Feeding the world: Uncovering a key regulator of flower head development in rice

Unfortunately, in these MS lines, the panicle (flower head) often remains enclosed in the surrounding leaf sheath because the region of the stem that supports it (the uppermost internode) is short, leading to blocked pollination and reduced seed production. To allow panicles to elongate and emerge from the leaf sheath, breeders use rice plants with a mutation in the gene ELONGATED UPPERMOST INTERNODE1 (EUI1), which encodes an enzyme that deactivates the plant hormone gibberellin (GA). This deactivation allows GA to stimulate uppermost internode extension, leading to panicle extension, as well as increased plant height. Understanding what regulates the enzyme EUI1 in normal (wild type) plants is crucial, as according to Dr. Chengcai Chu of the Chinese Academy of Sciences, "A further understanding of the molecular mechanism through which EUI1 activity is regulated during development will provide a more flexible way to fine-tune panicle extension, which may greatly help breeders improve hybrid rice seed production."

By isolating and exhaustively analyzing a dwarf mutant with impaired panicle extension, Dr. Chu and colleagues uncovered a critical regulator of EUI1 gene expression in rice, as discussed in this week's issue of The Plant Cell. This regulator, HOX12, binds directly to regulatory elements of the EUI1 gene, functioning as a transcription factor, or central regulator. The scientists propose that HOX12 helps regulate plant growth in response to environmental cues through its effect on EUI1, which acts as a switch to regulate the migration of floral-derived GA from the panicle to the stem. The next step will be to determine the upstream initiators of the HOX12-EUI1 regulatory cascade and the physiological conditions under which these modulations occur.