Most agriculturally important traits are regulated by genes known as quantitative trait loci (QTLs) derived from natural allelic variations. We here show that a QTL that increases grain productivity in rice, Gn1a, is a gene for cytokinin oxidase/dehydrogenase (OsCKX2), an enzyme that degrades the phytohormone cytokinin. Reduced expression of OsCKX2 causes cytokinin accumulation in inflorescence meristems and increases the number of reproductive organs, resulting in enhanced grain yield. QTL pyramiding to combine loci for grain number and plant height in the same genetic background generated lines exhibiting both beneficial traits. These results provide a strategy for tailormade crop improvement.

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...