Annual Report 2003

AHd3a / RUBQ25.04.03.02.01.0(x 10-3)0Nipponbare (53)NIL(Hd1) (69)NIL(Hd3a) (48)(SD) condition. High-resolution linkagemapping located the Hd3a locus in a ~20-kbgenomic region. In this region, we found acandidate gene that shows high similarity tothe FLOWERING LOCUS T (FT) gene,which promotes flowering in Arabidopsisunder long day (LD) condition. Introductionof the candidate gene caused an early-headingphenotype in rice. Based on theseresults, it was concluded that Hd3a encodesa protein with high similarity to the FT inArabidopsis.Hd3a transcripts were detected early andgradually increased with time under SD condition(Fig. 1). The expression level of Hd3ain NIL(Hd3a), in which chromosomal regionof Hd3a was substituted with Kasalath inthe genetic background of Nipponbare, washigher than that in Nipponbare under SDcondition. NIL(Hd3a) showed earlier headingthan did Nipponbare under SD condition.This result indicated that the higher expressionlevels of Kasalath Hd3a allele resultedin promotion of heading under SD condition(Fig. 1). To investigate whether Hd1 regulatesHd3a, we also quantified the expressionlevels of Hd3a in an NIL for Hd1[NIL(Hd1)]. The Hd1 has been identified asan ortholog of the Arabidopsis CONSTANS(CO), which is involved in promotion of floweringand up-regulates expression of FTunder LD condition. In NIL(Hd1), theNipponbare functional allele was replacedwith a Kasalath loss of function allele.8 18 28 38 48 61 68Days after sowingFig. 1RT-PCR assay for the Hd3a expression in different genotypes (A) andgenetic control pathway for the photoperiodic response in rice andArabidospsis (B). Abundance of Hd3a mRNA was measured using plantsgrown under SD (10-hr light) condition. Plants were sampled at one hourafter lights on. The vertical bar on each plot represents standard deviation.Days to heading in each line is given in parentheses.BRiceSD conditionHd1Hd3aHeadingArabidopsisLD conditionCOFTFloweringNIL(Hd1) showed later heading than didNipponbare under SD condition. The expressionlevels of Hd3a were reduced inNIL(Hd1), indicating that the functionalallele of Hd1 up-regulates the expression ofHd3a (Fig. 1). The results obtained in thisstudy suggest that the function of Hd3a andFT and the regulation of their expression byHd1 and CO, respectively, are conservedbetween rice (an SD plant) and Arabidopsis(an LD plant). However, this study alsodemonstrated a difference in the expressionprofiles of the key flowering time genes Hd3aand FT in response to day-length betweenrice and Arabidopsis. This raises a simplequestion: what kind of gene(s) or mechanism(s)is involved in generating the completelyopposite reaction to the photoperiodbetween rice and Arabidopsis? Further comparativestudies between Arabidopsis andrice will be required to clarify conservedand/or diverse features in flowering, animportant and complex developmental systemin plant.2) Three distinct rice cellulose synthasecatalytic subunit (CesA) genes arerequired for cellulose synthesis in thesecondary wallCellulose, a homopolymer of -1,4-glucan,is the most abundant and economicallyimportant biopolymer. Cellulose is alsoimportant, because it controls plant morphologyby determining cell shape and thus tissuemorphology. However, there are many importantquestions remaining to be answeredabout the biosyntheis of cellulose. Progressin genomic sequencing has revealed the existenceof multiple isoforms of cellulose synthase(CESA) , at least ten in rice, raising theimportant question of why plants have manygenes for cellulose synthesis. One explanationis that two or more distinct CESA proteinsare required to form a functional CESAunit. To address this issue, mutants ofCESA genes of rice were analyzed.Brittle culm mutations causing fragility ofplant tissues have been identified geneticallyin several monocot species, and some of the96 Annual Report 2003