIM, HYUNGJUNG - University Of California, Davis; Tai, Thomas; AHN, SANG-NAG - Chungnam National University

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2021
Publication Date: 1/5/2022
Citation: Kim, S., Ji, S., Lee, H., Jeon, Y., Shim, K., Adeva, C., Luong, N., Yuan, P., Kim, H., Tai, T., Ahn, S. 2022. A novel embryo phenotype associated with interspecific hybrid weakness in rice is controlled by the MADS-domain transcription factor OsMADS8. Frontiers in Plant Science. 12. Article 778008. https://doi.org/10.3389/fpls.2021.778008.
DOI: https://doi.org/10.3389/fpls.2021.778008

Interpretive Summary: Hybrid weakness, also known as hybrid breakdown, is a form of hybrid incompatibility that occurs during the postembryonic stage of plant development in the second (F2) generation or later of interspecific or intraspecific crosses. In rice, hybrid weakness is exemplified by various developmental problems including dwarfing, yellowing, stunted growth, and poor fertility. In this study, a novel trait, dark tip embryo (DTE), was identified in a line derived from an interspecific cross between cultivated Asian rice (Oryza sativa subspecies japonica) and a wild relative Oryza rufipogon. Using classical genetics, positional cloning, and transgenic plants (overexpression and knockdown lines), one of the genes responsible for the DTE trait and other hybrid weakness characteristics including reduced fertility, grain weight, and yield as well as altered floral organs was identified. This gene is OsMADS8, a member of the MADS-box transcription factor gene family, which regulates expression of floral development genes and the content of the plant hormone abscisic acid (ABA) in seeds. The DTE trait appears to be related to altered ABA content in the seed. These findings provide a link between plant hormones and hybrid weakness and contribute to the knowledge base for this phenomenon. A better understanding of hybrid weakness is important for unlocking the full potential of genes introduced through both inter and intraspecific crosses.

Technical Abstract: A novel hybrid weakness gene, DTE9, associated with a dark tip embryo (DTE) trait, was observed in CR6078, an introgression line derived from a cross between the Oryza sativa spp. japonica ‘Hwayeong’ (HY) and the wild relative Oryza rufipogon. CR6078 seeds exhibit protruding embryos and flowers have altered inner floral organs. DTE9 was also associated with several hybrid weakness symptoms including decreased grain weight. Map-based cloning and transgenic approaches revealed that DTE9 is and allele of OsMADS8, a MADS-domain transcription factor. Genetic analysis indicated that two recessive complementary genes responsible for expression of the DTE trait. No sequence differences were observed between the two parental lines in the OsMADS8 coding region; however, numerous single nucleotide polymorphisms were detected in the promoter and intronic regions. We generated overexpression (OX) and RNA interference (RNAi) transgenic lines of OsMADS8 in HY and CR6078, respectively. The OsMADS8-OX lines showed the dark tip embryo phenotype, whereas OsMADS8-RNAi recovered the normal embryo phenotype. Changes in gene expression, including of ABCDE floral homeotic genes, were observed in the OsMADS8-OX and OsMADS8-RNAi lines. Overexpression of OsMADS8 downregulated the expression of the OsEMF2b and ABA signaling-related genes including OsVP1/ABI3. HY seeds showed higher ABA content than CR6078 seeds, consistent with OsMADS8/DTE9 regulating the expression of genes related ABA catabolism in CR6078. Our results suggest that OsMADS8 is critical for floral organ determination and seed germination and that these effects are the result of regulation of the expression of OsEMF2b and its role in ABA signaling and catabolism.