Rice LEAFY COTYLEDONE1 regulates embryonic envelope development and chlorophyll biogenesis in embryo

The scutellum, coleoptile, coleorhiza, and epiblast (if it exists) consist of a complex embryonic envelope to protect the plumule and radicle inside a grass embryo. Controversies have been provoked for centuries regarding homologies of the grass embryonic structures. Here we found that the rice LEAFY COTYLEDONE1 (LEC1) gene, OsNF-YB7, is vital for embryo development. A leaf-like structure (LL) was developed from the scutellum of osnf-yb7 to replace the embryonic envelope that formed in wild-type. Additionally, osnf-yb7 developed chloroembryos due to overactivated chlorophyll biosynthesis. Thus, OsNF-YB7 likely plays a dual role in chlorophyll biogenesis in rice embryos: (1) OsNF-YB7 directly represses genes, such as rice GOLDED-LIKE1 (OsGLK1), involving chlorophyll biosynthesis; (2) OsNF-YB7 binds to OsGLK1 to repress the downstream genes of OsGLK1. Parallel phenotypes shown in osnf-yb7 and lec1 suggest functional conservation of the LEC1-type genes in plants. Both lec1 cotyledons and osnf-yb7 LL displayed true leaf characteristics. Our morphological and transcriptional evidence implied that LL replaces the embryonic envelope in osnf-yb7, raising the hypothesis that the grass embryonic envelope is an analog of Arabidopsis cotyledon. This study demonstrates that OsNF-YB7 acts as a negative regulator in chlorophyll biogenesis and is important for embryonic envelope formation.

Chaperone-like protein DAY plays critical roles in photomorphogenesis

Photomorphogenesis, light-mediated development, is an essential feature of all terrestrial plants. While chloroplast development and brassinosteroid (BR) signaling are known players in photomorphogenesis, proteins that regulate both pathways have yet to be identified. Here we report that DE-ETIOLATION IN THE DARKANDYELLOWING IN THE LIGHT (DAY), a membrane protein containing DnaJ-like domain, plays a dual-role in photomorphogenesis by stabilizing the BR receptor, BRI1, as well as a key enzyme in chlorophyll biosynthesis, POR. DAY localizes to both the endomembrane and chloroplasts via its first transmembrane domain and chloroplast transit peptide, respectively, and interacts with BRI1 and POR in their respective subcellular compartments. Using genetic analysis, we show that DAY acts independently on BR signaling and chlorophyll biogenesis. Collectively, this work uncovers DAY as a factor that simultaneously regulates BR signaling and chloroplast development, revealing a key regulator of photomorphogenesis that acts across cell compartments.

Map-Based Cloning and Functional Analysis of YE1 in Rice, Which Is Involved in Light-Dependent Chlorophyll Biogenesis and Photoperiodic Flowering Pathway

Light is one of the most important environmental factors that affect many aspects of plant growth, including chlorophyll (Chl) synthesis and flowering time. Here, we identified a rice mutant, yellow leaf and early flowering (ye1), and characterized the gene YE1 by using a map-based cloning method. YE1 encodes a heme oxygenase, which is localized to the chloroplasts. YE1 is expressed in various green tissues, especially in leaves, with a diurnal-rhythmic expression pattern, and its transcripts is also induced by light during leaf-greening. The mutant displays decreased Chl contents with less and disorderly thylakoid lamellar layers in chloroplasts, which reduced the photosynthesis rate. The early flowering phenotype of ye1 was not photoperiod-sensitive. Furthermore, the expression levels of Chl biosynthetic genes were downregulated in ye1 seedlings during de-etiolation responses to light. We also found that rhythmic expression patterns of genes involved in photoperiodic flowering were altered in the mutant. Based on these results, we infer that YE1 plays an important role in light-dependent Chl biogenesis as well as photoperiodic flowering pathway in rice.