Rice has diverse SEP-like genes, with at least five members in the genome 24. More interestingly, another C-function gene, DROOPING LEAF ( DL), orthologous to Arabidopsis gene, CRABS CLAW ( CRC), encoding a YABBY domain protein in rice has been identified, playing roles in carpel specification, floral meristem determinacy and the antagonistic function with class B genes 23. OsMADS13 and OsMADS21 are grouped as D class genes on the basis of their expression pattern and functional analyses 22. Two class C genes OsMADS3 and OsMADS58 in rice have been shown to play distinct roles in specifying the identity of lodicules, stamens and carpels 21. spw1 mutants display homeotic conversion of stamens to carpels and lodicules to palea/lemma-like structures 20. In rice, the B class gene SUPERWOMEN1 ( SPW1 or OsMADS16) that is orthologous to AP3 is crucial for stamen and lodicule specification. Despite the economic importance of grass flowers in producing grains, the underlying mechanism of grass floral organ specification still remains poorly understood 18, 19. Each flower contains characteristic floral organs of the lemma, the palea and lodicules, as well as stamens and pistil(s) 19. Each grass spikelet consists of glumes and one to several flowers. Evolutionary adoptions in organization and structure of grass flowers resulted in their unique shape, which is apparently distinct from those of higher eudicots and even other monocots 15, 16, 17, 18. Grass (Poaceae) is one of the largest flowering plant families of angiosperms with ∼10 000 species, including many important crops such as rice ( Oryza sativa), barley ( Hordeum vulgare) and maize ( Zea mays) 12, 13, 14. Investigations in eudicot Arabidopsis and petunia demonstrated that SEP genes may redundantly function as key regulators that either control the mRNA expression of other floral homeotic genes 8 or interact with these floral homeotic regulators to specify the identity of each floral whorl and regulate floral meristem determinacy 9, 10, 11. In Antirrhinum, the orthologs of AP3, PI and AG are DEF, GLO and PLE, respectively 7. D specifies the ovule 2, while E class genes ( SEPALLATA1/2/3/4, SEP1/2/3/4 formerly AGL2/4/9/3) determine the identity of all four whorls of floral organs and regulate floral meristem determinacy 3, 4, 5, 6. Later, two additional classes of genes (D and E) are added in the ABC model. In Arabidopsis, A (APETALA1, AP1 APETALA2, AP2) alone determines sepals, A and B (APETALA3, AP3 PISTILLATA, PI) together specify petals, B and C (AGAMOUS, AG) specify stamens and C alone determines the carpel 1. Studies in two model eudicot plants Arabidopsis thaliana and Antirrhinum majus have led to the classic genetic ABC model that explains how three classes of genes (A, B and C) work together to specify floral organ identity 1. Plant flower morphological formation is closely associated with changes in the number, expression pattern and interaction of MADS-box genes. Our finding, therefore, suggests that the ancient OsMADS6 gene is able to specify “floral state” by determining floral organ and meristem identities in monocot crop rice together with OsMADS1. Furthermore, the osmads1-z osmads6-1 double mutants developed severely indeterminate floral meristems.
Strikingly, mutation of a SEPALLATA ( SEP)-like gene, OsMADS1 ( LHS1), enhanced the defect of osmads6 flowers, and no inner floral organs or glume-like structures were observed in whorls 2 and 3 of osmads1-z osmads6-1 flowers. Compared to wild type plants, osmads6 mutants displayed altered palea identity, extra glume-like or mosaic organs, abnormal carpel development and loss of floral meristem determinacy. Subsequently, OsMADS6 transcripts were mainly detectable in paleas, lodicules, carpels and the integument of ovule, as well as in the receptacle. OsMADS6 was strongly expressed in the floral meristem at early stages. Here, we show the biological role of the AGL6-like gene, OsMADS6, in specifying floral organ and meristem identities in rice ( Oryza sativa L.). Although AGAMOUS-LIKE6 ( AGL6) MADS-box genes are ancient with wide distributions in gymnosperms and angiosperms, their functions remain poorly understood.
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