A major quantitative trait locus (QTL) controlling response to photoperiod, Hd1 , was identified by means of a mapbased cloning strategy. High-resolution mapping using 1505 segregants enabled us to define a genomic region of ‫ف‬ 12 kb as a candidate for Hd1 . Further analysis revealed that the Hd1 QTL corresponds to a gene that is a homolog of CON-STANS in Arabidopsis. Sequencing analysis revealed a 43-bp deletion in the first exon of the photoperiod sensitivity 1 ( se1 ) mutant HS66 and a 433-bp insertion in the intron in mutant HS110. Se1 is allelic to the Hd1 QTL, as determined by analysis of two se1 mutants, HS66 and HS110. Genetic complementation analysis proved the function of the candidate gene. The amount of Hd1 mRNA was not greatly affected by a change in length of the photoperiod. We suggest that Hd1 functions in the promotion of heading under short-day conditions and in inhibition under long-day conditions. INTRODUCTIONThe transition of the apical meristem from vegetative to reproductive growth is a critical event in the life cycle of a plant. In rice, the timing of this transition affects the timing of heading. This timing, or heading date, is one of the critical traits considered for adapting rice to different cultivation areas and cropping seasons. Rice is a short-day (SD) plant; its heading is promoted by short photoperiods. The response of the plant to length of day (referred to as photoperiod sensitivity [PS]) and its basic vegetative growth determine the heading date of rice. Many genetic studies of heading date have been performed, and several genes controlling PS in rice have been genetically identified, including Se1 ( Lm ), Se3 to Se7 , and E1 to E3 (Yokoo et al., 1980;Yamagata et al., 1986;Poonyarit et al., 1989;Sano, 1992;Yokoo and Okuno, 1993;Tsai, 1995; Kinoshita, 1998). However, only one gene involving photoperiod response in rice has been cloned, Se5 (Izawa et al., 2000). Little is known about the structure and function of PS genes in rice at the molecular level. In contrast, several genes involved in flowering time have been isolated, allowing clarification of part of the genetic control pathway in Arabidopsis (reviewed by Levy and Dean, 1998; Fowler et al., 1999; Kobayashi et al., 1999;Sheldon et al., 1999). Identification of the genes involved in flowering time has made it possible to determine the genetic control pathways for the response to photoperiod and vernalization in Arabidopsis (reviewed by Levy and Dean, 1998;Samach and Coupland, 2000). In addition, homologs of Arabidopsis genes for flowering time also function in Brassica napus (Robert et al., 1998).The major genes or quantitative trait loci (QTLs) for heading date have been mapped by using DNA markers in rice (Mackill et al., 1993; Li et al., 1995;Xiao et al., 1996;Lin et al., 1998;Tamura et al., 1998). Four QTLs for heading date involved in PS were mapped precisely as single Mendelian factors by using advanced backcross progeny (Yamamoto et al., , 2000Lin et al., 2000). A major gene controlling PS, Se1 , first was ide...

Heading date 3a (Hd3a) has been detected as a heading-date-related quantitative trait locus in a cross between rice cultivars Nipponbare and Kasalath. A previous study revealed that the Kasalath allele of Hd3a promotes heading under short-day (SD) conditions. High-resolution linkage mapping located the Hd3a locus in a approximately 20-kb genomic region. In this region, we found a candidate gene that shows high similarity to the FLOWERING LOCUS T (FT) gene, which promotes flowering in Arabidopsis: Introduction of the gene caused an early-heading phenotype in rice. The transcript levels of Hd3a were increased under SD conditions. The rice Heading date 1 (Hd1) gene, a homolog of CONSTANS (CO), has been shown to promote heading under SD conditions. By expression analysis, we showed that the amount of Hd3a mRNA is up-regulated by Hd1 under SD conditions, suggesting that Hd3a promotes heading under the control of Hd1. These results indicate that Hd3a encodes a protein closely related to Arabidopsis FT and that the function and regulatory relationship with Hd1 and CO, respectively, of Hd3a and FT are conserved between rice (an SD plant) and Arabidopsis (a long-day plant).

A major quantitative trait locus (QTL) controlling response to photoperiod, Hd1 , was identified by means of a mapbased cloning strategy. High-resolution mapping using 1505 segregants enabled us to define a genomic region of ‫ف‬ 12 kb as a candidate for Hd1. Further analysis revealed that the Hd1 QTL corresponds to a gene that is a homolog of CON-STANS in Arabidopsis. Sequencing analysis revealed a 43-bp deletion in the first exon of the photoperiod sensitivity 1 (se1) mutant HS66 and a 433-bp insertion in the intron in mutant HS110. Se1 is allelic to the Hd1 QTL, as determined by analysis of two se1 mutants, HS66 and HS110. Genetic complementation analysis proved the function of the candidate gene. The amount of Hd1 mRNA was not greatly affected by a change in length of the photoperiod. We suggest that Hd1 functions in the promotion of heading under short-day conditions and in inhibition under long-day conditions.

A rice semidwarfing gene, sd-1, known as the "green revolution gene," was isolated by positional cloning and revealed to encode gibberellin 20-oxidase, the key enzyme in the gibberellin biosynthesis pathway. Analysis of 3477 segregants using several PCR-based marker technologies, including cleaved amplified polymorphic sequence, derived-CAPS, and single nucleotide polymorphisms revealed 1 ORF in a 6-kb candidate interval. Normal-type rice cultivars have an identical sequence in this region, consisting of 3 exons (558, 318, and 291 bp) and 2 introns (105 and 1471 bp). Dee-Geo-Woo-Gen-type sd-1 mutants have a 383-bp deletion from the genome (278-bp deletion from the expressed sequence), from the middle of exon 1 to upstream of exon 2, including a 105-bp intron, resulting in a frame-shift that produces a termination codon after the deletion site. The radiation-induced sd-1 mutant Calrose 76 has a 1-bp substitution in exon 2, causing an amino acid substitution (Leu [CTC] to Phe [TTC]). Expression analysis suggests the existence of at least one more locus of gibberellin 20-oxidase which may prevent severe dwarfism from developing in sd-1 mutants.

Summary Rice blast, caused by the fungal pathogen Magnaporthe grisea, is one of the most serious diseases of rice. Here we describe the isolation and characterization of Pib, one of the rice blast resistance genes. The Pib gene was isolated by a map‐based cloning strategy. The deduced amino acid sequence of the Pib gene product contains a nucleotide binding site (NBS) and leucine‐rich repeats (LRRs); thus, Pib is a member of the NBS‐LRR class of plant disease resistance genes. Interestingly, a duplication of the kinase 1a, 2 and 3a motifs of the NBS region was found in the N‐terminal half of the Pib protein. In addition, eight cysteine residues are clustered in the middle of the LRRs, a feature which has not been reported for other R genes. Pib gene expression was induced upon altered environmental conditions, such as altered temperatures and darkness.