Heading date gene, dth3 controlled late flowering in O. Glaberrima Steud. by down-regulating Ehd1

Heading date in rice is an important agronomic trait controlled by several genes. In this study, flowering time of variety Dianjingyou 1 (DJY1) was earlier than a near-isogenic line (named NIL) carried chromosome segment from African rice on chromosome 3S, when grown in both long-day (LD) and short-day (SD) conditions. By analyzing a large F2 population from NIL × DJY1, the locus DTH3 (QTL for days to heading on chromosome 3) controlling early heading date in DJY1 was fine mapped to a 64-kb segment which contained only one annotated gene, a MIKC-type MADS-box protein. We detected a 6-bp deletion and a single base substitution in the C-domain by sequencing DTH3 in DJY1 compared with dth3 in NIL, and overexpression of DTH3 caused early flowering in callus. Quantitative real-time PCR revealed that the transcript level of dth3 in NIL was lower than that DTH3 in DJY1 in both LD and SD conditions. The Early heading date 1 (Ehd1) which promotes the RFT1, was up-regulated by DTH3 in both LD and SD conditions. Based on Indel and dCAPs marker analysis, the dth3 allele was only present in African rice accessions. A phylogenetic analysis based on microsatellite genotyping suggested that African rice had a close genetic relationship to O. rufipogon and O. latifolia, and was similar to japonica cultivars. DTH3 affected flowering time and had no significant effect on the main agronomic traits.

Molecular analysis of an additional case of hybrid sterility in rice (Oryza sativa L.)

Hybrid sterility hinders the exploitation of the heterosis displayed by japonica × indica rice hybrids. The variation in pollen semi-sterility observed among hybrids between the japonica recipient cultivar and each of two sets of chromosome segment substitution lines involving introgression from an indica cultivar was due to a factor on chromosome 5 known to harbor the gene S24. S24 was fine mapped to a 42 kb segment by analyzing a large F(2) population bred from the cross S24-NIL × Asominori, while the semi-sterility shown by the F(1) hybrid was ascribable to mitotic failure at the early bicellular pollen stage. Interestingly, two other pollen sterility genes (f5-Du and Sb) map to the same region (Li et al. in Chin Sci Bull 51:675-680, 2006; Wang et al. in Theor Appl Genet 112:382-387, 2006), allowing a search for candidate genes in the 6.4 kb overlap between the three genes. By sequencing the overlapped fragment in wild rice, indica cultivars and japonica cultivars, a protein ankyrin-3 encoded by the ORF2 was identified as the molecular base for S24. A cultivar Dular was found to have a hybrid-sterility-neutral allele, S24-n, in which an insertion of 30 bp was confirmed. Thus, it was possible to add one more case of molecular bases for the hybrid sterility. No gamete abortion is caused on heterozygous maternal genotype with an impaired sequence from the hybrid-sterility-neutral genotype. This result will be useful in understanding of wide compatibility in rice breeding.