This study characterizes a brittle culm (bc88) mutant of rice (Oryza sativa L.) obtained by ethylene methylsulfonate (EMS)-induced mutagenesis of Wuyunjing 7. The bc88 mutant exhibits a diversity of pleiotropic phenotypes, including brittle culm at the whole-plant growth stages, withered leaf tips at the seedling stage, and 18-d delay in heading date at the mature stage. Genetic analysis indicates that the bc88 mutant is controlled by a single recessive nuclear gene. The mutated bc88 gene isolated by map-based cloning contains only one point mutation in the 5th exon relative to its wild-type BC88 (LOC_Os09g25490 and Os09g0422500), leading to an amino acid change from P to L in bc88 plants. Alignment of the putative protein sequence with its homologs indicates that the mutation is located in the conserved region of the sequence. Detection of the transcription level of BC88 in rice plants shows that the expression level of BC88 is higher in spikes and culms than in leaves, roots, and leaf sheaths. These contribute to understanding of the molecular mechanism of cellulose synthesis. The target gene BC88 can be a useful tool in molecular marker-assisted selection for rice culm trait breeding.
RAO YuChunYANG YaoLongXIN DeDongLI XiaoJingZHAI KaiEnMA BoJunPAN JianWeiQIAN QianZENG DaLi
Brassinosteroids （BRs） are important hormones that regulate plant development and physiology. Substantial progresses have been made in BR-related studies, and especially an increasing number of new genes involved in BR biosynthesis have been identified. Here, we characterize a BR-related rice mutant, small grain 4 （sg4）, obtained from callus culture of japonica cultivar Nipponbare. This mutant showed multiple phenotypes such as dark green, rugose erect leaves and small round grains. It was higher than the wild type, different from the majority of BR- and gibberellin-related mutants. Genetic analysis showed that the mutant phenotypes are controlled by a single recessive locus. The gene was fine-mapped to a 90.7-kb interval with 1,100 F2 recessive individuals by means of map-basedcloning. Totally 11 open reading frames were found in this interval, only one of which was detected with an 8-bp in- sertion in the 5rUTR region by sequencing. Functional complementation test revealed that a DWARFll allele, LOC_OsO4g39430, is answer for the mutant phenotype of sg4. Tissue-specific response to BR and decreased expression levels of BR biosynthetic genes suggest that sg4 is a weak BR-deficient mutant. These results are beneficial to understanding the physiological action of sg4 in a more comprehensive way.
Plant height and tillering are crucial factors determining rice plant architecture and influencing rice grain production. In this study, rnulti-tillering dwarf1 （mtdl）, a stable multi-tiller and dwarf mutant, was screened from the ethylmethane sulfonate-treated japonica rice variety Wuyunging7. Compared with the wild type, mtdl mutant exhibited pleiotropic phenotypes, including dwarf- ism, more tillers, brittle culms and delayed heading date. By employing map-based cloning strategy, the gene MTD1 was finally mapped to an approximately 66-kb region on the short arm of chromosome 9. Sequencing results showed that the gene LOCOsO9g02650 （BC12） in mtdl mutant had a single nucleotide substitution （G to A）, which gen- erated a premature translation stop. Over-expressing MTD1/BC12 coding sequ（nce rescued all the phenotypes of mtdl mutants including plant height and tillers, which confirms that BC12 is the mutated gene in mtdl mutant. Quantitative reverse tran,-eription-PCR analysis showed that MTDI/BCI2 could negatively regulate the expression of MONOCULM 1, IDEAL PLANT ARCHITECTURE1 and Tillering and Dwarf 1, and control rice tillering. Remark- ably, a-amylase activity analysis and gibberellic acid （GA） treatment showed that the dwarf phenotype of mtdl mutant was dependent on GA biosynthesis pathway. These results facilitated to further uncover the molecular mechanism of the growth and development in rice.
